Tofu Samples Research Articles

A Detection Method for Recombinant DNA from Genetically Modified Soybeans and Processed Foods Containing Them

A method using polymerase chain reaction (PCR) was designed for the detection of food or food ingredients derived from genetically modified soybeans (GMS), imported from the United States, in a mixture with conventional non-genetically modified soybeans (non-GMS). The presence of recombinant deoxyribonucleic acid (DNA) in the soybeans could be detected with three different pairs of specific oligonucleotide primers designed from the sequences of the introduced genes. The soybean intrinsic lectin Le1 gene was used as an internal control. The results of the PCR amplification indicated that a method using cetyltrimethylammonium bromide (CTAB) was most suitable for DNA extraction from soybeans and the processed foods. The recombinant DNA could be detected in dry soybeans containing 0.05% GMS and tofu made from soybeans containing 0.5% GMS. Of 41 commercial tofu samples, recombinant DNA was detected from 27 tofu samples. It is, however, difficult to carry out PCR on DNA extracted from soybeans steamed at 131°C or on fermented natto, although the Le1 gene was detected from soybeans steamed at 115°C and in the fermented natto when a nested PCR technique was employed.

Microbiological evaluation of tofu and tempeh during processing and storage.

Tofu samples consisting of 'Fresh Tofu', 'Herb Tofu', 'Tofu Paste', 'Fried Tofu', 'Tofu Burger', 'Smoked Tofu' and different types of herbs used for making 'Herb Tofu' were analysed for their microbiological load. Tempeh samples consisted of cook water, inoculated beans and fresh tempeh. 'Fresh Tofu' and 'Herb Tofu' had high bacterial counts (> or = 10(5) cfu/g). 'Tofu Burger' and 'Smoked Tofu' had counts around 10(4) cfu/g. 'Tofu Paste' and 'Fried Tofu' had low counts (< 10(2) cfu/g). Cutting board, knife and frequent hand contact with product were the major sources of contamination for tofu. Herbs used for making 'Herb Tofu' were highly contaminated (> 10(5) cfu/g). Cook water and cooked beans for tempeh production had lower counts (< or = 10(4) cfu/g). Initial count of fresh tempeh was high (about 10(8) cfu/g). Most products had counts of > 10(6) cfu/g after 7 days of cold storage. The microbiological quality of the product may improve if producers avoid unnecessary contamination during processing.

Studies on the Process of Produced and Quality of Tofu Part I. Physical Properties of Tofu Produced Using Citrus Fresh Fruit Juice and Ume-zu as Coagulation Agent.

Fresh fruit juice including Sudachi, Yuzu, lemon and Kabosu and Ume-zu containing organic acids and Nigari components were using by applying the acid coagulation properties of soybean protein to examine their availability as a coagulation agent in the production of tofu. Tofu products being marketed using a chemically synthesized coagulation agent (calcium sulfate) were also tested as a control. The physical properties of the control products and test products were compared and examined. Production was carried out according to a common method employing automated production on a large-scale. Each fresh fruit juice was added at an amount of from 1.0-3.0% based on the weight of the soybean milk. In the 1.0-2.0% lots, the physical properties of the tofu products were somewhat insufficient, but the 3.0% lot showed good physical properties. The coagulation effects (rates) of the soybean milk prepared using fresh fruit juice as a coagulation agent were elevated with an increase in the addition ratio. They depended on the pH values of the fresh fruit juice per se. Namely, the coagulation effects were elevated under strongly acidic condition and the strongly acidic Ume-zu containing Nigari components showed further enhanced coagulation effects. When the coagulation effects were increased, the physical properties (hardness, penetration, elasticity and sectility) were increased and the products thus obtained scarcely differed from the marketed and control ones. In total, soft tofu products were obtained in these cases. The 3.0% fresh fruit juice lots showed the best results. In particular, excellent tofu products could be obtained in the Ume-zu and Yuzu lots, followed by the Sudachi, Kabosu and lemon lots. These results agreed with the strongly acidic pH values of the fresh fruit juice per se. The surface color tones of tested tofu samples differed greatly from those of the marketed products and the control products in numerical value. However, little difference was observed with the naked eye. Based on the these results, it is concluded that fresh citrus fruit juice are sufficiently applicable to the production of common tofu products from the viewpoint of physical properties.

Microbiological Quality of Tofu and Related Products in Canada

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 aureus, and psychrotrophs. Although S. aureus counts were generally less than 250 cells per g and Salmonella was not detected, levels of psychrotrophs exceeded 106 per g in more than 45% of finished tofu and okara samples, and levels of coliforms exceeded 103 per g in more than 35% of these samples. Yersinia enterocolitica was also isolated from four samples of finished tofu.

Microbiological Quality of Commercial Tofu

A study was done to investigate the microbiological quality of commercial tofu available in local retail outlets. A sampling method was first developed to obtain accurate and representative microbial counts of individual pieces of tofu. Plate count determination of total aerobic organisms, psychrotrophs, coliforms, sporeformers, yeasts and molds, and staphylococci were made on 60 tofu samples (representing three lots each of four different brands) obtained within 24 h after delivery to the retail store. In addition, for two brands that provided manufacturer's pull dates, the same microbial counts were obtained for samples stored in the laboratory at 10°C until the pull date. Of the tofu sampled immediately after purchase, 83% of the lots tested had total counts greater than 106 colony-forming units (CFU)/g and psychrotrophic counts greater than 104 CFU/g. In addition, 67% of the lots tested had confirmed coliform counts greater than 103 CFU/g. Very low levels (less than 10 CFU/g) of all other microbial groups tested for were found in the majority of lots. Samples held until the manufacturer's pull date contained higher total and psychrotrophic counts but lower or stable counts of other organisms compared with samples tested immediately after purchase. To improve the microbiological quality of tofu, processors need to reduce initial loads by improving sanitation and processing techniques, and retailers should provide more consistent and colder refrigerated storage.

Calcium and Phosphorus Contents and Ratios in Tofu as Affected by the Coagulants Used

Analyses of three commercial brands of tofu (soybean curd) were conducted to determine the calcium and phosphorus contents and the calcium/phosphorus ratios. The firm tofu, precipitated with calcium sulfate, contained over four times as much calcium and over twice as much phosphorus as the soft tofu precipitated with glucono delta lactone. The calcium/phosphorus ratios of the firm and soft tofu samples were 2.0–2.5/1 and 1/1, respectively.Tofu, a high protein food, is a good source of dietary calcium. Since the average American diet has a high phosphorus content and a low calcium/phosphorus ratio, tofu, particularly the firmer kind, also would result in a better calcium/phosphorus ratio.