Mechanical Food Processes Enhancing Preservation

Mechanical food-preservation methods are characterized by separation operations, which tend to affect food quality factors less than other methods, as exemplified by the production of sterile draft beer by filtration.

Packaging is the primary mechanical preservation method; barrier materials protect preserved foods from external spoilage factors during storage and distribution. Other mechanical food-processing methods include sieving, filtration, air classification, washing, sorting, ultrafiltration, microfiltration, centrifugation, extraction, stripping, flotation, and so on. Size grading, defect sorting, and washing are routinely used to reduce or eliminate microbes, enzymes, chemicals, insects, and other matter. Virtually all food materials require repetition of these operations between harvest and preservation.

Washing equipment is designed to accommodate the type of food being washed. Leafy vegetable washers, for example, use a series of rotating paddles that transfer the material along the water surface in a tank so that sand can settle to the bottom of the tank. Fruit washers have reels or belts fitted with high-pressure water sprays that mechanically loosen dirt and debris. For tomatoes, tree fruit,

-Lipid oxidation

-Moisture content isotherm

---Nonenzymatic browning

—o— Enzymatic activity —*— Mold growth —°— Yeast growth —•— Bacteria growth

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Water activity, aw

Figure 1. Relative rate of food deterioration factors as a function of water activity aw.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Water activity, aw

Figure 1. Relative rate of food deterioration factors as a function of water activity aw.

and root vegetables washing units have spinning soft-rubber disks that are mounted in rows to give an intensive wiping action at the surface of the food as it rolls over the disk array. Disk washers reduce water consumption in tomato washing from 1,500 L/t (360 gal/short ton) to as little as 20 L/t (5 gal/short ton). Other food washing equipment uses combinations of sprays, brushes, and mechanical agitators. For example, froth flotation removes foreign particulate organic matter from cut corn and peas. Approved wetting agents (eg, sodium lauryl sulfate) are often incorporated in wash water to improve the efficiency of the washing operation. The proper chlorination of wash and transport water also is extremely important in maintaining low levels of microbes. Break-point chlorination ensures adequate but not excessive residual free chlorine (because >5 ppm of free chlorine can spoil flavors of some foods). This method of chlorination is implemented by adding sufficient chlorine to oxidize organic materials in the water and leave a residual concentration to ensure antimicrobial activity. Water used in the final washing of food materials processed under FDA jurisdiction must be suitable for drinking.

Liquid foods free of suspended solids may be sterilized by filtering spoilage microbes. The filtered product, for example, wine, beer, and certain fruit juices, is packaged under aseptic conditions in a sterilized container. Centrifu-gation can be used to reduce microbial contamination and to remove extraneous organic matter in liquid foods such as milk.

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