| Traditional Chinese soybean paste,produced through long fermentation,has been increasingly welcomed by consumers due to its unique flavours and rich nutrition. However, the long fermentation period limits the output in the industrial production, thus increases the manufacturing cost and supresses the competitiveness. The nature of soybean koji fermentation is the metabolic activities of koji microorganisms as well as the catalytic reactions. If the metabolic acitivities and/or the catalytic reactions are enhanced or accelerated, the time for fermentation can be reduced. High pressure processing (HPP) has been approved to have the ability to regulate the acitivity of enzyme, i.e. low pressure activating the activity while high pressure inactivating enzyme acitivity.This research involves HPP into the production of traditional Chinese soybean paste by treating prepared koji before the aging period.The pressure induced changes of related enzymes acitivities were analyzed and the kinetics parameters were evaluated. The optimum HPP parameters, which activated the enzymes acitivities, were chosen for analyzing the quality attributes of the fermentated soybean paste, including color, composition of amino acids and aroma characteristics. At the same time, two commercial soybean pastes were compared with HPP-soybean pastes produced in the lab with respect to quality attributes. The purpose of this research was to investigate the possibility of shortening the fermentation period of soybean paste by applying HPP. It was noted that the adiabatic heating affected the change of enzyme activity induced by pressure. However, current technologies of monitoring and regulating the temperature variations during HPP were limited, therefore, new monitoring and insulating systems using thermocouple were designed in this research.The main results obtained in the research were shown as follows:1.Low pressure treatment could activate the PPO activity in Soybean paste koji. The relative PPO activities increased to116～135%after HPP treatment between200～300MPa with holding time of10min.While pressures higher than350MPa inactivated PPO activities significantly (p<0.05). Kinetic analysis indicated that the patterns of koji PPO activation or inactivation expressed in a different way at different pressure level. The PPO activation pattern following HPP at100-200MPa can be described well by a two-step first-order model(R2≥0.970); The survival curves with250～300MPa HPP treatment followed the fractional conversion model with the coefficient of determination higher than0.994; The biphasic model was proved to be suitable to describe the kinetics of koji PPO inactivation treated by HPP between350and500MPa (R2≥0.995).2.Through the color analysis of fermentated soybean paste, HPP treatment of koji between the range of200and300MPa accelerated the formation of pigment of soybean paste during fermentation process.3.Neutral (pH=7.5) and alkaline (pH=10.0) proteases expressed the similar trend towards pressure treatment, both fitted the two-step first-order models between150～500MPa. The activities of both proteases increased after HPP at100～300MPa, but were significantly inactivated at higher pressure (350～500MPa,0-20min). Paticularly, the activities of neutral and alkaline increased from39.7, and48.68U mL-1(untreated control) to above120U mL-1, with an increase of about240%.4.The total content of amino acids formed and accumulated during fermentation increased using the pressure-treated koji as fermentative starter. After HPP treatment of250or300MPa for5or10min, the total amino acids (dry basis) in soybean pastes ranged between7.0817and9.5332g100g-1, which were much higher than those obtained from commercial products and that required by GB/T24399-2009. Those results showed that pressure treated soybean paste featured a higher content of total amino acids, in other words, the fermentation process was enhanced and accelerated, due to the increases in proteases activities induced by pressure.5.As compared to250MPa,300MPa (5min) HPP treatment was better to accumulate Glu, Asp and Ser during fermentation, which featured a strong umami taste. While at the pressure level of300MP, a longer holding time (10min) showed disadvantages to5min in accumulating the content of amino acids, but increased the content of GAB A, which increased from0.0606g100g-1to0.0613g100g-1by1.16%.6.Commercial and pressure treated soybean pastes can be differentiated by aroma analysis through E-nose and LDA. Pressure significantly affected the aroma responses in E-nose.As compared to commercial samples, pressure treated soybean pastes featured a common characteristics, which was dependent on the level of pressure applied on koji.7.About sixty aroma compounds were detected in soybean paste by HS-SPME-GC-MS, those compounds can be classified into alcohols (8), aldehydes (10), acids (4),ester (22), alkane (8) and other7kinds. HPP treatment affected the composition of soybean paste aroma significantly. At the pressure holding for5or10min, higher presure resulted in more alcohos, aldehydes and acids during koji fermentation, but reduced the formation of esters. While at the same pressures, longer pressure holding time resulted in more concentration of aroma compounds. Specifically, more alcohols, acids, esters and alkane were produced during the aging period, but less aldehydes.8. High pressure auxiliary devices, including temperature measuring (thermocouple sensor) and monitoring (adiabatic insulator) systems were well designed and installed in the HPP unit. The devices were verified to be well performed in the conditions of high pressures (0.1～500MPa) as well as room and low temperatures. |
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