Fermentation of stinky brine and production of Chaw-Tofu, a Chinese traditional fermented soybean curd in Taiwan

JUSTIFICATION:

        Chaw-Tofu, also known as stinky soybean curd, is a traditional fermented food made from tofu. When the brine medium comprising several vegetables, tofu and shrimp becomes stinky, tofu squares are dipped in it for a few hours before deep-fried or steamed. This gives special odors to and produces a sponge-like structure inside the tofu through the action of microorganisms. Traditionally the stinky brine is produced from natural fermentation which takes more than six months. Moreover, it is easy to be contaminated with pathogens and maggots during the open-type fermentation.

OBJECTIVES:

        This study was designed to improve the traditional methods to produce sanitary Chaw-Tofu. The fermenting strains for stinky brine production were screened to shorten the time for fermentation and the process was controlled to avoid undesirable contamination.

METHODS:

        Samples of stinky brine produced by traditional methods were collected from different locations in Taiwan. They were transferred to sterilized brine medium and used to screen the microorganisms for production of stinky odors. Chaw-Tofu produced by the stinky brine resulted from inoculation with the selected strains was evaluated for sensory preference.

RESULTS:

        The results showed that A2 group was the best for fermenting the brine medium used in this study. Isolates of A2 group were identified by the Microbial Identification System and Bacillus sphaericus and lactic acid bacteria were found dominant. The pH values were increased due to the production of ammonia during the stinky brine fermentation. The optimal percentages of shrimp and tofu in brine medium were 5 % and 30 %, respectively. One-month fermentation time was best for A2 group to produce stinky brine. Tofu immersed in two-fold dilution of stinky brine for eight hours is the best condition for Chaw-Tofu production.

SIGNIFICANCE:

        The developed process of stinky brine fermentation can be used for large-scale production of sanitary Chaw-Tofu.

51H-2
        Opitmal conditions for production of stinky tofu and identification of microflora in stinky brine

        Stinky tofu, also known as stinky soybean curd, is a traditional fermented food made from tofu. When the brine medium comprising several vegetables, tofu and shrimp becomes stinky after fermentation, tofu squares are dipped in it for a few hours before deep-fried or steamed. This gives special odors to and produces a sponge-like structure inside the tofu through the action of microorganisms. Traditionally the stinky brine is produced from natural fermentation which takes more than six months. Moreover, it is easy to be contaminated with pathogens and maggots during the open-type fermentation. This study was designed to study the optimal conditions for production of stinky brine and to identify the fermenting strains. In this study, A2 microbial group was used to ferment for stinky brine. Production of ammonia was used as an indicator of the progress of fermentation. Changes of ammonia concentration in stinky brine were determined at different conditions. Fermentation isolates from stinky brines were identified by four rapid identification systems- Biolog system, API kit, VITEK system, and Microbial Identification System(MIDI), and conventional physiological and biochemical tests (Micro-IS system). Increase the inoculum of A2 microbial group would accelerate the fermentation at the beginning. The amount of ammonia was the highest when A2 microbial group was incubated at 35ˇăC for one month. It was the best that the stinky brine was made by submerged fermentation and the caps of the fermentation bottles were loosely capped. Microorganisms for stinky brine fermentation can be preserved by freezing and freeze-drying methods. Identification results showed that Bacillus sphaericus and lactic acid bacteria (such as Enterococcus sp. and Lactobacillus sp.) were dominant in A2 microbial group. This study determined the best condition for fermenting the brine medium to improve traditional process and identified the fermenting isolates.

        Tofu has an image problem in the west. Even chefs will sometimes claim it lacks flavor. And it's true that the custard-like white substance - also known as soybean curd - doesn't look very appetizing.

        Still, tofu has a lot going for it. Nutritionally speaking, it's high in calcium and vitamins, but low in fat and sodium. Tofu is cholesterol-free. It makes a great meat substitute, not only for vegetarians, but also for individuals who have trouble digesting meat, or suffer from medical conditions such as chronic heartburn. And if that isn't enough, tofu has been credited with offering protection against diseases such as cancer and osteoporosis.

        Of course, that does leave the not so small matter of flavor. There is no question that, served alone, tofu tastes rather bland. But tofu - or doufu to use its Chinese name - is not designed to be eaten alone. The beauty of tofu is that it absorbs the flavors of the food it is cooked with. Picture a large white sponge and you've got the basic idea.

        Besides blandness, another common complaint about tofu is the texture. However, today you can choose from a wide variety of tofus that vary from firm to extra firm, which is denser and more solid, to soft tofu, which is more jello-like. There is also silken tofu, which is creamier and more custard-like, and also comes in varying degrees of firmness. The firmer tofus are recommended for stir-fries and grilling, while the soft tofu works well in soups and silken tofu is great for blended dishes like pudding. But there are no rules - it all depends on your own preference. Recipes normally specify which type of tofu to use, but if they don't, it's safest to stick with a medium firm brand.

        Normally located in the refrigerated section of the grocery store, tofu comes in all sorts of packaging - from individual cakes to larger blocks. One company even packages their tofu in a convenient, plastic-wrapped cylinder - all you need to do is peel back the plastic and slice. If you live near an Asian market, try the Chinese-style tofu packaged in water. It is softer than Japanese tofus, but still firm enough to hold its shape during cooking. I find I get the best results when using it to cook Chinese dishes.

In the Kitchen
        How do you store tofu? Like any perishable product, you need to check for an expiry date. (If the tofu smells sour, throw it out). Also, depending on the type of packaging, it may need to be refrigerated immediately. Either way, once you've opened it, cover the leftover tofu with water and store it in the refrigerator, being sure to change the water daily. (If possible, use distilled instead of regular tap water). The tofu should last for up to a week.

        Tofu can also be frozen, which gives it a more meaty texture. The regular to extra firm tofus are better for freezing, as the softer tofus do not hold their shape as well. But no matter what type of tofu you're working with, you can add extra firmness by draining it before freezing. Simply place the tofu between paper towels or tea towels and lay a heavy pot or board on top of it for fifteen to thirty minutes. Frozen tofu will last for at least three, and up to five, months.

        Many recipes also call for tofu to be drained before cooking. This will increase its capacity to absorb other flavors, making for a tastier dish. Another way to increase tofu's flavor is to marinate it. There are no hard and fast rules here, but the longer the tofu is marinated, the more flavorful the result. After marinating, you can either fry it or add it to a soup or salad. You don't even need to use it immediately - stored in a sealed container in the refrigerator, marinated tofu should last for several days.