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cure color in processed meats, and aids the peelability of frankfurters.

Gelatin desserts are generally adjusted to an average pH of 3.5 for proper flavor and good gel strength. However, the pH can range from 3.0-4.0. Adipic and fumaric acids are used in gelatin desserts that are packaged for retail sales. Their low hygroscopicity allows use of packaging materials that are less moisture resistance and less expensive.

In jams and jellies, the firmness of pectin gel is dependent on rigid pH control. Slow set pectin attains maximum firmness at pH 3.05-3.15 while rapid set pectin reaches maximum firmness at pH 3.35-3.45 (8). The addition of buffer salts such as sodium citrate and sodium phosphate assist in maintaining the pH within the critical pH range for the pectin type. These salts also delay the onset of gelation by lowering the gelation temperature. The acid should be added as late as possible in the process. Premature acid addition will result in some pectin hydrolysis and weakening of the gel in the finished product. The acid is added as a 50% stock solution and thus soluble acids are required. Citric is generally used in this application but malic and tartaric are also satisfactory.

The United States Federal Standards of Identity (9) provide for the direct acidification of cottage cheese by the addition of phosphoric, lactic, citric, or hydrochloric acid as an alternate procedure to production with lactic acid producing bacteria. Milk is acidified to a pH of 4.5-4.7 without coagulation, and then after mixing, is heated to a maximum of 120°F without agitation to form a curd. Glucono-delta-lactone is also permitted for this application. It is added in such amounts as to reach a final pH value of 4.54.8 and is held until it becomes coagulated. GDL is preferred for this application because it must undergo hydrolysis to gluconic acid before it can lower pH. Thus the rate at which the pH is lowered is slowed, avoiding local de-naturation.

The activity of antimicrobial agents (benzoic acid, sorbic acid, propionic acid) is due primarily to the undissociated acid molecule (10). On the basis of undissociated acid concentration, Giannuzzi et al. (11) have shown that citric acid is more effective than ascorbic or lactic acids in inhibiting Listeria monocytogenes in a trypticase soya broth containing yeast extract. Under refrigerated temperatures, higher inhibition indices were obtained in the presence of lower concentrations of citric acid. Activity is therefore pH dependent and theoretical activity at any pH can be calculated. Table 3 shows the effect of pH on dissociation. It can be seen why acidification improves preservative perfor-

Table 3. Effect of pH on Dissociation"

pH

Sorbic

Benzoic

Propionic

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