Effect of thermal and non-thermal techniques for microbial safety in food powder: Recent advances

Food Research International 126 (2019) 108654

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Review

Effect of thermal and non-thermal techniques for microbial safety in food powder: Recent advances

T

E.J. Rifnaa, Sushil Kumar Singha, Snehasis Chakrabortyb, Madhuresh Dwivedia,

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a b

Department of Food Process Engineering, National Institute of Technology Rourkela, Rourkela, Odisha, India Department of Food Engineering and Technology, ICT, Mumbai, India

ARTICLE INFO

ABSTRACT

Keywords: Food powder Microbial inactivation Log-reduction Nutrient retention Non-thermal plasma Pulsed light Instant control pressure drop

Food powders are appreciated worldwide, as it enables food to be preserved for an extended period without significant loss of quality, even under the ambient storage condition. However, it is evidenced that the development of resistant microbial spore and viable microbial cells is a matter of concern even in low moisture foods like food powders. For microbial inactivation, the strategy generally applied is the implication of conventional preservation methods, such as heat treatment which is greatly accompanied by degradation of nutritional organoleptic properties. To overcome the shortcomings of conventional thermal processing, a set of advanced or emerging technologies are being developed which can inactivate the microbial spores and viable microbial cells capable of surviving with maximum retention in the nutritional or organoleptic profile. The examples include infrared heating, microwave heating, radiofrequency heating, instant control pressure drop technology, highpressure processing, pulsed electric field, pulsed light, ozone processing, and cold plasma. In this review, the potential of different advanced thermal and non-thermal technologies towards the inactivation of spores and viable cells of microorganisms in food powders has been highlighted precisely along with their mechanism of action. The summary of the literature encompassing the use of different processing techniques will help the readers to understand the underlying mechanism of microbial inactivation associated with each processing techniques applied to powders. Eventually, this information will help them to select the suitable technique (individual or in combination with another counterpart) to inactivate spores and viable cells in a specific food powder.

1. Introduction The advancement of formulation engineering notion in food processing and the demand for diversity in food products have driven a considerable market boost for food ingredients. Majority of ingredients are produced in powdered form, and therefore powder technology is a progressively more important issue both to food producers and food ingredient manufacturers (Hickey & Giovagnoli, 2018). There are several reasons for this, such as low bulk weight, easiness to transport and store food powders, usage conveniences and diverse applications. Furthermore, the major reason for food powders to represent the largest proportion of the total processed food is their low moisture content, thus, reducing the rate of quality degradation. As per the definition of Food and Agricultural Organization (FAO), low moisture foods or low water activity (aw) foods are the products with very low aw of equal to or less than 0.7 (Blessington, Theofel, & Harris, 2013). Food powders prepared by mechanical drying or dehydration process hold a final aw

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falling in the range ≤0.7, making it a low-moisture food. In India, a wide variety of solid or liquid food products are dehydrated or mechanically converted to powder form in regards to its long term stability and usability ranging from spice powder to dairy powder, infant formula powder, powdered egg, tea and coffee powder, fruit and vegetable powder, rice flour related powder and culinary powder. The minimum aw necessary for the growth of the majority of bacteria, yeast and mould are 0.9, 0.85 and 0.65, respectively (Hayman & Podolak, 2017). Similarly, the physiological activity required for microbial cell division to occur was observed to cease at aw of 0.6 or less (Young et al., 2015). Therefore, food in powder forms was presumed to be microbial safe owing to it's low aw. Later, it was understood that most food powders were not directly consumed by humans and are usually mixed with water and other liquids to produce wet formulations, which were directly consumed. It was studied by a number of researchers that rehydration of powder foods or ingredients containing microorganisms aided spores to undertake injury repair making final

Corresponding author. E-mail address: [email protected] (M. Dwivedi).

https://doi.org/10.1016/j.foodres.2019.108654 Received 21 June 2019; Received in revised form 29 August 2019; Accepted 31 August 2019 Available online 03 September 2019 0963-9969/ © 2019 Elsevier Ltd. All rights reserved.

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food unsafe for consumption. Recently, few deadly outbreaks of foodborne microorganisms appeared regards with food powders; Enterococcus in dairy powder (Pal et al., 2016), Cronobacter sakazakii in powdered infant formula (Heperkan, Dalkilic-Kaya, & Juneja, 2017), severe microorganism in vegetable powder (Wang et al., 2015), Bacillus aureus in rice powder and spice powder resulting in number of clinical cases in infants, young children and adults establishing safety issue of food powders to be a subject of major concern. It has been noticed that the missing link for these outbreaks was the survivability of food-borne pathogens: Their growth can be retarded, but they still survive for a long period once the food gets contaminated. Since a multitude of food powders are used as ready to eat, or by recombination or reconstitution (Oliveira et al., 2000) without additional heating, outbreaks in food powders stand to hack the reputation of food powders as safe. Several sterilization techniques are already used by the food industry, aimed at microbial cell inactivation. The most commonly used technology for denaturation of microbial DNA and cell destruction in both solid and liquid food products is the use of high heat at 71.06 °C for 15 s followed by drying with air inlet temperature ranging from 135 to 205 °C for 5–6 s (Li & Farid, 2016). However, these conventional thermal treatments at injudicious temperature though inactivated microbes and enhanced the shelf life of foods exerted a detrimental effect on organoleptic and nutritional properties of powders, for example, loss of the desirable flavour, volatile oils, vitamins, pungency, bioactive and antioxidant compounds. Fumigation was also initially considered as a successful method for microbial destruction in food powders. Despite, later it was demonstrated that fumigation left some carcinogenic and mutagenic residue in treated powder making it a banned product in the European Union, affecting the export of Indian spice powders to the European market (Golden, Berrang, Kerr, & Harrison, 2019). With this regard, in search of advanced alternative technologies to decontaminate food powders by preserving all its quality with respect to nutritional value and sensory properties, the potential of novel thermal and non-thermal technologies were introduced. In addition, safety and environmental disquiets trigger the introduction of emerging food preservation methods such as infrared heating, microwave heating, radiofrequency heating, instant control pressure drop technology, high-pressure processing, pulsed electric field, pulsed light, ozone processing, and cold plasma. There are several literature encompassing the outcome of novel thermal and non-thermal technologies on various food and agricultural applications (Cullen, Tiwari, & Valdramidis, 2012; Misra, 2015; Rawson et al., 2011; Rifna, Ramanan, & Mahendran, 2019; Van Impe et al., 2018). During past decades, various works have also revealed the inactivation of main foodborne pathogens using these emerging techniques in various food products (Barba, de Souza Sant'Ana, Orlien, & Koubaa, 2017; Barba, Koubaa, do Prado-Silva, Orlien, & de Souza Sant’Ana, 2017; Horita et al., 2018; Portela et al., 2019; Roohinejad, Koubaa, Sant’Ana, & Greiner, 2018). The chief aspect of powder is to conserve the stability of each ingredient functionality until it is necessitated for utilization, which is mostly in some wet formulation. The important functionalities of food powders that demands adoption of a suitable technique for microbial inactivation have been classified as: physical/chemical (pH, water activity, moisture content, gelation, foaming, emulsification); nutritional (vitamins, minerals, neutraceuticals); organoleptic (color, smell, flavor, texture). Since the main purpose of these powder forms is to uphold the constancy of the ingredient functionality by maintaining its quality by preventing the chemical, physical and microbial deteriorations until it is requisite for utilization, application of emerging thermal and non-thermal techniques was studied to be significant. However, it was observed that there has been no review works till date relating to the investigation of effect of advanced thermal and nonthermal technologies on the microbial decontamination of food powders though reports strongly revealed that it is often complicated or even impossible to eradicate pathogens from foods with very low

moisture by conventional thermal processes that work very well for high-moisture foods. Henceforth, this review was inducted to summarize the efficacy of these nine technologies for microbial inactivation in food powders. Mechanism of microbial decontamination corresponding to each novel technology cited above and their effects on powder properties has also been discussed. 2. Novel thermal processing technologies The conventional thermal methods like steaming, sterilization and pasteurization under high temperature have guaranteed microbiological safety of food powders by achieving desired limits of 3 log reductions though 5 log reductions has to be ensured as per 21 Code of Federal Regulation Part 114 for any food products to be microbial safe (Dufort, Etzel, & Ingham, 2017). The negative effect on nutritional and organoleptic properties which arises on the application of high temperature is the reason which limits the use of conventional thermal processing to achieve only 3 log reduction despite the complete inactivation levels. Furthermore, the application of conventional thermal methods in food industries stands limited, owing to its non-uniform heating, decreased heat transfer efficiency and loss of final product quality. Presently, electromagnetic technologies in food processing have acquired increased interest and proved the potential to replace, at least partially, the conventional established processes. Infrared heating, microwave heating, radiofrequency heating and instant control pressure drop technology (DIC) are assured alternatives to conventional techniques of heat processing. These novel thermal technologies are considered as a volumetric phase of heating where thermal energy is produced directly into the food sample. This pattern of heat production permits overcoming surplus treatment time and consequently can have direct involvement in regards to quality attributes, heating and energy efficiency. In particular, the section summarizes the novel thermal technologies for food powder decontamination, mechanism of action, effects on microorganism and parameter altered in food powder after exposure to chosen technologies. Table 1. provides a summary of the outcomes of studies on microbial reduction levels achieved for powdered foods and their effect on powder properties after exposure to thermal treatment applications. 2.1. Infrared heating Infrared is the portion of an electromagnetic spectrum with the wavelength ranging between ultraviolet and microwave radiation (0.78–1000 μm). Atoms or molecules falling in IR range possess both vibration state and rotational motion, leading to heat generation (Dagerskog & Osterstrom, 1979). Contradicting conventional heating method, where food is primarily heated by convection on the surface followed by conduction on inside of the product, infrared heating necessitates radiation heating of surface followed by conduction heating inside (Trivittayasil, Tanaka, & Uchino, 2011). A schematic diagram of infrared heating system is shown in Fig. 1(a). Various studies have confirmed the germicidal effects of IR in cottage cheese, honey, shell eggs, strawberries, shelled corn, milk and other liquid products (Eser & Ekiz, 2018; Krishnamurthy, Jun, Irudayaraj, & Demirci, 2008; Wilson, 2016). Interestingly suitability of infrared radiation for powdered food (mostly spice powders) decontamination has also been assessed. Various works reported over the past years revealed that the effectiveness of IR treatments on microorganism inactivation relies on various factors. Infrared = f (IR power, IR temperature of food sample, effect of peak wavelength, effect of sample depth, water activity of sample, effect of bandwidth and moisture content) (Abdul-kadir, Bargman, & Rupnow, 2000). Despite, it can be inferred that of the several factors mentioned above sample water activity, IR temperature and IR wavelength accounts to be the chief factors in achieving significant microbial log reduction regards to food powders. 2

Total bacterial count

Salmonella spp., Shigella spp. and E. coli Cronobacter sakazakii

Aspergillus niger

Total bacterial colonies

Aerobic bacteria

Microbial count

Aspergillus flavus

Aspergillus flavus

Black pepper

Powdered infant formula

Garlic powder

Ashitaba leaf powder

Rice powder

Bay leaf powder

Dried rice

Peanut

Grape seed powder

Chilli pepper

TMAB, TMY

Rice powder

TMAB, TMY

Mold and bacteria

Shredded almond

Black pepper

Enterococcus faecium

Shredded garlic

Mesophilic bacteria

Aspergillus niger

Oregano Cumin powder

Paprika powder

Salmonella typhimurium, E. coli Bacillus cereus TMAB & TMY

Red pepper

MW

Bacillus cereus

Paprika powder

IR

Microbial targets

Food powder

Technology

3 915 MHz frequency and power levels 5, 10, and 15 kW Power densities in the range 32.14–142.85 Wg-1 was maintained constant at 150 s 915 MHz frequency and power levels 5, 10, and 15 kW Microwave heating was performed at power levels 360, 480 and 600 W

The MW power density was varied from 1.03 to 2.67 Wg−1 at 1.5 and 2.0 pulsation ratios MW power of 300 W applied for 1 h

Treated at MW power of 800 and 900 W

Exposed to 800 W for 5 min

95 °C for exposure period of 100 s

IR heating at 135 °C for 108 s and subsequent holding at 75 °C for 60 min Exposure to MW at 650 W for holding time of 20 min Exposed to 800 W for 5 min

IR wavelengths of 3.2, 4.5 & 5.8 was applied at10, 20 and 30 s gaps

At 70 °C for 1 h

Treated at IR wavelength of 3.3 μm

At 90 °C for 10 min Treated at IR radiation of wavelength 2.5 μm

Microbial count decreased by 59–67%

At 15 kW fungal loads reduced to 2.93 log CFU

Total log reduction ranged from 0.115 to 1.069 log cycles

At 15 kW fungal loads reduced to 4.56 log CFU

bacterial count reduced from 3.1 × 106 to 2.7 × 106 cfu/g All achieved 6-log reduction at MW power of 800 W for 5 min Microbial reduction levels higher or equal to 5 log10 cycles were reached Average log reduction between 1.12 and 1.16 was achieved. Microbial colonies decreased by 2 log cycles

TMAB achieved log reduction to 4.1 ± 0.19

The population sizes showed the largest size reductions of 4.69 ± 0.71 log CFU Wavelength 3.2 for 30 s showed the highest reduction in mold and bacterial load by 3.11 and 1.09 log reduction Inactivation rates of TMAB & TMY were 1.97 ± 0.12 and 0.62 ± 0.09 log CFU/g Achieved 4.8 log units reduction

Spore reduction of 5.6 log unit was achieved 90% reduction rate of TMAB was achieved at end of 2h Inactivation > 4 log reduction achieved

Attained reduction of 2.78 log CFU

Reduction of 0.7 and 1.6 log10 CFU/g was obtained

IR power of 5 and 11 kW/m2 respectively Power of 500 W for 5 min

Salient results

Treatment condition

Table 1 Microbial log reduction levels in powdered foods and their effect on powder quality after novel thermal treatments.

↓ moisture content, water activity ↔ free fatty acid

↑ Drying efficiency

↑ Browning index, ΔE value

↑ Chlorophyll, flavanoids, color ↔ free fatty acid

↑ Pore size

↓ Volatile oil content, piperine, resin _____

↓ Volatile oil content, piperine ↓ Capsaicin

↑ oil yield, free fatty acid, peroxide value ↑ Powder aw at center

↓ Moisture, volatile oil content, weight & Vitamin C ↓ moisture content of almonds to 7% ↓ tempering period

↔ Color ↓ Powder agglomeration ↔ Color, pungency ↓ Volatile oil ↓ Volatile oil Significant weight loss

Parameters affected

(continued on next page)

Patil, Shah, Hajare, Gautam, and Kumar (2019)

Smith and Atungulu (2018)

Kapoor and Sutar (2018)

Cao, Zhang, Mujumdar, and Wang (2019) Smith and Atungulu (2018)

Kar et al. (2018)

Pina-Pérez et al. (2014)

Jeevitha et al. (2016)

Dababneh (2013)

Jeevitha et al. (2016)

Eliasson et al. (2015)

Fu, Xiao, Pan, and Wang (2019)

Oduola, Bowie, Shad, Wilson, and Atungulu (2019)

Venkitasamy et al. (2018)

Feng et al. (2018)

Eliasson et al. (2014) Erdoğdu and Ekiz (2011)

Ha and Kang (2013)

Staack et al. (2008)

Reference

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Food Research International 126 (2019) 108654

Salmonella typhimurium

S. Typhimurium & E. coli O157:H7 Bacteria

Mesophilic aerobic count

Enterococcus faecium

Staphylococcus aureus

E. coli & Aspergillus

Enterococcus faecium

Ground black pepper

Aspergillus niger

Salmonella spp. & Staphylococcus aureus Total bacteria

Initial bacterial flora.

Bacterial flora.

Bacterial colonies

Red pepper

Red pepper & black pepper Broccoli powder

Pepper powder

Wheat flour

Shredded in-shell walnut

Barley grass powder

Wheat flour

Salmonella spp. and Enterococcus faecium Apple pomace powder.

Seaweed & skim milk powder Onion powder

Onion powder

Dried carrot powder

Banana flour

4 Exposed to 3–5 kPa

Exposed to 0.4 MPa DIC steam pressure for 15 s Exposed to 3 kPa at 70 °C for 3 h

Saturated steam pressure of 0.44 ± 0.02 MPa for a time of 40 s Exposed to 0.35 MPa for 15 s

RF heating at 80–85 °C followed by 10–25 min cooling Drying for 40 min at 27 MHz between an electrode gap of 19.0 cm RF heating at 6 kW, 27.12 MHz with electrodes regulated between 120 and 240 mm RF heating at 27 MHz for 39 min with electrodes regulated between 150 mm RF heating at 6 kW, 27.12 MHz with electrodes regulated between 10.5 cm Exposed to 0.2–0.6 MPa

RF treatment at temperature 115 °C

RF treatment at 6 kW

RF heating to 70 °C with holding time over 60 s RF frequency of 27 MHz for 80 s.

Treated at 27 MHz for 39 min

Treatment condition

5.98 log CFU/g reduction for Salmonella spp. and the reduction of 3.89 log CFU/g for E. faecium Significant log reduction (4D) of patulin mycotoxin attained Decontamination ratio of > 87% for skim milk powder and 100% for seaweed powder Reduced bacterial count to 100 from initial 8,75,000 bacteria/g Showed desired range of inactivation of 4 log reduction Significant log reduction (4D) of bacterial flora attained IPD treatment was superior in reducing the number of bacteria in the banana

D75°C of E. faecium was 29.35 ± 1.10 min

RF treatment produced > 4-log reduction of S. aureus 5-log reduction of E. coli and 7-log reductions of Aspergillus

Microbial count was decreased from 3.0 log CFU/g to < 30 CFU/g Possessed a moderate decrease from 4.6 × 106 to 3.1 × 106 CFU/g Resulted in 2.5–3.7 log10 reduction

7-log reductions was achieved

D85°C was calculated to be 18 min respectively to achieve 1 log reductions Achieved 5 log reductions

Salient results

↑ Volume ratio ↔ Texture ↔ Flour quality ↑ Surface area

↔ Vitamins ↑ Quercetin ↑ Quercetin

↔ Piperine, total phenolic, volatiles ↑ Flavanoid, quercetin & drying kinetics ↑ Surface area

↔ Flour quality

↔ Walnut quality, moisture content, water activity ↔ Antioxidant, odor of barley grass

↓ Moisture content

↓ Total carotenoid content

↔ Color

↓ Moisture content

↓ Color

_____

Parameters affected

Setyopratomo, Fatmawati, Savitri, Sutrisna, and Allaf (2019)

Peng et al. (2018)

Albitar et al. (2011)

Mounir et al. (2011)

Allaf et al. (2011)

Wei, Lau, Stratton, Irmak, and Subbiah (2019) Mounir et al. (2009)

Liu et al. (2018)

Cao et al. (2019)

Xu, Yang, Jin, Barnett, and Tang (2019) Zhang et al. (2019)

Molnár et al. (2018)

Zhao et al. (2017)

Jeong and Kang (2014)

Hu et al. (2018)

Liu et al. (2018)

Reference

Where (↓) refers to decrease in the level; (↑) refers to increase in the level; (↔) refers to neither increase nor decrease in the level. IR: infrared heating; MW: microwave heating; RF: radiofrequency heating; DIC: instant control pressure drop technology; TMAB: total mesophilic aerobic bacteria; TMY: total mold and yeast.

DIC

S. enteritidis & E. faecium

Wheat flour

RF

Microbial targets

Food powder

Technology

Table 1 (continued)

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Fig. 1. Schematic diagram of thermal decontamination techniques (a) Infrared drying (b) Microwave drying (c) Radiofrequency drying (d) Instant control pressure drop.

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2.1.1. Mechanism of action Inactivation of microorganisms by infrared heating may comprise inactivation mechanism alike to that of microwave heating (induction heating) and ultraviolet light (DNA damage) in addition to thermal effects (Hamanaka et al., 2000). Thermal inactivation can destroy DNA, RNA, ribosome, cell envelope, and proteins in the microbial cell. The order of magnitude of microbial damages after infrared treatment was as follows: protein > RNA > cell wall > DNA. To demonstrate the effect of infrared heating and its mechanism of microbial decontamination, infrared spectroscopic and fluorescent molecular probes techniques were used for paprika powder in a study conducted by Staack, Ahrné, Borch, and Knorr (2008). The main effect of infrared heating was studied to be the inhibition of microbial RNA polymerase by the emission of generated infrared waves, which binds the ribosomal subunits and gradually inhibits the peptidyl transferase reactions. It was observed that treatment with infrared radiation resulted in total damage to cells by disintegrating cell membrane and mesosomes of microorganisms. The higher inactivation in IR treatment was due to the emission of infrared waves at higher energy levels compared to the distributed energy in any other conventional mode of heating as revealed by the fluorescent molecular probe analysis. Compared to traditional microscopy technique, these digital microscopic image analyses helped in studying the relationship between microenvironment and targeted organism non-invasively and in real time.

period (Feng et al., 2018). The authors observed that exposure to a wavelength of 3.3 μm achieved above required log reduction (say 4-log reduction) in TMY. However, in all works discussed above that elucidates the effect of infrared treatment for microbial decontamination in various spice powders the spore population was predicted using a numerical model of 1st order inactivation kinetics. Moreover, it has been found as in common that no researchers have mentioned about the powder sample depth range and initial physiological phase of the target organism that can be subjected for treatment to produce effective sterilization, which can possess a profound impact on the decontamination rate. Food and microbial components take up certain wavelengths of infrared radiation. Therefore, it is useful to study the absorption pattern of key components to ensure microbial inactivation and minimize variation in food quality. It would be reasonable to selectively heat the microorganisms present in food powders without adversely increasing the temperature of sensitive food components. Jun, Irudayaraj, Demirci, and Geiser (2003) applied selective infrared heating in the wavelength range of 5.88–6.66 μm using optical band pass filters for inactivation of Fusarium proliferatum and Aspergillus niger in cornmeal. The selected wavelength was observed to denature the protein in microorganisms, leading to a 40% increase in inactivation of A. niger and F. proliferatum compared to normal heating. In general, it can be concluded that although the sample temperatures after selective or nonselective infrared heating were identical, absorption of energy by microbial cells or spores were studied to increase in infrared heating, leading to a higher lethal rate (Jun & Irudayaraj, 2003).

2.1.2. Microbiological safety of food powders The effect of infrared radiation at 5 kW/m2 and 11 kW/m2 (95 °C) were studied on flora of Bacillus cereus on paprika powder at aw of 0.5, 0.80 and 0.96 using total plate count (TPC) and spore plate count (SPC) method (Staack et al., 2008). It was studied that zero microbial reduction was visualized in powder at aw 0.5. However, at aw 0.8, for treatment at 5 kW/m2 and 11 kW/m2 an overall reduction of 0.7 and 1.6 log10 CFU/g was obtained. At water activity of 0.96, no microbial survivors were visualized in the sample. Through this study, it was concluded that B. cereus flora showed to be highly sensible on infrared heating preserving powder colour, but its effect on preserving attributes like water loss, particle size and agglomeration percentage stands unstudied. Likewise, evaluation of disinfestations of Bacillus cereus spores in oregano powder was investigated by infrared heating (Eliasson, Libander, Lövenklev, Isaksson, & Ahrné, 2014). An acceptable reduction of B. cereus spore count, 5.6 log unit was achieved for IR heating conditioned at 90 °C for holding period of 10 min. A group of monoterpenoids including thymol, carvacrol and isoboreneol are responsible for its characteristic aroma. These compounds also possessed significant antimicrobial activity which was also synergistically acting with the thermal stress in view of spore inactivation in the powder by infrared heating. The authors hypothesized that a lower spore inactivation at 100 °C might be due to loss in antimicrobial activity arising from these volatile loss while exposed to infrared. Ha and Kang (2013) investigated the efficiency of application of infrared heating of 500 W (3–5 min) for eliminating population of Salmonella typhimurium and E. coli in red pepper. TPC of infrared treated red pepper powdered sample revealed the presence of 0.23 and 0.32 and 0.23–0.49 log CFU/g injured cells of S. Typhimurium and E. coli respectively, maintaining it'scolour and pungency components. However, comparing above all three works it was observed that the volatile oil content change at selected infrared wavelength were not considered though preservation of it is vital as volatile oil contains the antimicrobial properties. Another outcome was observed by Erdoğdu and Ekiz (2011)) in cumin powder on total mesophilic aerobic bacteria (TMAB) and total mould yeast (TMY). On exposure to infrared radiation of 2.5 μm wavelength 90% reduction rate was achieved at the end of 2 h demonstrating the possible use of infrared in the industrial application. Future experiments evaluated the effect of shredded garlic inoculated with TMY when exposed to infrared heating at wavelength 3.3 μm followed by holding at the inactivation wavelength for a particular

2.2. Microwave heating Microwaves are generally set to operate at a frequency of 2450 MHz, as this frequency according to the resonance of water molecules is a free-space wavelength in air or vacuum and results in more uniform distribution of hot and cold spots inside the oven. The heating principle of the microwave is on the interaction between the sample and electric field, resulting in ionic and dipole interaction, a movement finally converted into heat (Lew, Krutzik, Hart, & Chamberlin, 2002). A schematic diagram of microwave heating system is shown in Fig. 1(b). According to FAO/WHO Expert Committee on Food Safety report, “the radiation of any food product up to a total average dose of 1000 W has proven to present no radiological hazard” (WHO, 1972). Microwave radiation has been used as an efficient alternative in microbial load reduction of a few food powders. However, despite the advantages of microwave treatment the formation of “cold spots” within food powder where harmful bacteria initiates its growth under favorable condition limited its industrial application. Numerous researches over the years on microwave discovered that the effectiveness of microwave treatments on microorganism decontamination depends on important attributes, that is Microwave = f (microwave power, microwave temperature, sample thickness and treatment time) (Jiang, Liu, & Wang, 2018). 2.2.1. Mechanism of action The application of microwave treatment to microorganisms denatures the cell protein structure and results in extrusion of the microbial cellular matrix, gradually killing the microorganisms. Inactivation of cells is dependent on many microwave and microbial factors. There are two modes of action of microwave that have been investigated by researchers, which include thermal effect and nonthermal effect. It was later studied that non-thermal effect plays a dominant role in microbial decontamination compared to thermal effects (Banik, Bandyopadhyay, & Ganguly, 2003). The mechanism of the non-thermal act of microwaves is still not wholly understood. However, it seems that rotation and lining-up of the molecules was produced as a result of microwave radiations. This ultimately results in unfolding of enzymes or proteins, and breakdown of ribosomes; thus eventually 6

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resulting in complete or partial denaturation of cell components. According to the research carried out by Shamis et al. (2011), it was explained that the microbial cells displayed varied cell morphology rapidly following microwave radiation. Confocal laser scanning microscopy also revealed that the MW treated cells might take up FITC conjugated dextran probes, reporting that temporary pores had formed within the cell membrane leading to an explosion of microbial cellular compounds (Shamis et al., 2011). However, more studies involving specific modelling analyses and theoretical calculations to establish the optimized MWconditions for complete microbial inactivation of food powders are important directions for future expansion.

the basic advantage of this technique that makes it applicable to all unit operations in food industries (Wang, Wig, Tang, & Hallberg, 2003). A schematic diagram of radiofrequency heating system is shown in Fig. 1(c). Effectiveness of RF treatments on microorganism inactivation relies on few chief factors say, radiofrequency = f (RF temperature, frequency, effect of sample depth, sample moisture, capacity of equipment, microbial targets). The recent works revealed that the frequencies applied in radiofrequency heating fall within the region to which bound water is robustly heated. Thus, the frequency range of radiofrequency waves makes it favourable to decontaminate microbes that exist in low moisture foods. As a result, this chemical-free electro thermal pasteurization method explores the efficiency of radiofrequencytechnology for inactivation of microbes in dry powders.

2.2.2. Microbiological safety of food powders Through an experiment, the effect of MW treatment at a power level of 650 W and temperature of 98 °C on mesophilic bacteria in the paprika powder of 1 cm thickness was investigated (Eliasson, Isaksson, Lövenklev, & Ahrné, 2015). The microbial load before and after treatment was studied with thermal images captured using Thermal Tracer TH7700IR camera. From the recorded images, it was inferred that the microwave treatment at 650 W for holding time of 20 min achieved the 4.8-log reduction. Similarly, (Jeevitha, Sowbhagya, & Hebbar, 2016) evaluated the microbial log reduction in black pepper in terms of TMAB, TMY, Salmonella, Shigella and E. coli. Microwave power was 800 W, with initial temperature 50 °C for 2 min and then programmed at 5 °C min−1 to 250 °C and hold for 3 min for sample thickness of 3 cm. Salmonella, Shigella and E. coli achieved complete acceptable sterilization of 6-log reduction whereas TMAB and TMY reduced to 4.11 and 4.02 only in set power and time. However, on comparing above two works it was studied that, in paprika powder, digital images were taken based on the hot spot and cold spot created by mesophilic bacteria using thermal imaging technology, whereas, in black pepper TMAB, TMY, and other bacterial load was determined using viable count on Trypton Soy Agar. Pina-Pérez, Benlloch-Tinoco, Rodrigo, and Martinez (2014) demonstrated the effect of MW treatment on powdered infant formula inoculated with Cronobacter sakazakii. It was observed that microbial reduction levels higher or equal to 5 log10 cycles were reached at 800 and 900 W. Recently, MW rotary drum dryer was used to study the rate of Aspergillus niger inactivation on dried garlic powder (Kar, Mujumdar, & Sutar, 2018). Microwave treatment resulted in an average log reduction of A. niger between 1.12 and 1.16 at microwave temperature of 50 °C. Weibull, Page and Bigelow models were used to predict microbial inactivation data, and Page model possessed to be the best model of fit with R2 values > 0.99. Henceforth, through the conduction of this research authors suggested page model be the best fitting predictive model for prediction of A. niger decontamination rate. Through the above-reviewed studies, use of microwave treatment can be recommended as a promising technology for simultaneous microbial load reduction and maintaining the quality of food powders in domestic as well as at industrial scale, especially for spice powders. However, it is necessary to conduct further experiments to check its effect on microbial decontamination on other food powders such as milk powder, fruit powder, vegetable powders and cereal flours whose demand have been found to scale up largely in the market recently.

2.3.1. Mechanism of action The mechanism of microbial decontamination by radiofrequency heating is based on the principle that when the heat is developed at a faster rate within the microbial cell than in another medium, the cells get thermally destroyed at a low heating rate (Marra, Zhang, & Lyng, 2009). Since in radiofrequency heating the mode of heat transfer is radiation, energy applied is directly absorbed by microbial DNA and essential proteins leading to physical variation in the microbial cell structure and functionality. So, in general, it can be concluded that the effect of radiofrequency treatment for microbial log reduction in food powders depends on the species, cell wall structure of the targeted organisms, applied RF frequency and uniformity of heating. 2.3.2. Microbiological safety of food powders The influence of radiofrequency heating on wheat flour was performed to develop a microbial validation for radiofrequency heating on food powders (Liu et al., 2018). D value of Salmonella enteritidis and Enterococcus faecium in flour with an initial population of 8.6 ± 0.1 CFU/g and 7.8 ± 0.1 CFU/g respectively were investigated. The D85°C was calculated to be 18 min (S. enteritidis) and 25 min (E. faecium) respectively, to achieve 5 log reductions. Likewise, (Hu, Zhao, Hayouka, Wang, & Jiao, 2018) experimented the effect of RF dielectric heating to inactivate Salmonella typhimurium in powdered red pepper. It was assessed that as aw was in the range 0.57–0.71, the effect of RF heating was highest, resulting in 2–3 log reduction. Comparing the above two works, it was inferred that microbial kinetics explained using Weibull model was more effective as it described both upward and downward part of survival curves along with the log-linear model. According to the experiment carried out by Jeong and Kang (2014) the influence of radiofrequency at 27 MHz at 90 °C for 80 s on dried red and black pepper powder resulted in 7-log reductions of S. Typhimurium and E. coli O157: H7. This reduction was calculated to be the maximum comparing reductions achieved of the targeted organism by infrared and microwave radiations. Despite, the moisture content of the final powder reduced significantly due to the unpredictable heating pattern, which demands the need for future work. Another research was intended to study the effect of microorganisms in broccoli powder when pasteurized using radiofrequency treatment of 6 kW. The results showed that microbial inactivation was greatly decreased by 4.2 log CFU/g with no colour degradation (Zhao, Zhao, Yang, Singh Sidhu, & Kong, 2017). Contradicting findings of above four works, radiofrequency treatment performed on pepper powder at varying temperature revealed that microbial count on the samples possessed only a moderate decrease from 4.6 × 106 to 3.1 × 106 CFU/g (for mesophilic aerobic count) and from 5.2 × 103 to 3.1 × 102 CFU/g (for total moulds) even after the most severe temperature of 115 °C (Molnár et al., 2018). Henceforth, it can be generalized that except for mesophilic bacterium use of radiofrequency was effective in achieving microbial log reduction to an acceptable level for all other studied strains. However, the adverse impacts developed on the treated powder samples resulted in decreasing powder quality in terms of nutrition and sensorial aspects with significantly reducing

2.3. Radiofrequency heating Radiofrequency is a part of the electromagnetic spectrum with frequency in the range of 30–300 MHz. It is an indirect electro heating technique where initially, the electrical energy is transformed to electromagnetic radiation and slowly released as heat into the desired food sample depending on the dielectric properties of the food product (Piyasena, Dussault, Koutchma, Ramaswamy, & Awuah, 2003). Since radiofrequency produce non-ionizing radiations, the molecules of food remain unaltered even at the molecular level, making the key mechanism of inactivation as heat. Rapid heating with less-heating time is 7

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color value, moisture content, total carotenoid content and flavanoids. Henceforth, future studies are needed that aids to understand the interaction between food and microorganism resistance when exposed to radiation, which would aid in developing radiofrequency technique more applicable to food powder decontamination

The combination of DIC condition and hot air treatment (of temperature 50 °C) was studied to be significant in inactivating total bacteria on onion powder (Mounir, Besombes, Al-Bitar, & Allaf, 2011). It was reported that at DIC condition of 0.35 MPa for 15 s a level of 100 bacteria/g of total flora was achieved compared to 875,000 bacteria/g of total flora in case of the non-DIC treated onion powder sample. Researchers also revealed that a combination of DIC and hot air drying preserved the vitamins, thus proving its high possibility in producing a quality product at the end. So, from findings of the abovework, it can be inducted that the combination of DIC with mild heat is an effective method to decontaminate the powdered food sample. Through another work, (Albitar, Mounir, Besombes, & Allaf, 2011) reported inactivation of initial flora load accumulated on onion powder. The treatment was conducted at 0.4 MPa DIC steam pressure for 15 s. The result showed that the desired range of inactivation of 4 log reduction was achieved in DIC treated sample compared to log reduction of one as shown by conventional hot air dried powder.

2.4. Instant controlled pressure drop technology (DIC) The Détente Instantanée Contôlée (DIC), French for Instant Controlled Pressure-Drop, is based on the concept of thermodynamics of instantaneity and auto-vaporization. DIC is considered as a thermomechanical process initiated by subjecting the sample to saturated steam for a short period. This is followed by a sudden pressure drop towards vacuum (nearly about 3–5 kPa). The abrupt pressure drop induces notable mechanical stress resulting in instantaneous cooling of product and auto-vaporization of water (Hamoud-Agha & Allaf, 2019). A schematic diagram of DIC system is shown in Fig. 1(d). The DIC technology has also been reported as a robust tool in debacterization and decontamination of food powders, noteworthy the increased surface area of food powders being the chief cause to attain the desired microbial log reduction for food powders. However the efficacy of DIC technology on microorganism decontamination depends on critical factors say, DIC = f (product expansion property, particle size, vacuum level developed, thermal stress, mechanical stress, pressure drop within treatment system, volatile content percentage within sample, type of microorganism)

3. Non-thermal processing technologies Till date, due to the ability to inactivate targeted enzymes and microorganisms, use of heat is the most common method used for decontamination of food powders. However, it was studied that heat processing under extreme condition induces various chemical and physical changes in powder resulting in loss of organoleptic properties and bioavailability. Hence, there was a need for the application of nonthermal technologies in food powder processing to attain high-quality powder with no microbial load. Hence, with the rising interest of obtaining a high-quality powder with zero microbial load and improved functionalities, the non-thermal technologies found its application in food powder decontamination. However, due to the release of internal energy, some of the non-thermal techniques may also generate heat, despite; they are categorized as mild heat treatment, contrarily to novel thermal technologies, since they can remove the application of very high temperature to destroy microorganisms, keeping away the detrimental effect of high temperature on colour, volatile oil, favour and nutritive constituent of powders (Zhang et al., 2006). The extensively promising non-thermal techniques for decontamination of food powders appears to be high pressure processing (HPP), pulsed electric field (PEF), pulsed light (PL), ozone processing (OP), and non-thermal plasma (NTP) of which most have got established, and few like PEF and HPP are on the verge of commercialization. Table 2. provides a summary of the outcomes of studies on microbial reduction levels achieved for powdered foods and their effect on powder properties after exposure to non-thermal treatment applications.

2.4.1. Mechanism of action The effective microbial decontamination using DIC technology is due to its thermo-mechanical impact resulting in an irreversible modification in the cellular component of microorganisms. The mechanism of microbial inactivation has been attributed to two key mechanisms: controlled thermo-mechanical process and pressure relaxation highly stressed upon the targeted organism cell. The exposure of contaminated powder to vacuum stage, followed by homogenization, instant pressure drop to vacuum and abrupt release to atmospheric pressure results in the development of mechanical stress within a microbial cell causing cell explosion (Allaf, Besombes, Mih, Lefevre, & Allaf, 2011). Moreover, in addition to the mechanical stress, the thermal stress generated was observed to be highly effective for microbial decontamination. The process of auto-vaporization has also been reported to create a mechanical restriction that acts on the targeted organism cell wall and more precisely on the spore wall. However, more studies are demanded to design a proper system for microbial spore inactivation in food powders using DIC, as this technology in food powder application contribute to preserving organoleptic quality, nutritional content and powder specific surface area efficiently.

3.1. High-pressure processing (HPP)

2.4.2. Microbiological safety of food powders Mounir, Besombes, and Allaf (2009) evaluated successful decontamination of patulin mycotoxin produced by Aspergillus niger in apple pomace powder. The treatment was performed using DIC at 0.2–0.6 MPa. Experimental results showed that a notable hike had been observed in the major apple flavonoid, quercetin to a maximum of 500–800% after DIC application. The specific surface area of the powder was also reported to enhance by a factor 2 during treatment, aiding improvement in drying kinetics and significant log reduction of the mycotoxin. In addition,Allaf et al. (2011) investigated the response of instant control pressure drop for inactivation of Salmonella and Staphylococcus aureus on powdered seaweed and skim milk powder. The treatment was performed at a saturated steam pressure of 0.44 ± 0.02 MPa for a treatment time of 40 s. Enumeration of the targeted organism was attained using the LASAT technique. It was studied that the STEAM-DIC treatment provided a good relevant result in inactivating Salmonella and S. aureus with a decontamination ratio of > 87% for skim milk powder and 100% for seaweed powder.

High-pressure processing (HPP) is a modern non-thermal treatment for both food preparation and food preservation. HPP treatment involves the process of subjecting food powders packed or unpacked to pressure that generally can range from 300 to 800 MPa. A schematic diagram of high pressure processing system is shown in Fig. 2(a). HPP in food process applications is governed by two pertinent laws. Primarily, it is in agreement with the isostatic principle, which states that pressure is applied instantaneously and uniformly throughout the material. Secondly, it obeys Le chatelier’s principle which explains that as the pressure of a system under equilibrium is varied, it tries to regain its state of stability by adjusting the volume (Oey, Lille, Van Loey, & Hendrickx, 2008). The foremost application of HPP was reported for milk pasteurization (Yuste, Capellas, Pla, Fung, & Mor-Mur, 2001). Later researches were steered to find the effect of HPP on microbial inactivation of solid food forms. The mechanism of inactivation of microorganisms in food samples using HPP can be elucidated as function of: HPP = f (treatment pressure, treatment temperature, sample moisture, sample acidity, cellular protein structure, equilibrium 8

PL

PEF

E. coli

Native microorgamisms

Dried blueberries

9

Cronobacter sakazakii

Bacterial load

Listeria species

Salmonella Enteritidis

Powdered infant formula

Dried sesame seeds

Powdered infant formula

Dried almonds

PL treatment at a total fluence of 17 mJ/cm2 Sample exposed to pulsed light at 3000, 3400 and 3800 V at distances of 14.1 or 19.1 cm for 20 or 60 s

PL treatment at a total fluence of 39.85 J/cm2

Powder exposed to 57.5 ± 0.7 °C for 28 s

Bacillus subtilis

Salmonella typhimurium & Escherichia coli

Treated at at intensities of 4 and 6 pulses respectively Sample exposed to light intensity of 20.4 kJ/m2 for 10 min

Bacillus subtilis

Ground caraway, black pepper powder Peppercorn powder

Red pepper

Treated at energy level of 5.6 J/cm2 PL treatment for 5000, 600, 300 and 100 μs at 25 kV Exposed to PL of 10 J/cm2

Aspergillus niger Listeria monocytogenes

Corn meal Powdered infant formula

Treated at energy level of 31.12 J/cm

2

Scharomyces cerevisiae

PEF voltage of 2 kV/cm

20 kV/cm, 270 μs

PEF voltage of 33 kV/cm and 30 pulses Treatment performed at PEF voltage of 26, 40 and 28 kV/cm Low intensity PEF treatment of 15 kV/ cm applied for 3000 μs PEF processing at 70 °C for 16 kV/ cm–100 kJ/L

Exposure to HPP at 400, 500, and 600 MPa for 5 min

Black pepper & wheat flour

Onion, basil and dill powder Infant milk formula

Stevia & Ginseng powder

Naturally occurring microorganisms

Red bean powder

Aerobe, yeast/mold, and coliform

Total aerobic bacteria and yeasts and mold

Garlic powder

Tangor juice powder

Cronobacter sakazakii

Infant formula

Escherichia coli & Bacillus subtilis Total coliform, E. coli, yeast and mold Cronobacter sakazakii

Cronobacter sakazakii

Infant formula

Pea soup mix powder

Cronobacter sakazakii

Infant formula

HPP at 600 MPa for 5 min

HPP at 0.06 MPa 400 MPa for treatment period from 10 to 225 min Exposed to pressure of 200, 400 & 500 MPa Treated at 100–200 MPa for 10–20 min Exposed to pressure of 400 & 600 MPa

Bacterial strains Bacillus cereus

0.86 log microbial reduction was attained and at 44.46 J/cm2 reduction enhanced to 1.02 log reduction Inactivated > 99% of the vegetative cell populations of targeted organism PL reduced Salmonella populations by 0.44–4.14 log CFU/almond at 3800 V, 14.1 cm distance, 60 s

Exhibited maximum inactivation of 3.18 log10 CFU/g

Inactivation levels of 4.4 log10 cycles was obtained Reduced the aerobe, yeast/mold, and coliform counts of MH juice by 3.9, 4.3, and 0.8 log CFU/mL Highest inhibition zone diameter of 15.11 ± 0.11 mm was obtained Significant log reduction (5D) of microbial flora attained Log reduction of 2.93 and 0.7 was achieved for black pepper & wheat flour 4.93 log reduction was achieved 4–5 log reduction of cell was attained under treatment for 5000, 600, 300 & 100 μs at 25 kV 0.8 & 1 log reduction in caraway and pepper was attained A decimal reduction of 2.1 & 2.6 log CFU was achieved Achieved 0.22 CFU/g log reduction

A maximum of one-log reduction was achieved

Probabilistic growth model for predicting Cronobacter spp. inactivation by HPP developed Exhibited reduction in total aerobes count to 1.62 CFU/g and yeasts and molds count by 1.43 log CFU/g Microbial count in final RBP were reduced to 1.83, 1.55, and 1.05 log CFU/g using HHP 400, 500, and 600 MPa A decrease of 5.3D were observed

Inactivation level of 2, 4, 6 log10 cycles were achieved Inactivation level of 7 log10 cycles achieved

Heat resistant spore colony count reduced to significant level Achieved 5 log reduction Enhanced spore inactivation by 0.5 log cycles

Pressure of 6.8 × 104 N/cm

Bacillus cereus

Corn flour, fennel powder & Chinese herbs Licorice powder Olive powder

HPP

Salient results

Microbial targets

Treatment condition

Food powder

Technology

Table 2 Microbial log reduction levels in powdered foods and their effect on powder quality after novel non-thermal treatments.

↑ Sample surface temperature

↔ Nutrients & minerals

High significance (p > 0.05) was observed between extractable color value and CIE color value ↔ Amino acid, particle physical appearance, and volatile compounds _____

↔ Color, Vitamin A &B6

Nicorescu et al. (2013)

↑ Mixing index

(continued on next page)

Oner (2018)

Arroyo et al. (2018)

Hwang, Cheigh, and Chung (2018)

Chen et al. (2018)

Cheon et al. (2015)

Moreaua et al. (2011)

Fine and Gervais (2004) Jun et al. (2003) Choi et al. (2010)

Pina-Pérez, Rivas, Martínez, and Rodrigo (2018) Yu, Jin, Fan, and Xu (2017)

Lee, Bang, Choi, and Min (2018)

Pina-Pérez et al. (2013)

Keith et al. (1997)

Vega-Mercado et al. (1996)

Lee et al. (2018)

Park et al. (2019)

M. Pina-Pérez, Silva-Angulo, Rodrigo, & López (2012)

Arroyo et al. (2012)

Pina-Pérez et al. (2011)

Marco et al. (2011) Black et al. (2008)

Tsujimoto et al. (2004)

Reference

↔ Total phenolics, antioxidant activity, anthocyanins ↓ Flavor & color, ↓ Piperine content from 5.12% to 3.4% _____ _____

↔ Functional properties

↔ Ascorbic acid, antioxidant capacity

↔ Powder color, nutrient, taste and flavor ↔ Nutrients & minerals

↔ Total phenols, flavonoids, and total antioxidant capacity ↓ Phytic acid ↔ Vitamin A & C

↓ Pungent odour, alliinase activity

↔ Particle size, nutrition, ↑ Color, flavor

↑ Freshness

↔ Color, flavor

↓ Particle size ↑ Sensorial attributes

↓ Gelatinization ratio by 10%

Parameters affected

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Fusarium

Fungus

Aspergillus flavus

Enterobacteriacea

Rice flour

Barley powder

Peanut flour

Oregano, thyme, mountain tea, lemon verbena and chamomile powder Paprika, pepper &oregano powder

10

Bacillus cereus spore, Escherichia coli & Aspergillus brasiliensis Aspergillus flavus & Bacillus cereus Fusarium oxysporum Listeria monocytogenes, E. coli and Salmonella enterica Bacillus subtilis,E. coli, Salmonella Enteritidis

Onion powder

Exposed to NTP at 1000 W for 90 s Exposed to electrical discharge plasma of 10 kHz and power of 80 W

Exposed to DCSBD of power density 80 W cm−3

MCPT at 900 W for 20 min

Plasma was generated at atmospheric pressure in ambient air for 300 s

Plasma treatment led to reduction of B. subtilis from 7.36 to 2.30 CFU/g, E. coli and Salmonella Enteritidis to below detection level B. cereus and A. flavus spores were reduced by 1.4 ± 0.3 & 1.5 ± 0.2 log spores cm−2 A reduction of 3.79 log (CFU/g) in F. culmorum, 4.21 log (CFU/g) in A. flavus and 3.22 log (CFU/g) in A. alternata Inhibited the fungal growth by 50%. Corresponding to E. coli and S. aureus reduction D-values of 0.73 and 0.67 min were obtained

Reduced spore count by0.7 ± 0.1 & 1.5 ± 0.2 log spores/cm2. Inhibited the fungal growth by 50%. Resulted in 3.1 log CFU/cm2 reduction

MCPT at 0.17 W/m2 and 0.15 W/m2 NTP at 1000 W for 90 s Exposure to 15 kHz helium NTP

Reduced the count of A. brasiliensis, by 1.6 log spore/cm2

Achieved 2.5 ± 0.3 log spores/g reduction

Exhibited a 4 log reduction, from around 6.5–2.5 in the case of oregano, and only a 1–2 log reduction for other four herb powders Reduced bacterial flora of paprika, pepper & oregano by more than 3 log10, 3 log10 & 1.6 log10 respectively Effectively reduced E. coli to 9 log CFU/g

Achieved a degradation of 80% and 77%

2 log reduction of trichothecene mycotoxin achieved Achieved 96% spore inactivation in 5 min

Achieved maximum inactivation of 5 log reduction Achieved up to 2 log reduction

Salient results

MCPT at 400 W for 40 min

The frequency of the microwave plasma was 2.45 GHz and was ignited for 7 s DBD-PT & RF-TT at 1000 W and 1500 W NTP at 900 W for 20 min

O3 dose of 1 ppm and 5 ppm for 360 min Exposed to 18 ppm ozone concentration for 8 h Exposed to 10 ppm ozone concentration for 9 h Ozone dose of 0.16 mg/gm was applied Ozone dose of 5 ppm with treatment time (15–10 min) was applied Exposed to ozone treatment of 4 ppm for 30 or 60 min

Treatment condition

↑ Powder morphology ↔ Quality paramaters

↑ Wettability, growth parameters

↔ Powder quality

↑ Storage stability ↔ Powder quality. ↑ Powder morphology ↑ Antioxidant activity, quercetin content ↔ Product surface morphology

↓ aw ↑ Storage stability ↔ Antioxidant activity, quercetin content & color

↓ Moisture, aw, color

↓ Color (ΔE value)

_____

↔ Surface area

_____

↔ Particle size ↓ Flavor & color ↓ Flavor & acidity ↑ Saturated fatty acid ↓ pH

Parameters affected

Go et al. (2019) Chingsungnoen, Maneerat, Chunpeng, Poolcharuansin, and Nam-Matra (2018)

Zahoranová et al. (2018)

Kim et al. (2019)

Mošovská et al. (2018)

Go et al. (2019) Kim and Min (2018)

Kim, Oh, Song, and Min (2019)

Kim et al. (2017)

Kim et al. (2014)

Choi et al. (2018)

Hertwig, Reineke, Ehlbeck, Knorr, and Schlüter (2015)

Proctor, Ahmedna⁎, Kumar, and Goktepe (2014) Kazi, Parlapani, Boziaris, Vellios, and Lykas (2018)

Tiwari et al. (2010)

Young et al. (2016)

Byun et al. (1998)

Akbas and Ozdemir (2008)

Reference

Where (↓) refers to decrease in the level; (↑) refers to increase in the level; (↔) refers to neither increase nor decrease in the level. HPP: high pressure processing; PEF: pulsed electric field; PL: pulsed light; OP: ozone processing; NTP: non-thermal plasma; DBD-PT: dielectric barrier discharge plasma treatment; RF-TT: radiofrequency thermal treatment; MCPT: microwave powered cold plasma treatment; DCSBD: Diffuse Coplanar Surface Barrier Discharge.

Paprika powder Herbal tea powder

Dried maize seeds

Red pepper flakes

Black pepper powder

Paprika powder Onion powder

Bacillus cereus & Aspergillus flavus Aspergillus flavus, Alternaria alternata and Fusarium culmorum Fusarium oxysporum E. coli & Staphylococcus aureus

Aspergillus flavus

Red pepper

Red pepper

E. coli

Pepper powder

Bacterial flora

Total aerobic bacteria & yeast

Red ginseng powder

NTP

E. coli & Bacillus cereus

Flaked red pepper

OP

Microbial targets

Food powder

Technology

Table 2 (continued)

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Fig. 2. Schematic diagram of non-thermal decontamination techniques (a) High pressure processing (b) Pulsed electric field (c) Pulsed light (d) Ozone processing (e) Non-thermal plasma.

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Fig. 2. (continued)

constant). However alike to treatment pressure for liquid foods, the strong dependence of inactivation of microorganisms on water activity was observed as the main factor that determines the inactivation rate in food powders during HPP application.

and vegetative bacteria when exposed to pressure, heat or both. However, for successful decontamination of spores, the optimization of the HPP treatment conditions or the combination with other agents or treatments may be needed.

3.1.1. Mechanism of action The main effect of HPP treatment is destruction caused to the noncovalent bonds present within microbial cells such as proteins, nucleic acid and lipids resulting in inactivation of microorganisms. The mechanism of microbial inactivation in HPP is as follows. Exposure to high pressure for a fraction of few seconds induces membrane damage and a decrease of intercellular pH in microbial cell. Abe (2007) assessed that HPP pressure as low as 50–100 MPa is adequate for ceasing protein synthesis and for ribosomal changes in a microorganism. It has also been observed that quick volume expansion results in unfolding of proteins, thus triggering protein denaturation (Domitrovic, Fernandes, Boy-Marcotte, & Kurtenbach, 2006). These effects finally cause complete or partial denaturation of cell components. To contradict, minute molecules like vitamins, amino acids, aroma compounds and flavour aiding to the functional properties and sensorial quality of food powder was found to remain less affected. Overall, it can be stated that HPP treatment has enormous potential on inactivation of both spore forming

3.1.2. Microbiological safety of food powders Tsujimoto et al. (2004) developed a HPP equipment comprised of a roller compactor capable of sterilizing powdered food at a low cost. The experiment was performed on three food powders: corn flour, fennel powder and Chinese herbs inoculated with Bacillus cereus. It was detailed by the researchers that when the compaction rate was above 3 with a linear press force of 6.8 × 104 N/cm, the heat resistant spore colony count falls nearly to detection value. The kinetics of Bacillus cereus inactivation in above study was explained using the Weibull model. The shortcoming of the above method was acknowledged as the size reduction of the treated sample, making it less attractive for manufacturer for the products with medium particle size. So, modification in the developed design has to be performed, making it suitable for microbial inactivation of food powders without modifying its powder properties. In another work, (Marco et al., 2011) reported inactivation of Bacillus cereus spores in food powder reference media through the treatment of HPP combined with the effect of the olive 12

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powder. Statistically greater levels of inactivation were found at 200 and 500 MPa than compared to HPP pressure of 400 MPa (only by 0.5 log cycles) for 10 min. Microscopic analysis explained that, any microbial spores had to be germinated to inactivate as studied by Black et al. (2008). It was observed that HPP pressure of 200 and 500 MPa promoted this spore germination compared to the HPP pressure of 400 MPa, making it less effective. Numerous works have revealed the potential application of HPP technology to Cronobacter sakazakii inactivation on infant formula. The first work on C. sakazakii decontamination by HPP was performed by Gonzalez, Flick, Arritt, Holliman, and Meadows (2006). An inactivation level of 2, 4, 6 log10 cycles was obtained. Pina-Pérez, Rodrigo, and Martínez-López (2011) and Arroyo, Cebrián, Condón, and Pagán (2012) demonstrated that at a low pressure of 100–200 MPa for 10–20 min the inactivation level of C. sakazakii in infant formula ranged between 5 and 7 log10 cycles. Also, based on the inactivation kinetics observed through the above works (Pina-Pérez, Silva-Angulo, Rodrigo & López, 2012) developed a probabilistic growth Weibull model for predicting Cronobacter spp. inactivation by HPP. Furthermore, regards to above works where HPP technique was used for Cronobacter spp. inactivation, it can be noticed that only the most generally used Weibull model was used to study the microbial growth curve. Henceforth it can be suggested that future works need to be performed to study the application of other models such as Log-logistic model and Biphasic-linear model which has proven to be more capable in relating the survival curves of Cronobacter spp. Despite the advantages that HPP inactivates microbial load to desired levels, there are some defects that might slightly affect the food treated quality. As studied by Erdoğdu and Ekiz (2011), HPP promoted chemical reactions that are related to a loss in functional and nutritional properties of the cellular materials, especially proteins. The losses in nutritional components have been reported as a result of prolonged holding time (Houška et al., 2006). Similar outcome was observed by Windyga et al. (2008) on microbiological inactivation of caraway and coriander with a 30 min treatment of HPP at 800 and 1000 MPa. The authors observed 2-log reduction of mesophilic bacteria and total elimination of coliform, molds and yeast. However, the strong dependence of inactivation of microorganisms on water activity is still a considerable obstruction to the use of HPP as an inactivation method in spice and food powder production. Henceforth, to keep our food powder quality to meet standards of export quality it is very vital to optimize the HPP treatment conditions for which more works are requisite to be performed.

inactivate microorganisms present in low moisture foods such as powdered foods, dry fruits etc comparing to other non-thermal techniques in regards to log reduction attained. 3.2.1. Mechanism of action The chief effect of PEF technique is electroporation of the microbial cell membrane, that is, enhanced cell membrane permeability as a consequence of the development of pores in the cell membrane under the influence of an electric field developed surrounding the cell. Use of short pulses for microseconds at varying intensity levels (10–40 kV/cm) has been evaluated in a few of food powders for inactivation of microorganisms without damaging the nutritional and sensorial status of the food powders (Roohinejad et al., 2018). As a result of a collision with direct current pulses of a sample placed between the electrodes in the PEF chamber, an electric field is developed whose intensity depends on the voltage delivered and the gap between the electrodes. The mechanism causing the inactivation of microorganism by PEF has not been completely explored as yet. Moreover, the major reason for the dielectric breakdown of the microbial cell membrane is the development of local instability around cell due to applied field (Rowan, Macgregor, Anderson, Fouracre, & Farish, 2000). According to the intensity of the electric field applied, the rate of structural changes occurring in the microbial cell membrane and the rate of pore formation has been studied to vary. 3.2.2. Microbiological safety of food powders To achieve microbial inactivation from powder surface non-thermally, microbial cell disruption without application of heat is considered as the chief unit operation. The first work on microbial inactivation using PEF technology was to assess the effect of PEF on Escherichiacoli and Bacillus subtilis on prepared pea soup mix when present individually (Vega-Mercado et al., 1996). A decrease of 6.5D and 5.3D were observed at PEF voltage of 33 kV/cm and 30 pulses for Escherichia coli and Bacillus subtilis respectively. The inactivation kinetics of microbes in this study proceeded in the way resembling ultraviolet light and heating. However, the above work deprived in elucidating the rate of inactivation and best model with both targeted organism together. Keith et al. (1997) investigated the outcome of PEF treatment for microbial log reduction of total coliform, E. coli, yeast and mould. At 26, 40 and 28 kV/cm a maximum of less than one-log reduction (Gompertz growth model) was only achieved for the targeted organisms in onion, basil and dill powder sample respectively. Moreover, powder colour, nutrient, taste and flavour were preserved to significant levels even after treatment. Another work was performed by Pina-Pérez et al. (2013), to study the impact of PEF technology (15 kV/cm–3000 μs) in inactivating Cronobacter sakazakii cells present in cocoa flavoured infant milk formula. Experimental results using biphasic model revealed that recommended inactivation levels of 4.4 log10 cycles, was obtained. This was owing to the antimicrobial effect possessed by cocoa naturally, which enhanced the applied PEF resulting in lysis of microbial protoplast at low voltage. However, unlike wide use of PEF for microbial inactivation in liquid and semi-solid food, its application for food powder inactivation was very limited since microbial resistance to decontamination was possibly linked to the occurrence of bacterial spores.

3.2. Pulsed electric field processing (PEF) Pulsed electric field is one of the most extensively studied nonthermal technologies that have been applied to food products for microbial inactivation as well as to preserve product quality significantly till end use. PEF holds the process of exhibiting the food material sandwiched between two electrodes at very high voltages (kV/cm) and short pulses (μs), making electropermeabilization as its chief principle. A schematic diagram of pulsed electric field system is shown in Fig. 2(b). Broad microbial inactivation works have been performed to validate the notion of PEF as a non-thermal food pasteurization process. It was observed that pore formation was the chief cause for cellular rupture of microbial cell after PEF treatment. However as cellular rupture critically depend on the moisture present within the food, food powders that fall under low-moisture food category was not found to be suited for inactivation to significant levels by PEF technique. As a result microbial reductions of only up to maximum of four log reduction have been achieved under PEF technique on food powders say, pea soup mix powder, onion powder, dill powder and PIF (Keith, Harris, Hudson, & Griffiths, 1997; Pina-Pérez, Martínez-López, & Rodrigo, 2013; VegaMercado, Martin-Belloso, Chang, Barbosa-CcAnovas, & Swanson, 1996). These authors showed that PEF was not a potential technique to

3.3. Pulsed light Pulsed light treatment is a more novel non-thermal technology, in which short-time pulses of a heavy power polychromatic flash, ample in UV-C are repeatedly and quickly released on the food powder surface by a xenon lamp. A schematic diagram of pulsed light system is shown in Fig. 2(c). Pulsed light contains a wide spectrum of white light with peak emission between 400 and 500 nm (Kao, Saxena, Wang, Sancar, & Zhong, 2005). The microbial inactivation effects of pulsed light wavelength owing to the absorption of the energy by strongly conjugated 13

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double bonded carbons in nucleic acid and proteins of microorganisms, which rupture cellular metabolism was discovered primarily. Since then, interestingly, the effect of pulsed light on inactivation of microbial load in liquid and powdered food has been studied. Several works published over the past years revealed that the effectiveness of pulsed light treatments on microorganism inactivation relies on various factors such as: Pulsed light = f (number of flashes, pulsed energy level, the distance between sample and lamps, the voltage applied, the spectral range of flashes, the time between contamination and exposure to treatment, the kind of sample treated and the amount and type of microbial contamination) (Koch, Wiacek, & Braun, 2019). However, it can be observed that of the aforementioned several factors pulsed light energy level and number of flashes accounts to be the chief parameters in achieving desired microbial inactivation regards to food powders.

pulsed light. The results revealed that exposure to 2, 4 and pulses of light aided to a reduction of 0.9–1.1, 2.1 and 2.6 log CFU. Similar outcome was observed by, Cheon, Shin, Park, Chung, and Kang (2015) through an experiment to study the effectiveness of pulsed light treatment combined with UV-C in inactivation of Salmonella typhimurium and Escherichia coli on the powdered red pepper. It was reported that reduction range of S. Typhimurium and E. coli on the sample when exposed to the light intensity of 20.4 kJ/m2 for 10 min was 0.29 and 0.22 CFU/g respectively preserving color significantly. Preliminary studies performed for pulsed light technique have revealed that pulsed light had no effect on the sensory properties and nutritional quality of treated samples, thus enabling it to be a safe technology providing high quality and safe product at the end. However, more experiments on the inactivation of spores in food powder using pulsed light need to be performed in the future.

3.3.1. Mechanism of action The mechanism of pulsed light technology has been widely researched. The DNA damage in the microorganisms followed by cellular structure destruction has been considered as the key factor for lethal effect. The destruction of microorganisms initiates with the DNA absorbing UV radiation emitted by pulsed light, followed by the formation of cross-linked pyrimidine nucleoside bases, causing a mutation in the DNA. This practice consequently affects DNA functioning and stops reproduction of the microorganism. Energy level of pulsed light treatment of at least 35 J/cm2 is enough for microbial inactivation, however, there are other factors affecting the efficacy of pulsed light treatment, such as time of exposure, the voltage applied, microorganism species, and thickness of food. Treatment with pulsed light alters the DNA of the microorganism by creating thymine dimmers and finally, results in DNA possessing structure unzipped for reproduction (Keklik, Krishnamurthy, & Demirci, 2012). The photoproducts formed during emission of PL results in DNA breakage, hydration of pyramidins and cross linking of strands resulting in complete inactivation of organism cell (Gómez-López, Koutchma, & Linden, 2012). The final results of these reactions are loss of physiological functions, leading to the inhibition of microbial growth with significantly preserving the initial powder properties to significant levels.

3.4. Ozone processing (OP) Ozone (O3) is a molecule that exists in a gaseous state and is generated from the oxygen molecule owing to the effect of electric discharges in the atmosphere leaving no residues. A schematic diagram of ozone processing system is shown in Fig. 2(d). The antimicrobial property of ozone has recognized several benefits to the food industry, due to its potential over traditional antimicrobial agents (Garud, Negi, & Rastogi, 2019). With the recent FDA approval of ozone as a safe additive, the potential of ozone in food application increased, particularly among fluid foods. Also, the effect of ozone technology on microbial inactivation of food powders has also been investigated effectively. Number of researchers initially performed to investigate the effectiveness of ozone treatment on inactivation of microorganisms in food applications were function of following parameters, OP = f (inlet gas composition, produced ozone concentration, ozone yield, exposure time and storage period of ozone treated samples) (Brodowska, Nowak, & Śmigielski, 2018). However through preparation of this review paper it was studied that treatment proved to be the main parameters to achieve desired level of microbial log reduction for food powders 3.4.1. Mechanism of action The inactivation of microorganisms starts with the microbial cell absorbing ozone molecule, and the mechanism of microbial inactivation is chiefly attributed to the anti-microbial property of ozone. The cell component in the cell wall gets oxidized, owing to the high oxidation potential of ozone. Once ozone molecule starts entering the cell of a targeted organism, all essential components, proteins, DNA, RNA and enzymes undergo complete oxidation, resulting in complete cellular rupture (Brodowska et al., 2018). Giese and Christensen (1954) recommended that the bacterial cell surface is the primary target of ozone activity. Scott and Lesher (1963)identified the leakage of microbial cell contents during ozone treatment. They proposed the double bonds of unsaturated lipids in the cell envelope as the primary site of the attack. Murray, Steed, and Elson (1965) suggested that lipopolysaccharide and lipoprotein layers of gram-negative bacteria would be subjected first to attack by ozone that results in a change in cell permeability, eventually leading to lyses.

3.3.2. Microbiological safety of food powders In regards to microbial inactivation of food powders, black pepper and wheat flour was researched for decontamination of Saccharomyces cerevisiae by Fine and Gervais (2004). At a pulse energy level of 31.12 J/cm2, a log reduction of 2.93 and 0.7 was studied using Baranayi logistic model for black pepper and wheat flour respectively. Despite, it was observed that flavour and colour attributes (ΔE = 4.5 for pepper and ΔE = 4.3 for wheat flour) of the above powders were greatly altered before attaining the set pulsed energy level, owing to overheating. According to the experiments carried the oxidation effect combined with over-heating resulted in a reduction of piperine content from 5.12% to 3.4% in treated black pepper powder. Effect of pulsed light treatment on corn meal has also been investigated. At an energy level of 5.6 J/cm2(100 s pulses) in the 3 cm distance apart, (Jun et al., 2003) reached 4.93 log reduction in Aspergillus niger spore inoculated with cornmeal. In another work, the inactivation effect of pulsed light treatment for Listeria monocytogenes present in infant powdered milk was investigated (Choi, Cheigh, Jeong, Shin, & Chung, 2010). Through this study it was revealed that from an initial density of 105 CFU/g, about 4–5 log reduction of the cell was attained with pulsed light treatment for 5000, 600, 300 and 100 μs at 10, 15, 20 and 25 kV. Researchers also reported that the highest inactivation (power law model) with no critical treatment time was achieved at 25 kV. In addition Nicorescu et al. (2013) assessed decrease of 0.8 log reduction in Bacillus subtilis cells on ground caraway and black pepper powder when exposed to PL treatment of 10 J/cm2. Similarly, (Moreaua et al., 2011) through another work, peppercorn samples inoculated with Bacillus subtilis spore were exposed to 2, 4, 6, 8 and ten flashes of

3.4.2. Microbiological safety of food powders Through an experiment, microbial inactivation achieved on flaked red pepper after exposure to ozone was researched by Akbas and Ozdemir (2008). Ozone concentrations of 1, 5, 7 and 9 ppm for 360 min were applied on Bacillus cereus spore, while 0.1, 0.5 and 1.0 ppm were applied to spores of E. coli. Results using negative Gompertz equation revealed that exposure to O3 concentration of 1 ppm and 5 ppm for 360 min was found as the best combination in achieving maximum inactivation rate of E. coli and Bacillus cereus respectively, maintaining powder size quality appreciably. In way back, (Byun et al., 1998) investigated the comparative effect of gamma irradiation (7.5 kGy) and 14

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ozone treatment (18 ppm) on inactivation effect on total aerobic bacteria and yeast with an initial count of 1.8 x 105 CFU/g and 6.7 × 102 CFU/g of red ginseng powder. It was noticed that compared to ozone treatment, gamma radiation proved to be a better technology achieving up to 3 log reduction. However, it was also studied that gamma radiation and ozone treatment markedly increased the saturated fatty acid concentration of treated ginseng powder. The destruction of double-bonded compounds in fatty acids after treatment was studied as the factor responsible for final increased saturated fatty acid value, reducing its consumer acceptance. Young, Zhu, and Zhou (2016) demonstrated a quick degradation (2 log reduction) of trichothecene mycotoxin produced by Fusarium in rice flour. It was observed that though the influence of pH is not important as far as ozonation of food powder is concerned, trichothecene mycotoxin showed a rapid degradation at pH 4–6 compared to higher pH of 7–8. Proctor et al. (2014), observed a higher degradation of aflatoxin produced by Aspergillus flavus in powdered peanut flour at a higher temperature. The ozone dose of 5 ppm with decreased treatment time ranging from 15 min to 10 min for a temperature enhancement from 25 to 75 °C achieved a degradation of 80% for AFG1 and 77% for AFB1 respectively. Further, the increase in decontamination kinetics in peanut flour was most likely due to the increased surface area compared to kernels. Similarly, (Tiwari et al., 2010) studied that an ozone application of 0.16 mg/gm barley powder resulted in achieving 96% inactivation of fungal spore within 5 min. In contrast, it was observed that generally when levels of ozone dose increased than the recommended range, it negatively affected the organoleptic, nutritional and physical attributes of food. Researches conducted on food powders revealed that, when ozone dose, its application time and type deviated from the critical range, it resulted in discolouration, surface oxidation and unfavourableodour development and Vitamin C degradation, decreasing the consumer acceptance value of the product.

& Thompson, 2012). It was elucidated that photon-induced desorption emitted from UV photons of plasma damaged the chemical bonds in microorganism material and resulted in the development of volatile compound within atoms intrinsic to microorganisms. Henceforth, it can be summarized that the inactivation effect of plasma treatment is attributed to the synergistic mechanism of UV radiation, surface etching and oxidation produced by reactive species, charged particle and ozone. 3.5.2. Microbiological safety of food powders Hertwig et al. (2015) evaluated the feasibility of microwave driven plasma discharge to decontaminate microbial flora inoculated onto crushed oregano, pepper seeds and paprika powder samples. To study the log reduction plates were incubated at 25 °C for 7 days. It was found that plasma treatment decreased the microbial flora of the paprika powder, pepper and oregano powder by more than 3 log10, 3 log10 and 1.6 log10 respectively. The obtained decontamination kinetics was modelled with GInaFit using Weibull model by Geeraerd and Van Impe Inactivation Model Fitting Tool. However, it was studied that plasma treatment > 5 min resulted in considerable loss of redness in paprika powder ((ΔE⁎ = 46.9) and a minor effect on oregano and pepper powder color (ΔE⁎ up to 20.0 and 10.2 respectively). Through another work, dielectric barrier discharge plasma technology (DBD-PT) on combination with radiofrequency thermal technology (RF-TT) at 1000 W and 1500 W was studied to be effective in reducing the microbial count of E. coli to 1 log CFU/g in pepper powder (Choi, Yang, Park, & Chun, 2018). It was demonstrated that combined treatment of DBD-PT and RF-TT caused a negligible change in moisture content, extractable color value and water activity. Kim, Lee, and Min (2014) studied that the number of Aspergillus flavus on red pepper powder was reduced by 2.5 ± 0.3 log spores/g by cold plasma technology operating in nitrogen source at 900 W for an exposure time of 20 min. Weibull model and Fermi’s model was used in this study to adequately describe the inhibition of A. flavus on the red pepper powder. Despite, it was also reported that aw of red pepper decreased when exposed to plasma at 650 and 825 W. The drop in water activity was primarily owing to the reduced moisture content sample, which was found to benefit the product by enhancing storage stability and microbiological safety. Similar outcome was observed by, Kim, Oh, Won, Lee, and Min (2017) on evaluating effect of microwave-integrated cold plasma treatment (MCPT) for inactivating Bacillus cereus spore, E coli and Aspergillus brasiliensis. MCPT at 400 W for 40 min reduced the count of A. brasiliensis, Bacillus cereus spore and E. coli by 1.6 log spore/cm2, 2.1 log spores/cm2 and 1.9 CFU/cm2 respectively. Researchers also revealed that contradicting NTP, MCPT treatment did not alter the antioxidant activity, quercetin content and colour of treated onion powder owing to the synergistic effect produced by electromagnetic radical species in MCPT. According to studies conducted by Kim et al. (2018), red pepper powder inoculated with spores of Aspergillus flavus and Bacillus cereus was treated with MCPT at power density 0.17 W/m2 and 0.15 W/m2. The count of A. flavus and B. cereus reduced by 0.7 ± 0.1 log spores/cm2 and 1.5 ± 0.2 log spores/cm2. Thus the researchers concluded that MCPT at higher power density provided effective microbial inactivation, improved stability, microbial safety and maintained the powder quality. More recently, (Go, Park, Kim, Choi, & Jeong, 2019) demonstrated the successful inactivation of Fusarium oxysporum in paprika powder by non-thermal atmospheric plasma. In vivo assays revealed that NTP treatment at 1000 W for 90 s inhibited the growth of targeted fungal pathogen by 50%. In the above study to evaluate the morphological changes, FS-SEM analysis was performed, and relevant structural modifications were observed in the sample treated with plasma when compared to control samples. Another experiment was performed to evaluate the effect of treatment time and frequency on the inactivation of three bacteria species critically found on onion powder (Kim & Min, 2018). The cold plasma treatment with helium gas was performed for 2–20 min at 15–35 kHz. The exposure of onion powder to 15 kHz

3.5. Non-thermal plasma (NTP) Plasma, the fourth state of matter, refers to a completely and partially ionized gas composed necessarily of ions, atoms, nuclei and molecules making plasma electrically neutral. The food product to be treated is exposed directly or indirectly to the gas species ionized by an external energy source or electric field (Bourke, Ziuzina, Boehm, Cullen, & Keener, 2018). Plasma was studied to depend on few equipment and sample parameters for its significant microbial inactivation; NTP = f (inlet gas composition, voltage, treatment time and relative humidity). A schematic diagram of non-thermal plasma system is shown in Fig. 2(e). Presently, NTP technology was found to be used widely in food and chemical industries for enzyme inactivation, surface structure modification, pesticide degradation, seed germination and volatile oil extraction. It was observed that since the mid-twentieth century though microbial inactivation property of NTP was explored, its application was limited. However, recently, the development in research has shown the great potential of plasma technology as a novel decontamination method producing a lethal effect on a wide range of microorganisms in solid as well as liquid foods. 3.5.1. Mechanism of action The lethal effect of non-thermal plasma is either through direct or indirect exposure of food product to feed gas. In direct exposure: the plasma generated damages DNA of a targeted organism, consequently restricting cell division by ceasing DNA synthesis and eroding cell membrane (Mandal, Singh, & Singh, 2018). The indirect effect occurs owing to the chemical interaction of generated plasma radicals (O, OH, etc.), or charged particles (electrons, and atomic or molecular ions); excited or reactive molecules (O2, O3, NO, etc.) with the cell membrane of microorganisms. The exact mechanism of plasma technology for inactivating bacterial spores was studied to rely on the elementary process of, erosion of microorganism through photo-desorption (Fernández 15

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resulted in a high reduction of Listeria monocytogenes, E. coli and Salmonella enteric by 1.1 ± 0.3,1.4 ± 0.1 and 3.1 ± 0.1 logs CFU/cm2, respectively. The microbial inactivation of all targeted pathogens was studied using the Fermi model and the Weibull model. Moreover, it was concluded that the Fermi model precisely explained the microbial inactivation with R2 value nearing to 0.84–0.94. Powders’ being very fine and dispersive with high probability to take up moisture on exposure to ambient conditions, the packaging is a crucial unit operation to be performed. In-package decontamination by NTP has enormous potential and received increasing attention from the food industry recently (Patil et al., 2014). It is achieved by applying an electric field to ionizing the gas inside the package (Misra et al., 2014). Hereafter, for future directions of powder industry In-package cold plasma treatment has a huge potential to scale up the commercial application and also to inhibit the recontamination during the packaging step (Misra, Tiwari, Raghavarao, & Cullen, 2011). Henceforth, further works on the effect of in-package cold plasma on the quality of food powders are considered necessary.

ultraviolet (UV-C) irradiation and mild heating (heating at 65 °C for 10 min) for foodborne pathogens, including Escherichia coli O157:H7 and Salmonella Typhimurium on powdered red pepper (Cheon et al., 2015). The inactivation levels of S. Typhimurium and E. coli O157:H7 on powdered red pepper when exposed to UV-C irradiation alone at 20.4 kJ/m2 for 10 min was 0.29 and 0.22 log CFU/g, respectively. While, additive treatment with mild heating at 65 °C decreased the surviving numbers of each pathogens by 3.06 and 2.88 log CFU/g, respectively. Extractable color value and CIE color value were not significantly (p > 0.05) different between non-treated and combination treated samples. The capsaicinoids contents were significantly increased by UV-C treatment, thus enhancing the pungency and flavor of final red pepper powder. These variation of capsaicinoids content could be owing to drying of samples followed by combined UV-C treatment and mild heating at 65 °C. Therefore, these observations suggest that UV-C irradiation with mild heating could be applied by the food industries in order to efficiently inactivate S. Typhimurium and E. coli O157:H7 with a minimum quality deterioration of powdered red pepper. Commercial heat treatment aided to quality changes in powdered red pepper, such as flavor and color degradation. To reduce the quality degradation of spices (Rico et al., 2010) observed that it is more effective to merge ultraviolet radiation with mild temperature heating techniques for inactivation of E. coli O157:H7 and S. Typhimurium than to treat with UV-C technology alone. The combined effect of UV-C (12.5 mW/cm2) + irradiation (10 kGy) resulted in a 5-log reduction with minimal effects on the physicochemical properties, except for the decreased content of capsanthin. In addition, (Ha & Kang, 2014) determined the synergistic effects of the application of IR heating and UV irradiation in powdered infant formula against Cronobacter sakazakii. Corresponding NIR-UV (near infrared-ultraviolet) combined application for 7 min attained a 2.79-log-unit CFU reduction of targeted organism. The color values and sensory characteristics of NIR-UV-treated PIF were not appreciably different from those of the control. Henceforth, from the above research it can be suggested that the NIR-UV inactivation system can be implied as an alternative to other interventions in PIF. However, one of the limitation of above work is that the pilot instrument used in the current research was batch type with comparatively small capacity, therefore future work has to be performed that expands the practical industrial scale by using it in the form of continuous line processing. Several non-thermal techniques, say, non-thermal plasma, pulsed light, ozone processing have been greatly studied for decontamination of food powders, whereas additive effects of non-thermal technology combinations with a thermal technique or another non-thermal technique though are very promising in tackling highly resistant species in food powders, still lack proper research to prove and apply it for industrial applications. Henceforth, more studies are still needed to develop new combinations of hurdle technologies and to scale them up so as to satisfy the requirements of powder industry.

4. The application of the hurdle or synergistic technologies for prolonging the shelf life of powdered foods As stated by Leistner (2000), food preservation applies putting the microorganisms in hostile environments to inhibit their multiplication or to cut down their survival and cause their complete inactivation. This target could be attained through one powerful stress or with various hurdles, acting at the same period of time through a combination (Leistner, 2005) or in an individual manner. For a wide period, the scientific community has concentrated on the multi-target preservation. It has been reported that different hurdles (pH, aw, antimicrobials, modified atmosphere) in food samples could act synergistically, i.e. if two technologies are applied to one food (A and B), the antimicrobial effect is not simply A + B, but each element could be powered by the other. The multi-target preservation system is based on the notion that many hurdles or techniques would result in multi-target disturbance of DNA, cell membrane, inner cellular components and enzymes thus making it more difficult for the microorganisms to restore damages and to reactivate of partially destroyed proteins (Lambert & Bidlas, 2007). Novel thermal and non-thermal techniques showed to have a good potential for attaining these goals individually, but the further approach combining two or more emerging thermal treatments, nonthermal treatments and a thermal-non thermal techniques has been proven to gain higher efficacy at the industrial scale. Table 3 provides a summary of the outcomes of studies on microbial reduction levels achieved for powdered foods and their effect on powder properties after exposure to hurdle technology applications. Combined superheated steam (SHS) and infrared heat treatments have been researched for Salmonella inactivation on dried almonds and grated/powered almonds (Bari et al., 2010). The combination was found to deliver advantages like accelerated drying rate and high heat transfer. It was found that heat treatment on grated/powered almonds followed by infrared treatment for 70 s was able to decrease Salmonella population by 5.73-log CFU/g (compared to 1.56- log CFU/g and 2.83 -log CFU/g under SHS and IR respectively when applied individually), and no survivors were found in the enrichment medium thereafter. The quality attributes, along with the microbiological parameter, propose that SHS treatment proceeded by exposure to infrared is effective in eliminating the population of pathogens on almonds without significantly affecting their overall quality (Bari et al., 2010). Erdoğdu and Ekiz (2011) investigated the effect of infrared and UV-C treatment (10.5 mW/cm2 for 2 our) on microbial inactivation and quality of cumin seed powder. Total mesophilic bacteria of the cumin powders were reduced to the target level of 104 CFU/g after 1.57, 2.8, and 4.8 min of infrared treatment at 300, 250, and 200 °C, respectively, followed by 2 h ultraviolet-C (UV-C) treatment. Another work was performed to study the efficacy of combined

5. Comparison of efficacy of thermal and non-thermal techniques The application of novel thermal and non-thermal technologies was demonstrated to hold high potential for producing food powders of high quality and safety. Infrared, radiofrequency, microwave and DIC were revealed to be the novel thermal technologies that proved its effectiveness in achieving desired log reduction on food powders. Out of aforementioned four technologies, first three were studied to be widely applied for inactivation of bacterial spores (Bacillus cereus, Salmonella typhimurium, E. coli) present mostly in spice powders while, latter was found promising in tackling the most resistant bacterial and fungal spores (Aspergillus niger, TMY) present in spice and vegetable powders with inactivation levels around 5 log10 cycles (Pina-Pérez et al., 2014). However, it was established that contact to electromagnetic heat waves through infrared, microwave and radiofrequency treatment resulted in powder quality degradation. The thermo-mechanical impact, coupled 16

17

UV-C+ Heating

IR+UV-C

SHS+IR

UV-C+ Irradiation

IR+UV

IR heating + UV irradiation RF-TT+DBD-PT

IR+ Haeting

IR+ MW

Powdered red pepper

Cumin seed powder

Dried grated/powdered almonds Powdered red pepper

Dried fig powder

PIF

Redpepper powder

Dried grape seed powder

Paprika powder

2

IR heating at 135 °C followed by conventional heating at 75 °C IR at 300 °C & MW at 800 W for 100 s

Combination of 1500 W RF heating and 1000 W DBD-PT

UV-C irradiation at 12.5 kJ/m and irradiation at 10 KGy IR (5.6 mW/cm2/nm; 5, 10, 20, and 30 s) and UV (20 mW/cm2; 1, 3, 5 s) NIR-UV applied application for 7 min

2

IR treatment at 300, 250, and 200 °C,respectively, followed by 2 h UV-C treatment at 10.5 mW/cm2 SHS (for 70 s) and IR (90 °C for 70 s)

UV-C irradiation at 20.4 kJ/m and heating at 65 °C

Treatment condition

Mesophilic aerobic bacteria & Mould

TMAB & TMY

E. coli & Staphylococcus aureus

Cronobacter sakazakii

E. coli O157:H7 and S. Typhimurium Rhodotorula mucilaginosa

Salmonella

Total mesophilic bacteria

Escherichia coli O157:H7 and Salmonella Typhimurium

Microbial targets Additive treatment decreased the surviving numbers of each pathogens by 3.06 and 2.88 log CFU/g A complete elimination for total mesophilic bacteria were reported Decreased Salmonella population by 5.73-log CFU/g Reduced surviving numbers of each targeted organism by 5 log reduction Effective inactivation by achieving > 3 log CFU/cm2 Attained a 2.79-log-unit CFU reduction of targeted organism Microbial counts reduced below the detection limit (1 log CFU/g) after 2 repetitions of the treatment cycle TMAB & TMY reduced by 1.97 ± 0.12 and 0.62 ± 0.09 log CFU/g Less effective in the reduction of the mesophilic aerobic bacteria & levels of moulds were significantly reduced (5D)

Salient results

↔ Chemical parameters ↓ Color

↓ Iodine value

↔ Color values and sensory characteristics ↓ Extractable color, moisture content, water activity

↔ Color values

↓ Capsanthin

↔ Volatile oil content and color ↔ Overall product quality

↔ Extractable and CIE color

Parameters affected

Molnár et al. (2018)

Fu et al. (2019)

Ha and Kang (2014) Choi et al. (2018)

Rico et al. (2010)

Erdoğdu and Ekiz (2011) Bari et al. (2010)

Cheon et al. (2015)

Reference

Where (↓) refers to decrease in the level; (↑) refers to increase in the level; (↔) refers to neither increase nor decrease in the level. SHS: super heated steam; IR:infrared: UV-C: ultraviolet- C; NIR-UV: near infrared ultraviolet; RF-TT: radiofrquency thermal treatment; DBD-PT: dielectric barrier discharge plasma treatment; MW: microwave; TMAB: total mesophilic aerobic bacteria; TMY: total mold and yeast.

Hurdle technology

Food powder

Table 3 Microbial log reduction levels in powdered foods and their effect on powder quality after hurdle technology applications.

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with an instant pressure drop to vacuum in DIC technology aided to achieve high contamination levels relevantly, preserving powder colour, expansion ratio, porosity and functional properties to significant levels. In recent years, special attention has been paid to DIC technique as a future potentially effective method for treating food powders and granules at non-intensive treatment condition, retaining the maximum nutritional quality, enhancing the powder surface area and preserving food powder. In, pulsed light, pulsed electric field and ozone processing the structural changes that occurred within the macromolecular components of organism cell owing to photoproduct formation, electroporation and antimicrobial effect of feed gas respectively restricted further spore proliferation. During last decades, various works have confirmed the inactivation effect of pulsed light, pulsed electric field and ozone processing in various food powders. In contrast, exposure to HPP though retarded growth of targeted organism to significant levels was found to affect the nutritional quality, extractable colour and micromolecular structures within the food. Among the emerging technologies overviewed, NTP seems to be the most competitive considering the rapid inactivation of microbial spore. This is owing to the synergistic effect of UV radiation and reactive species generated using plasma production. Additionally, the electric field produced by plasma in direct mode aided complete inactivation of microorganisms from 2 to 9 log10 cycles through various food powders. However, all of these nonthermal techniques are still in an infant stage of expansion concerning their application in food powder industry. The most suitable techniques (combining both thermal and nonthermal) regards to preserving the nutritional, sensorial and physical properties of food powders can be categorized as: DIC > non-thermal plasma > pulsed light > pulsed electric field > high hydrostatic pressure > radio frequency > ozone processing > microwave > infrared. However, the most promising technologies, regards to overall food microbial safety, quality and practical point of view, to be directly treated on food powders, can be ranked as follows: non-thermal plasma > pulsed light > infrared > high hydrostatic pressure > DIC > pulsed electric field > radio frequency. Henceforth it can be generalized that, the use of emerging thermal technologies in addition to nonthermal technologies materialize to afford better treatment duration. Use of these novel technologies was found proven to be able to manufacture powders with superior nutritional qualities and sensory properties, better than those after conventional thermal processes, with attain the same level of safety useful for products that are not sensitive to heat.

of the treatment, process parameters, food medium and load of contamination. Regards to the product, advancement of efficient treatment systems capable of preserving powder nutrition property and microbial safety owing to high-temperature exposure are the important consideration regards to the industry. Incorporation of a cooling system or an efficiently advanced technology fluence approach is vital to decrease temperature hike when taking into account scaling up. The current statistical reports disclosed by industries in the area of food powder decontamination indicates that microbial safety can be enhanced at a reduced operational cost by use of these emerging technologies (Nath, Kale, & Bhushan, 2019). Henceforth, a thorough assessment of costs associated with initiating, operating and maintaining the process line and product line leading to the practical application of these emerging technologies to food powder industries are still necessary. 7. Conclusion Advanced thermal and non-thermal technologies have proven to be able to produce food powders with greater nutritional qualities and sensory properties, better than those after conventional thermal processes, and attain augmented level of safety. Moreover, it is vital to perform out studies on a large scale to commence this disinfection technique at an industrial level. Despite the ability to reduce microbial counts, to improve quality parameters, and to extend shelf life, the application of non-thermal technologies in the powder industry is still limited due to the high initial investment and the requirement of large production volumes that cannot be accommodated by the accessible present processing lines. In certain cases, regards to the legal issues reported in few above technologies, interest among consumers are arising in the incorporation of natural antimicrobial products extracted from plant or animal source. However, it is probable that most of these technologies, owing to their particularities, will find robust niche application in the food industry, substituting conventional processing technologies, either through their individual application or synergistic interaction (hurdle technology). Several emerging technologies, for example, non-thermal plasma, pulsed light, DIC, PEF, infrared and radiofrequency have been widely studied for inactivation of Cronobacter sakazakii, Salmonella typhimurium, E. coli and Bacillus cereus and spoilage yeast. Furthermore, hurdle effects of novel technology combinations though was found very promising for tackling the more resistant spores and species, but further research is needed to prove its additional benefits on maintaining food powder quality. Further works are still required to make the technologies more reasonable and to scale them up to suit the needs of the powder industry. It is emphasized that the processes professed to be the most environmentally friendly will be more expected to succeed in industrial applications. Conclusively, more research is needed to ensure that these emerging technologies could provide complete sterilization of food powders preserving its quality and ensure that the spores do not undertake injury repair during storage or powder reconstitution.

6. Future trends of thermal and non-thermal techniques in food powder industry To authenticate the matching of technology and product, the next pace would be to scale up the same to an industrial level. Within past decade high pressure processing, ozone processing, microwave, infrared has been commercialized in most of the countries around the globe, while pulsed electric field (PEF) and DIC have marked narrow commercialization chances (Morales-de La Peña, Salvia-Trujillo, RojasGraü, & Martín-Belloso, 2010). However, most other sterilization nonthermal technologies have not been commercialized for powdered food yet. Therefore, the application of these emerging technologies to decontaminate food powder still demands more efforts to gain industrial scale for direct food decontamination. All the techniques discussed in this work seems worthy for microbial inactivation, with each of them possessing specific merits and demerits. Therefore, it is vital to assess and establish the specific combination of processing technique and product (food powder), taking into account the powder characteristics and the lethality of the stress towards microbial spore. The existing constraint of these technologies in the industrial level is related to high initial investment costs and lack of data regarding process control parameters. To eradicate these problems, it is important to optimize the treatment condition and considering the productiveness

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