Microbial Spoilage of Tofu (Soybean Curd)

Abstract

Plate counts and pH measurements were performed on tofu from different aged lots. The pH declined from to 5.8 to 5.2 with age. Aerobic plate counts of 1- and 30-d-old samples were approximately 106 CFU/g. Older samples had counts of about 108 CFU/g. Anaerobic counts rose from 106 CFU/g in the 1-d-old lot to a high of 109 CFU/g in the 30-d-old lot. The major species from the different age lots were lactic acid bacteria, enteric bacteria, and Pseudomonas species. Representatives from each of these groups were inoculated into autoclaved tofu and incubated at 5°C for 23 d. CFU/ml and pH of both water and cake were measured. Turbidity, mg protein per ml, and mg NH4+ per mi of the water were measured. All species tested increased in numbers in the tofu and caused changes in at least some of the characteristics measured. Samples were taken during the manufacture of tofu, and all organisms found in each sample were characterized. All organisms that were shown capable of causing spoilage in tofu were present in large numbers early in manufacture but were no longer found in samples subsequent to pressure cooking. All of the spoilage organisms reappeared later during manufacture. These data indicate that tofu is spoiled by enteric bacteria and Pseudomonas species, as well as by lactic acid bacteria. The spoilage organisms are probably introduced by equipment and personnel with which the product comes in contact after it is pressure cooked. In addition, aerobic plate counts can underestimate the bacterial load in tofu, and therefore, anaerobic plate counts should also be performed.