Development of high-quality protein tofu with dietary fiber supplementation as an alternative for dietary protein

Abstract

Tofu is a plant-based product consumed worldwide as a dietary protein alternative. Tofu production is based on protein extraction by water and precipitation and is considered rich in protein nutrients. Although there is a relatively high protein content in tofu, it is limited by certain essential amino acids, such as methionine, and dietary fibers. Therefore, this study aimed to develop a high-quality protein tofu formula, which is rich in dietary fiber, by adding cow milk and oyster mushroom. The researcher hoped that the developed tofu could be added to the market as an alternative to the dietary protein source. The experimental design started with optimizing the conditions for tofu production on the laboratory scale and produced from a standard method for household consumption. Then, the nutritional and physical properties of the tofu were analyzed. In the optimization step, the optimal water volume for soymilk production was initially obtained from 50 g of dry soybeans, and the number of extraction repeats was investigated. Next, the suitable coagulant for soymilk and cow milk protein precipitation was explored. Finally, the amounts of cow milk and oyster mushroom were investigated for tofu production. After all optimal conditions were revealed, four types of tofu were produced accordingly to the household consumption scale for nutritional analyses, including proximate analysis and amino acid contents. The physical analyses included water holding capacity (WHC), texture profile analysis, and microstructure investigation. The results were compared between types of tofu, and the relationship between nutritional data and physical characteristics was established. Furthermore, the lactose content was assured due to the addition of cow milk during tofu production. The results revealed that the suitable water volume for extracting 50 g of soybean protein to produce soymilk was 1,000 ml (1:20, dried soybeans to water). The extraction was repeated 4 times within the optimal water volume to obtain the highest protein in the soymilk. The most suitable coagulant is 0.2% (w/v) citric acid, which precipitated both soymilk and cow milk proteins. The optimal percentage of cow milk added to soymilk was 5% (w/v) using whole cow milk, and the optimal percentage of oyster mushroom added to soymilk was 10% (w/v). Four types of tofu in this study were the tofu made from solely soymilk (SPT), the tofu made from soymilk and cow milk (SCT), the tofu made from soymilk and oyster mushroom (SMT), and the tofu made from soymilk, cow milk, and oyster mushroom, known as hybrid tofu (HBT). The general characteristics of the four types of tofu were similar. Nutritional analysis by the proximate analysis revealed that tofu with oyster mushroom (SMT=16.99±1.60 g) and soy protein tofu (SPT = 14.60±0.24 g) contained lower protein contents than those of the tofu containing cow milk protein (SCT = 22.46±0.15 g and HBT = 21.12±0.51 g). Moreover, this resulted in the amino acid content results in which tofu without oyster mushrooms (SPT = 12.068 mg and SCT = 19.733 mg) and SMT (19.076 mg) contained higher protein than HBT (6.740 mg). However, the essential amino acid to non-essential amino acid ratio of HBT was highest at 113.3%, while other types of tofu ranged from 78.70% to 84.87%. In addition, methionine content in HBT (0.038 mg) was the highest compared to other tofu types without cow milk. The absolute content of dietary fiber in tofu with oyster mushroom (SMT = 28.13±2.21 g and HBT = 25.67±2.17 g) was higher than that in tofu without oyster mushroom (SPT = 1.06±0.11 g and SCT = 0.54±0.12 g). The moisture of the tofu with oyster mushroom (SMT = 127.79±0.29g) and HBT = 133.11±0.32 g) was relatively high. However, fat content was lower in tofu without oyster mushroom (SPT = 17.64±1.01 g and SCT = 25.37±5.69 g) and SMT (12.22±3.35 g) while HBT (49.88±2.53 g) contained the highest fat content. The texture analyses of tofu, including the hardness, cohesiveness, and springiness of the tofu with cow milk and oyster mushrooms, were balanced to maintain the original tofu. Nevertheless, it did not affect the ability to swallow (gumminess) and chew (chewiness). The microstructure of tofu without oyster mushroom (SPT and SCT) showed a smooth and homogeneous protein surface, while the tofu with oyster mushroom (SMT and HBT) appeared rough surfaces and the fine fibers were visible in a good combination with the protein. Pearson’s correlation between nutrients and texture characteristics of tofu indicated protein and fat content were positively correlated to cohesiveness (r = 0.63 and r = 0.57, respectively), while protein was negatively correlated to crude fiber (r = -0.76) and water-holding capacity; WHC) (r = -0.87). A negative association was found between fat and WHC (r = -0.61). Moreover, lactose content analysis of four types of tofu showed that all tofu in this study was lactose-free. To summarize, HBT demonstrated general characteristics similar to conventional tofu, although the protein and amino acid contents were significantly less than that of the conventional one. However, the ratio of essential amino acids to non-essential amino acids was higher in HBT and was rich in dietary fiber. HBT might be considered an alternative dietary protein source for specific groups, such as limited dietary protein-consuming people and restricted dietary fuel-consuming people.