The use of W/O/W controlled-release coagulants to improve the quality of bittern-solidified tofu
Graphical abstract
Introduction
Tofu is one of the most important and popular traditional food products in most East and Southeast Asian countries, including China, Japan, and Korea. Tofu has a history lasting more than 2000 years and has great nutritional value because it is cholesterol free, low in saturated fat, and high in protein. Western countries have also shown an increasing interest in tofu as a result of its nutritional value and health benefits (Hui, Henning, Park, Heber, & Go, 2001). Normally, tofu is either classified as firm, soft, silken, or packed or as a tofu derivative, such as fried tofu or frozen tofu (Saio, 1979), depending on the preparation method, the coagulant type, and the moisture content. The yield and quality of tofu are influenced by the soybean variety, soybean quality (which is dependent on the growth and storage conditions), and processing conditions.
The coagulation of soymilk is the key process that determines tofu quality and yield during processing. In addition, coagulation is the most difficult step to control because it relies on the complex interactions of a number of variables, such as the soybean chemistry; the cooking temperature, solid content, and pH of the soymilk; the coagulant type and concentration; the methods used to add and mix the coagulant; and the coagulation temperature and time (Beddows and Wong, 1987, Cai and Chang, 1998, Gandhi and Bourne, 1998, Li et al., 2013b, Shen et al., 1991, Shih et al., 1997, Shurtle and Aoyagi, 1990, Sun and Breene, 1991). Of these variables, the coagulant type is the most important and directly affects the tofu's taste and sensory qualities. Many different coagulants, such as calcium sulfate and glucono-delta-lactone (GDL), are used to produce tofu, but bittern (magnesium chloride) was the first coagulant used in tofu processing. Bittern creates a more natural flavor and allows the taste of the soybean to be retained. These qualities make bittern one of the most popular coagulants in China, and both European and American consumers also prefer bittern-coagulated tofu because it retains the sweet taste of the original soybeans and does not spoil the soybean oil. However, bittern is a quick-acting coagulant, so its use is limited because it promotes extremely rapid solidification, making the coagulation step very difficult to control. Moreover, rapid solidification at the local scale causes the tofu to become hard and nonuniform and to have a low WHC (water-holding capacity) (Watanabe, 2000, pp. 104–112). In addition, nutritional components such as proteins and soy isoflavones are lost along with the expelled water.
Our previous research demonstrated the potential use of water-in-oil (W/O) emulsions containing a bittern solution as the dispersed phase in tofu processing. These emulsions were considered promising due to the good controlled-release properties of W/O emulsions. As a consequence, solidification using this emulsion resulted in a significant delay in coagulation and yielded firm tofu with greater water content, compared with solidification using traditional coagulants. The tofu produced also had a finer microstructure and texture when W/O emulsion coagulants were used (Li et al., 2013a, Li et al., 2013b, Li et al., 2013a, Li et al., 2013b). However, one disadvantage was that because these emulsions are insoluble in water, the W/O emulsion coagulants must be sheared at high speed to disperse and release the bittern in the soymilk. This need for shearing makes the coagulation process more complex.
Hence, the use of multiple water-in-oil-in-water (W/O/W) emulsion coagulants was considered. W/O/W emulsions are compartmented systems comprising aqueous droplets dispersed in oil globules that are themselves dispersed in a continuous aqueous phase. There are two interfaces in these complex systems; a hydrophobic emulsifier with a low HLB value stabilizes the first interface, and a hydrophilic emulsifier with a high HLB value is present at the second interface (Benichou et al., 2004, Florence and Whitehill, 1982, Garti and Aserin, 1996, Garti and Bisperink, 1998, Owusu et al., 1992). Over the last 30 years, W/O/W emulsions have proven useful as systems for the sustained and slow release of active compounds. These structured emulsions have also been of interest to the food industry due to their ability to entrap water-soluble substances, e.g., NaCl (Garti, 1997a) and vitamins (Benichou et al., 2004, Fechner et al., 2007) in the internal compartments. Therefore, in our current study, W/O/W emulsions with a bittern solution as the internal aqueous phase were prepared based on W/O emulsions. The aim of our present research was to investigate the effect of the preparation parameters on the stability of the W/O/W emulsions and to evaluate the controlled-release properties of these emulsions during the production of bittern-solidified tofu (firm tofu).
Section snippets
Materials
Bittern (MgCl2·6H2O) was purchased from Tianjin Tanggu Jinlun Salinization Co., Ltd. (Tianjin, China), and contained up to 47% MgCl2 by mass. The bittern solution used as the internal aqueous phase was prepared by dissolving two units of bittern in one unit of distilled water. This solution was then filtered to remove impurities. The polyglycerine ester (containing one type of polyglycerol polyricinoleate, PGPR, trade name 818SK, and one type of polyglycerol fatty acid ester, PGFE, trade name
Preparation of stable W/O/W emulsion coagulants
Double W/O/W emulsions have many potential applications, but few commercial products based on these emulsions are currently available in the market place, primarily because of the inherent instability of the preparation and the uncontrolled release of the entrapped matter (both on the shelf and during their application) (Garti & Aserin, 1996). Thus, regardless of the desired application, the preparation of a stable W/O/W emulsion must be achieved first. The stability of a W/O/W emulsion depends
Conclusions
This study investigated several factors that influence the preparation of W/O/W emulsions using a basic two-step procedure. The concentration of hydrophobic emulsifier in the internal aqueous phase and the mass ratio of the primary W/O emulsion to the external aqueous phase are the key factors necessary to create stable W/O/W emulsions. If the influence of viscosity is ignored, higher concentrations of hydrophobic emulsifier produce more stable W/O/W emulsions. The optimal mass ratio of the
Acknowledgments
This research was conducted with the financial support of the National Science Foundation of China (project no. 31171648) and the National Key Technologies R&D Program (no. 2011BAD23B04). The corresponding author also acknowledges support from the Program for New Century Excellent Talents in University (NCET-09-0741) and the Chinese Universities Scientific Fund (2012YJ087).
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