Raw stinky tofu
Drip-dry at 4 °C overnight i
Frying or steaming i
Figure 4 Flowchart for making stinky tofu. (From Ref. 4.)
increased gradually to about 7.5. The total bacterial count fluctuated between a light decrease after 10 weeks of fermentation.
In the first week of the fermentation period, the pH value of tofu slices in the stinky tofu brine dropped from 6.5 to about 4.6 due to the production of lactic acid and growth of the lactic acid bacteria. Thereafter, the protein in the tofu was hydrolyzed by the microbial proteases to form amino acids, followed by the action of deaminase to form ammonia. The pH value therefore increased gradually to about 7.5 in week 10. Ammonia content continues to increase, favoring the growth of more alkali-tolerant bacteria instead of the lactic acid bacteria, with a decrease in total bacterial count (4). Because of this phenomenon, stinky tofu is considered an alkaline, fermented food (5).
It is understood that types of bacteria for various formulations will differ. From this particular fermentation, Bacillus sp. such as B. megaterium, B. polymyxa, B. pumilus, B. subtilis, and B. sphaericus, and lactic acid bacteria such as Enterococcus, Streptococcus, and Lactobacillus sp. were isolated from the brine and from the tofu surface. At the beginning of the fermentation, the lactic acid bacteria such as Enterococcus, Streptococcus and Lactobacillus prevailed. As fermentation progressed, the ammonia resulting from the hydrolysis of protein caused the pH value to increase. Therefore, at the later stage of fermentation, the more alkali-tolerant Bacillus sp. dominated. It was found that quality of stinky tofu is better from mixed cultures as compared to single cultures (6).
In the traditional method of making stinky tofu brine, it is common to use salted leaf mustard brine or rotted, salted leaf mustard, shrimp, and brine for making thousand year old egg or pedan (brine composed of salt, tea leaves, wine, and potash). Fermentation relies on the microorganisms from these ingredients. Therefore, differences will occur from different batches of raw materials with inconsistent quality. In addition, the use of large chunks of raw materials makes digestion by microorganisms a very slow process, up to 6 months. During this period, because of the use of an open system, the brine is vulnerable to insect infestation. In the stinky tofu brine made by the household method, the pH value is between 5 and 7, with 1-3% salt, and has a total bacterial count of 107-109 cfu/mL. Quality is not consistent. With the use of pure cultured stinky tofu starter to make the brine, the making of stinky tofu is sanitary and safe, with consistent good quality.
Deep-fried stinky tofu has the following nutrient composition (per 100 g): total solids, 30.37 g; protein, 15.22 g; fat, 11.94 g; ash, 1.07 g; vitamin B1, 0.048 mg; vitamin B2, 0.03 mg; niacin, 0.43 mg; iron, 2.9 mg; potassium, 94 mg; sodium, 58 mg; calcium, 131 mg; magnesium, 47 mg; and phosphorus, 220 mg (7).
When mixing new and used brine, the soaking period of tofu in the brine has to be adjusted accordingly. Soaking too long will cause easy damage to the integrity of the final product, and too short a soaking period will result in a low quality product. Temperature of deep-frying should not be too high; it should be maintained at medium heat. The center of the tofu pieces must be fully cooked with the surface golden yellow. If not, diarrhea may occur.
In the manufacturing of stinky tofu, the traditional procedure for preparing the stinky brine is to use raw salted vegetables. These vegetables carry the microorganisms that come with them, and thus the process is a natural fermentation. Consequently, the quality is not stable and varies among batches. In addition, this lengthy, open fermentation method to induce fermentative microbial growth attracts flies and insects, which lay eggs that can change into maggots. Pathogenic microorganisms can also grow and are harmful to the consumers. It is therefore important that in the preparation of stinky brine, the medium should be sterilized and followed by inoculation with pure cultures of Bacillus sp. and other lactic acid bacteria to prepare sanitary brine for making stinky tofu. The tofu blocks used should also be as fresh as possible. After soaking in the stinky brine, they should be washed with clean water and left at 4°C for drip-drying. The pretreated tofu can then be deep-dried to become sanitary stinky tofu.
Stinky tofu belongs to the alkaline fermented food category. The amino acids in them will not be decarboxylated and form amines. Amino acids will form amines through decarboxylation only under acidic condition. In neutral and alkaline conditions, the deamination process will produce ammonia. Therefore, stinky tofu has a strong ammonia odor. During deep-frying, the ammonia volatilized does not post a health problem.
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