Food Processing Technology - Principles and Practice PDF

3 Raw material preparation At the time of harvest or slaughter, most foods are likely to contain contaminants, to have components which are inedible or to have variable physical characteristics (for example shape, size or colour). It is therefore necessary to perform one or more of the unit operat...

3 Raw material preparation At the time of harvest or slaughter, most foods are likely to contain contaminants, to have components which are inedible or to have variable physical characteristics (for example shape, size or colour). It is therefore necessary to perform one or more of the unit operations of cleaning, sorting, grading or peeling to ensure that foods with a uniformly high quality are prepared for subsequent processing. It is not possible to produce high quality processed foods from substandard raw materials and these mechanical separation procedures, which are applied near the beginning of a process, are a highly cost-effective method of improving the quality of the raw material. Further details are given by Leniger and Beverloo (1975) and applications of these techniques to fruit processing are described by Woodroof (1975). Other separation operations are described in Chapter 6. 3.1 Cleaning Cleaning is the unit operation in which contaminating materials are removed from the food and separated to leave the surface of the food in a suitable condition for further processing. A classification of the type of contaminants found on raw foods is shown in Table 3.1. Peeling fruits and vegetables (Section 3.4), skinning meat or descaling fish may also be considered as cleaning operations. In vegetable processing, blanching (Chapter 10) also helps to clean the product. The presence of contaminants (or foreign bodies) in processed foods is the main cause of prosecution of food companies. An analysis of the types of contaminants is given in Fig. 3.1. Methods that are used to remove foreign bodies are discussed below. Cleaning should take place at the earliest opportunity in a food process both to prevent damage to subsequent processing equipment by stones, bone or metals, and to prevent time and money from being spent on processing contaminants which are then discarded. In addition, the early removal of small quantities of food contaminated by micro-organisms prevents the subsequent loss of the remaining bulk by microbial growth during storage or delays before processing. Cleaning is thus an effective method 84 Food processing technology Table 3.1 Contaminants found on raw foods Type of contaminant Examples Metals Ferrous and non-ferrous metals, bolts, filings Mineral Soil, engine oil, grease, stones Plant Leaves, twigs, weed seeds, pods and skins Animal Hair, bone, excreta, blood, insects, larvae Chemicala Fertiliser, pesticides, herbicides Microbial cells Soft rots, fungal growth, yeasts Microbial products Colours, flavours, toxins a Not to be confused with adulterants (chemicals intentionally added to food which are forbidden by law) or additives (chemicals added to food to improve eating qualities or shelf life). Adapted from Brennan et al. (1990). of reducing food wastage, improving the economics of processing and protecting the consumer. Equipment for cleaning is categorised into wet procedures (for example soaking, spraying, flotation washing and ultrasonic cleaning) and dry procedures (separation by air, magnetism or physical methods). The selection of a cleaning procedure is determined by the nature of the product to be cleaned and by the types of contaminant to be removed. In general, more than one type of cleaning procedure is required to remove the variety of contaminants found on most foods. 3.1.1 Wet cleaning Wet cleaning is more effective than dry methods for removing soil from root crops or dust and pesticide residues from soft fruits or vegetables. It is also dustless and causes less damage to foods than dry methods. Different combinations of detergents and sterilants at different temperatures allow flexibility in operation. However, the use of warm cleaning water may accelerate chemical and microbiological spoilage unless careful control is exercised over washing times and subsequent delays before processing. Furthermore wet procedures produce large volumes of effluent, often with high Fig. 3.1 Types of contaminants leading to prosecution in UK (1988–1994). (From Graves et al. (1998).)  Raw material preparation 85 concentrations of dissolved and suspended solids (measured as biological oxidation demand (BOD) or chemical oxidation demand (COD)). There is then a requirement both to purchase clean water and to either pay for high effluent disposal charges or build in- factory water treatment facilities (also Chapter 26). To reduce costs, recirculated, filtered and chlorinated water is used whenever possible. Examples of wet-cleaning equipment include spray washers, brush washers, drum or rod washers, ultrasonic cleaners and flotation tanks. They are described in detail by Brennan et al. (1990). 3.1.2 Dry cleaning Dry cleaning procedures are used for products that are smaller, have greater mechanical strength and possess a lower moisture content (for example grains and nuts). After cleaning, the surfaces are dry, to aid preservation or further drying. Dry procedures generally involve smaller cheaper equipment than wet procedures do and produce a concentrated dry effluent which may be disposed of more cheaply. In addition, plant cleaning is simpler and chemical and microbial deterioration of the food is reduced compared to wet cleaning. However, additional capital expenditure may be necessary to prevent the creation of dust, which not only creates a health and explosion hazard but also recontaminates the product. The main groups of equipment used for dry cleaning are: air classifiers magnetic separators separators based on screening of foods (Section 3.2.1). Classifiers (for example Fig. 3.2) use a moving stream of air to separate contaminants from foods by differences in their densities. They are widely used in harvesting machines to separate heavy contaminants (for example stones) and light contaminants (for example leaves, stalks and husks) from grain or vegetables. Calculation of the air velocity required for separation is described in Chapter 1 (Equation (1.11)). 3.1.3 Removing contaminants and foreign bodies Physical separation of contaminants from foods is possible when the food has a regular well-defined shape. For example round foods (peas, blackcurrants and rapeseed) are Fig. 3.2 Separation of chaff from grain by aspiration cleaning. 86 Food processing technology separated from contaminants by exploiting their ability to roll down an inclined, upward moving conveyor belt. Contaminants, such as weed seeds in rape-seed or snails in blackcurrants, are carried up the conveyor and separated. A disc separator, used to separate grain from weed seeds, consists of a series of vertical metal discs with precisely engineered indentations in the sides that match the shape of the grain. As the discs rotate, the grain is lifted out and removed (Brennan et al., 1990). Screens (Section 3.2.1) are also used to remove contaminants from foods. Contamination by metal fragments or bolts from machinery is a potential hazard in all processing. Raw materials may pass through metal detectors before processing and all packaged foods are checked for contamination. Details are given in Chapter 25. Ferrous metals are removed by either permanent magnets or electromagnets. Electromagnets are easier to clean (by switching off the power supply) but permanent magnets are cheaper. However, unless regularly inspected, permanent magnets may build up a mass of metal which is lost into the food all at once to cause gross recontamination (Anon., 1991). Small-particulate foods may be automatically checked for contaminants using microprocessor controlled colour sorting equipment (also Section 3.2.2). For example, coffee beans are viewed in ultraviolet light to cause bacterial contaminants to fluoresce (Anon., 1982). More recent developments enable contaminants having the same colour, but a different shape to the product, to be removed (for example green stalks from green beans). X-rays are used to detect metals and other types of solid contaminant in both raw materials and inside packaged foods. The X-rays pass through the food as it passes on a conveyor and are converted to visible light by a phosphor strip or screen. The light is magnified and transmitted by fibre optic cables to an image intensifier and video camera. The final image is displayed on a television monitor. The system activates a warning and also automatically rejects the contaminated item (Anon., 1984a; Williams et al., 1983). The system is limited to products that are larger than 15 cm and to date has been mainly applied to glass jar inspection. New developments include the use of solid-state, X-ray sensitive elements which collect a sample of information as the product passes over the Table 3.2 Summary of techniques used in foreign body food inspection Technique Wavelength Food product Foreign bodies Magnetic N/A Loose and packaged Metals foods Capacitance N/A Products