To investigate the effect of preparation methods of cottonseed meals on protein properties, the physicochemical and functional properties of proteins isolated from hot-pressed solvent extraction cottonseed meal (HCM), coldpressed solvent extraction cottonseed meal (CCM) and subcritical fluid extraction cottonseed meal (SCM) were investigated. Cottonseed proteins had two major bands (at about 45 and 50 kD), two X-ray diffraction peaks (8.5° and 19.5°) and one endothermic peak (94.31 °C–97.72 °C). Proteins of HCM showed relatively more β-sheet (38.3%–40.5%), and less β-turn (22.2%–25.8%) and α-helix (15.8%–19.5%), indicating the presence of highly denatured protein molecules. Proteins of CCM and SCM exhibited high water/oil absorption capacity, emulsifying abilities, surface hydrophobicity and fluorescence intensity, suggesting that the proteins have potential as functional ingredients in the food industry.
Cottonseed, which is available in many temperate and tropical countries, is one of the richest sources of oilseeds mostly processed to extract oil that is used as edible fat (Zhou, Zhang, Gao, Wang, & Qian, 2015). Cottonseed meal is a co-product of the cottonseed oil processing industry. With processing, typical yields from cottonseed are 50% meal, 22% hulls, 16% oil and 7% linters, with a 5% loss (Hinze et al., 2015). The commonly used methods of lipid extraction from oil seeds are pressing and extraction with organic solvents (cold or hot). Pressing is the process of mechanically pressing liquid out of liquid containing solids, whereas extraction refers to the process of separating a liquid from a liquid-solid system (Anderson et al., 2016). Subcritical fluid extraction is one of the newly emerging clean and environment-friendly technologies for food products (Zheng, Ren, Su, Yang, & Zhao, 2013). During the oil extraction from cottonseeds, a portion of free gossypol binds with the epsilon amino group of lysine, thereby reducing the availability of lysine. Free gossypol in cottonseed meal depends on the variety of cultivars, methods of oil extraction and proportion of kernel to husk (Nagalakshmi, Rama Rao, & Panda, 2007). According to the Food and Drug Administration (FDA), a protein food product made from cottonseed is considered edible if it contains less than 0.045% free gossypol (FDA regulations, 1974). Several edible products have been developed, and cottonseed flours and protein concentrates have been accepted as functional and nutritional additives for meat products, baked goods, and cereals (Zhuge, Posner, & Deyoe, 1988). The use of cottonseed as protein source for humans does not depend only on the nutritional value of cottonseed, but also on their ability to be used as, or to be incorporated into, foods. Therefore, the functional properties of proteins rather than their nutritional value largely determine their acceptability as ingredients in various foods (Tsaliki, Pegiadou, & Doxastakis, 2002).