The manufacturing process physical and chemical characteristics

The manufacture of cooked chilled foods follows many different processes, which includes cooking at one or several stages of processing (Fig. 11.1). The time needed to cook vegetables, that is, to achieve the optimal texture, ranges from 4 to 13min for root vegetables, or ranges for potato from 35 to 50min at 90°C and from 4 to 12min at 100°C (Harada et al., 1985; Harada and Paulus, 1987). Products may receive an additional heat-treatment in the final packaging after vegetable cooking at a temperature sometimes lower than 70°C, and rarely higher than 100°C. This is quite common in the manufacture of sous vide foods and may therefore be compared to a 'pasteurisation' treatment.

Cooked chilled foods are mostly prepared without the additives and the preservatives used in traditional food processing. The pH and the water activity (aw) of the foods are therefore close to those of raw material. pH is between 5.0 and 7.0,

Fig. 11.1 Flow diagram describing the main processing operations in the manufacture of cooked chilled foods made from vegetables.

and mainly between 6.0 and 7.0 and the only exceptions are fruit such as tomato (Lund, 1992). The aw is high (>0.95, often >0.98) because of high moisture content.

Cooked chilled foods are commonly packaged under low oxygen (O2) atmospheres obtained by exclusion of air (and then O2) as in vacuum-packaging or in a oxygen-free modified atmosphere (usually a mixture of CO2 and N2) used to prevent either enzymatic oxidation of food material and organoleptic deterioration or microbial growth and spoilage. These contribute to creating conditions close to anaerobiosis, which is favourable to microaerophilic and strict anaerobic microorganisms. In addition, the redox potential of cooked chilled foods, vegetables or processed vegetables may be quite low, that is between -400 and 0mV (Montville and Conway, 1982; Adams and Moss, 1995; Snyder, 1996), which also creates conditions favourable for these bacteria.

Cooked chilled foods are sold under refrigeration at temperatures mostly lower than 10°C. The shelf-lives of cooked chilled foods are very variable. They range from one week to three months and are commonly close to one month. This depends on product, process and also on national regulations or chill chain conditions.

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