| Traditional soy products-soymilk, tofu and Yuba, favorite Chinese foodstuffs of high nutritional value and a long history form a pillar of China’s soybean industry. Traditional soy industry is generally in small scale and contains little modern technology. In addition, several quality problems, such as oil-water stratification, precipitation, flavor instability and other quality problems occur during the processing and storage of soymilk. Therefore, development of an advanced technology suitable for industrial scale production, which also cable of producing products with sufficient and stable quality, maintaining the original flavor and texture of traditional soybean, is an urgent need currently.Ultra high pressure homogenization technology is with great development potential which have already widely used in food industry, pharmaceutical industry and the cosmetics industry. Ultra high pressure homogenization is cable of producing soymilk with reduced particle size, improved uniformity and stability, and with antibacterial effects and reduced enzymatic activity. In addition, the high pressure homogenization technology is suitable for industrial scale production due to the typical features such as short processing time and continuously operating.In this study, traditional soybean products-soymilk, tofu and Yuba treated with heat and high-pressure homogenization were studied for the physical-chemistry properties changes using a laser particle size analyzer, rheometer, SDS-PAGE analysis. The objective of this study was to reveal the typical characteristics changes during high-pressure homogenization and heat treatment of soymilk, tofu and Yuba, as to improve the overall quality and yield of the three traditional soybean products. The study will also provide data basis and theoretical guidance for future elevation of soybean processing techniques. 1. Effects of ultra high pressure homogenization on the physicochemical properties, micro-organisms and enzymes of soymilkThe temperature of soymilk significantly increased along with the improving pressure in the range of0-140Mpa; Two cycles of soymilk at Tin=20℃,140MPa resulted a temperature rise up to55.2℃. With the increasing of homogenization pressure, decreased soymilk color differences observed. A decrease of3.44%obtained in140MPa soymilk AE*than the untreated samples, in which the color turned milky white to light gray. In addition, Ultra high pressure homogenization soymilk resulted viscosity decreasing. Centrifugal sedimentation rate measurements showed that physical stability increased along with the homogenization pressure. DSC measurement results demonstrated that the higher homogenization pressure gave higher Td and smaller AH value of both7S and11S soy protein, which indicated enhanced degree of denaturation and thermal stability.Soymilk particle size increased significantly with the pressure decreased; at140MPa, the soymilk (water:soybean ratio of10:1) particle size D4,3, D3,2, and d0,5were respectively0.28μm,0.28μm,0.27μm, which have achieved submicron size level. The soymilk particle size reached submicron level when the homogenization pressure≥1OOMpa using: water:soybean ratio among6:1to12:1. Inlet temperature and homogeneous times affected soymilk particle sizes. Excessive homogenization induced soymilk particles repolymerisation, thus resulted in an unstable soymilk system.Compared with heat treatment (95℃,5min), ultra high pressure homogenization was less effective in enzyme inhibition and microorganism inhibition. After2cycles (Tin=20℃) of high pressure homogenization at140MPa the total number of bacteria decreased1.77magnitude, coliform group was less than3, and the relative activity of urease reduced19.91%. Increasing of the inlet temperature and homogenization cycles could increase the effects of inhibitions of microorganisms and enzymes, yet not comparable to heat treatment.2. Effects of ultra high high pressure homogenization and heat treatment on soy protein solubility and its mechanismUltra high pressure homogenization in the range of0-140MPa can increase the solubility of raw soy protein; At140MPa, the soymilk protein solubility improved63.8%than the untreated ones; Thermal processing after homogenization further improved the solubility and reached91.1%(140MPa). Homogenization reduced viscosity and particle size of soymilk; Heating after homogenization further enhanced these properties. TEM observation demonstrated that a combination of thermal treatment and ultra high pressure homogenization increased the dispersion and uniformity of soymilk particles, decreased the particle size; particle size decrease improved the soymilk hydration, thereby increased the solubility of the soy protein. Internal fluorescence analysis showed that homogeneous and heating changed soy protein conformation, improved the polarity of tryptophan microenvironment, enhanced hydration of protein molecules.SDS-PAGE results showed that ultra high pressure homogenization mechanical shear force could degrade protein aggregates, dissociation of protein subunits, improved soymilk protein solubility; After homogenization, the raw soymilk had higher contents for A1, A2, A3and B subgroup, with reduced a and P subunits contents. Increasing homogenization pressure not only dissociate large protein molecules, small proteins7S and11S protein might also dissociate (α,β,A, B subunit content decreased). Heating after homogenization induced protein aggregates larger than130kD, which improved the soymilk protein solubility.3. Effects of homogenizing pressure on homogenized (0-140MPa), heating after homogenization, and homogenization after heating soymilk protein emulsification propertiesAll three approaches could improve soymilk protein emulsification properties. With higher homogenization pressure, all three treatment had increased soymilk protein emulsifying activity; emulsifying stability of homogenization and heating after homogenization samples indicated trends of first increases and then decreases curves, and maximized at lOOMpa, whereas homogenization after heating samples increases. Homogenization pressure could improve the raw milk proteins hydrophobicity and influence the free sulfurdryl group contents.Emulsifying activity and stability was significantly positively correlated with homogenization pressure and hydrophobicity, but significantly negatively correlated with particle sized, and with no correlation with free sulfhydryl groups. Samples of heating after homogenization had emulsifying activity and emulsion stability significantly positively correlated with homogenization pressure, hydrophobicity and free sulfhydryl groups, significantly negatively correlated with the particle sizes. Whereas the samples heating before homogenization had emulsifying activity and emulsion stability significantly positively correlated with homogenization pressure, hydrophobicity, significantly negatively correlated with the particle sizes. The emulsion stability significantly positively correlated with free sulfhydryl groups; however, the emulsion activity had no correlation with free sulfhydryl groups.4. The effect of homogenization (140MPa), heating before homogenization and heating after homogenization on the soymilk flavorSensory evaluations showed that the soymilk homogenization had better taste, and heating increased the nice bean flavor. Seven compounds attributed to beany flavor was identified using GS-MS (hexanal, hexanol,1-octen-3-ol, trans-2-hexenal,1-octen-3-one, benzaldehyde, trans, trans-2,4-decene aldehyde) and three non-beany flavor compounds (trans-2-octyl aldehyde, nonyl aldehyde, trans-2-nonenal). After heating and homogenization of raw soymilk, increase of n-amyl alcohol,2-n-pentyl furan, and hexanoic acid was observed. Homogenization barely changed the raw or cooked soymilk flavor compounds composition; heating and homogenization have reduced the total content of flavor compounds, but heating increased the ratio of non-beany and beany flavor compounds. Heating before or after homogenization did not change composition of the flavor compounds, however, heating after homogenization had higher ratio for the nice bean flavor/non-beany flavor compounds.5.Effect of different homogenization pressure (0-140MPa) on the tofu propertiesThe gel formation curves between non-homogenized soymilk and homogenized soymilk was the same, but the gel formation time was shorter in the140MPa treated soymilk; increased homogenization pressure improved G’, G" significantly, and G’> G" This means tofu had a nature of more solid than colloid. Scanning electron microscopy showed that the internal structure of tofu was a honeycomb structure. As higher the homogenization pressure was, the network had higher uniformity, enhanced three-dimensional properties, and changed density properties. TPA analysis showed that tofu hardness, elasticity, cohesiveness and chewiness were improved with homogenization pressure increases.140MPa processed tofu chewiness were112.7%,87.9%higher than that of non-homogenization and20MPa treated samples, respectively. The tofu water retention properties increased significantly when homogenization pressure reached lOOMPa; Compared to non-homogenization samples, water retention properties of tofu underwent1OOMPa and140MPa increased1.47%and2.45%, respectively.6. Effect of different homogenization pressure (20-140MPa) on the Yuba propertiesAs increase of homogenization pressure, Yuba became changed from light golden yellow to golden; when homogenizing pressure was greater than1OOMPa, the Yuba forming speeded, higher thermal stability obtained, and more stable Yuba quality observed. TPA analysis showed that high pressure homogenization improved tensile strength, elongation at break and yield of Yuba. The yield of140MPa treated Yuba was improved31.38%than that20MPa. Scanning electron microscopy showed that the internal structure of the Yuba was protein-fat complex which formed superimposed layers of mesh structure;20MPa and60MPa treated Yuba in presence of free fat globules, whereas the free fat globules were degraded when using homogenization pressure higher than1OOMPa;140MPa treated Yuba had no significant changes in the microstructure before and after defatted treatments, the protein-fat complex was more stable. This structure reduced the "oil-exuding" phenomenon and prevented its oxidation "rancid smell", which eventually helped extend product shelf life. |
