The Growth of Yeasts

The grape juice industry has shifted almost entirely from the old method of storing juice in 5-gallon carboys to cool storage in tanks at 22 to 28 F (-5.5 to -2.2 C). Excess tartrates are precipitated during storage. When juice was stored in carboys, the carboys wNere filled at processing temperature, and either spray-cooled or allowed to cool slowly at the cellar storage temperature. For the modern cool storage, the juice is flash heated at 175 to 185 F (70 to 85 C) and flash cooled to about 32 F (0 C) before pumping it into storage tanks. After the tartrates and other argols are precipitated, the juice is repasteurized and bottled for retail sale. By this method of storage, heating effects are minimized. A general improvement in quality has been effected by this change in method of storage. However, problems of yeast and mold contamination and growth have been serious at times. Since quality is retained better when stored at the cool temperature, processors would like to hold juice in tanks until bottled juice is needed. They would also like to distribute bottling throughout the year and, at times, hold juice into the following season to equalize differences in supply resulting from variations in production of grapes. They would prefer to use a cool storage temperature high enough to prevent the juice from freezing solidly. The mere presence of yeast is unimportant in itself. However, if yeasts grow and multiply sufficiently to ailter the product, they will cause an economic loss to the producer. Although it was originally assumed that yeasts would not grow at low storage temperatures, it is now well known by all processors that they will grow. In fact, it has been demonstrated that some of the varieties of yeast will grow better at a low temperature of 1 C than at room temperature, 21 C (Lawrence et al., 1959). The temperature used for pasteurization is sufficient to kill all yeasts. It is possible to pack juice without having viable yeast present (Pederson et al., 1959). This study was instituted in an effort to determine the degree and the sources of contamination in the handling and storage of grape juice in order that