Tahun 2000. Part IV processing by the removal of heat

19.4 Effect on foods
The process of chilling foods to their correct storage temperature causes little or no reduction in the eating quality or nutritional properties of food. The most significant effect of chilling on the sensory characteristics of processed foods is hardening due to solidification of fats and oils. Chemical, biochemical and physical changes during refrigerated storage may lead to loss of quality, and in many instances it is these changes rather than micro-biological growth that limit the shelf life of chilled foods. These changes include enzymic browning, lipolysis, colour and flavour deterioration in some products and retrogradation of starch to cause staling of baked products (which occurs more rapidly at refrigeration temperatures than at room temperature). Lipid oxidation is one of the main causes of quality loss in cook–chilled products, and cooked meats in particular rapidly develop an oxidised flavour termed ‘warmed-over flavour’ (WOF), described in detail by Brown (2000). Physico-chemical changes including migration of oils from mayonnaise to cabbage in chilled coleslaw, syneresis in sauces and gravies due to changes in starch thickeners, evaporation of moisture from unpackaged chilled meats and cheeses, more rapid staling of sandwich bread at reduced temperatures and moisture migration from sandwich fillings may each result in quality deterioration (Brown, 2000). Vitamin losses during chill storage of selected fresh and processed foods are shown in Table 19.6 and details are given by Bognar (1990).

In cook–chill systems, nutritional losses are reported by Bognar (1980) as insignificant for thiamine, riboflavin and retinol, but vitamin C losses are 3.3–16% day ¹ at 2ºC. The large variation is due to differences in the chilling time, storage temperature, oxidation (the amount of food surface exposed to air) and reheating conditions. Vitamin C losses in cook–pasteurise–chill procedures are lower than cooked–chilled foods (for example

Table 19.6 Loss of vitamins during chilled storage of selected foods
Food Losses (% per day)

_{Ascorbic Thiamin}² _Riboflavin² _Pyridoxine² _Carotene³

_acid¹
Fruit and vegetables

Apples 0.1–0.5

Brussels sprouts

(cooked) 4.6 0.3

Cabbage (white) 0.1–0.2

Carrots 0–0.6 0 0 1.6 0.2–0.8

Cauliflower 0.1–0.2

French beans 1.9–10.0* 0 0 1.8 1.8–2.2

Lettuce 4.8–9.7* 4.7 5.4 2.9

Oranges 26.0

Parsley 2.2–4.5* 8.2 3.9 1.8 1.0–3.0

Peas 1.0–2.0

Pineapples 18.0

Potatoes (boiled) 10.7 1.3

Strawberries 0

Spinach (cooked) 6.4

Tomatoes 41

Meats

Pork liver (fried) 10.3 0.7 0.7 0

Roast pork 0.1

¹ _{Storage at 0–2ºC and relative humidity 76–98%, storage time: 2–21 days}

² _{Storage at 1ºC and relative humidity 50 10%, storage time 3–14 days}

³ _{Storage at 7ºC and relative humidity 60–80%, storage time 2–21 days}

^* Rapid wilting at low storage humidity

Adapted from Ezell and Wilcox (1959 and 1962), Adisa (1986) and Bognar (1980).

spinach lost 66% within 3 days at 2–3ºC after cook–chilling compared with 26% loss within 7 days at 24ºC after cook–pasteurising–chilling.


19.5 Acknowledgements
Grateful acknowledgement is made for information supplied by: Air Products plc, Basingstoke, Hampshire RG24 8YP, UK; BOC Gases, London SW19 3UF, UK; Frigoscandia Equipment AB, S-251 09 Helsingborg, Sweden.


19.6 References
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ANON. (1996) Micro-organisms in Foods (5): Characteristics of microbial pathogens. Int. Committee on Microbiological Specifications for Foods, Int. Union of Biological Sciences (ICMSF), Blackie Academic & Professional, London.
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20

Controlled- or modified-atmosphere storage and packaging

A reduction in the concentration of oxygen and/or an increase in carbon dioxide concentration of the storage atmosphere surrounding a food reduces the rate of respiration of fresh fruits and vegetables and also inhibits microbial and insect growth. When combined with chilling (Chapter 19), modified or controlled atmospheres are an increasingly important method of maintaining high quality in processed foods during an extended shelf life. Modified atmospheres are often included with other minimal processing methods (Chapters 1 and 9) as an important area of future development of mild processed, convenient and ready-to-eat foods that have good nutritional properties and a ‘natural’ image.

There remain differences in, and some confusion over, the terminology used. Modified-atmosphere storage (MAS) and packaging (MAP) are the use of gases to replace air around non-respiring foods without further controls after storage or packing. In controlled-atmosphere storage (CAS) and packaging (CAP), the composition of gas around respiring foods is monitored and constantly controlled, but with advances in ‘active’ packaging systems (Section 20.2.4) the distinction between MAP and CAP is no longer clear. In this book, MAP is used to refer to all methods to change atmospheres in packed food regardless of whether or not the atmosphere changes over time. It also includes vacuum packing (VP), equilibrium- modified-atmosphere (EMA) packaging, passive atmosphere modification (PAM), vacuum-skin packaging (VSP) and gas-exchange preservation (GEP). For definitions of these terms, see Section 20.2 or Church (1994).

In commercial operation, controlled-atmosphere storage (CAS) and modified-atmo- sphere storage (MAS) are mostly used with apples and smaller quantities of pears and cabbage. Modified-atmosphere packaging (MAP) is used for fresh foods and an increasing number of mildly processed foods, and is gaining in popularity as new applications are developed. Examples of MAP products include raw or cooked meats, poultry, fish, seafood, vegetables, fresh pasta, cheese, bakery products, sandwiches, sous vide foods, potato crisps, coffee and tea (Davies, 1995), and with new products including prepared salads, part-baked bread, croissants, pizzas, peeled fruits and prepared vegetables with dressing (Church, 1994).

The normal composition of air is 78% nitrogen and 21% oxygen, with the balance made up of carbon dioxide (0.035%), other gases and water vapour. An increase in the proportion of carbon dioxide and/or a reduction in the proportion of oxygen within specified limits (Section 20.2) maintains the original product quality and extends the product shelf life. This is achieved by:
 inhibiting bacterial and mould growth

 protecting against insect infestation

 reducing moisture loss

 reducing oxidative changes

 controlling biochemical and enzymic activity to slow down senescence and ripening.
CO₂ inhibits microbial activity in two ways: it dissolves in water in the food to form mild carbonic acid and thus lowers the pH of the product; and it has negative effects on enzymic and biochemical activities in cells of both foods and micro-organisms. However, close control over the degree of atmospheric modification is necessary to prevent physiological disorders in the living tissues and secondary spoilage by anaerobic micro- organisms in non-respiring foods. The effects of CO₂ on microbial growth are discussed by Dixon and Kell (1989) and reviewed by Farber (1991).


20.1 Modified- and controlled-atmosphere storage (MAS and CAS)
In MAS, the store is made airtight, and respiratory activity of fresh foods is allowed to change the atmosphere as oxygen is used up and CO₂ is produced (Chapter 19). In CAS, the concentrations of oxygen, carbon dioxide and sometimes ethylene (ethene) are monitored and regulated. Oxygen concentrations as low as 0%, and carbon dioxide concentrations of 20% or higher can be produced in for example grain storage, where these conditions destroy insects and inhibit mould growth. When storing fruits, a higher oxygen concentration is needed to prevent anaerobic respiration which would risk producing alcoholic off-flavours. Different types of fruit, and even different cultivars of the same species, require different atmospheres for successful storage and each therefore needs to be independently assessed. Examples of atmospheres for apple storage are 8% CO₂, 13% O₂ and 79% N₂ for Bramley’s Seedling; and 5% CO₂, 3% O₂ and 92% N₂ for Cox’s Orange Pippin at 3.5ºC to produce an increase from 3 months storage in air to 5 months under CAS. This can be further increased to 8 months using a CAS atmosphere of

1% CO₂, 1% O₂ and 98% N₂ although such low oxygen levels risk anaerobic respiration in other fruits. Refrigerated storage of winter white cabbage in 5% CO₂, 3% O₂ and 92% N₂ enables the crop to be stored until the following summer (Brennan et al., 1990). Details of the atmospheric composition required for other products, building construction, equipment and operating conditions are described by Ryall and Lipton (1979). Safety measures for operators when using increased concentrations of CO₂ are described in Chapter 19 and by Anon. (undated).

Storage is achieved using gas-tight stores, sealed using metal cladding and carefully sealed doorways. When CO₂ and oxygen levels change due to respiration in MAS, or when adjustment to atmospheric composition is needed in CAS, solid or liquid CO₂ can be used to increase gas concentration; controlled ventilation is used to admit oxygen or

‘scrubbers’ may be used to remove CO₂ and thus maintain a constant gas composition in the atmosphere. CO₂ scrubbers operate either by passing the atmosphere from the store over bags of hydrated calcium hydroxide (lime), under sprays of sodium hydroxide or

over activated carbon, to absorb the CO₂. Individual gases may be added from pressurised cylinders in MAS stores that are not completely gas-tight, to speed up the creation of the required atmosphere rather than relying on the action of the fruit alone.

The CO₂ content in the atmosphere can be monitored using sensors to measure differences in the thermal conductivity between CO₂ (0.015 W m ¹ K ¹) and N₂ (0.024

W m ¹ K ¹) and O₂ (0.025 W m ¹ K ¹) or by differences in infrared absorption. Gas composition is automatically controlled by microprocessors using information from the sensors (Chapter 2) to control air vents and gas scrubbers, to maintain a pre-determined atmosphere.

MAS and CAS are useful for crops that ripen after harvest, or deteriorate quickly; even at optimum storage temperatures CA stores have a higher relative humidity (90–95%) than normal cold stores and therefore retain the crispness of fresh foods and reduce weight losses.

The main disadvantages of MAS and CAS are economic: crops other than apples (and to a lesser extent cabbage and pears) have insufficient sales to justify the investment. Short season crops, which increase in price out of season, justify the additional costs of MAS or CAS, but the plant cannot be used throughout the year. Also plant utilisation cannot be increased by storing crops together, because of the different requirements for gas composition, and the risk of odour transfer. Other limitations of MAS and CAS are as follows:
 the low levels of oxygen, or high levels of carbon dioxide, which are needed to inhibit bacteria or fungi, are harmful to many foods

 CAS conditions may lead to an increase in the concentration of ethylene in the

atmosphere and accelerate ripening and the formation of physiological defects

 an incorrect gas composition may change the biochemical activity of tissues, leading to development of off-odours, off-flavours, a reduction in characteristic flavours, or anaerobic respiration