بررسی تاثیر تیمارهای فراصوت و آنزیم ترانس‌گلوتامیناز بر میزان راندمان، ویژگی‌های فیزیکوشیمیایی و ریزساختار پنیر سویا

پنیر سویا (توفو)، فرآورده شبه‌‌لبنی است که از طریق انعقاد شیرسویا به کمک مواد منعقدکننده تهیه می‌شود. پژوهش پیش‌رو با هدف بررسی تاثیر تیمارهای آنزیمی ترانس‌گلوتامیناز (tg) و فراصوت بر میزان راندمان تولید، ویژگی‌های فیزیکوشیمیایی و ریزساختار توفو انجام شد. پس از خیساندن و خرد کردن لوبیا با آب (نسبت 1:6)، شیرابه در سه سطح زمانی (0، 15 و 30 دقیقه) تحت تیمار فراصوت (فرکانس 37khz، توان 520w و دمای °c 40) قرار گرفت. تیمار آنزیمی شیرسویا در سه سطح (0، 0.35 و 0.7 واحد آنزیم tg به ازای هر گرم پروتئین شیرسویا) انجام پذیرفت. پس از تهیه توفو توسط ماده منعقدکننده سولفات کلسیم، تاثیر تیمارهای مذکور بر ویژگی‌های مورد بررسی طی مدت دو ماه نگهداری در یخچال (0، 30 و 60 روز) بررسی گردید. نتایج نشان داد که با افزایش غلظت آنزیم، مقادیر رطوبت و راندمان افزایش معنی‌داری یافت اما پروتئین و چربی تفاوت چندانی نکرد (0.05

evaluation of effect of ultrasound and transglutaminase enzyme treatments on yield,, physicochemical properties and microstructure of soy cheese

introduction: soy products consumption reduces the hazards of numerous chronic disorders such as cancer, arteriosclerosis, and osteoporosis syndromes. among diverse soy products, soymilk has gained much popularity as an outstanding source of nutrients which is normally suggested as a less-expensive supernumerary instead of cow's milk. soymilk is water extract of soybeans which is rich in functional components such as isoflavones and high quality proteins and it is lactose and cholesterol free as compare with cow's milk. the extent of extracted soybean components into the soymilk could be improved by ultrasound technique. in recent years, this non-thermal procedure has many applications in food industry such as extraction (plant substances), filtration, destroy of microorganisms, enzyme inactivation, emulsification, etc. high-power ultrasound thorough the shock waves and shear forces can improve the liquid/solid extraction of intracellular plant constituents by cavitation and bubbles interruption. on the other hand, the quality of food proteins like soymilk or soy cheese (tofu) could be improved by microbial transglutaminase (tg). tg-induced crosslinking handling in different food proteins have much studied. transglutaminase (ec 2.3.2.13) is an acyltransferase that catalyzes the cross-linking of most food proteins including milk proteins through formation of ε-(γ-glutamyl)-lysine intra- and intermolecular isopeptide bonds. tg enzyme can form a gel with an appropriate structure by developing covalent connections between glutamine and lysine in protein systems. soy-cheese is a quasi-dairy product that is prepared by coagulants through coagulation of soy milk. although some investigations have studied the individual effect of tg enzyme or ultrasound on tofu quality, the simultaneous effects of both treatment on tofu has not yet been studied. therefore, present study was conducted to investigate the effect of transglutaminase and ultrasound enzymatic treatments on the yield and physicochemical characteristics of tofu. materials and methods: after washing, soaking and grinding soybeans with water (1: 6 ratio), the mixture was sonicated (frequency of 37 khz with power of 520 w and temperature of 40 °c) at three time levels (0, 15 and 30 minutes). the mixture was filtered through a cheese cloth to separate the soymilk material from the residue and heated at 95 °c for 5 min. afterward, soymilk was cooled to 95 °c and enzymatic treatment of transglutaminase (tg) was performed at three levels (0, 0.35 and 0.7 unit/g soymilk protein) for 150 min. for enzyme inactivation, treated soymilk heated at 95 °c for 1 min. after tofu preparation with 0.4% calcium sulfate solution as coagulant, the effect of these treatments on the cheese yield and physicochemical parameters (ph, moisture, fat, protein, color values) of tofu were investigated during one month storage (0, 30 and 60 days) under refrigeration conditions. microstructure evaluation of cheese samples were measured by scanning electron microscopy (sem) at the end of 60 days of the cold storage. tofu samples were cut into cubes of approximately 5 mm3 and dipped in 2.5% (w/w) gluteraldehyde for 3 h. the cheese blocks were fixed and washed with distilled water and dehydrated using ethanol. the samples were then defatted three times in chloroform and were covered with ethanol and kept refrigerated. film samples were cryo-fractured by immersion in liquid nitrogen and fixed on stubs. the samples were coated with gold. photomicrographs were recorded at different magnification levels operated at 20 kv. results and discussion: the results showed that with increasing enzyme concentration, moisture content and production yield of tofu meaningfully increased while protein and fat contents slightly increased (p>0.05). the yield of tofu at 0, 0.35 and 0.7 unit/g protein were recorded as 19.80, 21.08 and 22.44%, respectively. increasing the ultrasound time also significantly increased moisture content and yield of cheese making while considerably reduced the protein and fat content of the cheese analogues. the yield of tofu at different ultrasound time (0, 15 and 30 minutes) were recorded as 19.37, 21.24 and 23.26%, respectively. moreover, the storage time had not significant effect on ph and chemical components of tofu samples. evaluation of the color indexes of the product with a colorimeter showed that with increasing enzyme concentration, the amount of lightness (l*) increased and redness (a*) decreased significantly while yellowness (b*) insignificantly changed. also, with increasing the ultrasound period and storage time, the amount of redness (a*) and yellowness (b*) parameters increased and the lightness (l*) decreased significantly. microstructure of soy cheeses showed that tg treatment caused a compact structure in protein matrix of tofu samples while ultrasound process led to less compact and more voids form. the compact structure and rougher surface of tofu samples treated with tg might have resulted from microscopic pools of free water within the sample due to the increased hydration capacity of protein matrix. conclusions: the results of this study showed that by using 30 minute ultrasound time and 0.7 transglutaminase concentration (unit/g protein), a tofu with the highest yield (23.73%) as compare with control sample (18.17%) could be produced. however, as the time of ultrasound increased, the cheese quality reduced significantly. therefore to obtain a tofu sample with satisfactory quality and acceptable cheese yield (22.34%) treatment with 0.7 tg (unit/g protein) and 15 minute ultrasound time is suggested.