31-08-2016, 12:46 PM
PREPARATION OF SOYABEAN MILK AND ITS COMPARISION WITH THE NATURAL MILK WITH RESPECT TO CURD FORMATION AND TEMPERATURE
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INTRODUCTION
Soymilk is essentially a water extract of soybeans, and contains all of the components of the bean, except for some insoluble fiber removed during processing (called okara). Soymilk composition varies depending on processing conditions and bean variety (Kwok and Niranjan, 1995; Mullin et al., 2001; Akintunde and Akintunde, 2002; Lakshmanan et al., 2006) and in general contains about 8-10% total solids, 3.6% protein, 2.0% fat, 2.9% carbohydrates and 0.5% ash (Liu, 1997). Due to its many components, the physicochemical characteristics of soymilk are a result of complex interactions occurring between the various colloidal molecules and particles. Some research has been conducted on the bulk properties of soymilk, as well as the requirements to produce a safe, stable and nutritious food product; however, very little is understood about the molecular and macromolecular changes occurring during its manufacture and processing. Heat treatment of soymilk during and after extraction is necessary to achieve the highest possible quality of soymilk.
Adequate heat treatment was shown to increase the colloidal stability of soymilk, maximize yield in terms of total solids and protein recovery (Shimoyamada et al., 2008), improve nutritional value by inactivation of anti-nutritional factors such as trypsin inhibitors and spoilage enzymes such as lipoxygenases, increase protein digestibility, increase shelf-life by destruction of spoilage microorganisms, and improve sensory qualities by minimizing off-flavours and improving colour (Kwok and Niranjan, 1995; Kwok et al., 2002; Iwuoha and Munnakwe, 1997). Adequate heat treatment is a balance between inactivation of anti-nutritional factors and satisfactory retention of nutrients. Excessive heat treatment, indeed, can cause adverse effects interms of sensory attributes such as development of browning and cooked flavour, and destruction of essential amino acids and vitamins (Kwok
Soymilk Characteristics
The colloidal properties of soymilk particles need to be followed during processing, to determine possible changes in quality and stability of the soymilk. Particle size distribution is a useful parameter to follow such changes. Ono and others (1991)showed that the distribution of particles in raw soymilk consists of 40% large particles (>120 nm in diameter), 20% medium particles (40-120 nm) with the remainder being small soluble particles (<40 nm). The same study showed that heat treatment decreasesthe fraction of large particles by disrupting aggregates and increasing the fraction of medium size particles, thereby decreasing the average particle size to around 40-100 nm. Particle size distribution of defatted soymilk was reported to range between 40-200 nm in diameter, with only few particles above 200 nm (Ren et al., 2009). High pressure treatment following extraction of beans at 80oC was shown to also reduce particle size; in this case, ultra high temperature treatment did not further reduce average size (Cruz et al., 2007). Homogenization after heating results in a narrow size distribution with a decrease in the average particle diameter, from about 200 nm to about 130 nm (Malaki Nik et al.,2008). Soy proteins contain two main fractions, glycinin and b-conglycinin, accounting for 40% and 30% of total protein, respectively. Their structure and physicochemical characteristics are discussed in detail in the next section. Previous research showed that large particles in unheated soymilk are composed mostly of glycinin proteins, which precipitate upon centrifugation, and that the ratio of b-conglycinin/glycinin in the supernatant increases with subsequent centrifugation steps (Ono et al., 1991; Malaki Niket al., 2008). Glycinin subunits precipitation decreases when heating and/or homogenization are applied to soymilk (Malaki Nik et al., 2008). It has been established that during the making of soymilk, the heat-induced denaturation of soy proteins causes them to rearrange and form aggregates. The aggregates contain varying ratios of all subunits present in glycinin and b-conglycinin (Ono et al., 1991; Malaki Nik et al., 2008; Ren et al., 2009). Heat treatment of soymilk causes protein denaturation, which exposes reactive amino acid side groups (nonpolar and sulfhydryl groups) normally buried in the hydrophobic core of the proteins (Doi, 1993). This was reported to occur in soy protein isolate suspensions, where an increase in the surface hydrophobicity was observed after heat denaturation (Takagi et al., 1979; Nakai, 1983; Sorgentini et al., 1995; Shimoyamada et al., 2008). Surface hydrophobicity plays a crucial role in soy protein solubility and the tendency for soy proteins to aggregate. Generally, as surface hydrophobicity decreases, so does the solubility of soy proteins (Wagner et al., 2000).This is due to the unfolding of protein, exposing the hydrophobic core, which increases surface hydrophobicity. However, partially denatured or totally denatured soy protein also show higher solubility, and hence surface hydrophobicity (Wagner et al., 2000)Once denatured, the protein subunits rearrange and associate to form soluble aggregates, predominantly via non covalent interactions such as hydrophobic interactions and hydrogen bonding, although some disulphide interchange also occurs (Ono et al., 1991; Lakshmanan et al., 2006; Malaki Nik et al., 2008; Ren et al., 2009). The b subunit of b-conglycinin was said to interact with the basic subunit of glycinin predominantly through electrostatic interactions (Utsumi et al., 1984), while the acidic polypeptide of glycinin and the a and a subunits of b-conglycinin tend to interact to form soluble aggregates (Ono et al., 1991; Guo et al., 1997; Ren et al., 2009).Generally soymilk contains about 2% fat, mainly in the form of triglycerides, with a fatty acid composition of poly-and mono-unsaturated hydrocarbon chains (Liu, 1997). Its presence and concentration plays an important role in the texture and sensory quality of soy products such as soymilk and tofu (Liu, 1997). Lipid-protein interaction in soymilk has been investigated by several authors (Ono et al., 1996; Guo et al., 1997; Taha and Mohamed, 2004; Toda et al., 2008). In raw soymilk, about 60% of total lipids are found in the protein particles, while only a small portion (about 3% of the total)remains following heat treatment (Ono et al., 1996). Once released from raw protein particles, lipids form droplets in soymilk of 200-400 nm in diameter (Ono, 2000), and are emulsified by small proteins such as oleosins, and some glycinin and b-conglycinin, as well as some triglycerides (Guo et al., 2002; Toda et al., 2008).The majority of neutral lipids moves from the particles to the floating (or creaming) fraction, while about half of the phospholipids remains in the particles. It has been suggested that phospholipids are able to bind to proteins in the raw soymilk protein particles, while the neutral lipid are released during heating of soymilk (Ono et al., 1996). The migration of lipids during heat treatment was suggested to occur in three stages: 1) at 65-75oC, some of the lipids are released from the raw lipid-protein particles into the soluble (supernatant) fraction, as the complexes undergo disruption, 2) at 75oC, the lipids begin to migrate from the soluble phase to the floating (creaming) fraction and finally 3) above 90oC, almost all the lipids migrate to the floating fraction (Ono et al., 1996).However, some lipids are visible in complexes with protein aggregates in the supernatant
after centrifugation of soymilk at 40,000 g (Malaki Nik et al., 2008). The interactions between lipids and proteins are reported to occur during heat induced protein denaturation (Taha and Mohamed, 2004).