Gelation Behaviour, Rheology, Microstructure And Sensory Characteristics Of Acid And Salt-induced Soy Proteins Soft-tofu Type Gels

Nowadays, a growing interest in food of plant origin, especially plant protein foods has become evident; as well as the consumption of dairy free cheeses containing soy protein has increased in the western world in recent years. The healthy characteristics of soy products have aroused the attention of many researchers and consumers. One of the most common dairy free cheeses made from vegetable proteins is tofu also called “soy cheese”. The development of tofu made from soy proteins isolate(SPI) with similar taste, texture and composition to a dairy cheese would be desirable, due to its reduced saturated fat and cholesterol content. The aim of this comparative study was to investigate the gelation behavior and rheology characteristics of soft tofu type gels made from soy proteins as affected by coagulant, raw material, oil volume fraction, coagulation temperature, protein concentration and coagulation time. Finally, the sensory characteristics of the prepared “soy cheeses” were determined and their relation to the volatile compound was assessed. A better understanding of the gelation mechanism, small and large scale deformations of soy protein gels, is industrially relevant, as it will allow for better process design, monitoring and quality control, thus potentially lead to the development of novel soybased and dairy free cheeses products.Soft tofu-type gels coagulated with calcium sulphate(Ca SO4) or glucono-?-lactone(GDL) were made from soybean and SPI at 5 % protein concentration. The gels were subjected to small and large deformation tests in order to compare their gelation behaviour and rheology characteristics. Oscillatory tests data were analyzed using the power law and the weak gel models. Acid-induced gels had higher storage modulus(G?) than salt-induced gels. During gelation, both moduli increased gradually but never reached a steady state, with G? being higher than G". Under applied parameters, acid-induced gels coagulated faster than salt-induced gels with the ones made from soybean having the lowest gelation time. Syneresis rate in salt-induced gels was signi?cantly lower than in acid-induced gels, with SPI acid-induced gel having the highest rate(36.72%) and soybean salt-induced gel having the lowest(14.77%). Furthermore, salt-induced gels resulted in a reduction in shear deformation at fracture and an increase in the shear stress at yielding, whereas the shear stress at yielding was reduced and the shear deformation at fracture was increased in both acid-induced gels. It was observed on cryoscanning micrographs that gels made from soybean had a more uniform and denser network with smaller network pore size than the ones made from SPI. Therefore, the use of Ca SO4 and GDL in soybean and SPI tofu-type gel production led to different gelation behavior and rheology properties.The coagulant and protein concentration, effects on the gelation behavior and rheological properties of acid and salt-induced soft tofu-type gels formed with heated soy protein isolate solutions were investigated. All gels showed a clear gel-like behavior, with acid-induced gels having the highest G' at all coagulant concentrations used. Increase in coagulant concentration resulted in higher G? and shorter gelation time(Tgel). The dependence of Tgel on the coagulant concentration could be scaled with a power law model with R2 varying between 0.9535 and 0.9787. Also, the dynamic moduli changes over angular frequency fitted well the models: G?~ G0? ?n' and G??~ G0???n??. The increase in shear deformation at fracture and shear stress yielding was directly proportional to the increase in coagulant concentration. Both gels were found in the transition regime with varying protein concentration, when experimental data were scaled with a fractal model. Salt-induced gels had more uniform network pore size, finer and smoother surface texture than acid-induced gels. Dynamic moduli analysis of the gels was significantly affected by coagulant type and concentration(p ? 0.05). Consequently, the preparation of soft tofu-type gels using different concentrations of Ca SO4 and GDL at varied conditions including temperature, p H and protein concentration results into gels of different rheological properties, gelation behavior and microstructure.The oil volume fraction effects on the rheological properties of acid and salt-induced soft tofu-type gels formed at different temperatures were investigated. The emulsions were prepared prior heating at 95 °C for 10 min. Emulsions with the highest oil volume fraction were characterized with the lowest protein adsorption, emulsifying stability and emulsifying activity index but they had the highest average droplets diameter viscosity. For salt-induced gels, high oil volume fraction resulted in high gel point(Pgel) temperature and Pgel G? while gel time(Tgel) decreased. A power low relation between Tgel, Pgel temperature and oil concentration was found and the plot of G? versus oil concentration fitted well the model: G?~CA. The exponent “A” was positive in salt-induced gels but negative in acid-induced gels, indicating a change in the ratio between the stiffness(modulus) of the oil droplets and that of the gel matrix after addition of coagulants. Soy oil acted as active filler in salt-induced gels; however it acted as inactive filler in acidic gels. The weak gel model was used to analyse and describe the network extension and its strength. Acid-induced gels without oil were characterized by the highest strength and a drastic reduction of the network extension whereas G? of salt-induced gels containing various oil volume fractions were not significantly different. Salt-induced gels made in situ were more resistant to fracture than acid-induced gels. After 24 h storage at 4°C, reduction in oil volume fraction in acid-induced gels increased the fracture stress but decreased the fracture strain, while salt-induced gels resulted in an increment in shear deformation at fracture when the oil volume fraction was increased up to 15%.It is generally known that sensory characteristics can be influenced by a wide range of factors. In the last part of our study, the effects of coagulants and raw materials on the sensory characteristics of acid and salt-induced soft tofu were investigated and compared. Significant differences were observed in the colour indices for all the samples. The volatile compounds composition was more affected by the raw materials than the coagulants. The total amino acids content significantly decreased during both acid and salt-induced coagulations while the free amino acids content did not present a significant change during coagulation. It was also established that Ca SO4 produced softer gels with higher elasticity than GDL. The salt-induced soybean gels registered higher scores in all the flavour attributes evaluated. Additionally, the salt-induced gels showed higher surface smoothness, homogeneity and firmness than acidinduced gels, with soybean gels registering the highest scores. The Partial least square regression correlation results depicted that the volatile compounds neither had a positive nor a negative significant impact on mouthfeel of soft tofu gels; however they were significantly correlated to the beany and bitter taste. Furthermore, all free amino acids were positively correlated to beany, bitter taste and mouthfeel except for cysteine. Glutamic acid, alanine, tyrosine, methionine and isoleucine free amino acids showed a significant influence on beany taste; while, glutamic acid, tyrosine, methionine and isoleucine showed a significant impact on bitter taste of soft tofu gels. It was concluded that the sensory characteristics of soybean and soy protein isolate soft tofu type gels mostly depended on their raw materials rather than the added coagulants and the relationship between volatile compounds and sensory characteristics was established.