Incoming vegetables or fruits, which are covered with soil, mud and sand, should be carefully cleaned before processing. A second wash must usually be done after peeling and/or cutting (Wiley, 1994; Ahvenainen and Hurme, 1994). For example,
IH Fresh (control)
S 0.5% citric acid
Fig. 14.2 Effect of washing solution and storage time on the odour of grated carrots packed in air and stored at 5°C.
Chinese cabbage and white cabbage must be washed after shredding, whereas carrot must be washed before grating (Hurme et al., 1994; Ahvenainen et al., 1994). Washing after peeling and cutting removes microbes and tissue fluid, thus reducing microbial growth and enzymatic oxidation during storage. Washing in flowing or air-bubbling water is preferable to dipping into still water (Ohta and Sugawara, 1987). The microbiological quality of the washing water used must be good and its temperature low, preferably below 5°C. The recommended amount of water used is 5-10 lkg-1 of product before peeling/cutting (Huxsoll and Bolin, 1989) and 3 lkg-1 after peeling/cutting (Hurme et al., 1994; Ahvenainen et al., 1994).
Preservatives can be used in washing water to reduce microbial numbers and to retard enzymatic activity, thereby improving the shelf-life. 100-200 mg of chlorine or citric acid per litre is effective in washing water before or after peeling and/or cutting to extend shelf-life (Wiley, 1994; Kabir, 1994; Hurme et al., 1994; Ahvenainen et al., 1994; O'Beirne, 1995). The relative effects of differing washing solutions are shown in Fig. 14.2. However, when chlorine is used, vegetable material should be rinsed. Rinsing reduces the chlorine concentration to the level of that in drinking water and means that sensory quality is not compromised (Hurme et al., 1994). The effectiveness of chlorine can be enhanced by using a combination of low pH, high temperature, pure water and correct contact time (Wiley, 1994; Kabir, 1994). It seems that chlorine compounds reduce counts of aerobic microbes at least in some leafy vegetables such as lettuce (Wiley, 1994; Garg et al., 1990), but not necessarily in root vegetables or cabbages (Garg et al., 1990; Ahvenainen et al., 1994). Chlorine compounds are of limited effectiveness in suppressing growth of Listeria monocytogenes in lettuce and cabbage (Skytta et al., 1996; Francis and O'Beirne, 1997). In addition, the use of some preservatives (e.g. chlorine compounds) is not necessarily allowed in all countries. Alternatives to chlorine include chlorine dioxide, peracetic acid ozone, trisodium phosphate and hydrogen peroxide (Sapers and Simmons, 1998). Hydrogen peroxide vapour treatment, for example, appears to reduce microbial counts on freshly cut cucumber, bell peppers and zucchini, extending shelf-life without leaving significant residues or compromising product quality. However, more research is still required to validate these treatments.
Washing water should be removed gently from the product (Wiley, 1994). A centrifuge seems to be the best method. The centrifugation time and rate should be chosen carefully (Zomorodi, 1990; Bolin and Huxsoll, 1991) so that the process removes free water but does not damage vegetable cells.
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