Plasma modification of starch

Accepted Manuscript Plasma modification of starch Fan Zhu PII: DOI: Reference:

S0308-8146(17)30586-1 http://dx.doi.org/10.1016/j.foodchem.2017.04.024 FOCH 20894

To appear in:

Food Chemistry

Received Date: Revised Date: Accepted Date:

31 January 2017 11 March 2017 4 April 2017

Please cite this article as: Zhu, F., Plasma modification of starch, Food Chemistry (2017), doi: http://dx.doi.org/ 10.1016/j.foodchem.2017.04.024

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Plasma modification of starch

Fan Zhu * School of Chemical Sciences, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand * Corresponding author, email: [email protected]

Abstract Plasma is a medium of unbound negative and positive particles with the overall electrical charge being roughly zero. Non-thermal plasma processing is an emerging green technology with great potential to improve the quality and microbial safety of various food materials. Starch is a major component of many food products and is an important ingredient for food and other industries. There has been increasing interest in utilizing plasma to modify the functionalities of starch through interactions with reactive species. This mini-review summarises the impact of plasma on composition, chemical and granule structures, physicochemical properties, and uses of starch. Structure-function relationships of starch components as affected by plasma modifications are discussed. Effect of plasma on the

1

properties of wheat flour, which is a typical example of starch based complex food systems, is also reviewed. Future research directions on how to better utilise plasma to improve the functionalities of starch are suggested.

Keywords: starch modification, physicochemical property, structure, application, nonthermal processing, wheat flour 1. Introduction Plasma processing is a physical treatment that induces chemical changes. Non-thermal plasma is suitable for processing thermo labile materials without generating industrial waste, making it a novel green technology. It has great potential to be used in food and other industries (Scholtz et al., 2015; Thirumdas et al., 2015; Mir et al., 2016; Sarangapani et al., 2016). A notable application of plasma is the disinfection and sterilization to ensure microbial safety for food and medical purposes (Moreau et al., 2008; Scholtz et al., 2015). Compared with some traditional disinfecting methods, plasma processing has the advantage of little affecting the quality attributes of food material (Scholtz et al., 2015). Plasma based technology is now commercially available for disinfecting (Scholtz et al., 2015). Apart from ensuring microbial safety, non-thermal plasma may influence other quality attributes of food material, which has been reviewed recently (Thirumdas et al., 2015; Mir et al., 2016). For example, a low-pressure plasma decreased the cooking time of rice while improving the eating quality; a cold argon plasma decreased the enzyme activity of peroxidase and polyphenoloxidase in a model food system; a microwave processed air plasma better retained the color of freshly cut kiwifruit; a surface discharge reactor air plasma increased the germination rate of wheat seeds; a cold atmospheric pressure argon plasma enhanced the anthocyanin concentration in sour cherry; a cold atmospheric air plasma 2

improved the dough strength of wheat flour; and a cold oxygen plasma decreased the emulsifying and foaming capacity of whey protein isolate (Thirumdas et al., 2015; Mir et al., 2016). The above mentioned effects are due to the interactions of food components with the reactive species in the plasma. Therefore, understanding the plasma-food component interactions provides a basis for its further development of this green food processing technique. Starch is a major component of many food products. It is widely used in diverse food and non-food industries. Starch is consisted of amylose and amylopectin molecules, which are naturally assembled in granular forms with the size ranging from 1 to 100 µm (Pérez & Bertoft 2010). Native starch has limited functionalities and is commonly modified to expand the functional range (BeMiller & Whistler, 2009). There has been a trend of using green technology to modify the starches without generating any waste products, and plasma treatment falls into this category (Chaiwat et al., 2016). However, there is a lack of systematic knowledge on the effect of plasma processing on starch properties. This mini-review firstly briefs the plasma generation and chemistry to provide basic background information. Then, the impacts of plasma processing on the chemical composition, chemical structure, granular and crystalline structure, physicochemical properties, and applications of starch are summarised. Structure-function relationships of starch components affected by plasma processing are discussed. The influence of plasma treatment on the functional properties of wheat flour (a typical starch based food system) is also reviewed, with an aim to illustrate the contributions from the changes in non-starch components (e.g., protein) to the overall modification of complex food systems. Lastly, research gaps are pointed out to better understand the plasma-starch interactions. The basics of starch have been well reviewed previously and are, therefore, not covered here (BeMiller

3

& Whistler, 2009; Pérez & Bertoft, 2010). This review may stimulate further interest in utilising plasma for food ingredient processing to create novel functionalities in a green way.

2. Plasma generation William Crookes firstly identified and described the plasma as “radiant matter” in a Crookes tube in 1879 (Crookes, 1879). The term “plasma” was introduced by Irving Langmuir in the middle 1920s (Mott-Smith et al., 1971). Plasma, together with gas, liquid, and solid, are the four basic states of matter. It is a medium of unbound negative and positive particles with the overall electrical charge being roughly zero (Moreau et al., 2008; Scholtz et al., 2015). The particles of plasma include electrons, photons, negative and positive ions, free radicals, atoms, and non-excited or excited molecules (Moreau et al., 2008). There are two types of plasma, namely thermal and non-thermal, based on the conditions in which they are generated. The former is generated at high pressure (≥ 105 Pa) with high power input (up to 50 MW). The temperature can be very high (up to 2 × 10 4 K) (Moreau et al., 2008). In contrast, non-thermal plasmas are produced at lower pressures with much less power input. The non-thermal plasmas are of interest for food processing (Thirumdas et al., 2015). Plasma can be produced by the use of energy in various forms such as thermal, magnetic or electric fields, and microwave or radio frequencies (Thirumdas et al., 2015). The treatment increases the thermodynamic energy of the electrons and the collisions of particles, resulting in the formation of plasma (Thirumdas et al., 2015). A range of techniques have been used to obtain non-thermal plasmas. They include corona discharge, dielectric barrier discharge, microwave discharge, gliding arc, and plasma jet (Scholtz et al., 2015) (Supplementary Fig. 1). These discharges differ in the instrumental design and settings, and may have different impact on the food quality. Various plasma processing systems have been set up (e.g., 4

Supplementary Figure 2). Currently, plasma jet is the mostly used in practical applications because of the easy maintenance and simple design (Scholtz et al., 2015). A range of gas such as helium, nitrogen, argon, oxygen, hydrogen, air, and their mixtures, have been used to generate the plasmas. The majority of reactive species in plasmas from common sources are vibrationally and electronically excited nitrogen N2 and oxygen O2, reactive nitrogen species (e.g., atomic nitrogen N, nitric oxide NO•, excited nitrogen N2(A)), active forms of oxygen molecules and atoms (reactive oxygen species) (e.g., ozone O3, superoxide anion O2−, singlet oxygen 1O2, and atomic oxygen O), and OH− anion, H2O+, OH• radical or hydrogen peroxide (H2O2) if moisture is present (Scholtz et al., 2015). There are other agents associated with plasmas such as UV radiation, heat, and electric field (Guo et al., 2015). All these plasma components interact with food components and microbes in food products, altering food properties and ensuring food safety (Mir et al., 2016). It should be noted that the composition of these reactive agents differs greatly among various types of plasma and experimental conditions. The molecular mechanisms for the interactions of these agents with food biomolecules remain to be better studied (Guo et al., 2015; Scholtz et al., 2015; Mir et al., 2016). All the reactive species immediately disappear when the power is off (Mir et al., 2016). Therefore, plasma processing is ecologically and environmentally green.

3. Effect of plasma treatment on starch composition The chemical composition of starch can be greatly affected by the plasma treatments (Supplementary Table 1). The plasma treatment decreased the moisture content of starch (Lii et al., 2002a; Chaiwat et al., 2016). For example, a low-pressure argon plasma (180 min) reduced the moisture content of cassava starch from 13.5 to 6.2% (Chaiwat et al., 2016). This is probably due to the interactions of the reactive species with the water and starch molecules surrounding the water molecules. The interactions result in the release of bound water 5

molecules from the starch, and the released water is subsequently removed by the vacuum (Chaiwat et al., 2016). Amylose is a major component of starch. Plasma treatment reduced the amylose content of starch (Thirumdas et al., 2017). For example, a cold air plasma (60 W) decreased the apparent amylose content (quantified by iodine binding-spectrophotometry based method) of rice starch from 30 to 23% (Thirumdas et al., 2017). This may be due to the extensive degradation of the amylose as a result of interactions with the reactive species. The decreasing molecular size of starch by plasma treatment is described in the following section 4.2. Plasma treatment reduced the pH of starch solution (Lii et al., 2002a and 2002b; Thirumdas et al., 2017). For example, a low-pressure glow plasma reduced the pH of starches from different sources by 1.4−2.8 (Lii et al., 2002b). The reactive species of the plasma interact with starch molecules to introduce new acidic functional groups (e.g., carboxyl group) attached to the starch molecules as well as the formation of new compounds, reducing the pH of the solution (Khorram et al., 2015). The newly formed functional groups can be identified by various techniques [e.g., FTIR (Fourier transform infrared) spectroscopy] as described in the following section. The extent of changes in the chemical composition depends on the type of starch and plasma as well as the experimental conditions (e.g., power input) (Supplementary Table 1). For example, in a comparative study, a low pressure oxygen plasma reduced the pH of rice starch solution to a greater extent (by ~2.6) than that of maize starch (by ~1.4) (Lii et al., 2002b). This could be attributed to the different types and concentrations of reactive species from different plasmas as well as the differences in starch composition and structure. How the starch composition and type may affect their susceptibility to plasma treatment remains to be systematically established. Other minor components such as proteins, lipids, and phosphorus-containing compounds in starch granules may also be affected by the plasma treatment through their interactions with 6

reactive species (Mir et al., 2016; Thirumdas et al., 2015). However, no studies on these aspects have been reported yet.

4. Effect of plasma treatment on starch structures 4.1. Iodine binding property A low-pressure air glow plasma generated from DC supply or AC inductor increased, had little effect, or decreased the λmax and the adsorption intensity of iodine-starch solution, and the extent of changes depended on the starch type (Lii et al., 2002a). Another study showed that the plasma treatment (cold air plasma at 40 W and 60 W for 5 and 10 min) increased the blue value (640 nm) of rice starch solution from 0.1 to 0.13 (Thirumdas et al., 2017). The changes in iodine binding property indirectly reflect the changes in starch composition and structure. The decrease in λmax and adsorption intensity may be due to the amylose degradation, whereas the increase may be due to an increased amount of amylose released from the disruption of amylose-lipid inclusion complexes. The altered amylose and amylopectin structures may also contribute to the altered iodine binding behaviours. The changes in the chemical structure of starch are further described in the following section.

4.2. Molecular structure Plasma induces various chemical changes to the starch, which include de-polymerization, cross-linking, and formation and grafting of new functional groups. Various analytical techniques have been employed, which include HPSEC-MALLS-RI (high-performance sizeexclusion chromatography coupled with multi-angle laser light scattering and differential refractometry detection), FTIR spectroscopy, NMR (nuclear magnetic resonance) spectroscopy, and XPS (X-ray photoelectron spectroscopy) (Table 1). 7

Plasma treatment decreased the molecular weight and radius of gyration of starches to various extents, depending on the type of starch and plasma (Lii et al., 2002a, 2002b, and 2002c; Zhang et al., 2014; Bie et al., 2016a). For example, a dielectric barrier discharge air plasma (treatment time: 10 min) reduced the weight-averaged molecular weight of maize starch from 19.34 × 10 6 to 0.98 × 106 g/mol (Bie et al., 2016a). It appeared that larger molecules were more susceptible to plasma degradation than smaller molecules (Lii et al., 2002b). Potato starch appeared to be more susceptible to plasma degradation than maize starch (Zhang et al., 2014). This may be due to a higher amount of water trapped in potato starch granules (Zhang et al., 2014). Also, potato starch tends to have a large molecular weight than maize starch, making them more susceptible to molecular alteration (Swinkels, 1985). In contrast to the above reports, one study showed that nitrogen and helium glowplasma treatments increased the molecular weight and radius of gyration of potato starch (Zhang et al., 2015). For example, the helium glow-plasma treatment for 60 min increased the weight-averaged molecular weight of potato starch from 6.11 × 107 to 10.42 × 107 g/mol (Zhang et al., 2015). The increase in molecular size could be mainly due to crosslinking/polymerization of starch molecules. The type of plasma may greatly affect the molecular changes of starch (Lii et al., 2002b). For example, ammonia and hydrogen plasmas reduced the molecular weight of cassava starch (9.35 × 107 g/mol) to 1.59 × 107 and 5.79 × 107 g/mol, respectively (Lii et al., 2002b). This discrepancy could be readily attributed to the differences in the composition of reactive species in different plasmas. Reactive species may induce cross-linking among starch molecules, which can be detected by FTIR spectroscopy (Wongsagonsup et al., 2013; Deeyai et al., 2013; Khorram et al., 2015; Chaiwat et al., 2016; Bie et al., 2016a). The content of C–O–C bonding, which may reflect the degree of cross-linking in starch, was increased by plasma treatment (Deeyai et al., 2013). Furthermore, 1H-NMR analysis revealed the loss of OH groups in starch treated by a glow 8

discharge argon plasma, which may be due to cross-linking (Zou et al., 2004). NMR analysis revealed that an argon plasma induced the cross-linking at the C-2 position of wheat starch (Khorram et al., 2015). Prolonged plasma treatment may induce the molecular degradation as just described in the above paragraph, which may functionally counteract the effect of crosslinking on starch (Wongsagonsup et al., 2013; Chaiwat et al., 2016) (Supplementary Fig. 3). New structural units may be formed by plasma treatment (Khorram et al., 2015; Bie et al., 2016a). Carbonyl groups were formed by oxygen/air plasma treatment due to oxidation, and NMR analysis showed that the oxidation occurred at the C-6 position of starch (Khorram et al., 2015). Grafting reactions may occur between the starch and plasma. The ethylene was grafted onto rice and sweet potato starches in a low-pressure ethylene glow plasma. However, the grafting did not occur on cassava, potato, normal and waxy maize starches (Lii et al., 2002c). The reasons are not clear, and confirmative studies are certainly needed in the future. XPS analysis showed that no new elements were introduced on the surface of maize starch by an air dielectric barrier discharge plasma (Bie et al., 2016a). Therefore, changes in the molecular structure of starch granules appear to depend on the type of starch and plasma as well as the treatment length. However, there are many questions that remain to be explored. They may include, but are not limited to, the fine, internal and cluster structure of amylopectin as affected by the plasma treatment; the limit of the extent of modification in starch structure that plasma treatment can achieve; the treatment conditions in relation to the degree of starch structural modification; the impact of starch moisture content on the modification outcome; and the effect of starch type on the outcome of plasma modification.

4.3. Granule morphology 9

Plasma treatment may alter the granule morphology of starch to various extents, depending on the treatment conditions as well as the type of plasma and starch (Table 2). Various microscopic techniques [color-meter, scanning electron microscopy (SEM), confocal laser scanning microscopy, and optical microscopy] have been used to probe the morphological changes. A low-pressure argon plasma increased the b* (yellowness) of cassava starch. The a* (redness) increased before decreasing with increasing plasma treatment time. Plasma had no effect on the L (lightness) (Chaiwat et al., 2016). Similar results were observed on the color of starch treated by a low-pressure glow ethylene plasma (Lii et al., 2002c). The nature of the pigments formed on starch surface is not clear. On the granule surface, plasma may induce the formation of fissures, cavities, corrosions, and tiny deposits (Thirumdas et al., 2017; Lii et al., 2002a). A glow discharge argon plasma induced the partial merging of the granules (Zou et al., 2004). An air dielectric barrier discharge plasma also enlarged the size of the channels in the granules while partially fracturing the granules (Bie et al., 2016a). However, plasma treatment may not induce any change on the granule surface if the treatment conditions are mild enough (Bie et al., 2016b; Thirumdas et al., 2017; Zhang et al., 2015). A nitrogen plasma had no effect on granule morphology, whereas a helium plasma created surface corrosion (Zhang et al., 2015). This difference could be readily attributed to differences in the plasma composition. A cold air plasma treatment increased the number of visible “granular” remnants in the cooked starch paste (Wongsagonsup et al., 2013). This may be due to cross-linking induced by plasma as described in the section 4.2.

4.4. Crystallinity

10

The nature of granular crystallinity of starch affected by plasma treatment has been studied by various techniques including WAXS (wide-angle X-ray scattering), SAXS (small-angle X-ray scattering), NMR spectroscopy, and polarized light microscopy (Table 3). Plasma treatment had no effect on the polymorph type of starch (Lii et al., 2002c; Zou et al., 2004; Thirumdas et al., 2017; Zhang et al., 2014; Zhang et al., 2015). The relative degree of crystallinity was decreased by plasma treatment as revealed by WAXS (Thirumdas et al., 2017; Zhang et al., 2015; Bie et al., 2016b). For example, a cold air plasma (10 min) reduced the degree of crystallinity of rice starch from 43 to 37% (Thirumdas et al., 2017). Birefringence of maize starch granules became less obvious after an air dielectric barrier discharge plasma, indicating the partial disruption of crystallinity (Bie et al., 2016a). The decreased degree of order in starch was also reflected by NMR analysis, which showed that the plasma treatment decreased the proportions of both single (V-type) and double helices (Zhang et al., 2015). The decreased degree of crystallinity could be readily attributed to the interactions of reactive species with the starch molecules, which may lead to molecular scission and granular corrosion as described in the sections 4.2 and 4.3. One report showed that plasma treatment increased, had little effect, or decreased the degree of crystallinity of starch (Lii et al., 2002b). This discrepancy may be due to the differences in the reaction conditions, plasma composition, as well as the quantification methods. Confirmative studies are certainly needed. SAXS analysis showed that the lamellae structure of starch granules may be affected by the plasma treatment (Zhang et al., 2014; Zhang et al., 2015; Bie et al., 2016b). Plasma decreased the compactness of the scattering objects in the granules, while increasing their surface roughness (Zhang et al., 2014). Plasma slightly increased the repeating distance of the semicrystalline lamellae (Zhang et al., 2015; Bie et al., 2016b). The changes in the lamellae structure of starch appeared to depend on the types of both starch and plasma. For example, 11

potato starch was more susceptible to plasma treatment than maize starch in this aspect (Zhang et al., 2014). This may be due to the fact that the former has more water molecules trapped inside the crystalline regions (Pérez & Bertoft, 2010). An oxygen glow-plasma induced more alteration in crystalline lamellae than in the amorphous lamellae, and produced similar changes to the amorphous background and the amorphous lamellae. Compared with an oxygen glow-plasma, a helium glow-plasma induced more changes in crystalline lamellae than in the amorphous lamellae of cassava starch and more changes in the amorphous background than in the amorphous lamellae (Bie et al., 2016b). This discrepancy may be due to the difference in the plasma composition and experimental conditions, though the exact reasons remain to be studied. Overall, the effect of plasma on starch lamellae structure was rather small under the experimental conditions of the reported studies, though the molecular basis responsible for these changes remains to be explored.

5. Effect of plasma treatment on starch physicochemical properties 5.1. Swelling and solubilisation Opposite results of the impact of plasma on starch swelling and solubilisation properties have been observed (Nemtanu & Minea, 2006; Thirumdas et al., 2017). A corona electrical discharge plasma decreased the water solubility of maize starch (Nemtanu & Minea, 2006). The decrease may be mostly due to the cross-linking among starch molecules as described in the section 4.2. Another study showed that a cold air plasma increased the water absorption index from 9 to 11 g/g, water solubility index by 1−2%, and water and fat absorption by 0.1−0.2 g/g, while increasing the swelling power from 9.6 to ~12 g/g (Thirumdas et al., 2017). The increase may be due to the molecular degradation and granular corrosion as described in the sections 4.2 and 4.3, resulting in a more open granular structure and better water

12

adsorption. The different results between the two studies could be readily attributed to the differences in the plasma composition, starch type, and treatment conditions (e.g., length and power input). 5.2.Rheological properties Pasting, flow, and dynamic oscillatory rheology of starch affected by plasma treatment have been studied (Table 4). Nitrogen and helium glow-plasma treatments reduced the viscosity, BD (breakdown), and SB (setback) of potato starch during the pasting event (Zhang et al., 2015) (Supplementary Fig. 4). A cold air plasma increased the starch viscosity during pasting event (Thirumdas et al., 2017) (Supplementary Fig. 5). A low pressure argon plasma (treatment time: 60 min) increased the HPV and CPV while decreasing the BD of cassava starch during the pasting event. Additional plasma treatment (treatment time: 180 min) increased the BD and decreased the HPV and SB (Chaiwat et al., 2016). The apparent viscosity of starch was either increased or decreased by plasma treatment, depending on the types of starch and plasma as well as the treatment conditions (Bie et al., 2016a; Nemtanu & Minea, 2006). Flow analysis showed that the K (consistency coefficient) of starch paste was decreased by plasma treatment, while the n (flow coefficient) was increased or decreased, depending on the experimental conditions (Wongsagonsup et al., 2013; Bie et al., 2016a). Dynamic frequency sweep test showed that plasma treatment increased or decreased G' and G'' of starch paste, depending on the treatment length (Chaiwat et al., 2016; Thirumdas et al., 2017). The opposite influence of plasma treatment on starch rheological properties observed in different studies could be attributed to the different plasma, treatment conditions (power input and length), and starch type. The plasma treatment may stabilise the starch paste/granules through cross-linking, or de-stabilise them through molecular degradation and granular

13

corrosion (sections 4.2 and 4.3). Prolonged treatment more de-stabilised the starch systems, reducing the viscosity (Thirumdas et al., 2017; Chaiwat et al., 2016). The introduced functional groups may also contribute to the altered starch rheology (e.g., through pH alteration). 5.3. Thermal properties Plasma treatment altered the thermal behaviours of glass transition, gelatinization, and thermal degradation of starch to various extents (Table 5). Plasma increased, had no effect, or decreased the glass transition temperature of starch (Lii et al., 2002b; Lii et al., 2002a). Plasma treatment of starch increased or decreased the weight loss of thermal decomposition (e.g., 200 to 400 oC) (Wongsagonsup et al., 2013; Lii et al., 2002c; Lii et al., 2002b). Similarly, plasma treatment increased, had little effect, or decreased the gelatinization temperatures and enthalpy change (∆H) as measured by differential scanning calorimetry (DSC) (Bie et al., 2016b; Zhang et al., 2015; Lii et al., 2002a). Therefore, the plasma impact on starch thermal properties appears to depend on the types of starch and plasma as well as the experimental conditions (e.g., power input and treatment length) (Lii et al., 2002a; Bie et al., 2016b). For example, an oxygen glow-plasma appeared to be more effective in changing the gelatinization properties of cassava starch than a helium plasma (Bie et al., 2016b). Plasma may degrade or cross-link starch molecules and cause granular corrosions as described previously. Extensive degradation and corrosion lead to decreased thermal stability, whereas the cross-linking stabilises the starch granules and counteracts the impact of degradation. The extent of changes in thermal properties of starch mostly depends on the sum/interplay of these two types of reactions induced by plasma. 5.4. Retrogradation

14

Retrogradation of starch as affected by plasma treatment has been mostly studied by clarity, light transmittance, and freeze-thaw stability of starch paste/gel (Supplementary Table 2). Plasma treatment increased or decreased the paste clarity, depending on the experimental conditions. For example, the light transmittance of cassava starch paste was decreased by a low-pressure argon plasma before increasing with increased treatment time (Chaiwat et al., 2016). Plasma-induced molecular degradation and cross-linking of starch molecules tend to have the opposite effects on starch retrogradation. Thus, the extent of changes in starch retrogradation mainly depends on the sum of these two types of reactions. Overall, there is a scarcity of information on how the plasma treatment can affect the starch retrogradation which can be quantified by various techniques (Wang et al., 2015). 5.5. Enzyme susceptibility Only two studies reported the impact of plasma treatment on the enzyme susceptibility of granular starch (Lii et al., 2002c; Thirumdas et al., 2017). A cold air plasma treatment decreased the extent of amyloglucosidase hydrolysis by up to 4% (Thirumdas et al., 2017). This may be attributed to the cross-linking effect of plasma reaction as described in the section 4.2. The cross-linkings formed in the starch molecules sterically hinders the enzymes from approaching the starch for hydrolysis. A low-pressure glow ethylene plasma increased the β-amylolysis of different starches to various extents, depending on the starch type (Lii et al., 2002c). This increase could be attributed to the molecular scission and granular corrosion as shown in the sections 4.2 and 4.3. Plasma may induce the formation of new functional groups on starch. It is still to be determined if the newly introduced functional groups may have any negative impact on human health when these modified starches would be used as food ingredients.

15

6. Applications Plasma has been employed to modify the surface properties of various starch based material including thermoplastic films and bio-composites (Table 6). A major drawback limiting the application of starch based thermoplastic films is their high hydrophilicity. Various plasma treatments increased the hydrophobicity of maize starch films (Andrade et al., 2005; Bastos et al., 2009; Santos et al., 2012; Pankaj et al., 2015). This could be due to the surface structuring, deposition and coating of polymerized molecules from the plasma, as well as molecular cross-linking and formation of new compounds. For example, an atmospheric dielectric barrier discharge plasma increased the surface oxygen concentration while giving new O=C−O groups to the high amylose maize starch film (Pankaj et al., 2015). A low pressure glow discharge SF6 plasma induced the incorporation of fluoride as well as sulfur on the surface of maize starch film (Bastos et al., 2009). As a result of the increased hydrophobicity, the water contact angle increased and the water adsorption capacity of the starch films decreased (Bastos et al., 2009; Santos et al., 2012) (Supplementary Fig. 6). The plasma treatment also increased the surface roughness of starch films due to the corrosion as described in the section 4.3 (Andrade et al., 2005; Pankaj et al., 2015) (Supplementary Fig. 7). Plasma treatment had little effect on the polymorph type of the film, had no effect on the thermal degradation profile, while decreasing the maximum degradation temperature (Pankaj et al., 2015). This decrease may be due to the molecular degradation and surface corrosion as described above. Similar effects of plasma treatment on starch based composite material [starch/poly(ε-caprolactone) film and starch foam tray reinforced with aspen wood fiber] were recorded (Arolkar et al., 2015; Han et al., 2011). Plasma treatment increased the surface hydrophobicity due to the incorporation of new material such as fluorine (Han et al., 2011). The surface roughness and free energy were also increased due to corrosion reactions (Arolkar et al., 2015). As a result of the modified surface properties, the biodegradability of 16

starch/poly(ε-caprolactone) film by Bacillus subtilis MTCC 121 was increased according to an indoor soil burial method (Arolkar et al., 2015). Plasma modified starch granules had increased hydrophobicity, which improved the compatibility in the polyethylene matrix (Szymanowski et al., 2005). This increased compatibility was mostly due to the deagglomeration and better dispersion of starch granules (rather than the filler–matrix interaction) in the matrix. The improved compatibility also resulted in increased tensile strength and elongation at break of the composite (Szymanowski et al., 2005). There has been growing interest of using starch-based artificial tissues as engineering scaffolds in medical applications (Table 6). Plasma modification increased the compatibility of starch-based bio-composite with proteins and cells (Elvira et al., 2005; Alves et al., 2007; Pashkuleva et al., 2010). The plasma treatment altered surface hydrophobicity, energy, and roughness of the starch based bio-composites, resulting in better adhesion and proliferation of cells such as MG63 osteoblast-like osteosarcoma cells (Alves et al., 2007). Compared with chemical- or UV-induced surface modification, plasma gave a more homogeneous surface grafting and a better effect for cell adhesion (Elvira et al., 2005; Pashkuleva et al., 2010). This further confirms the role of plasma as a green and efficient agent for surface modification of biomaterial. It should be pointed out that plasma treatment not only modifies the surface properties for enhanced cell adhesion and proliferation, but also sterilizes the biomaterial for medical applications (Pashkuleva et al., 2010). The above mentioned effects of surface modification on starch film/composite depend on the type of plasma, treatment conditions, as well as the starch/composite type. By carefully manipulating these factors, desired surface properties of starch based materials may be achieved.

7. Impact of plasma on physicochemical properties of starch based food systems

17

The above sections show that non-thermal plasma could greatly influence the composition, structures, and physicochemical properties of starch. It may be expected that plasma treatment could influence the functional properties of food systems (e.g., cereals) rich in starch. Wheat flour, as a typical example with starch as the major component, has been subjected to cold plasma treatments (Bahrami et al., 2016; Misra et al., 2015). Misra et al. (2015) used atmospheric pressure cold plasma (up to 70 kV and 10 min) to treat soft and hard wheat flour. Mixographic studies showed that the dough strength and optimum mixing time of both flours were increased. Dynamic rheological analysis showed that the elastic and viscous moduli of the flours increased with increasing voltage and treatment time. FTIR spectroscopy analysis on the protein structure revealed a decrease in β-sheets and increase in β-turns and α-helix for both flours (Misra et al., 2015). Bahrami et al. (2016) used atmospheric pressure cold plasma (up to 20 V and 120 s) to modify wheat flour. The plasma treatment had little effect on the total aerobic bacterial or mould count, and the concentrations of total non-polar and glycolipids or non-starch lipids. The treatment reduced the contents of total free fatty acids and phospholipids in a dose-dependent manner. The lipid oxidation in the flour was increased. The treatment had no impact on the protein content while oxidising the protein by increasing its molecular weight. The dough strength increased as a result of plasma treatment (Bahrami et al., 2016). These two studies clearly showed that the functional properties of non-starch components such as protein and lipids in wheat flour were much affected by the plasma treatments as a result of their interactions with reactive species. The changes in the physicochemical properties of wheat flour could be attributed to the changes in starch, protein, lipids, water, and so on. Therefore, the impact of plasma processing on model systems of individual components other than starch should also be studied to obtain a holistic picture for complex food systems.

18

8. Conclusions Plasma modification is an emerging green technology to create new functionalities of starch. Various reactive species in plasma interact with starch, inducing chemical changes. Plasma treatment decreases the moisture and amylose content of starch, and may induce the formation of new functional groups attached to the starch molecules. The polymorph type of starch is not affected. The starch granules are either unaffected or etched and disrupted. Plasma treatment either decreases, has no effect, or increases a range of structural and physicochemical parameters of starch. These properties include the molecular size, degree of crystallinity, lamellae structure of granules, swelling and solubilisation, viscosity during pasting/rheological testing, thermal parameters under a range of water contents and temperature, retrogradation, and enzyme susceptibility. Factors affecting the outcome of plasma treatment include the type and composition of plasma, method of plasma generation, plasma exposure time, power input, and type and composition of starch. It appears that plasma may not only stabilise the starch systems by cross-linking and functionalization (e.g., fluorination), but also may de-stabilise the systems by molecular degradation and granular corrosion/etching. The outcome of plasma modification on starch functional properties depends on the interactions of these two opposite types of forces. Plasma modification improves the hydrophobicity and alters the morphology of the surface of various starch based biomaterials such as thermoplastic films and bio-composite for different applications. The altered surface improves the biocompatibility of starch based bio-composites with proteins and cells by increasing cell adhesion and proliferation. This feature indicates that the plasmamodified starch-based material could be used as engineering scaffolds in medical applications. The improved adhesion with cells also enhances the biodegradability of starch thermoplastics for environmental applications. In starch based food systems such as wheat flour, apart from starch, plasma treatment also modifies other components such as proteins and lipids. Changes 19

in functional properties of starch based complex foods could be contributed by the plasmainduced changes of all the biopolymers present. Compared with other types of starch modification methods such as ultrasound and γirradiation, there is a scarcity of information on the plasma modification. The following research gaps should be tackled to improve our understanding of the plasma modified starches. Confirmative studies are in need as some opposite results were recorded in different studies. The chemistry of starch-plasma interactions remains largely unknown. There is a lack of comparative studies on the effect of the type of plasma generation (e.g., corona discharge vs dielectric barrier discharge) on starch properties. The roles of water and moisture content of starch in plasma-starch interactions are to be better studied. The limit in the extent of the modification of structure and physicochemical properties remains to be explored. For example, the maximum extent of starch dextrinization may be studied by prolonging treatment time and increasing power input, providing a basis for a waste-less dextrinization process of starch. Multiple starch modifications coupled with plasma treatment may be explored to create a range of starch functionalities. The toxicity of new functional groups and elements introduced onto the starch by plasma should be tested for food applications.

The author declares no conflicts of interest.

References Alves, C. M., Yang, Y., Carnes, D. L., Ong, J. L., Sylvia, V. L., Dean, D. D., et al. (2007). Modulating bone cells response onto starch-based biomaterials by surface plasma treatment and protein adsorption. Biomaterials, 28, 307–315.

20

Andrade, C. T., Simão, R. A., Thiré, R. M. S. M., & Achete, C.A. (2005). Surface modification of maize starch films by low-pressure glow 1-butene plasma. Carbohydrate Polymers, 61, 407–413. Arolkar, G. A., Salgo, M. J., Kelkar-Mane, V., & Deshmukh, R. R. (2015). The study of airplasma treatment on corn starch/poly(ε-caprolactone) films. Polymer Degradation and Stability, 120, 262−272. Bahrami, N., Bayliss, D., Chope, G., Penson, S., Perehinec, T., & Fisk, I. D. (2016). Cold plasma: A new technology to modify wheat flour functionality. Food Chemistry, 202, 247– 253. Bastos, D. C., Santos, A. E. F., da Silva, M. L. V. J., & Simão, R.A. (2009). Hydrophobic corn starch thermoplastic films produced by plasma treatment. Ultramicroscopy, 109, 1089– 1093. BeMiller, J., & Whistler, R., (Eds.). (2009). Starch: Chemistry and technology (Third Edition) (pp. 589−599). Cambridge: Academic Press. Bie, P., Li, X., Xie, F., Chen, L., Zhang, B., & Li, L. (2016b). Supramolecular structure and thermal behavior of cassava starch treated by oxygen and helium glow-plasmas. Innovative Food Science and Emerging Technologies, 34, 336–343. Bie, P., Pu, H., Zhang, B., Su, J., Chen, L., & Li, X. (2016a). Structural characteristics and rheological properties of plasma-treated starch. Innovative Food Science and Emerging Technologies, 34, 196–204. Chaiwat, W., Wongsagonsup, R., Tangpanichyanon, N., Jariyaporn, T., Deeyai, P., Suphantharika, M., et al. (2016). Argon plasma treatment of tapioca starch using a semicontinuous downer reactor. Food and Bioprocess Technology, 9, 1125–1134.

21

Crookes, W. (1879). On radiant matter. A lecture to the British Association for the Advancement of Science, in Sheffield, UK on Friday, 22th August, 1879. Deeyai, P., Suphantharika, M., Wongsagonsup, R., & Dangtip, S. (2013). Characterization of modified tapioca starch in atmospheric argon plasma under diverse humidity by FTIR spectroscopy. Chinese Physics Letters, 30, 018103. Elvira, C., Yi, F., Azevedo, M. C., Rebouta, L., Cunha, A. M., Román, J. S., & Reis, R. L. (2003). Plasma- and chemical-induced graft polymerization on the surface of starch-based biomaterials aimed at improving cell adhesion and proliferation. Journal of Materials Science: Materials in Medicine, 14, 187−194. Guo, J., Huang, K., & Wang, J. (2015). Bactericidal effect of various non-thermal plasma agents and the influence of experimental conditions in microbial inactivation: A review. Food Control, 50, 482−490. Han, Y. S., Manolach, S. O., Denes, F., Rowell, R. M. (2011). Cold plasma treatment on starch foam reinforced with wood fiber for its surface hydrophobicity. Carbohydrate Polymers, 86, 1031–1037. Khorram, S., Zakerhamidi, M. S., & Karimzadeh, Z. (2015). Polarity functions’ characterization and the mechanism of starch modification by DC glow discharge plasma. Carbohydrate Polymers, 127, 72–78. Lii, C. Y., Liao, C. D., Stobinski, L., & Tomasik, P. (2002a). Behaviour of granular starches in low-pressure glow plasma. Carbohydrate Polymers, 49, 499−507. Lii, C. Y., Liao, C. D., Stobinski, L., & Tomasik, P. (2002b). Effects of hydrogen, oxygen, and ammonia low-pressure glow plasma on granular starches. Carbohydrate Polymers, 49, 449−456.

22

Lii, C. Y., Liao, C. D., Stobinski, L., & Tomasik, P. (2002c). Exposure of granular starches to low-pressure glow ethylene plasma. European Polymer Journal, 38, 1601–1606. Mir, S. A., Shah, M. A., & Mir, M. M. (2016). Understanding the role of plasma technology in food industry. Food and Bioprocess Technology, 9, 734–750. Misra, N. N., Kaur, S., Tiwari, B. K., Kaur, A., Singh, N., & Cullen, P.J. (2015). Atmospheric pressure cold plasma (ACP) treatment of wheat flour. Food Hydrocolloids, 44, 115−121. Moreau, M., Orange, N., & Feuilloley, M. G. J. (2008). Non-thermal plasma technologies: New tools for bio-decontamination. Biotechnology Advances, 26, 610–617. Mott-Smith, H. M. (1971). History of "plasmas". Nature, 233, 219. Nemtanu, M. R., & Minea, R. (2006). Functional Properties of Corn Starch Treated with Corona Electrical Discharges. Macromolecular Symposia, 245–246, 525–528. Pankaj, S. K., Bueno-Ferrer, C., Misra, N. N., O'Neill, L., Tiwari, B. K., Bourke, P., & Cullen, P. J. (2015). Dielectric barrier discharge atmospheric air plasma treatment of high amylose corn starch films. LWT−Food Science and Technology, 63, 1076−1082. Pashkuleva, I., Marques, A. P., Vaz, F., & Reis, R. L. (2010). Surface modification of starch based biomaterials by oxygen plasma or UV-irradiation. Journal of Materials Science: Materials in Medicine, 21, 21–32. Pérez, S., & Bertoft, E. (2010). The molecular structures of starch components and their contribution to the architecture of starch granules: a comprehensive review. Starch/Stärke, 62, 389–420.

23

Santos, A. E. F., Bastos, D. C., da Silva, M. L. V. J., Thiré, R. M. S. M., & Simão, R. A. (2012). Chemical analysis of a cornstarch film surface modified by SF6 plasma treatment. Carbohydrate Polymers, 87, 2217–2222. Sarangapani, C., Thirumdas, R., Devi, Y., Trimukhe, A., Deshmukh, R. R., & Annapure, U. S. (2016). Effect of low-pressure plasma on physico-chemical and functional properties of parboiled rice flour. LWT−Food Science and Technology, 69, 482−489. Scholtz, V., Pazlarova, J., Souskova, H., Khun, J., & Julak, J. (2015). Nonthermal plasma - A tool for decontamination and disinfection. Biotechnology Advances, 33, 1108–1119. Swinkels, J. J. M. (1985). Composition and properties of commercial native starches. Starch/Stärke, 37, 1–5. Szymanowski, H., Kaczmarek, M., Gazicki-Lipman, M., Klimek, L., & Woźniak, B. (2005). New biodegradable material based on RF plasma modified starch. Surface & Coatings Technology, 200, 539–543. Thirumdas, R., Sarangapani, C., & Annapure, U. S. (2015). Cold plasma: A novel nonthermal technology for food processing. Food Biophysics, 10, 1–11. Thirumdas, R., Trimukhe, A., Deshmukh, R. R., & Annapure, U. S. (2017). Functional and rheological properties of cold plasma treated rice starch. Carbohydrate Polymers, 157, 1723– 1731. Wang, S., Li, C., Copeland, L., Niu, Q., & Wang, S. (2015). Starch retrogradation: a comprehensive review. Comprehensive Reviews in Food Science and Food Safety, 14, 568– 585. Wongsagonsup, R., Deeyai, P., Chaiwat, W., Horrungsiwat, S., Leejariensuk, K., Suphantharika, M., Fuongfuchat, A., & Dangtip, S. (2014). Modification of tapioca starch by 24

non-chemical route using jet atmospheric argon plasma. Carbohydrate Polymers, 102, 790– 798. Zhang, B., Chen, L., Li, X., Li, L., & Zhang, H. (2015). Understanding the multi-scale structure and functional properties of starch modulated by glow-plasma: A structurefunctionality relationship. Food Hydrocolloids, 50, 228−236. Zhang, B., Xiong, S., Li, X., Li, L., Xie, F., & Chen, L. (2014). Effect of oxygen glow plasma on supramolecular and molecular structures of starch and related mechanism. Food Hydrocolloids, 37, 69−76. Zou, J. J., Liu, C. J., & Eliasson, B. (2004). Modification of starch by glow discharge plasma. Carbohydrate Polymers, 55, 23–26.

25

Table 1 Effect of plasma treatment on starch chemical structure Parameter

Starch type Plasma type

Molecular size Various starches

Low-pressure

Exposure Analytical

Major findings

Reference

Lii et al.,

time

method

30 min

HPSEC-

Plasma treatment decreased the starch molecular weight and radius of

MALLS-RI

gyration to various extents (e.g., up to 83% for the former), depending on 2002a

glow air plasma from a DC

the starch type

supply Molecular size Various starches

Low-pressure

Up to 30

HPSEC-

Plasma treatment decreased the starch molecular weight and radius of

Lii et al.,

hydrogen,

min

MALLS-RI

gyration to various extents, depending on the starch and plasma type.

2002b

oxygen, and

Larger molecules were more susceptible to degradation than smaller

ammonia glow

molecules

plasma from a DC supply Molecular size Various

Low-pressure

30 min

HPSEC-

Plasma treatment decreased the starch molecular weight and radius of

Lii et al.,

26

starches

glow ethylene

MALLS-RI

gyration to various extents, depending on the starch type. The ethylene

plasma from a

was graft-polymerized onto rice and sweet potato starches, while having

AC inductor

homo-polymerization onto cassava, potato, normal and waxy maize

2002c

starches Molecular size Maize, potato

Oxygen glow

Up to 60

HPSEC-

Plasma treatment decreased the starch molecular weight and radius of

Zhang et al.,

plasma

min

MALS-RI

gyration to various extents, depending on reaction time. Potato starch is

2014

more susceptible to plasma treatment than maize starch Molecular size Potato

Nitrogen and

Up to 60

HPSEC-

Plasma treatment increased the starch molecular size and radius of

Zhang et al.,

helium glow-

min

MALS-RI

gyration to various extents, depending on reaction time. Helium glow-

2015

plasma

plasma was more effective in increasing the molecular size than the nitrogen glow plasma

Structure of

Commercial Glow discharge 45 min

Solution 1H-

13

glucosyl

soluble

argon plasma

and 13C-NMR whereas 1H-NMR analysis showed the loss of OH groups in starch

2004

groups

starch

Structure of

Cassava

Jet atmospheric 5 min

Solution 1H- The relative area of C–O–C peak (measured by FTIR) and the content of

Wongsagon

glucosyl

(cooked and argon plasma

NMR, FTIR OH groups (measured by 1H-NMR) in starch either increased or

sup et al.,

C-NMR spectrum of the starch was not affected by plasma treatment,

Zou et al.,

27

groups, content granular)

(50 or 100 W)

decreased, depending on the plasma power input and starch form

2013

The content of C–O–C bonding of starch reflects the degree of cross-

Deeyai et

of C–O–C bonding Content of C– Cassava

Atmospheric

O–C bonding

argon plasma

linking. Plasma treatment increased the degree of cross-linking of starch. al., 2013

(40 W)

The increase was more profound in starch with less moisture content

Structure of

Wheat

Argon and

30 min

FTIR

0−45 min Solution 1H- Argon plasma treatment induced the cross-linking of starch at the C-2

glucosyl

oxygen DC glow

and 13C-

groups,

discharge

NMR, FTIR, starch at the C-6 position to form the carbonyl groups. Increasing

polarity

plasma

Kamlet-

properties

(pressure: 0.07–

Abboud-Taft oxidation in the respective systems

0.11 Torr)

polarity

Khorram et

position. Oxygen plasma treatment induced the oxidation of OH groups of al.,2015

exposure time and pressure of plasma increased the cross-linking and

functions Content of C– Cassava

Low-pressure

30 to 180 FTIR

The relative area of C–O–C peaks (reflecting the degree of cross-linking

Chaiwat et

O–C

argon plasma

min

and depolymerisation) increased before decreasing with increasing plasma al., 2016 exposure. The initial increase could be due to the cross-linking and

28

subsequent decrease could be due to depolymerisation of starch molecules

Molecular size, Maize

Air dielectric

functional groups, new

Up to 10

HPSEC-

Plasma treatment greatly reduced the molecular weight of starch in a

Bie et al.,

barrier discharge min

MALS-RI,

treatment-length dependent manner by molecular scission. FTIR analysis 2016a

plasma

FTIR, XPS

showed that short-range order in the granule surface decreased with increasing plasma treatment time. The carboxyl peak (at 1720 cm−1) was

elements

developed by plasma treatment and the intensity increased with increasing treatment time. No new elements were introduced on starch surface HPSEC-MALLS-RI, high-performance size-exclusion chromatography coupled with multi-angle laser light scattering and differential refractometry detection; FTIR, Fourier transform infrared spectroscopy; XPS, X-ray photoelectron spectroscopy

Table 2 Effect of plasma treatment on starch granule morphology Starch type

Plasma type

Exposure time Technique

Various starches

Low-pressure glow air 30 min

SEM

Major observations

Reference

Irregularly distributed deposits were noted on granule surface Lii et al., 2002a

plasma from a DC

supply and AC inductor

29

Various starches

Low-pressure glow

30 min

SEM

Visible and abundant tiny deposits were noted on granule

ethylene plasma from a

surface. The treated starches had a light yellow to brownish

AC inductor

color 45 min

SEM

Lii et al., 2002c

Commercial

Glow discharge argon

Starch granules tend to physically linked into each other after Zou et al., 2004

soluble starch

plasma

plasma treatment

Rice

Cold air plasma (40 W 5 and 10 min SEM

Plasma treatment (60 W-10 min) gave some fissures and

Thirumdas et al.,

and 60 W)

cavities on the surface, whereas treatment at other conditions

2017

had no effect Cassava (cooked Jet atmospheric argon and granular)

plasma (50 or 100 W)

5 min

Optical microscopy Number of visible “granular” remnants in the cooked starch

Wongsagonsup et

paste became more with increasing plasma input power (may al., 2013 be due to cross-linking). Little remnants were observed for the pastes from granular starch

Potato

Nitrogen and helium glow plasma

Up to 60 min Optical microscopy Nitrogen plasma treatment had no effect on granule

Zhang et al., 2015

morphology, whereas helium plasma treatment created surface corrosions

30

Maize

Air dielectric barrier

Up to 10 min Optical microscopy, Plasma treatment increased the size of the channels in the

discharge plasma

Cassava

Oxygen and helium

CLSM

Bie et al., 2016a

granules and partially fractured the granules

Up to 60 min Optical microscopy Plasma treatment had no apparent effect on starch granules

Bie et al., 2016b

glow-plasma SEM, scanning electron microscopy; CLSM, confocal laser scanning microscopy

Table 3 Effect of plasma treatment on starch crystalline structure Starch type

Plasma type

Exposure time

Instrument

Major results

References

Various

Hydrogen,

Up to 30 min

WAXS

Plasma treatment increased, had little effect, or decreased the degree of Lii et al., 2002b

starches

oxygen, and

crystallinity, depending on the type of both starch and plasma. It had no

ammonia low-

effect on the polymorph type

pressure glow plasma from a DC supply

31

Various

Low-pressure

starches

glow ethylene

30 min

WAXS

Plasma treatment had no effect on the polymorph type

Lii et al., 2002c

45 min

WAXS

Plasma treatment had no effect on the polymorph type

Zou et al., 2004

5 and 10 min

WAXS

Plasma treatment had no effect on the polymorph type while decreasing Thirumdas et al.,

plasma from a AC inductor Commercial

Glow discharge

soluble starch argon plasma Rice

Cold air plasma (40 W and 60 W)

Potato, maize

Oxygen glow plasma

Up to 60 min

the degree of crystallinity from 43 to 37% when at 40 W for 10 min

2017

SAXS,

Plasma treatment had no effect on the polymorph type of starch. The

Zhang et al.,

WAXS

treatment decreased the compactness of the scattering objects in starch

2014

while increasing their surface roughness. Potato starch is more susceptible to plasma treatment than maize starch. Plasma treatment had little effect on the repeating distance of the semi-crystalline lamellae. Both the crystalline and amorphous lamellae were disrupted, and the latter suffered a greater extent of disruption at the initial stage of treatment (may be due to more water molecules in it)

32

Potato

Nitrogen and

Up to 60 min

SAXS,

Plasma treatment had no effect on the polymorph type of starch while

Zhang et al.,

helium glow-

WAXS,

decreasing the degree of crystallinity by up to 5%. The treatment

2015

plasma

solid-state

somewhat increased the crystalline lamellae thickness while having no

13

C CP/MAS effect on that of amorphous lamellae (may due to the former having

NMR

water molecules trapped). NMR analysis showed that the plasma treatment decreased the proportion of both single (V-type) and double helices. Helium glow-plasma was more effective in modifying the structural and physicochemical properties of starch

Maize

Air dielectric

Up to 10 min

PLM

barrier discharge

Birefringence of starch granules became less obvious after plasma

Bie et al., 2016a

treatment, suggesting the disruption of the granules

plasma Cassava

Oxygen and helium glow plasma

Up to 60 min

PLM, SAXS, Birefringence of starch granules was apparently not affected by the WAXS

Bie et al., 2016b

treatment. Plasma treatment slightly increased the repeating distance of the lamellae. Oxygen plasma induced more alteration in crystalline lamellae than in amorphous lamellae, and produced similar changes to amorphous background and amorphous lamellae. Helium plasma

33

induced more alteration in crystalline lamellae than in amorphous lamellae and more changes in amorphous background than in amorphous lamellae. Plasma treatment had no effect on polymorph type while decreasing degree of crystallinity (up to 3%). Oxygen plasma appeared more effective in altering the structure than helium plasma WAXS, wide-angle X-ray scattering; SAXS, small-angle X-ray scattering; CP/MAS NMR, cross polarization/magic angle spinning nuclear magnetic resonance spectroscopy; PLM, polarized light microscopy Table 4 Impact of plasma treatment on rheological properties of starch Starch type

Plasma type

Maize

Corona electrical Up to 30 min

Apparent

The treatment increased the apparent viscosity of starch while decreasing the gel

Nemtanu &

discharge

viscosity, gel

consistency

Minea, 2006

plasma

consistency

Jet atmospheric 5 min

Dynamic

Plasma treatment on granular starch at 50 W increased or had little effect on G' and Wongsagonsup

(cooked and argon plasma

rheology, flow

G'' of starch from dynamic frequency sweep test, while the treatment at 100 W

granular)

property

decreased them. For flow analysis, plasma treatment of cooked and granular

Cassava

(50 or 100 W)

Exposure time Starch property Major results

References

et al., 2013

34

starches at 100 W decreased the consistency coefficient (K) and flow coefficient (n) of both starch pastes Potato

Nitrogen and

Up to 60 min

Pasting property Plasma treatment decreased the viscosity, BD, and SB of starch during pasting

helium glow

event

Zhang et al., 2015

plasma Cassava

Low-pressure

30 to 180 min Pasting property, Plasma treatment for 60 min increased the HPV and CPV of starch, while

Chaiwat et al.,

argon

dynamic

decreasing BD. Treatment for 180 min increased the BD of starch while decreasing 2016

plasma

rheology, flow

HPV and SB. Treatment for 30 min increased both G' and G'' of starch from

property

dynamic frequency sweep test and increased shear stress of starch from the flow test. Prolonged treatment decreased them

Maize

Rice

Air dielectric

Up to 10 min

Flow

Plasma treatment decreased the apparent viscosity and shear stress in a treatment

barrier discharge

length dependent manner. The K and n of flow properties were decreased and

plasma

increased by the treatment, respectively

Bie et al., 2016a

Cold air plasma 5 and 10 min

Dynamic

Plasma treatment increased the starch viscosity during pasting event. It increased

Thirumdas et al.,

(40 W and 60

rheology

(60 W, 5 min) or decreased (60 W, 10 min) the G' and G'' of starch from the

2017

W)

dynamic rheology (temperature sweep), depending on the treatment conditions 35

Table 5 Effect of plasma treatment on starch thermal properties Starch type

Plasma type

Exposure Starch property

Major results

References

Glass transition by

Plasma treatment either increased or had no effect on

Lii et al.,

thermogravimetry

glass transition temperature, depending on the starch

2002a

time Various starches Low-pressure glow air plasma 30 min from a DC supply and AC inductor Various starches Hydrogen, oxygen, and ammonia low-pressure glow

type Up to 30

Glass transition and

min

thermal degradation by heating from 260 to 350 oC, while decreasing the glass 2002b

plasma from a DC supply Various starches Low-pressure glow ethylene

thermogravimetry 30 min

plasma from a AC inductor

Plasma treatment increased the weight loss during

transition and thermal decomposition temperatures

Thermal degradation by Plasma treatment increased the weight loss during thermogravimetry

Lii et al.,

heating from 260 to 350 oC, while decreasing the

Lii et al., 2002c

thermal decomposition temperatures Various starches Low-pressure glow air plasma 30 min from a DC supply and AC

Gelatinization by DSC Plasma treatment either increased or decreased the

Lii et al.,

gelatinization temperatures and ∆H, depending on the 2002a

36

inductor

starch type

Cassava (cooked Jet atmospheric argon plasma 5 min

Gelatinization by DSC Plasma treatment had little effect on gelatinization

and granular)

and thermal degradation properties of starch. Plasma treatment either increased et al., 2013

(50 or 100 W)

by thermogravimetry

Wongsagonsup

or decreased the weight loss (200 to 400 oC) of starch, depending on the starch form and plasma power input

Potato

Nitrogen and helium glow

Up to 60

plasma

min

Gelatinization by DSC Plasma treatment decreased the gelatinization

Zhang et al.,

temperatures (by ~1 to 3 oC) and ∆H (by ~1 to 1.5 J/g), 2015 while having little effect on temperature range

Cassava

Oxygen and helium glow

Up to 60

plasma

min

Gelatinization by DSC Plasma treatment increased the gelatinization temperatures and narrowed the temperature range by

Bie et al., 2016b

~1 oC, while decreasing the ∆H up to 3 J/g. Oxygen plasma appeared to be more effective in altering the properties than helium plasma Rice

Cold air plasma (40 W and 60 5 and 10 W)

min

Gelatinization by DSC Treatment at 60 W for 10 min decreased To, Tc, and ∆H Thirumdas et by 3 oC, 2 oC, and 1.5 J/g, respectively, whereas

al., 2017

37

treatment at other conditions had little effect

DSC, differential scanning calorimetry; To and Tc are onset and conclusion temperatures and ∆H is enthalpy change of gelatinization Table 6 Effect of plasma treatment on starch applications Uses

Starch film

Targeted

Starch

property

type

Plasma type

Exposure

Major results

References

Plasma treatment increased the hydrophobicity of the film surface by

Andrade et al., 2005

time

Film surface Maize

Low-pressure

Time to

properties

radio frequency produce

polymerisation coating. The water adsorption and contact angle decreased. Less

glow 1-butene

film

homogeneous surfaces gave a higher water adsorption rate. Water absorption

plasma

thickness of capacity of films made from fully gelatinized starch decreased before increasing 20–230 nm with increasing coating thickness (may be due to formation of micro-cracks in the film due to prolonged plasma treatment)

Starch film

Film surface Maize

Low pressure

Up to 900 s Fluoride was mostly incorporated onto film surface and the incorporation of both

properties

glow discharge

fluoride and sulfur depended on the plasma power. At −100V self-bias, fluoride

SF6 plasma

was preferably attached onto film surface. The surface contact angle depended on

Bastos et al., 2009

38

the surface morphology which depended on the treatment time. The increasing treatment time greatly increased the surface roughness of the film. Water contact angles up to 130o could be achieved through optimizing treatment conditions. The contact angle was over 100 o after the film surface was in contact with water Starch film

Film surface Maize

Low pressure

Up to 600 s The incorporation fluoride and sulfur on the starch film surface depended on the

properties

glow discharge

self-bias of plasma system. The former was more incorporated at > 100 V. An

SF6 plasma

amorphous layer was formed on the film. Water contact angles of over 120o

Santos et al., 2012

(increased hydrophobicity) could be achieved with optimal conditions Starch film

Starch

Film surface High

Atmospheric

60, 70 and

properties

amylose

dielectric

80 kV, 1−5 little effect on the thermal degradation profile and decreased the maximum

maize

barrier

min

starch

discharge

Surface

composite with properties of

Plasma treatment increased the surface roughness of starch film. The treatment had Pankaj et al.,

degradation temperature. The treatment (70 and 80 kV, 5 min) increased the surface oxygen concentration and introduced new O=C−O groups onto starch. The

(HYLON plasma

A-type polymorph of the film was not affected. The treatment increased the

VII)

hydrophilicity of film surface without affecting the water vapor permeability

Potato

Radio

2015

Up to 100

frequency glow W

Plasma treatment increased the hydrophobicity of starch granule surface. The

Szymanowski

modified starch was formulated with polyethylene to form the composite. Tensile

et al., 2005

39

polyethylene

starch and

discharge CH4

strength and elongation at break of the composite were improved by the starch

matrix

mechanical

plasma

modification. The de-agglomeration and better dispersion of starch granules (rather

properties of

than the filler-matrix interaction) as a result of plasma treatment contributed to the

composite

enhanced mechanical properties

Starch/poly(ε-

Surface and Maize

Radio

Up to 5 min Plasma treatment increased the hydrophilicity, roughness, and surface free energy Arolkar et al.,

caprolactone)

biodegradab

frequency air

of the film. The wettability, adhesion, and printability of the films were increased

film

le properties

plasma

by the treatment. The water barrier properties of the films were not affected by

2015

short treatment time (up to 2 min). The biodegradation of the film by Bacillus subtilis MTCC 121 was increased by plasma treatment (increased bacterial adhesion) by using an indoor soil burial method Starch foam tray Surface

n.a.

Cold radio

< 10 min

Fluorine was incorporated on the tray surface, and its concentration ranged from

Han et al., 2011

reinforced with properties

frequency CF4

46.8−60.2%, depending on the reaction conditions. Plasma increased the

aspen wood

and SF6 plasma

hydrophobicity of the tray. Water contact angle of up to 150 o was obtained. CF4

fiber

(pressure up to

plasma gave a higher fluorine incorporation than SF6. The permeability of water

350 mT, power

molecules (both liquid and gas forms) was restricted on the fluorine-rich surface of

up to 240 W)

the starch tray

40

Cell adhesion

Surface

and proliferation properties

Maize

Argon, oxygen 30 min

Starch based bio-composite materials were formulated with various components

Elvira et al.,

starch

radio frequency

and subjected to plasma/chemical treatment to induce the grafting of acrylic polar

2005

based

glow discharge

monomers. The treatment increased the adhesion and proliferation of goat bone

composite plasma

marrow cells to the composite. The plasma-induced grafting gave a better effect than the chemical-induced one by giving a more homogeneous distribution of grafted polymers

Cell adhesion

Surface

and proliferation properties

Maize

Oxygen radio

starch based

180 s

Starch based bio-composites were formulated with various components and

Alves et al.,

frequency glow

subjected to plasma treatment. The treatment increased the hydrophilicity and

2007

discharge

surface energy of the composite surface. Bovine serum albumin, vitronectin or

composite plasma

fibronectin were incubated with the composite before MG63 osteoblast-like osteosarcoma cells were added for cell adhesion and proliferation. Plasma treatment increased protein and cell adhesion as well as cell proliferation, and the extent depended on the type of composite material

Cell adhesion

Surface

and proliferation properties

Maize

Oxygen

15 min

Starch based bio-composite materials were formulated with various components

Pashkuleva et

starch

frequency glow

and subjected to plasma treatment or UV irradiation. Both the treatments increased al., 2010

based

discharge

the surface adhesion of osteoblast-like cells on the film by modifying the

41

composite

morphology and composition of the film surface. A moderate hydrophobicity of film surface favoured the cell adhesion. Plasma treatment appeared to be more efficient in surface modification than the UV irradiation

42

•

Various aspects of starch affected by plasma are reviewed

•

Starch may be cross-linked and degraded by plasma

•

Factors affecting the modification outcomes are discussed

•

Starch modification by plasma treatment is a green technology

43