Comparison of Two Small-Scale Processing Methods for Testing
Silken Tofu Quality
Aijun Yang1 & Andrew T. James1
Received: 23 April 2015 /Accepted: 18 May 2015 # Springer Science+Business Media New York 2015
Abstract In developing a simple, reliable, small-scale method to assess silken tofu quality in our soybean improvement program, we examined two processing methods and two coagulants, glucono-δ-lactone (GDL) or nigari (magnesium chloride) in two experiments. Silken tofu was prepared from a commercial soybean variety (expt 1) or seven soybean varieties (V1–V7) which were grown and harvested together (expt 2). The soybeans were soaked overnight (the soak method, with 55 g soybeans) or ground dry first (the dry method, with 60 g soybeans) before processing. The quality of the silken tofu was evaluated and compared among varieties and coagulant-processing methods and their interactions.
Moisture and protein content in soymilk and soybean seeds, soymilk yield and protein and solid recovery in soymilk were determined. Compared with the dry method, the soak method allowed faster soymilk extraction, produced soymilk with lower solid and higher protein content and firmer silken tofu with either GDL or nigari as coagulant. Depending onwhether nigari or GDL was used as coagulant, the soak method also produced silken tofu with the highest or the lowest water loss which correlated strongly and negatively with tofu hardness (r=−0.93***). Differences were detected among varieties for the key quality attributes. Taken together, the soak method with GDL as coagulant would be the preferred combination to use to assess tofu quality.
Keywords Tofu processingmethods . Coagulants . Silken tofu quality . Soybean varieties
Soybean (Glycine max) seeds are processed into an extraordinary range of food and feed ingredients and products. There is great variation in attributes of the seed and in the processing performance of seed into ingredients and products. Tofu is a traditional food product made from whole soybean and is a curd made by coagulation of the protein and oil in soymilk. It is effectively a water-based gel formed through the addition of calcium or magnesium salts or acids to heated soymilk, with soy lipids and proteins trapped in its gel networks (Kohyama et al. 1995). The two main types of tofu are silken which is set in a container, or hard which is pressed. There is wide variation in the basic procedure used to make tofu, but the main steps are producing a slurry of beans with water, then cooking and separating the soluble material from the insoluble. A coagulant such as glucono-δ-lactone (GDL), magnesium chloride or calcium sulphate is then added to the soluble ‘milk’ component to form a curd (Cui et al. 2004).
Tofu is, nonetheless, an elaborately transformed product manufactured via a range of traditional and modern methodologies. The diversity of manufacturing processes and their interaction with attributes of the raw soybean seed are a challenge which needs to be addressed in an improvement program targeting development of cultivars with improved tofu making potential. There has been much conjecture and some conflicting results published pertaining to variation in attributes of soybean seed and resultant variation in yield and quality of tofu manufactured. Some of this variation is due to non-genetic effects, such as insect damage during the maturity of the crop, adverse weather conditions at the time of grain maturity, damage caused during harvest, handling, storage or storage time. For example, there can be substantial year-to-year or batch-to-batch variation in tofu quality from a single cultivar (Kijima 1997). Pre- and post-harvest handling * Aijun Yang email@example.com 1 CSIRO Agriculture Flagship, 306 Carmody Road, St Lucia,
Brisbane, Qld 4067, Australia
Food Anal. Methods
DOI 10.1007/s12161-015-0205-8 and storage also affect tofu making potential of the soybean as it can affect protein solubility (Hou et al. 1997). Poor protein solubility may result from beans being stored at too high temperature and moisture content or too low moisture content (Saio 1997; Sasaki 1997), or from damage during harvesting resulting in many split beans (Hou et al. 1997).
There is strong evidence that choice of cultivar strongly affects tofu yield and quality (Cui et al. 2004; Jin and Gai 1995; Qian et al. 1999b). In the early stages of varietal improvement for tofu quality, desirable traits of soybean seeds such as yellow seedcoat and yellow hilum, large round seed, high protein content and moderate oil and sugar contents can be addressed reasonably easily (Cui et al. 2004; Gai et al. 2000; Qian et al. 1999a; Taira et al. 1990). Once these attributes are largely achieved, further improvements are less tangible and require tofu to be made from the beans and its quality to be evaluated. Seeds of differing cultivars need to be grown, harvested and stored as similarly as possible so that varietal differences can be measured with minimal confounding effects (Cui et al. 2004).
Variation in manufacture technique is also considered likely to affect quality and yield of tofu (Beddows and Wong 1987; Cai and Chang 1999). Generally, the traditional method for solubilising the material from soybean seeds is to soak the beans first, then grind the wet beans into a slurry, heat the slurry and separate the soluble material or milk from the remaining material. However, it was discovered that, as a result of reduced lipoxygenase activity, a more bland product could be produced if the beans were ground dry before being blended with hot water to solubilise the material, (Saio 1997). There are advantages in using the dry-grind method with GDL as the coagulant in small-scale evaluation (Evans et al. 1997). These are mostly associated with the convenience of grinding the beans in advance and in higher repeatability of GDL in coagulation. However, nearly all manufacturers of tofu in Australia use an overnight soak of beans as it gives a higher solubility of protein (Saio 1997) and use either nigari or a nigari-GDL mix to coagulate the soymilk because of the better flavour. Recent results from Australia have shown that both variety and processing conditions affect silken tofu quality (Yang and James 2013). In developing and refining a suitable, small-scale method to make silken tofu for use in our soybean improvement program for human consumption, we adapted the dry method (Evans et al. 1997) to our laboratory and compared it with a more commonly used soak method. Two main types of coagulants were also examined in order to determine an appropriate coagulant-processing method combination to use.