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Blanching
Blanching
9
Blanching serves a variety of functions, one of the main ones being to destroy enzymic activity in vegetables and some fruits prior to further processing. A few vegetables, e.g., onions and green peppers, do not require blanching to prevent enzyme activity during storage, but the majority suffer considerable loss in quality if they are not blanched or if they are underblanched. To achieve adequate enzyme inactivation, food is heated rapidly to a preset temperature, held for a preset time and then cooled rapidly to near ambient temperatures. As such, it is not intended as a sole method of preservation but as a pretreatment that is normally carried out between preparation of the raw material (see Sections 2.1 to 2.4) and later operations (particularly heat sterilisation, dehydration and freezing (see Sections 12.1, 14.1 and 22.1)). Blanching is also combined with peeling and/or cleaning of foods (see Sections 2.2 and 2.4) to achieve savings in energy consumption, space and equipment costs. Further details are given by Varzakas et al. (2015).
9.1
Theory
Blanching is an example of unsteady-state heat transfer (Section 1.8.4), involving convective surface heating by steam or hot water and conduction of heat from the surface to the interior of the food. Mass transfer of material into and out of the food (see Section 1.8.1) is also important for the yield of product and nutrient losses. An example of an unsteady-state heat transfer calculation is shown in sample problem (9.1) and further problems are given by Singh and Heldman (2014).
Sample Problem 9.1 Peas with an average diameter of 6 mm are blanched to give a temperature of 85 C at the centre. The initial temperature of the peas is 15 C and the temperature of the blancher water is 95 C. Calculate the time required, assuming that the heat transfer coefficient is 1200 W m22 C21 and, for peas, the thermal conductivity is 0.35 W m21 C21, the specific heat is 3.3 kJ kg21 C21 and the density is 980 kg/m3. (Continued)
Food Processing Technology. DOI: http://dx.doi.org/10.1016/B978-0-08-101907-8.00009-2 © 2017 Elsevier Ltd. All rights reserved.
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Sample Problem 9.1—cont’d Solution to sample problem 9.1
From Eq. (1.54), Bi 5 5
hδ k 1200ð3 3 1023 Þ 0:35
5 10:3 Therefore, k 5 0:097 hδ From Eq. (1.55), θh 2 θf 95 2 85 5 θh 2 θi 95 2 15 5 0:125 From the unsteady-state heat transfer chart for a sphere (Fig. 1.44) Fo 5 0:32 From Eq. (1.56), Fo 5
k t Cρ δ2
5 0:32 Therefore to calculate blanching time (t) 0:32 cρ δ2 t5 k 5
0:32ð3:3 3 103 Þ980ð3 3 1023 Þ2 0:35
5 26:6 s
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The maximum processing temperature in freezing and dehydration is insufficient to inactivate enzymes and does not substantially reduce the number of microorganisms in unblanched foods. If the food is not blanched, enzymes cause undesirable changes in sensory characteristics and nutritional properties during storage, and microorganisms are able to grow on thawing or rehydration. In canning, the time taken to reach sterilising temperatures, particularly in large cans, may be sufficient to allow enzyme activity to take place. It is therefore necessary to blanch foods prior to these operations. Underblanching may cause more damage to food than the absence of blanching does. This is because heat, which is sufficient to disrupt tissues and release intracellular enzymes, but not inactivate them, causes mixing of enzymes and substrates during subsequent storage. In addition, only some enzymes may be inactivated, which results in increased activity of others and accelerated deterioration. Enzymes that cause loss of colour or texture, production of off-odours and offflavours, or breakdown of nutrients in vegetables and fruits include lipoxygenase, polyphenoloxidase, polygalacturonase and chlorophyllase. Two heat-resistant enzymes that are found in most vegetables are catalase and peroxidase. Although they do not cause significant deterioration during storage, they are used as marker enzymes to determine the success of blanching. Peroxidase is the more heat-resistant of the two, so the absence of residual peroxidase activity indicates that other less heat-resistant enzymes are also destroyed. The following factors affect blanching conditions: G
G
G
G
The size and shape of the pieces of food; The thermal conductivity of the food, which is influenced by the type, cultivar and degree of maturity; The blanching temperature and method of heating; The convective heat transfer coefficient.
In practice, the timetemperature combinations used for blanching are evaluated for each raw material to achieve a specified temperature at the thermal centre of the food pieces, to achieve a specified degree of peroxidase inactivation, or to retain a specified proportion of vitamin C. Typical timetemperature combinations vary from 1 to 15 min at 70100 C.
9.2
Equipment
The two most widespread commercial methods of blanching involve passing food through an atmosphere of saturated steam, or a bath of hot water. Both types of equipment are relatively simple and inexpensive. There were substantial developments to blanchers during the 1980s and 1990s to reduce their energy and water consumption and also to reduce the loss of soluble components of foods. The aim was to reduce the volume and polluting potential of effluents (Section 1.7.1.3) and increases the yield of product (the weight of food after processing compared to the weight before processing).
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Table 9.1 Effect of blanching method on ascorbic acid losses in selected vegetables Treatment
Water blanch-water cool Water blanch-air cool Steam blanch-water cool Steam blanch-air cool
Loss (%) of ascorbic acid Peas
Broccoli
Green beans
29.1 25.0 24.2 14.0
38.7 30.6 22.2 9.0
15.1 19.5 17.7 18.6
Source: Adapted from Cumming, D.B., Stark, R., Sandford, K.A., 1981. The effect of an individual quick blanching method on ascorbic acid retention in selected vegetables. J. Food Process. Preserv. 5, 3137. doi:10.1111/j.1745-4549.1981.tb00617.x (Cumming et al., 1981).
Commercially, the yield of food after blanching is an important factor in determining the success of a particular method. In some methods the cooling stage may result in greater losses of product or nutrients than the blanching stage, and it is therefore important to consider both blanching and cooling when comparing different methods (Lin and Brewer, 2005). Steam blanching results in higher nutrient retention when cooling is by cold air or cold-water sprays. However, aircooling causes weight loss of the product due to evaporation, and this may outweigh any advantages gained in nutrient retention. Cooling with running water (fluming) substantially increases leaching losses (washing of soluble components from the food), but the product may gain weight by absorbing water and the overall yield is therefore increased. There are also substantial differences in yield and nutrient retention due to differences in the method of blanching and cooling (Table 9.1), the type of food and differences in the method of preparation, especially if foods are sliced or diced before blanching. Recycling of cooling water does not affect the product quality or yield but substantially reduces the volume of effluent produced. However, it is necessary to ensure adequate hygienic standards to prevent a build-up of bacteria in cooling water.
9.2.1 Steam blanchers The advantages and limitations of steam blanchers are described in Table 9.2. In general this is the preferred method for foods with a large area of cut surfaces as leaching losses are much lower than those found using hot-water blanchers. At its simplest, a steam blancher consists of a mesh conveyor that carries food through a steam atmosphere in an insulated tunnel. The residence time of the food is controlled by the speed of the conveyor and the length of the tunnel. A video of a steam blancher is available at www.youtube.com/watch?v 5 ASCImdcBiwE. Steam blanchers may have microprocessor control of the belt speed (residence time) and blanching temperature, which can be programmed with different blanching conditions for individual products.
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Table 9.2 Advantages and limitations of steam and hot-water blanchers Equipment
Advantages
Steam blanchers
G
G
G
G
Hot-water blanchers
G
G
G
Smaller losses of watersoluble components and higher product yield Smaller volumes of effluent and lower disposal costs than water blanchers, particularly with air cooling instead of water Better energy efficiency Better retention of product colour, flavour and texture Lower capital cost than steam blanchers More uniform product heating Use less floor space
Limitations G
G
G
G
G
G
G
G
Limited cleaning of foods so washers are also required Uneven blanching if food is piled too high on the conveyor Some loss of mass from the food Larger, more complex equipment with higher maintenance costs More difficult to clean Large volumes of dilute effluent result in higher costs for both purchase of water and effluent treatment Risk of contamination of foods by thermophilic bacteria Turbulence may cause physical damage to some products
Typically a tunnel is 1520 m long and 11.5 m wide (Fig. 9.1). The efficiency of energy consumption (i.e., amount of energy used to heat the food divided by the amount of energy supplied) is 19% when water sprays are used at the inlet and outlet to condense escaping steam. Alternatively, food may enter and leave the blancher through rotary valves or hydrostatic seals to reduce steam losses and increase energy efficiency to 27%; or steam may be reused by passing it through Venturi valves. Energy efficiency is improved to 31% using combined hydrostatic and Venturi devices (Scott et al., 1981). In older methods of steam blanching, there was often poor uniformity of heating in multiple layers of food. The timetemperature combination required to ensure enzyme inactivation at the centre of the bed resulted in overheating of food at the edges and a consequent loss of quality. Individual quick blanching (IQB), which involves blanching in two stages, was developed to overcome this problem. In the first stage the food is heated in a single layer to a sufficiently high surface temperature to inactivate enzymes. In the second stage (termed ‘adiabatic holding’) a deep bed of food is held for sufficient time to allow the temperature at the centre of each piece to increase to that needed for enzyme inactivation without the addition of more steam. This reduces heating times (e.g., 25 s for heating and 50 s for holding 1 cm diced carrot compared with 3 min for conventional blanching, or a reduction from 12 to 4.5 min for blanching whole sweetcorn cobs) (ABCO, 2016) (Table 9.3). Shorter
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Figure 9.1 Turbo-Flo steam blancher. Courtesy of Key Technologies Inc. (Johnson, S., 2011. Steam blanching vs water blanching: cost, efficiency and product quality. Key Technologies Inc. Available at: ,www.key.net/products/turbo-flo-blancher/default.html. (www.key.net . select ‘Equipment’ . ‘TurboFlos blancher’) (last accessed February 2016)). Table 9.3
Steam blanching times for selected vegetables
Product
Size (mm)
Blanching time (s)
Broad beans Broccoli, cut Brussels sprouts
2025 30 25 40 13 10 6 50 2050 1330 13 2030 4 25 10 10 25
90120 120180 150180 240300 6090 5070 90120 240300 180240 480720 7090 150180 6090 6090 180240 4560 6090 240300
Cabbage, cut Carrots, diced Carrots, sliced Carrots, whole, baby Cauliflower florets Corn on the cob Green beans, cut Leeks, cut Lima beans Mushrooms, sliced Mushrooms, whole Peas Potatoes, diced Potatoes, whole
Source: Adapted from Cabinplant A/S (Cabinplant, 2016. Blancher/cooler. Cabinplant A/S. Available at: www.cabinplant.com/fileadmin/user_upload/downloads/Product_sheets/Blancher_Type_BC_1036.pdf (www.cabinplant. com . search ‘blancher’ . select ‘Blancher type BC’) (last accessed February 2016)).
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Steam Insulated tunnel Preheat ing
Blanching
Cooling
Chilled water Waste water
Preheat water pump
Figure 9.2 Steam blancher with countercurrent cooling. Adapted from Cabinplant, 2016. Blancher/cooler. Cabinplant A/S. Available at: www.cabinplant.com/fileadmin/user_upload/downloads/Product_sheets/Blancher_Type_BC_ 1036.pdf (www.cabinplant.com . search ‘blancher’ . select ‘Blancher type BC’) (last accessed February 2016).
heating results in improvement in the energy efficiency to 8691%, retention of the product colour and flavour, and, compared with water-blanched products, lower losses of solids and nutrients and smaller volumes of effluent that has a lower COD (see Section 1.7.1.3). Traditional blanchers used 1 kg of steam to blanch 34 kg of vegetables, but more recent designs are capable of blanching up to 16 kg of vegetables per kg of steam at capacities up to 30 t h21 (Johnson, 2011). The low energy consumption is due to multistage countercurrent cooling, in which the chilled cooling water absorbs heat from the blanched product and is pumped to a preheating section where it heats incoming product (Fig. 9.2). The preheated product then requires only a small amount of additional heating in the blanching section. A cooling section employs a fog spray to saturate the cold air with moisture. This reduces evaporative losses from the food and reduces the amount of effluent produced. If required, used cooling water can be used to wash incoming product. The recirculation of water ensures very low water consumption and wastewater discharge (e.g., water consumption of 0.6 m3 h21 for a blancher processing 16,000 kg h21). Alternative designs that use chilled air to cool the product have extremely low water consumption (up to 75% less than traditional blanchers) and negligible wastewater production. However, evaporative air cooling does not permit heat recovery (Cabinplant, 2016). Nutrient losses during steam blanching are also reduced by exposing the incoming food to warm air (65 C) in a short preliminary drying operation (termed ‘preconditioning’). Surface moisture evaporates and the surfaces then absorb condensing steam during IQB. Weight losses and nutrient losses are reduced compared to conventional steam blanching, with no reduction in the yield of blanched food.
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Figure 9.3 Clean-flow rotary drum blancher. Courtesy of Lyco Manufacturing Inc. (Lyco, 2016. Clean-flows rotary drum blancher. Lyco Manufacturing Inc. Available at: http://lycomfg.com/equipment/blanchers (last accessed February 2016).
The equipment for IQB steam blanching consists of a heating section in which a single layer of food is heated on a conveyor and then held on a holding conveyor before cooling. Bucket elevators used to load/unload food are located in close-fitting tunnels and the blancher chamber is fitted with rotary valves, both of which minimise steam losses. Typically, a blancher has a 1.5 m 3 6 m chamber that is fully enclosed and insulated with hydrostatic water seals to prevent evaporation and improve thermal efficiency. It can process 13,000 kg h21, creating only 130 L h21 of wastewater, which is 10% of that created by water blanchers. Williams (2007) describes a rotary drum steam blancher with a compact design that reduces energy and water consumption and has lower capital and maintenance costs than conventional tunnel steam blanchers (Fig. 9.3). Fluidised-bed blanchers operate using a mixture of air and steam, moving at 4.5 m s21, which fluidises and heats the product simultaneously. The design of the blanching chamber promotes continuous and uniform circulation of the food until it is adequately blanched. Although these blanchers are not widely used at a commercial scale, they have advantages that include: (1) faster, more uniform heating and hence shorter processing times and smaller losses of vitamins and other soluble heat-sensitive components of foods, and (2) substantial reductions in the volumes of effluent.
9.2.2 Hot-water blanchers There are a number of different designs of blancher, each of which holds the food in hot water at 70100 C for a specified time and then passes it to a dewatering cooling section. The advantages and limitations of hot-water blanchers are described in Table 9.2. In the reel blancher, food is moved through a slowly rotating cylindrical
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mesh drum that is partly submerged in hot water. The speed of rotation and length of the drum determine the heating time. Pipe blanchers consist of a continuous insulated metal pipe fitted with feed and discharge ports. Hot water is recirculated through the pipe and food is metered in. The length of the pipe and the velocity of the water determine the residence time of food in the blancher. These blanchers have the advantage of a large capacity while occupying a small floor space and in some applications they may be used to simultaneously transport food through a factory. Rotary drum water blanchers have a similar design to the steam drum blancher in Fig. 9.3, with the auger conveying food through hot water. Developments in hot-water blanchers, based on the IQB principle, reduce energy consumption and minimise the production of effluent. For example, the blanchercooler, has three sections: a preheating stage, a blanching stage and a cooling stage, similar to the IQB steam blancher in Fig 9.2. The food remains on a single conveyor throughout each stage and therefore does not suffer physical damage caused by turbulence in conventional hot-water blanchers. A heat exchanger heats the preheat water and simultaneously cools the cooling water, leading to up to 70% heat recovery. A recirculated watersteam mixture is used to blanch the food and final cooling is by cold air. Effluent production is negligible and water consumption is reduced to approximately 1 m3 per 10 t of product. The mass of product blanched is 16.720 kg per kg of steam, compared with 0.250.5 kg per kg of steam in conventional hot-water blanchers. Other studies used low-temperature long-time (LTLT) blanching, typically at 5070 C, to improve the firmness of products, or high-temperature short-time (HTST) blanching to increase energy efficiency. In a combination of both, termed ‘stepwise’ blanching, foods are cooled following LTLT and then subjected to HTST blanching (Pan and Atungulu, 2010).
9.2.3 Newer blanching methods Microwave heating is described in Section 19.1. There have been many studies of microwave blanching (e.g., corn kernels (Boyes et al., 1997), mushrooms (RodriguezLopez et al., 1999; Devece et al., 1999), turnip greens (Osinboyejo et al., 2003) and peanuts (Schirack, 2006; Schirack et al., 2007)). Most have confirmed the advantages over conventional blanchers of faster heating and reduced energy costs, which lead to reduced processing times and lower nutrient losses. The main disadvantages of microwave blanching are the higher cost of the equipment compared to conventional blanchers, nonuniform energy distribution, and difficulties in predicting and monitoring the heating pattern. Microwave blanching has been used commercially in Europe and Japan, but not widely. It is reviewed by Dorantes-Alvarez and Parada-Dorantes (2005) and Ramesh et al. (2002). Ohmic heating is described in Section 19.2. It is not yet used commercially for blanching, but studies have indicated its potential for mushroom blanching (Sensoy and Sastry, 2004). Icier et al. (2006) applied ohmic blanching to pea pure´e and found that peroxidase was inactivated in a shorter time than using water blanching. Similar results were found in ohmically blanched artichokes (Icier, 2010). Guida et al. (2013) found that ohmic blanching (at 24 V cm21 and 80 C) inactivated peroxidase and polyphenoloxidase in artichoke heads more quickly than hot-water
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blanching at 100 C and better preserved their colour and texture. Lespinard et al. (2015) studied ultrasonic-assisted blanching of mushrooms and concluded that a combined treatment with conventional blanching could reduce the processing time and retain the colour of the blanched product. High-humidity hot-air impingement blanching (HHAIB) is a thermal technology that is under development. It combines the advantages of steam blanching and hot-air impingement technologies to produce a uniform, rapid and energy-efficient blanching process that causes minimum loss of water-soluble nutrients. It uses jets of highhumidity hot air that impinge on the product surface at high velocity to achieve a high rate of heat transfer (Du et al., 2006). Studies by Xiao et al. (2012) found that HHAIB pretreatment accelerated drying and improved the whiteness of yam slices, and Bai et al. (2013a,b) found that the process inactivated polyphenoloxidase and maintained the quality of Fuji apple and grapes respectively. Pan et al. (2005) evaluated the feasibility of using medium- and far-infrared heating in a catalytic infrared blancher/dryer for blanching and drying fruits and vegetables without water or steam. Pear cubes, baby carrots, sweetcorn and French fries were blanched with a radiation energy intensity of 5.7 kW m22 for between 1 and 3.5 min to inactivate peroxidase. When pear cubes were dried with radiant energy after blanching, the time was reduced by 44% and the texture and appearance of the dried pears was superior, compared to those produced by steam blanching and hot-air drying. This catalytic infrared processing system is commercialised for drying, peeling, toasting and disinfecting foods in addition to blanching (CDT, 2016). Details are given by Pan and Atungulu (2010). High-pressure processing is described in Section 7.2. There have been a number of studies of high-pressure blanching (e.g., Van Buggenhout et al., 2005) which have demonstrated greater nutrient retention than conventional blanching but Cheftel et al. (2002) concluded that there is insufficient inactivation of enzymes at high-pressure low temperatures and it is unlikely that this process will replace commercial thermal blanching.
9.3
Effect on foods
The heat received by a food during blanching inevitably causes some changes to sensory and nutritional qualities. Blanching causes physical and metabolic changes within food cells that result in cell death. Heat damages cytoplasmic and other membranes, which become permeable and result in loss of cell turgor (Fig. 9.4). Water and solutes pass into and out of cells, resulting in nutrient losses. Heat also disrupts subcellular organelles and their constituents become free to interact within the cell. Overblanching can cause excessive softening and loss of flavour in the food, but the heat treatment is less severe than, e.g., in heat sterilisation, and the resulting changes in food quality are less pronounced. Blanching removes intercellular gases from plant tissues, which together with removal of surface dust, alters the wavelength of reflected light of the food and hence brightens the colour of some vegetables. The time and temperature of
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Figure 9.4 Effect of blanching on cell tissues: S, starch gelatinised; CM, cytoplasmic membranes altered; CW, cell walls little altered; P, pectins modified; N, nucleus and cytoplasmic proteins denatured; C, chloroplasts and chromoplasts distorted.
blanching also influence changes to food pigments according to their D-value (Table 1.41). Changes in colour and flavour caused by blanching are described in more detail by Selman (1987). Sodium carbonate (0.125% w/w) or calcium oxide may be added to blancher water to protect chlorophyll and to retain the colour of green vegetables, although the increase in pH may also increase losses of ascorbic acid. Holding foods such as cut apples and potatoes in dilute (2% w/w) brine prior to blanching prevents enzymic browning. When correctly blanched, most foods have no significant changes to flavour or aroma. The timetemperature conditions needed to achieve enzyme inactivation may cause an excessive loss of texture in some types of food (e.g., some varieties of potato) and in large pieces of food. To reduce this, calcium chloride (12% w/w) is added to blancher water to form insoluble calcium pectate complexes and thus maintain firmness in the tissues. In canned foods, blanching softens vegetable tissues, which facilitates filling into containers. The removal of intercellular gases from plant tissues by blanching also assists the formation of a partial vacuum in the head-space of containers. This prevents expansion of air during processing and so reduces strain on the container seams. Removal of oxygen also reduces oxidative changes to the product during storage. The heat received by a food during blanching inevitably causes some changes to sensory and nutritional qualities. Some minerals, water-soluble vitamins and other water-soluble components are lost during blanching. Losses are mostly due to leaching, thermal destruction and to a lesser extent, oxidation. The amount of vitamin loss depends on a number of factors including: G
G
G
G
G
G
The variety of food and its maturity; Methods used in preparation of the food, particularly the extent of cutting, slicing or dicing (Section 4.1); The surface-area-to-volume ratio of the pieces of food; Method of blanching and cooling; Time and temperature of blanching (lower vitamin losses at higher temperatures for shorter times); The ratio of water to food (in both water blanching and cooling).
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Fat-soluble components (e.g., β-carotene) are largely retained (Rickman et al., 2007a). Puupponen-Pimia¨ et al. (2003) studied the effect of blanching on 20 common vegetables. They found that changes were plant species-dependent, but in general dietary fibre components were either not affected or increased slightly, carotenoids and sterols were not affected, and minerals were stable although there were some leaching losses of soluble minerals. Phenolic antioxidants and vitamins were more heat-sensitive and significant losses of antioxidant activity (2030%) were found in many vegetables. Phenolic compounds and other phytochemicals are water-soluble and therefore susceptible to leaching, but blanching inactivates enzymes that cause their oxidation. However, chemical degradation can occur during storage, depending on the available oxygen and exposure to light. Typical vitamin losses are 1520% for riboflavin, 10% for niacin and 1030% for ascorbic acid (Berry-Ottaway, 2002) and .50% for folic acid. There is a 30% loss of thiamine in spinach due to blanching before freezing and losses of 960% in the frozen product (Rickman et al., 2007b). Losses of ascorbic acid are used as an indicator of the severity of blanching and therefore of food quality. Rickman et al. (2007b) also report studies in which asparagus had the smallest losses during blanching and freezing, with retention averaging 90%, but note that losses of ascorbic acid can vary widely, from 1080%, with average values 50%, depending on the cultivar and processing conditions.
9.4
Effect on microorganisms
Blanching reduces the numbers of contaminating microorganisms on the surface of foods and hence assists in subsequent preservation operations. This is particularly important in heat sterilisation (see Section 12.1), as the time and temperature of processing are designed to achieve a specified reduction in cell numbers. If blanching is inadequate, a larger number of microorganisms are present initially and this may result in a larger number of spoiled containers after processing. The effect of blanching on microorganisms has been described by a number of authors including, e.g., Breidt et al. (2000) who found that blanching whole cucumbers for 15 s at 80 C reduced bacteria by 23 log cycles.
References ABCO, 2016. High efficiency blancher/cookers. ABCO Industries Ltd. Available at: www.abco.ca/blanchers.html (last accessed February 2016). Bai, J.W., Gao, Z.J., Xiao, H.W., Wang, X.T., Zhang, Q., 2013a. Polyphenol oxidase inactivation and vitamin C degradation kinetics of Fuji apple quarters by high humidity air impingement blanching. Int. J. Food Sci. Technol. 48, 11351141, http://dx.doi.org/10.1111/j.1365-2621.2012.03193.x. Bai, J.-W., Sun, D.-W., Xiao, H.-W., Mujumdar, A.S., Gao, Z.-J., 2013b. Novel high-humidity hot air impingement blanching (HHAIB) pretreatment enhances drying kinetics and color attributes of seedless grapes. Innov. Food Sci. Emerging Technol. 20, 230237, http://dx.doi.org/10.1016/j.ifset.2013.08.011. Berry-Ottaway, P., 2002. The stability of vitamins during food processing. In: Henry, C.J., Chapman, C. (Eds.), The Nutrition Handbook for Food Processors. Woodhead Publishing, Cambridge, pp. 247264.
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Boyes, S., Chevis, P., Holden, J., Perera, C., 1997. Microwave and water blanching of corn kernels: control of uniformity of heating during microwave blanching. J. Food Process. Preserv. 21 (6), 461484, http://dx.doi.org/ 10.1111/j.1745-4549.1997.tb00796.x. Breidt, F., Hayes, J.S., Fleming, H.P., 2000. Reduction of microflora of whole pickling cucumbers by blanching. J. Food Sci. 65 (8), 13541358, http://dx.doi.org/10.1111/j.1365-2621.2000.tb10611.x. Cabinplant, 2016. Blancher/cooler. Cabinplant A/S. Available at: ,www.cabinplant.com/fileadmin/user_upload/ downloads/Product_sheets/Blancher_Type_BC_1036.pdf. (www.cabinplant.com . search ‘blancher’ . select ‘Blancher type BC’) (last accessed February 2016). CDT, 2016. Catalytic infrared processing system. Catalytic Drying Technologies. Available at: ,www.catalyticdrying. com. (last accessed February 2016). Cheftel, C., Thiebaud, M., Dumay, E., 2002. Pressure-assisted freezing and thawing of foods: a review of recent studies. High Pressure Res. 22 (34), 601611, http://dx.doi.org/10.1080/08957950212448. Cumming, D.B., Stark, R., Sandford, K.A., 1981. The effect of an individual quick blanching method on ascorbic acid retention in selected vegetables. J. Food Process. Preserv. 5, 3137, http://dx.doi.org/10.1111/j.1745-4549.1981. tb00617.x. Devece, C., Rodriguez-Lopez, J.N., Fenoll, L.G., Tudela, J., Catala, J.M., De Los Reyes, E., et al., 1999. Enzyme inactivation analysis for industrial blanching applications: comparison of microwave, conventional, and combination heat treatments on mushroom polyphenoloxidase activity. J. Agric. Food Chem. 47 (11), 45064511, http://dx. doi.org/10.1021/jf981398 1 . Dorantes-Alvarez, L., Parada-Dorantes, L., 2005. Blanching using microwave processing. In: Schubert, H., Regier, M. (Eds.), The Microwave Processing of Foods. Woodhead Publishing, Cambridge, pp. 153173. Du, Z.L., Gao, Z.J., Zhang, S.X., 2006. Research on convective heat transfer coefficient with air jet impinging. Trans. Chinese Soc. Agric. Eng. 22, 14 (in Chinese with English abstract). Guida, V., Ferrari, G., Pataro, G., Chambery, A., Di Maro, A., Parente, A., 2013. The effects of ohmic and conventional blanching on the nutritional, bioactive compounds and quality parameters of artichoke heads. LWT Food Sci. Technol. 53 (2), 569579, http://dx.doi.org/10.1016/j.lwt.2013.04.006. Icier, F., 2010. Ohmic blanching effects on drying of vegetable byproduct. J. Food Process Engineering. 33 (4), 661683. Icier, F., Yildiz, H., Baysal, T., 2006. Peroxidase inactivation and colour changes during ohmic blanching of pea puree. J. Food Eng. 74 (3), 424429, http://dx.doi.org/10.1016/j.jfoodeng.2005.03.032. Johnson, S., 2011. Steam blanching vs water blanching: cost, efficiency and product quality. Key Technologies Inc. Available at: www.key.net/products/turbo-flo-blancher/default.html (www.key.net . select ‘Equipment’ . ‘TurboFlos blancher’) (last accessed February 2016). Lespinard, A.R., Bon, J., Ca´rcel, J.A., Benedito, J., Mascheroni, R.H., 2015. Effect of ultrasonic-assisted blanching on size variation, heat transfer, and quality parameters of mushrooms. Food Bioprocess Technol. 8 (1), 4153. Lin, S., Brewer, M.S., 2005. Effects of blanching method on the quality characteristics of frozen peas. J. Food Quality. 28 (4), 350360, http://dx.doi.org/10.1111/j.1745-4557.2005.00038.x. Lyco, 2016. Clean-flows rotary drum blancher. Lyco Manufacturing Inc. Available at: http://lycomfg.com/equipment/ blanchers (last accessed February 2016). Osinboyejo, M.A., Walker, L.T., Ogutu, S., Verghese, M., 2003. Effects of microwave blanching vs. boiling water blanching on retention of selected water soluble vitamins in turnip greens using HPLC. In: Annual Meeting of Inst. Food Technologists, Chicago, USA, July 15th. Abstract available at: nchfp.uga.edu/papers/2003/03aamuiftabstract.pdf (last accessed February 2016). Pan, Z., Olson, D.A., Amaratunga, K.S.P., Olsen, C.W., Zhu, Y., McHugh, T.H., 2005. Feasibility of using infrared heating for blanching and dehydration of fruits and vegetables. In: ASAE Annual International Meeting, Tampa, Florida, 17-20 July. Available at: www.catalyticdrying.com/featured-articles.html (last accessed February 2016). Pan, Z.P., Atungulu, G.G., 2010. Infrared dry blanching. In: Pan, Z.P., Atungulu, G.G. (Eds.), Infrared Heating for Food and Agricultural Processing. CRC Press, Boca Raton, FL, pp. 169202. Puupponen-Pimia¨, R., Ha¨kkinen, S.T., Aarni, M., Suortti, T., Lampi, A.-M., Eurola, M., et al., 2003. Blanching and long-term freezing affect various bioactive compounds of vegetables in different ways. J. Sci. Food Agric. 83 (14), 13891402, http://dx.doi.org/10.1002/jsfa.1589. Ramesh, M.N., Wolf, W., Tevini, D., Bognar, A., 2002. Microwave blanching of vegetables. J. Food Sci. 67 (1), 390398. Rickman, J.C., Bruhn, C.M., Barrett, D.M., 2007a. Review: nutritional comparison of fresh, frozen and canned fruits and vegetables, Part II. Vitamin A and carotenoids, vitamin E, minerals and fiber. J. Science Food Agriculture. 87 (7), 11851196, http://dx.doi.org/10.1002/jsfa.2824. Rickman, J.C., Barrett, D.M., Bruhn, C.M., 2007b. Review: nutritional comparison of fresh, frozen and canned fruits and vegetables, Part I. vitamins C and B and phenolic compounds. J. Sci. Food Agric. 87 (7), 930944, http://dx. doi.org/10.1002/jsfa.2825.
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Rodriguez-Lopez, J.N., Fenoll, L.G., Tudela, J., Devece, C., Sanchez-Hernandez, D., de Los Reyes, E., et al., 1999. Thermal inactivation of mushroom polyphenoloxidase employing 2450 MHz microwave radiation. J. Agric. Food Chem. 47 (8), 30283035, http://dx.doi.org/10.1021/jf980945o. Schirack, A.V., 2006. The Effect of Microwave Blanching on the Flavour Attributes of Peanuts (Ph.D. thesis). North Carolina State Univ. Available at: www.lib.ncsu.edu/theses . search ‘Schirack’ (last accessed February 2016). Schirack, A.V., Sanders, T.H., Sandeep, K.P., 2007. Effect of processing parameters on the temperature and moisture content of microwave-blanched peanuts. J. Food Process Eng. 30 (2), 225240, http://dx.doi.org/10.1111/j.17454530.2007.00110. Scott, E.P., Carroad, P.A., Rumsey, T.R., Horn, J., Buhlert, J., Rose, W.W., 1981. Energy consumption in steam blanchers. J. Food Process Eng. 5 (2), 7788, http://dx.doi.org/10.1111/j.1745-4530.1981.tb00263.x. Selman, J.D., 1987. The blanching process. In: Thorne, S. (Ed.), Developments in Food Processing, vol. 4. Elsevier Applied Science, London, pp. 205249. Sensoy, I., Sastry, S.K., 2004. Ohmic blanching of mushrooms. J. Food Process Eng. 27 (1), 115, http://dx.doi.org/ 10.1111/j.1745-4530.2004.tb00619.x. Singh, R.P., Heldman, D.R., 2014. Unsteady state heat transfer, Introduction to Food Engineering. fifth ed. Academic Press, San Diego, CA. Van Buggenhout, S., Messagie, I., Van Loey, A., Hendrickx, M., 2005. Influence of low-temperature blanching combined with high-pressure shift freezing on the texture of frozen carrots. J. Food Sci. 70 (4), S304S308, http://dx. doi.org/10.1111/j.1365-2621.2005.tb07207.x. Varzakas, T., Mahn, A., Pe´rez, C., Miranda, M., Barrientos, H., 2015. Blanching. In: Varzakas, T., Tzia, C. (Eds.), Handbook of Food Processing: Food Preservation. CRC Press, Boca Raton, FL, pp. 126. Williams, D., 2007. Breakthrough blanching technology combines benefits of steam and rotary drum design. Manufacturing Innovation Insider Newsletter, Lyco manufacturing Inc. Available at: http://foodtechinfo.com/foodpro/Efficiency/Blancher_Eff_Rotary_Steam_Lyco.pdf (last accessed February 2016). Xiao, H.W., Yao, X.D., Lin, H., Yang, W.X., Meng, J.S., Gao, Z.J., 2012. Effect of SSB (superheated steam blanching) time and drying temperatures on hot air impingement drying kinetics and quality attributes of yam slices. J. Food Process Eng. 35 (3), 370390, http://dx.doi.org/10.1111/j.1745-4530.2010.00594.x.
Recommended further reading Selman, J.D., 1987. The blanching process. In: Thorne, S. (Ed.), Developments in Food Processing, vol. 4. Elsevier Applied Science, London, pp. 205249. Varzakas, T., Mahn, A., Pe´rez, C., Miranda, M., Barrientos, H., 2015. Blanching. In: Varzakas, T., Tzia, C. (Eds.), Handbook of Food Processing: Food Preservation. CRC Press, Boca Raton, FL, pp. 126.
9
Blanching serves a variety of functions, one of the main ones being to destroy enzymic activity in vegetables and some fruits prior to further processing. A few vegetables, e.g., onions and green peppers, do not require blanching to prevent enzyme activity during storage, but the majority suffer considerable loss in quality if they are not blanched or if they are underblanched. To achieve adequate enzyme inactivation, food is heated rapidly to a preset temperature, held for a preset time and then cooled rapidly to near ambient temperatures. As such, it is not intended as a sole method of preservation but as a pretreatment that is normally carried out between preparation of the raw material (see Sections 2.1 to 2.4) and later operations (particularly heat sterilisation, dehydration and freezing (see Sections 12.1, 14.1 and 22.1)). Blanching is also combined with peeling and/or cleaning of foods (see Sections 2.2 and 2.4) to achieve savings in energy consumption, space and equipment costs. Further details are given by Varzakas et al. (2015).
9.1
Theory
Blanching is an example of unsteady-state heat transfer (Section 1.8.4), involving convective surface heating by steam or hot water and conduction of heat from the surface to the interior of the food. Mass transfer of material into and out of the food (see Section 1.8.1) is also important for the yield of product and nutrient losses. An example of an unsteady-state heat transfer calculation is shown in sample problem (9.1) and further problems are given by Singh and Heldman (2014).
Sample Problem 9.1 Peas with an average diameter of 6 mm are blanched to give a temperature of 85 C at the centre. The initial temperature of the peas is 15 C and the temperature of the blancher water is 95 C. Calculate the time required, assuming that the heat transfer coefficient is 1200 W m22 C21 and, for peas, the thermal conductivity is 0.35 W m21 C21, the specific heat is 3.3 kJ kg21 C21 and the density is 980 kg/m3. (Continued)
Food Processing Technology. DOI: http://dx.doi.org/10.1016/B978-0-08-101907-8.00009-2 © 2017 Elsevier Ltd. All rights reserved.
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Sample Problem 9.1—cont’d Solution to sample problem 9.1
From Eq. (1.54), Bi 5 5
hδ k 1200ð3 3 1023 Þ 0:35
5 10:3 Therefore, k 5 0:097 hδ From Eq. (1.55), θh 2 θf 95 2 85 5 θh 2 θi 95 2 15 5 0:125 From the unsteady-state heat transfer chart for a sphere (Fig. 1.44) Fo 5 0:32 From Eq. (1.56), Fo 5
k t Cρ δ2
5 0:32 Therefore to calculate blanching time (t) 0:32 cρ δ2 t5 k 5
0:32ð3:3 3 103 Þ980ð3 3 1023 Þ2 0:35
5 26:6 s
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The maximum processing temperature in freezing and dehydration is insufficient to inactivate enzymes and does not substantially reduce the number of microorganisms in unblanched foods. If the food is not blanched, enzymes cause undesirable changes in sensory characteristics and nutritional properties during storage, and microorganisms are able to grow on thawing or rehydration. In canning, the time taken to reach sterilising temperatures, particularly in large cans, may be sufficient to allow enzyme activity to take place. It is therefore necessary to blanch foods prior to these operations. Underblanching may cause more damage to food than the absence of blanching does. This is because heat, which is sufficient to disrupt tissues and release intracellular enzymes, but not inactivate them, causes mixing of enzymes and substrates during subsequent storage. In addition, only some enzymes may be inactivated, which results in increased activity of others and accelerated deterioration. Enzymes that cause loss of colour or texture, production of off-odours and offflavours, or breakdown of nutrients in vegetables and fruits include lipoxygenase, polyphenoloxidase, polygalacturonase and chlorophyllase. Two heat-resistant enzymes that are found in most vegetables are catalase and peroxidase. Although they do not cause significant deterioration during storage, they are used as marker enzymes to determine the success of blanching. Peroxidase is the more heat-resistant of the two, so the absence of residual peroxidase activity indicates that other less heat-resistant enzymes are also destroyed. The following factors affect blanching conditions: G
G
G
G
The size and shape of the pieces of food; The thermal conductivity of the food, which is influenced by the type, cultivar and degree of maturity; The blanching temperature and method of heating; The convective heat transfer coefficient.
In practice, the timetemperature combinations used for blanching are evaluated for each raw material to achieve a specified temperature at the thermal centre of the food pieces, to achieve a specified degree of peroxidase inactivation, or to retain a specified proportion of vitamin C. Typical timetemperature combinations vary from 1 to 15 min at 70100 C.
9.2
Equipment
The two most widespread commercial methods of blanching involve passing food through an atmosphere of saturated steam, or a bath of hot water. Both types of equipment are relatively simple and inexpensive. There were substantial developments to blanchers during the 1980s and 1990s to reduce their energy and water consumption and also to reduce the loss of soluble components of foods. The aim was to reduce the volume and polluting potential of effluents (Section 1.7.1.3) and increases the yield of product (the weight of food after processing compared to the weight before processing).
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Table 9.1 Effect of blanching method on ascorbic acid losses in selected vegetables Treatment
Water blanch-water cool Water blanch-air cool Steam blanch-water cool Steam blanch-air cool
Loss (%) of ascorbic acid Peas
Broccoli
Green beans
29.1 25.0 24.2 14.0
38.7 30.6 22.2 9.0
15.1 19.5 17.7 18.6
Source: Adapted from Cumming, D.B., Stark, R., Sandford, K.A., 1981. The effect of an individual quick blanching method on ascorbic acid retention in selected vegetables. J. Food Process. Preserv. 5, 3137. doi:10.1111/j.1745-4549.1981.tb00617.x (Cumming et al., 1981).
Commercially, the yield of food after blanching is an important factor in determining the success of a particular method. In some methods the cooling stage may result in greater losses of product or nutrients than the blanching stage, and it is therefore important to consider both blanching and cooling when comparing different methods (Lin and Brewer, 2005). Steam blanching results in higher nutrient retention when cooling is by cold air or cold-water sprays. However, aircooling causes weight loss of the product due to evaporation, and this may outweigh any advantages gained in nutrient retention. Cooling with running water (fluming) substantially increases leaching losses (washing of soluble components from the food), but the product may gain weight by absorbing water and the overall yield is therefore increased. There are also substantial differences in yield and nutrient retention due to differences in the method of blanching and cooling (Table 9.1), the type of food and differences in the method of preparation, especially if foods are sliced or diced before blanching. Recycling of cooling water does not affect the product quality or yield but substantially reduces the volume of effluent produced. However, it is necessary to ensure adequate hygienic standards to prevent a build-up of bacteria in cooling water.
9.2.1 Steam blanchers The advantages and limitations of steam blanchers are described in Table 9.2. In general this is the preferred method for foods with a large area of cut surfaces as leaching losses are much lower than those found using hot-water blanchers. At its simplest, a steam blancher consists of a mesh conveyor that carries food through a steam atmosphere in an insulated tunnel. The residence time of the food is controlled by the speed of the conveyor and the length of the tunnel. A video of a steam blancher is available at www.youtube.com/watch?v 5 ASCImdcBiwE. Steam blanchers may have microprocessor control of the belt speed (residence time) and blanching temperature, which can be programmed with different blanching conditions for individual products.
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Table 9.2 Advantages and limitations of steam and hot-water blanchers Equipment
Advantages
Steam blanchers
G
G
G
G
Hot-water blanchers
G
G
G
Smaller losses of watersoluble components and higher product yield Smaller volumes of effluent and lower disposal costs than water blanchers, particularly with air cooling instead of water Better energy efficiency Better retention of product colour, flavour and texture Lower capital cost than steam blanchers More uniform product heating Use less floor space
Limitations G
G
G
G
G
G
G
G
Limited cleaning of foods so washers are also required Uneven blanching if food is piled too high on the conveyor Some loss of mass from the food Larger, more complex equipment with higher maintenance costs More difficult to clean Large volumes of dilute effluent result in higher costs for both purchase of water and effluent treatment Risk of contamination of foods by thermophilic bacteria Turbulence may cause physical damage to some products
Typically a tunnel is 1520 m long and 11.5 m wide (Fig. 9.1). The efficiency of energy consumption (i.e., amount of energy used to heat the food divided by the amount of energy supplied) is 19% when water sprays are used at the inlet and outlet to condense escaping steam. Alternatively, food may enter and leave the blancher through rotary valves or hydrostatic seals to reduce steam losses and increase energy efficiency to 27%; or steam may be reused by passing it through Venturi valves. Energy efficiency is improved to 31% using combined hydrostatic and Venturi devices (Scott et al., 1981). In older methods of steam blanching, there was often poor uniformity of heating in multiple layers of food. The timetemperature combination required to ensure enzyme inactivation at the centre of the bed resulted in overheating of food at the edges and a consequent loss of quality. Individual quick blanching (IQB), which involves blanching in two stages, was developed to overcome this problem. In the first stage the food is heated in a single layer to a sufficiently high surface temperature to inactivate enzymes. In the second stage (termed ‘adiabatic holding’) a deep bed of food is held for sufficient time to allow the temperature at the centre of each piece to increase to that needed for enzyme inactivation without the addition of more steam. This reduces heating times (e.g., 25 s for heating and 50 s for holding 1 cm diced carrot compared with 3 min for conventional blanching, or a reduction from 12 to 4.5 min for blanching whole sweetcorn cobs) (ABCO, 2016) (Table 9.3). Shorter
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Figure 9.1 Turbo-Flo steam blancher. Courtesy of Key Technologies Inc. (Johnson, S., 2011. Steam blanching vs water blanching: cost, efficiency and product quality. Key Technologies Inc. Available at: ,www.key.net/products/turbo-flo-blancher/default.html. (www.key.net . select ‘Equipment’ . ‘TurboFlos blancher’) (last accessed February 2016)). Table 9.3
Steam blanching times for selected vegetables
Product
Size (mm)
Blanching time (s)
Broad beans Broccoli, cut Brussels sprouts
2025 30 25 40 13 10 6 50 2050 1330 13 2030 4 25 10 10 25
90120 120180 150180 240300 6090 5070 90120 240300 180240 480720 7090 150180 6090 6090 180240 4560 6090 240300
Cabbage, cut Carrots, diced Carrots, sliced Carrots, whole, baby Cauliflower florets Corn on the cob Green beans, cut Leeks, cut Lima beans Mushrooms, sliced Mushrooms, whole Peas Potatoes, diced Potatoes, whole
Source: Adapted from Cabinplant A/S (Cabinplant, 2016. Blancher/cooler. Cabinplant A/S. Available at: www.cabinplant.com/fileadmin/user_upload/downloads/Product_sheets/Blancher_Type_BC_1036.pdf (www.cabinplant. com . search ‘blancher’ . select ‘Blancher type BC’) (last accessed February 2016)).
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Steam Insulated tunnel Preheat ing
Blanching
Cooling
Chilled water Waste water
Preheat water pump
Figure 9.2 Steam blancher with countercurrent cooling. Adapted from Cabinplant, 2016. Blancher/cooler. Cabinplant A/S. Available at: www.cabinplant.com/fileadmin/user_upload/downloads/Product_sheets/Blancher_Type_BC_ 1036.pdf (www.cabinplant.com . search ‘blancher’ . select ‘Blancher type BC’) (last accessed February 2016).
heating results in improvement in the energy efficiency to 8691%, retention of the product colour and flavour, and, compared with water-blanched products, lower losses of solids and nutrients and smaller volumes of effluent that has a lower COD (see Section 1.7.1.3). Traditional blanchers used 1 kg of steam to blanch 34 kg of vegetables, but more recent designs are capable of blanching up to 16 kg of vegetables per kg of steam at capacities up to 30 t h21 (Johnson, 2011). The low energy consumption is due to multistage countercurrent cooling, in which the chilled cooling water absorbs heat from the blanched product and is pumped to a preheating section where it heats incoming product (Fig. 9.2). The preheated product then requires only a small amount of additional heating in the blanching section. A cooling section employs a fog spray to saturate the cold air with moisture. This reduces evaporative losses from the food and reduces the amount of effluent produced. If required, used cooling water can be used to wash incoming product. The recirculation of water ensures very low water consumption and wastewater discharge (e.g., water consumption of 0.6 m3 h21 for a blancher processing 16,000 kg h21). Alternative designs that use chilled air to cool the product have extremely low water consumption (up to 75% less than traditional blanchers) and negligible wastewater production. However, evaporative air cooling does not permit heat recovery (Cabinplant, 2016). Nutrient losses during steam blanching are also reduced by exposing the incoming food to warm air (65 C) in a short preliminary drying operation (termed ‘preconditioning’). Surface moisture evaporates and the surfaces then absorb condensing steam during IQB. Weight losses and nutrient losses are reduced compared to conventional steam blanching, with no reduction in the yield of blanched food.
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Figure 9.3 Clean-flow rotary drum blancher. Courtesy of Lyco Manufacturing Inc. (Lyco, 2016. Clean-flows rotary drum blancher. Lyco Manufacturing Inc. Available at: http://lycomfg.com/equipment/blanchers (last accessed February 2016).
The equipment for IQB steam blanching consists of a heating section in which a single layer of food is heated on a conveyor and then held on a holding conveyor before cooling. Bucket elevators used to load/unload food are located in close-fitting tunnels and the blancher chamber is fitted with rotary valves, both of which minimise steam losses. Typically, a blancher has a 1.5 m 3 6 m chamber that is fully enclosed and insulated with hydrostatic water seals to prevent evaporation and improve thermal efficiency. It can process 13,000 kg h21, creating only 130 L h21 of wastewater, which is 10% of that created by water blanchers. Williams (2007) describes a rotary drum steam blancher with a compact design that reduces energy and water consumption and has lower capital and maintenance costs than conventional tunnel steam blanchers (Fig. 9.3). Fluidised-bed blanchers operate using a mixture of air and steam, moving at 4.5 m s21, which fluidises and heats the product simultaneously. The design of the blanching chamber promotes continuous and uniform circulation of the food until it is adequately blanched. Although these blanchers are not widely used at a commercial scale, they have advantages that include: (1) faster, more uniform heating and hence shorter processing times and smaller losses of vitamins and other soluble heat-sensitive components of foods, and (2) substantial reductions in the volumes of effluent.
9.2.2 Hot-water blanchers There are a number of different designs of blancher, each of which holds the food in hot water at 70100 C for a specified time and then passes it to a dewatering cooling section. The advantages and limitations of hot-water blanchers are described in Table 9.2. In the reel blancher, food is moved through a slowly rotating cylindrical
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mesh drum that is partly submerged in hot water. The speed of rotation and length of the drum determine the heating time. Pipe blanchers consist of a continuous insulated metal pipe fitted with feed and discharge ports. Hot water is recirculated through the pipe and food is metered in. The length of the pipe and the velocity of the water determine the residence time of food in the blancher. These blanchers have the advantage of a large capacity while occupying a small floor space and in some applications they may be used to simultaneously transport food through a factory. Rotary drum water blanchers have a similar design to the steam drum blancher in Fig. 9.3, with the auger conveying food through hot water. Developments in hot-water blanchers, based on the IQB principle, reduce energy consumption and minimise the production of effluent. For example, the blanchercooler, has three sections: a preheating stage, a blanching stage and a cooling stage, similar to the IQB steam blancher in Fig 9.2. The food remains on a single conveyor throughout each stage and therefore does not suffer physical damage caused by turbulence in conventional hot-water blanchers. A heat exchanger heats the preheat water and simultaneously cools the cooling water, leading to up to 70% heat recovery. A recirculated watersteam mixture is used to blanch the food and final cooling is by cold air. Effluent production is negligible and water consumption is reduced to approximately 1 m3 per 10 t of product. The mass of product blanched is 16.720 kg per kg of steam, compared with 0.250.5 kg per kg of steam in conventional hot-water blanchers. Other studies used low-temperature long-time (LTLT) blanching, typically at 5070 C, to improve the firmness of products, or high-temperature short-time (HTST) blanching to increase energy efficiency. In a combination of both, termed ‘stepwise’ blanching, foods are cooled following LTLT and then subjected to HTST blanching (Pan and Atungulu, 2010).
9.2.3 Newer blanching methods Microwave heating is described in Section 19.1. There have been many studies of microwave blanching (e.g., corn kernels (Boyes et al., 1997), mushrooms (RodriguezLopez et al., 1999; Devece et al., 1999), turnip greens (Osinboyejo et al., 2003) and peanuts (Schirack, 2006; Schirack et al., 2007)). Most have confirmed the advantages over conventional blanchers of faster heating and reduced energy costs, which lead to reduced processing times and lower nutrient losses. The main disadvantages of microwave blanching are the higher cost of the equipment compared to conventional blanchers, nonuniform energy distribution, and difficulties in predicting and monitoring the heating pattern. Microwave blanching has been used commercially in Europe and Japan, but not widely. It is reviewed by Dorantes-Alvarez and Parada-Dorantes (2005) and Ramesh et al. (2002). Ohmic heating is described in Section 19.2. It is not yet used commercially for blanching, but studies have indicated its potential for mushroom blanching (Sensoy and Sastry, 2004). Icier et al. (2006) applied ohmic blanching to pea pure´e and found that peroxidase was inactivated in a shorter time than using water blanching. Similar results were found in ohmically blanched artichokes (Icier, 2010). Guida et al. (2013) found that ohmic blanching (at 24 V cm21 and 80 C) inactivated peroxidase and polyphenoloxidase in artichoke heads more quickly than hot-water
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blanching at 100 C and better preserved their colour and texture. Lespinard et al. (2015) studied ultrasonic-assisted blanching of mushrooms and concluded that a combined treatment with conventional blanching could reduce the processing time and retain the colour of the blanched product. High-humidity hot-air impingement blanching (HHAIB) is a thermal technology that is under development. It combines the advantages of steam blanching and hot-air impingement technologies to produce a uniform, rapid and energy-efficient blanching process that causes minimum loss of water-soluble nutrients. It uses jets of highhumidity hot air that impinge on the product surface at high velocity to achieve a high rate of heat transfer (Du et al., 2006). Studies by Xiao et al. (2012) found that HHAIB pretreatment accelerated drying and improved the whiteness of yam slices, and Bai et al. (2013a,b) found that the process inactivated polyphenoloxidase and maintained the quality of Fuji apple and grapes respectively. Pan et al. (2005) evaluated the feasibility of using medium- and far-infrared heating in a catalytic infrared blancher/dryer for blanching and drying fruits and vegetables without water or steam. Pear cubes, baby carrots, sweetcorn and French fries were blanched with a radiation energy intensity of 5.7 kW m22 for between 1 and 3.5 min to inactivate peroxidase. When pear cubes were dried with radiant energy after blanching, the time was reduced by 44% and the texture and appearance of the dried pears was superior, compared to those produced by steam blanching and hot-air drying. This catalytic infrared processing system is commercialised for drying, peeling, toasting and disinfecting foods in addition to blanching (CDT, 2016). Details are given by Pan and Atungulu (2010). High-pressure processing is described in Section 7.2. There have been a number of studies of high-pressure blanching (e.g., Van Buggenhout et al., 2005) which have demonstrated greater nutrient retention than conventional blanching but Cheftel et al. (2002) concluded that there is insufficient inactivation of enzymes at high-pressure low temperatures and it is unlikely that this process will replace commercial thermal blanching.
9.3
Effect on foods
The heat received by a food during blanching inevitably causes some changes to sensory and nutritional qualities. Blanching causes physical and metabolic changes within food cells that result in cell death. Heat damages cytoplasmic and other membranes, which become permeable and result in loss of cell turgor (Fig. 9.4). Water and solutes pass into and out of cells, resulting in nutrient losses. Heat also disrupts subcellular organelles and their constituents become free to interact within the cell. Overblanching can cause excessive softening and loss of flavour in the food, but the heat treatment is less severe than, e.g., in heat sterilisation, and the resulting changes in food quality are less pronounced. Blanching removes intercellular gases from plant tissues, which together with removal of surface dust, alters the wavelength of reflected light of the food and hence brightens the colour of some vegetables. The time and temperature of
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Figure 9.4 Effect of blanching on cell tissues: S, starch gelatinised; CM, cytoplasmic membranes altered; CW, cell walls little altered; P, pectins modified; N, nucleus and cytoplasmic proteins denatured; C, chloroplasts and chromoplasts distorted.
blanching also influence changes to food pigments according to their D-value (Table 1.41). Changes in colour and flavour caused by blanching are described in more detail by Selman (1987). Sodium carbonate (0.125% w/w) or calcium oxide may be added to blancher water to protect chlorophyll and to retain the colour of green vegetables, although the increase in pH may also increase losses of ascorbic acid. Holding foods such as cut apples and potatoes in dilute (2% w/w) brine prior to blanching prevents enzymic browning. When correctly blanched, most foods have no significant changes to flavour or aroma. The timetemperature conditions needed to achieve enzyme inactivation may cause an excessive loss of texture in some types of food (e.g., some varieties of potato) and in large pieces of food. To reduce this, calcium chloride (12% w/w) is added to blancher water to form insoluble calcium pectate complexes and thus maintain firmness in the tissues. In canned foods, blanching softens vegetable tissues, which facilitates filling into containers. The removal of intercellular gases from plant tissues by blanching also assists the formation of a partial vacuum in the head-space of containers. This prevents expansion of air during processing and so reduces strain on the container seams. Removal of oxygen also reduces oxidative changes to the product during storage. The heat received by a food during blanching inevitably causes some changes to sensory and nutritional qualities. Some minerals, water-soluble vitamins and other water-soluble components are lost during blanching. Losses are mostly due to leaching, thermal destruction and to a lesser extent, oxidation. The amount of vitamin loss depends on a number of factors including: G
G
G
G
G
G
The variety of food and its maturity; Methods used in preparation of the food, particularly the extent of cutting, slicing or dicing (Section 4.1); The surface-area-to-volume ratio of the pieces of food; Method of blanching and cooling; Time and temperature of blanching (lower vitamin losses at higher temperatures for shorter times); The ratio of water to food (in both water blanching and cooling).
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Fat-soluble components (e.g., β-carotene) are largely retained (Rickman et al., 2007a). Puupponen-Pimia¨ et al. (2003) studied the effect of blanching on 20 common vegetables. They found that changes were plant species-dependent, but in general dietary fibre components were either not affected or increased slightly, carotenoids and sterols were not affected, and minerals were stable although there were some leaching losses of soluble minerals. Phenolic antioxidants and vitamins were more heat-sensitive and significant losses of antioxidant activity (2030%) were found in many vegetables. Phenolic compounds and other phytochemicals are water-soluble and therefore susceptible to leaching, but blanching inactivates enzymes that cause their oxidation. However, chemical degradation can occur during storage, depending on the available oxygen and exposure to light. Typical vitamin losses are 1520% for riboflavin, 10% for niacin and 1030% for ascorbic acid (Berry-Ottaway, 2002) and .50% for folic acid. There is a 30% loss of thiamine in spinach due to blanching before freezing and losses of 960% in the frozen product (Rickman et al., 2007b). Losses of ascorbic acid are used as an indicator of the severity of blanching and therefore of food quality. Rickman et al. (2007b) also report studies in which asparagus had the smallest losses during blanching and freezing, with retention averaging 90%, but note that losses of ascorbic acid can vary widely, from 1080%, with average values 50%, depending on the cultivar and processing conditions.
9.4
Effect on microorganisms
Blanching reduces the numbers of contaminating microorganisms on the surface of foods and hence assists in subsequent preservation operations. This is particularly important in heat sterilisation (see Section 12.1), as the time and temperature of processing are designed to achieve a specified reduction in cell numbers. If blanching is inadequate, a larger number of microorganisms are present initially and this may result in a larger number of spoiled containers after processing. The effect of blanching on microorganisms has been described by a number of authors including, e.g., Breidt et al. (2000) who found that blanching whole cucumbers for 15 s at 80 C reduced bacteria by 23 log cycles.
References ABCO, 2016. High efficiency blancher/cookers. ABCO Industries Ltd. Available at: www.abco.ca/blanchers.html (last accessed February 2016). Bai, J.W., Gao, Z.J., Xiao, H.W., Wang, X.T., Zhang, Q., 2013a. Polyphenol oxidase inactivation and vitamin C degradation kinetics of Fuji apple quarters by high humidity air impingement blanching. Int. J. Food Sci. Technol. 48, 11351141, http://dx.doi.org/10.1111/j.1365-2621.2012.03193.x. Bai, J.-W., Sun, D.-W., Xiao, H.-W., Mujumdar, A.S., Gao, Z.-J., 2013b. Novel high-humidity hot air impingement blanching (HHAIB) pretreatment enhances drying kinetics and color attributes of seedless grapes. Innov. Food Sci. Emerging Technol. 20, 230237, http://dx.doi.org/10.1016/j.ifset.2013.08.011. Berry-Ottaway, P., 2002. The stability of vitamins during food processing. In: Henry, C.J., Chapman, C. (Eds.), The Nutrition Handbook for Food Processors. Woodhead Publishing, Cambridge, pp. 247264.
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Pan, Z., Olson, D.A., Amaratunga, K.S.P., Olsen, C.W., Zhu, Y., McHugh, T.H., 2005. Feasibility of using infrared heating for blanching and dehydration of fruits and vegetables. In: ASAE Annual International Meeting, Tampa, Florida, 17-20 July. Available at: www.catalyticdrying.com/featured-articles.html (last accessed February 2016). Pan, Z.P., Atungulu, G.G., 2010. Infrared dry blanching. In: Pan, Z.P., Atungulu, G.G. (Eds.), Infrared Heating for Food and Agricultural Processing. CRC Press, Boca Raton, FL, pp. 169202. Puupponen-Pimia¨, R., Ha¨kkinen, S.T., Aarni, M., Suortti, T., Lampi, A.-M., Eurola, M., et al., 2003. Blanching and long-term freezing affect various bioactive compounds of vegetables in different ways. J. Sci. Food Agric. 83 (14), 13891402, http://dx.doi.org/10.1002/jsfa.1589. Ramesh, M.N., Wolf, W., Tevini, D., Bognar, A., 2002. Microwave blanching of vegetables. J. Food Sci. 67 (1), 390398. Rickman, J.C., Bruhn, C.M., Barrett, D.M., 2007a. Review: nutritional comparison of fresh, frozen and canned fruits and vegetables, Part II. Vitamin A and carotenoids, vitamin E, minerals and fiber. J. Science Food Agriculture. 87 (7), 11851196, http://dx.doi.org/10.1002/jsfa.2824. Rickman, J.C., Barrett, D.M., Bruhn, C.M., 2007b. Review: nutritional comparison of fresh, frozen and canned fruits and vegetables, Part I. vitamins C and B and phenolic compounds. J. Sci. Food Agric. 87 (7), 930944, http://dx. doi.org/10.1002/jsfa.2825.
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Rodriguez-Lopez, J.N., Fenoll, L.G., Tudela, J., Devece, C., Sanchez-Hernandez, D., de Los Reyes, E., et al., 1999. Thermal inactivation of mushroom polyphenoloxidase employing 2450 MHz microwave radiation. J. Agric. Food Chem. 47 (8), 30283035, http://dx.doi.org/10.1021/jf980945o. Schirack, A.V., 2006. The Effect of Microwave Blanching on the Flavour Attributes of Peanuts (Ph.D. thesis). North Carolina State Univ. Available at: www.lib.ncsu.edu/theses . search ‘Schirack’ (last accessed February 2016). Schirack, A.V., Sanders, T.H., Sandeep, K.P., 2007. Effect of processing parameters on the temperature and moisture content of microwave-blanched peanuts. J. Food Process Eng. 30 (2), 225240, http://dx.doi.org/10.1111/j.17454530.2007.00110. Scott, E.P., Carroad, P.A., Rumsey, T.R., Horn, J., Buhlert, J., Rose, W.W., 1981. Energy consumption in steam blanchers. J. Food Process Eng. 5 (2), 7788, http://dx.doi.org/10.1111/j.1745-4530.1981.tb00263.x. Selman, J.D., 1987. The blanching process. In: Thorne, S. (Ed.), Developments in Food Processing, vol. 4. Elsevier Applied Science, London, pp. 205249. Sensoy, I., Sastry, S.K., 2004. Ohmic blanching of mushrooms. J. Food Process Eng. 27 (1), 115, http://dx.doi.org/ 10.1111/j.1745-4530.2004.tb00619.x. Singh, R.P., Heldman, D.R., 2014. Unsteady state heat transfer, Introduction to Food Engineering. fifth ed. Academic Press, San Diego, CA. Van Buggenhout, S., Messagie, I., Van Loey, A., Hendrickx, M., 2005. Influence of low-temperature blanching combined with high-pressure shift freezing on the texture of frozen carrots. J. Food Sci. 70 (4), S304S308, http://dx. doi.org/10.1111/j.1365-2621.2005.tb07207.x. Varzakas, T., Mahn, A., Pe´rez, C., Miranda, M., Barrientos, H., 2015. Blanching. In: Varzakas, T., Tzia, C. (Eds.), Handbook of Food Processing: Food Preservation. CRC Press, Boca Raton, FL, pp. 126. Williams, D., 2007. Breakthrough blanching technology combines benefits of steam and rotary drum design. Manufacturing Innovation Insider Newsletter, Lyco manufacturing Inc. Available at: http://foodtechinfo.com/foodpro/Efficiency/Blancher_Eff_Rotary_Steam_Lyco.pdf (last accessed February 2016). Xiao, H.W., Yao, X.D., Lin, H., Yang, W.X., Meng, J.S., Gao, Z.J., 2012. Effect of SSB (superheated steam blanching) time and drying temperatures on hot air impingement drying kinetics and quality attributes of yam slices. J. Food Process Eng. 35 (3), 370390, http://dx.doi.org/10.1111/j.1745-4530.2010.00594.x.
Recommended further reading Selman, J.D., 1987. The blanching process. In: Thorne, S. (Ed.), Developments in Food Processing, vol. 4. Elsevier Applied Science, London, pp. 205249. Varzakas, T., Mahn, A., Pe´rez, C., Miranda, M., Barrientos, H., 2015. Blanching. In: Varzakas, T., Tzia, C. (Eds.), Handbook of Food Processing: Food Preservation. CRC Press, Boca Raton, FL, pp. 126.