Microorganisms Involved in the Spoilage of Fermented Fruit Juices

Abstract

Bottled wines, ciders and perries that contain sugar are susceptible to refermentation by yeasts and the growth oflactic acid bacteria. The yeasts are strongly fermentative strains, often species of Saccharomyces, while the tactics are acid and ethanol tolerant species that grow slowly and are fastidious in their nutrient requirements. Spoilage is manifested by gas, haze and various flavor changes. Control procedures include filtration, pasteurization and the use of the preservatives sulfur dioxide and sorbic acid. Commercially important fermented fruit juice products are wines, ciders and perries (fermented pear juice). These foods have a low pH and a high alcohol content and, as a result, do not support growth of most microorganisms. Microbes that can be a problem are yeasts, molds and a few genera of aciduric bacteria. This list becomes even smaller if one considers only spoilage of the bottled product, since under anaerobic conditions the microorganisms of concern are mainly yeasts and lactic acid bacteria. A completely dry table wine is biologically stable because the growth of yeasts and lactic acid bacteria is dependent upon the presence of a utilizable carbohydrate. However many table wines, especially the white varieties, are not completely dry. This usually is intentional since the presence of a little sugar often makes the wine more palatable to American tastes and may help mask certain flavor defects. Wines also may contain low concentrations of pentose sugars that can serve as a carbon source for some yeasts (13). YEASTS Products containing less than 18 vol o/o ethanol are subject to growth and refermentation by yeasts, mainly species of Saccharomyces. Dessert wines are stable because of fortification alone or because of a synergism between the high levels of ethanol and sugar. Inhibitory concentrations can be estimated on the basis of Delle units (DU) which equal o/o sugar plus 4.5 times vol o/o ethanol. Most yeasts are inhibited when the DU values are 70 to 80 (JJ). Other means for preventing refermentation will be discussed later. LACTIC ACID BACTERIA The lactic acid bacteria, the principle spoilage bacteria, are gram-positive cocci and rods that belong to the families Streptococcaceae and Lactobacteriaceae. Many of the species that grow in fermented fruit products are heterofermentative (Table 1). In general, ciders and perries permit the growth of a greater number of species than do table wines, probably because of their lower ethanol content. (Most ciders contain 4-5 vol o/o alcohol compared to 11-12% in a typical table wine.) Properties shared by many of these wine isolates are a tolerance for ethanol, a preference for an acid medium, exacting nutrient requirements and a slow growth rate even under presumably optimal conditions. The slow growing lactobacilli, termed the "inactive group" (17), include Lactobacillus hilgardii, L. fructivorans and L. trichodes. They, as well as Leuconostoc oenos, are heterofermentative species (24). TABLE 1. Bacteria important in the spoilage of bottled and wines. Product Cider and perry Wine, table dessert Microorganism Lactobacillus plantarum (5) Lactobacillus mali (5) Lactobacillus collinoides (6) Pediococcus cerevisiae (4) Leuconostoc oenos (6) Zymomonas mobilis (3) Lactobacillus hilgardii (25) Lactobacil/usfructivorans (27) Leuconostoc oenos (12) Pediococcus sp. (26)