Tahun 2000. Part IV processing by the removal of heat
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- Bu səhifədəki naviqasiya:
- 19.1 Theory 19.1.1 Fresh foods
- Table 19.1
- Table 19.2
- Table 19.3
- 19.1.2 Processed foods
- Table 19.4
Tahun 2000. Part IV Processing by the removal of heat In the unit operations described in this section, a reduction in the temperature of foods slows the biochemical and microbiological changes that would otherwise take place during storage. Preservation by lowering the temperature of foods has important benefits in maintaining their sensory characteristics and nutritional value to produce high quality products. As a result these products have substantially increased in importance during the 1980s and 1990s. Many of the developments in minimal processing methods (Chapter 9) as well as storage of fresh foods rely on chilling (Chapter 19) as a main preservation component. Rapid expansion of ready-to-eat chilled foods, which may also be packed in modified atmospheres (Chapter 20) has been an important development over the last ten years. In general, the lower the storage temperature, the longer foods can be stored, and freezing (Chapter 21) continues to be an important method of processing to produce foods that have a long shelf life. Freeze drying and freeze concentration (Chapter 22) remain important processes for some high-value products, but the high operating costs of these technologies remain important deterrents to their more widespread adoption. Micro-organisms and enzymes are inhibited at low temperatures, but unlike heat processing they are not destroyed. Any increase in temperature can therefore permit the growth of pathogenic bacteria or increase the rate of spoilage of foods. Careful control is needed to maintain a low storage temperature and prepare foods quickly under strict hygienic conditions to prevent spoilage or food poisoning. The need to maintain chill- or frozen temperatures throughout the distribution chain is a major cost to producers and retailers, and this area has seen significant developments to improve efficiency, reduce costs and reduce the risk of spoilage and food poisoning.
Chilling is the unit operation in which the temperature of a food is reduced to between 1ºC and 8ºC. It is used to reduce the rate of biochemical and microbiological changes, and hence to extend the shelf life of fresh and processed foods. It causes minimal changes to sensory characteristics and nutritional properties of foods and, as a result, chilled foods are perceived by consumers as being convenient, easy to prepare, high quality and ‘healthy’, ‘natural’ and ‘fresh’. Since the 1980s there has been substantial product development and strong growth in the chilled food market, particularly for sandwiches, desserts, ready meals, prepared salads, pizza and fresh pasta (Jennings, 1997). Bond (1992), for example, describes the introduction of 1000 new chilled products per annum in the late 1980s, with product development still continuing at a rate of some 750 new products per year. Chilling is often used in combination with other unit operations (for example fermentation (Chapter 7) or pasteurisation (Chapter 11)) to extend the shelf life of mildly processed foods. There is a greater preservative effect when chilling is combined with control of the composition of the storage atmosphere (Chapter 20) than that found using either unit operation alone. However, not all foods can be chilled and tropical, subtropical and some temperate fruits, for example, suffer from chilling injury at 3–10ºC above their freezing point. Chilled foods are grouped into three categories according to their storage temperature range as follows (Hendley, 1985): 1. 1ºC to +1ºC (fresh fish, meats, sausages and ground meats, smoked meats and breaded fish). 2. 0ºC to +5ºC (pasteurised canned meat, milk, cream, yoghurt, prepared salads, sandwiches, baked goods, fresh pasta, fresh soups and sauces, pizzas, pastries and unbaked dough). 3. 0ºC to +8ºC (fully cooked meats and fish pies, cooked or uncooked cured meats, butter, margarine, hard cheese, cooked rice, fruit juices and soft fruits).
and Dade (1992). The successful supply of chilled foods to the consumer is heavily dependent on sophisticated and relatively expensive distribution systems which involve chill stores, refrigerated transport and retail chill display cabinets, together with widespread ownership of domestic refrigerators. Precise temperature control is essential at all stages to avoid the risk of food spoilage or food poisoning. In particular, low-acid chilled foods, which are susceptible to contamination by pathogenic bacteria (for example fresh and pre-cooked meats, pizzas and unbaked dough) must be prepared, packaged and stored under strict conditions of hygiene and temperature control. Details of legislation that affects temperature control of chilled foods in Europe and North America are given by Turner (1992) and Woolfe (2000). 19.1 Theory 19.1.1 Fresh foods The rate of biochemical changes caused by either micro-organisms or naturally occurring enzymes increases logarithmically with temperature (Chapter 1). Chilling therefore reduces the rate of enzymic and microbiological change and retards respiration of fresh foods. The factors that control the shelf life of fresh crops in chill storage include: the type of food and variety or cultivar the part of the crop selected (the fastest growing parts have the highest metabolic rates and the shortest storage lives (Table 19.1)) the condition of the food at harvest (for example the presence of mechanical damage or microbial contamination, and the degree of maturity) the temperature of harvest, storage, distribution and retail display the relative humidity of the storage atmosphere, which influences dehydration losses.
The rate of respiration of fresh fruits is not necessarily constant at a constant storage temperature. Fruits which undergo ‘climacteric’ ripening show a short but abrupt increase in the rate of respiration which occurs near to the point of optimum ripeness. Table 19.1 Botanical function related to respiration rate and storage life for selected products
From Alvarez and Thorne (1981).  Table 19.2 Heat produced by respiration in selected foods
Food Heat (W t 1) of respiration for the following storage temperatures  0ºC 10ºC 15.5ºC Apples 10–12 41–61 58–87
Bananas – 65–116 – Beans 73–82 – 440–580 Carrots 46 93 – Celery 21 58–81 – Oranges 9–12 35–40 68 Lettuce 150 – 620 Pears 8–20 23–63 – Potatoes – 20–30 – Strawberries 36–52 145–280 510 Tomatoes 57–75 – 78
Adapted from Leniger and Beverloo (1975) and Lewis (1990). Climacteric fruits include apple, apricot, avocado, banana, mango, peach, pear, plum and tomato. Non-climacteric fruits include cherry, cucumber, fig, grape, grapefruit, lemon, pineapple and strawberry. Vegetables respire in a similar way to non- climacteric fruits. Differences in respiratory activity of selected fruits and vegetables are shown in Tables 19.1 and 19.2. Undesirable changes to some fruits and vegetables occur when the temperature is reduced below a specific optimum for the individual fruit. This is termed chilling injury and results in various physiological changes (for example internal or external browning, failure to ripen and skin blemishes). The reasons for this are not fully understood but may include an imbalance in metabolic activity which results in the over-production of metabolites that then become toxic to the tissues (Haard and Chism, 1996). It is found for example in apples (less than 2–3ºC), avocados (less than 13ºC), bananas (less than 12– 13ºC), lemons (less than 14ºC), mangoes (less than 10–13ºC) and melons, pineapples and tomatoes (each less than 7–10ºC). The optimum storage temperature and relative humidity, and expected storage times are shown in Table 19.3 for a variety of fresh fruits and vegetables. Undesirable changes due to incorrect relative humidity are described by van den Berg and Lentz (1974). In animal tissues, aerobic respiration rapidly declines when the supply of oxygenated blood is stopped at slaughter. Anaerobic respiration of glycogen to lactic acid then causes the pH of the meat to fall, and the onset of rigor mortis, in which the muscle tissue becomes firm and inextensible. Cooling during anaerobic respiration is necessary to produce the required texture and colour of meat and to reduce bacterial contamination. Undesirable changes, caused by cooling meat before rigor mortis has occurred, are termed cold shortening. Details of these and other post-mortem changes to meat are described by Laurie (1998). To chill fresh foods it is necessary to remove both sensible heat (also known as field
20ºC and atmospheric pressure is given by equation (19.1).  C6 H12 O6 6O2 6CO2 6H2 O 2 835 106 J kmol 1 C6 H12 O6 19 1
The size of refrigeration plant and the processing time required to chill a crop are calculated using unsteady-state heat transfer methods (Chapter 1). The calculations are Table 19.3 Optimum storage conditions for some fruits and vegetables Food Temperature (ºC) Relative humidity (%) Storage life (days) Apricot 0.5–0 90 7–14
Banana 11–15.5 85–95 7–10 Bean (snap) 7 90–95 7–10 Broccoli 0 95 10–14 Carrot 0 98–100 28–42 Celery 0 95 30–60 Cherry 1 90–95 14–20 Cucumber 10–15 90–95 10–14 Eggplant 7–10 90–95 7–10 Lemon 10–14 85–90 30–180 Lime 9–10 85–90 40–140 Lettuce 0–1 95–100 14–20 Mushroom 0 90 3–4 Peach 0.5–0 90 14–30 Plum 1–0 90–95 14–30 Potato 3–10 90–95 150–240 Spinach 0 95 10–14 Strawberry 0.5–0 90–95 5–7 Tomato 4–10 85–90 4–7
(1998).
simpler when processed foods are chilled as respiratory activity does not occur. A number of assumptions are made to simplify calculations further; for example the initial temperature of a food is constant and uniform throughout the food, and the temperature of the cooling medium, respiratory activity and all thermal properties of the food are constant during cooling. Detailed derivations of theoretical considerations and examples of calculations of heat load and chilling rate are described by van Beek and Meffert (1981). Sample problem 19.1 Freshly harvested berries measuring 2 cm in diameter are chilled from 18ºC to 7ºC in a chiller at 2ºC, with a surface heat transfer coefficient of 16 W m 2 K 1. They are then loaded in 250 kg batches into containers and held for 12 h in a cold store operating at  2ºC, prior to further processing. The cold store holds an average of 2.5 t of food and measures 3 m high by 10 m 10 m. The walls and roof are insulated with 300 mm of polyurethane foam, and the floor is constructed from 450 mm of concrete. The ambient air temperature averages 12ºC and the soil temperature 9ºC. An operator spends an average of 45 min day 1 moving the containers in the store and switches on four 100 W lights when in the store. Each container weighs 50 kg. Calculate the time required to cool the berries in the chiller and determine whether a 5 kW refrigeration plant would be suitable for the cold store. (Additional data: the thermal conductivity of the berries is
0.127 W m 1 K 1, the thermal conductivity of the insulation is 0.026 W m 1 K 1, the thermal conductivity of the concrete is 0.87 W m 1 K 1 (Table 1.5), the specific heat of the berries is 3778 J kg 1 K 1, the specific heat of the container is 480 J kg 1K 1, the density of berries is 1050 kg m 3, the heat produced by the operator is 240 W, and the average heat of respiration of berries is 0.275 J kg 1 s 1.)
To calculate the time required to cool the berries, from equation (1.25) for unsteady- state heat transfer (Bi h k) for berries, 16 0 01 Bi 0 127 1 26 1 Bi 0 79 From equation (1.26) for cooling, h f 7 2 h i 18 2 0 45
From Fig. 1.10 for a sphere, Fo 0.38. From equation (1.27), k t 0 38 c 2 Therefore, t 0 38 3778 1050 0 01 2 0 127
19 8 min To determine whether the refrigeration plant is suitable as a cold store, assume that the berries enter the store at chill temperature. Total heat load Now heat of respiration sensible heat of containers heat evolved by operators and lights heat loss through roof and walls heat loss through floor
heat of respiration 2500 0 275 687 5 W Assuming that the containers have the same temperature change as the berries and the number of containers is 2500/250 10, 10 50 480 18 7 
Next heat removed from containers 12 61W
heat evolved by operators and lights 240 4 100 45 60  20 W
24 3600
From equation (1.12), for an area of 60 60 100 220 m2 0 026 220 12 2 Finally,
heat loss through roof and walls heat loss through floor (of area 100 m)2 0 3 267 W 2127 W
0 45
Therefore the total heat loss is the sum of the heat loads 687.5 W 61 W 20 W 2394 W 3162.5 W 3.2 kW.
A reduction in temperature below the minimum necessary for microbial growth extends the generation time of micro-organisms and in effect prevents or retards reproduction. This mechanism is described in detail in most microbiological texts (for example Frazier and Westhoff, 1978). There are four broad categories of micro-organism, based on the temperature range for growth (Walker and Betts, 2000): 1. thermophilic (minimum: 30–40ºC, optimum: 55–65ºC) 2. mesophilic (minimum: 5–10ºC, optimum: 30–40ºC) 3. psychrotrophic (minimum: 0–5ºC, optimum: 20–30ºC) 4. psychrophilic (minimum: 0–5ºC, optimum: 12–18ºC). Chilling prevents the growth of thermophilic and many mesophilic micro-organisms. The main microbiological concerns with chilled foods are a number of pathogens that can grow during extended refrigerated storage below 5ºC, or as a result of any increase in temperature (temperature abuse) and thus cause food poisoning (Kraft, 1992). Previously it was considered that refrigeration temperatures would prevent the growth of pathogenic bacteria, but it is now known that some species can either grow to large numbers at these temperatures, or are sufficiently virulent to cause poisoning after ingestion of only a few cells. Examples of these pathogens are Aeromonas hydrophilia, Listeria spp, Yersinia enterocolitica, some strains of Bacillus cereus, Vibrio parahaemolyticus and enter- opathogenic Escherichia coli (Marth, 1998). An example of the last (E.coli 0157:H7) may cause hemorrhagic colitis after ingestion of as little as ten cells (Buchanan and Doyle, 1997). A summary of the sources of these bacteria, types of infection or spoilage and typical high-risk foods is given in Table 19.4. Details of the taxonomy, pathogenicity, detection and distribution of important pathogens are given by Anon. (1996), Marth (1998) and Walker and Betts (2000). It is therefore essential that good manufacturing practice (GMP) is enforced during the production of chilled foods. Details of the hygienic design of chilling plants, cleaning schedules and total quality management (TQM) procedures are discussed in detail by Holah and Brown (2000), Holah (2000) and Rose (2000), respectively. Table 19.4 Pathogenic or spoilage bacteria in high–risk chilled foods
 Table 19.4 Continued
Micro-organism Source Minimum growth temperature (oC) Type of infection/spoilage and incubation period Typical high-risk foods
Clostridium perfringens Soil, dust, vegetation, raw, dried and cooked foods Listeria monocytogenes Ubiquitous (soil, healthy humans or animals, food processing surfaces) 12 Acute diarrhoea, nausea but little fever or vomiting (8–24 h) 0.4–3 Gastro-enteritis. Individuals having compromised immune systems are especially vulnerable (24–96 h) Raw meats, poultry, fish, dairy products, dried foods, soups, spices, pasta
Milk, seafoods, ready-to-eat sandwiches and salads, especially those containing meat, coleslaw, soft cheeses
Spoilage micro-organisms Brochothrix thermosphacta – Sliminess, off-odours or flavours Vacuum packed beef, pork, lamb, sliced cured meats, corned beef Lactic acid bacteria Ubiquitous 0–5 Production of either lactic acid, acetic acid, formic acid, ethanol, carbon dioxide Pseudomonas spp – 3–0 Development of bitterness and rancidity, green colouration Milk, milk products, meats, fruit juices, vegetables, alcoholic beverages, sugar products Most chilled foods Yeasts (e.g. Candida spp), and moulds (e.g. Mucor spp, Rhizopus spp)
Ubiquitous 0 Fermentation by yeasts causing yeasty, fruity or alcoholic off- flavours and odours Visible mould growth, softening, flavour and aroma changes and mycotoxin production Fruit juices, meat products, vegetables, dairy products Adapted from Marth (1998), Frazier and Westhoff (1988), Anon. (1996) and Walker and Betts (2000). The shelf life of chilled processed foods is determined by: the type of food the degree of microbial destruction or enzyme inactivation achieved by the process control of hygiene during processing and packaging the barrier properties of the package temperatures during processing, distribution and storage.
‘hurdles’ to microbial growth and further details of this concept are given in Chapter 1. Packaging of chilled foods is described in Chapter 24. Details of correct storage conditions for specific chilled products are listed by Anon. (1979), and procedures for the correct handling of chilled foods are described by Anon. (1982). Yüklə 1,14 Mb. Dostları ilə paylaş:
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