Microbiological Quality of Tofu and Related Products in Canada

Abstract

One hundred and fifty-three samples of tofu, related products , and environmental samples comprising 346 sample units were collected from 14 manufacturers across Canada. They were analyzed for coliforms, Salmonella, Yersinia, Staphylococcus au-reus, and psychrotrophs. Although S. aureus counts were generally less than 250 cells per g and Salmonella was not detected, levels of psychrotrophs exceeded 10 6 per g in more than 45% of finished tofu and okara samples, and levels of coliforms exceeded 10 3 per g in more than 35% of these samples. Yersinia entero-colitka was also isolated from four samples of finished tofu. Tofu, or soybean curd, is made from soy (bean) milk. Soybeans soaked in water and heated (mash) are pressed, yielding soymilk and a fibrous residue (okara). Soymilk is heated and coagulated with calcium or magnesium chloride or sulfate. The curd formed is molded into cakes which may be kept either in their whey or in water. Tofu may be soft, hard, dry, fried, or canned. Manufacture of tofu involves two heating steps. The mash is cooked at 100-110°C for about 10 min; the coagu-lant is added, then the soymilk-coagulant mixture is heated and held at 75-85°C for 15-30 min. These heat treatments are sufficient to destroy vegetative cells of bacteria. Despite this, tofu has been implicated in a food poisoning episode in 1982, when 87 persons in Seattle, WA, developed yersini-osis after eating tofu (2). This study was designed to determine the microbio-logical quality of tofu and related products at the production level. MATERIALS AND METHODS Sampling Fourteen manufacturers of tofu and related products were visited. Samples were obtained of regular and fried tofu, unfinished product, coagulation solution, okara, mash, and the environment. Each sample of finished product was drawn at random from a different production lot, and normally consisted of three sample units. Microbiological methods General. Sample units were analyzed for psychrotrophs, coliforms, Staphylococcus aureus, Salmonella spp., and Yersinia spp. using Health Protection Branch (HPB) methods. For Salmonella determination, 25 g from each of the sample units were combined to form composite analytical units. The composites were blended with equal volumes of nutrient broth for 2 min and then 8 additional volumes of nutrient broth were added (7). For the other determinations, 11 g portions comprised the analytical unit. Decimal dilutions were made in 0.1% (w/v) peptone water. If the pH of any homogenate fell outside the range of 6.0 to 7.6, it was adjusted to 7.0 ± 0.1 with IN NaOH or IN HC1 to improve recoverability of the target organism. Environmental swabs taken on site were applied to the primary media described in the following methods. Psychrotrophs. Duplicate 1 ml volumes of appropriate dilutions were plated by the pour plate method using Standard Plate Count Agar (3). Plates were incubated at 7°C + 1°C for 10 d. Coliforms. Coliforms were determined by a 5-tube most probable number procedure (MPN). Lauryl Sulfate Tryptose Broth (LST) in tubes containing inverted gas vials were incubated at 35°C for up to 48 h. Inocula from gas-positive LST tubes were transferred to tubes of Brilliant Green Bile Broth containing gas vials. Gas production within 48 h at 35°C confirmed the presence of coli-forms, and the MPN was computed from a 5-tube MPN table (4). Fecal coliforms and Escherichia coli. From gas-positive LST tubes, inocula were transferred to tubes of EC broth. Gas production at 44.5 + 0.2°C within 48 h confirmed the presence of fecal coli-forms. Portions from gas-positive EC tubes were streaked onto plates of Endo Agar and incubated for 24 h at 35°C. Typical colonies were picked and E. coli was confirmed by IMVIC tests and production of gas at 44.5°C in EC broth (4). Staphylococcus aureus. Two-tenths ml volumes of appropriate dilutions were spread in duplicate on Baird-Parker agar; the plates were incubated at 35°C for 48 h. Typical black colonies with or without clear halos were counted and a representative number cultured in Brain Heart Infusion Broth. S. aureus was confirmed by the tube coagulase and thermostable nuclease tests (5).