This makes in-line clarification possible. The use of kieselsol, a synthetic silica sol, in combination with gelatin, or pectinase enzyme mixtures, increases the speed of the clarification process. Such tests are frequently necessary because of the variation in the natural tannin and protein contents of the juice. Laboratory procedures are frequently conducted by adding graduated amounts of gelatin and tannin to samples of juice and noting the amounts required for greatest clarity. The amount and type of gelatin added to form a floc must be carefully controlled because an excess of gelatin may cause a haze. To overcome tannin precipitates, gelatin is sometimes added to the juice during enzyme digestion. Haze also may result from the denaturation of proteins. Other sources of colloidal haze in filtered juices are polymerized tannins or compounds formed by the combination of tannins and proteins. This facilitates filtration and minimizes subsequent precipitation or flocculation of pectic substances. In the production of clear juices, natural pectin present is degraded with added pectolytic enzymes. Ascorbic acid addition and pasteurization of the juice as soon as possible after pressing minimize oxidation of polyphenols, which cause browning and contribute to pulp flocculation. This is to retain more of the apple flavor. In the production of opalescent or natural-type apple juice, ascorbic acid is often added to the fruit pulp before pressing or to the juice as it comes from the press. Particle size may be adjusted by screening, homogenization, colloid milling, or selective filtration. The size of the suspended particles is another factor influencing flocculation. This has been found to be particularly important in providing a stable cloud. Preservation of the natural pectin content by rapid heating and cooling of juice as soon after pressing as possible is necessary to inactivate the natural enzymes. In some juices, such as citrus, many of the flavoring constituents adhere to the pulp, and the clarified juices lack flavor. By contrast, cranberry, grape, and many apple juices have little or no suspended solids and are translucent. Citrus and pineapple juices typically have high contents of insoluble pulpy solids and are almost always sold as cloudy juices. The appearance of fruit juices varies widely. Ashurst, in Encyclopedia of Food and Health, 2016 Clarification Biological variability between cultivars of fruit, as well as length of time in storage, cultivation conditions, and extraction methods can all affect the level of bioactive components in juice. The juice making process may or may not result in a loss of phenolic antioxidants. The level of endogenous antioxidants and bioactive compounds is a current area of interest and much work is being conducted to determine what the profile of juice pigments and antioxidants is and how this may be nutritionally beneficial. Other ingredients, such as pigments, phenolics, and volatile compounds also contribute to the sensory and nutritional qualities of fruit juices. Also, vitamins, dietary essential fatty acids, phytochemicals, antioxidants, and proteins are added to increase the nutritional functionality of juices. New juice based drinks are appearing on the market which have been fortified with calcium, specifically citrus juices. The profile of minerals in fruit juice can be used for juice authentication ( Fry, 1990). The mineral content of reconstituted juice may differ from fresh juice, since the water used for juice reconstitution may add some minerals to the product. For example, iron content may be higher in canned juice than fresh juice. The major inorganic ion is potassium.The mineral content may change during fruit juice processing and storage.
Table 13.2 shows the mineral content of some common fruit juices. Fruit juice is not a major source of dietary vitamins and minerals, although orange juice is rich in vitamin C.
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