摘要
梨汁饮料营养丰富,口感鲜美,备受消费者青睐。目前果汁加工中广泛采用热力杀菌技术进行杀菌灭酶,但热加工对果汁新鲜度和品质影响很大,对营养和功能性成分也有一定破坏。非热加工技术(Non-thermal Processing, NTP)最大限度保持食品新鲜度,是目前研究热点。本文以鲜榨梨汁为原料,研究了高密度二氧化碳(Dense Phase Carbon Dioxide, DPCD)技术不同工艺参数对梨汁中菌落总数、多酚氧化酶(Polyphenol Oxidase, PPO)和主要品质指标影响,筛选出有效工艺参数处理梨汁,并置于4℃下贮藏,对主要品质指标进行了跟踪观测。主要结论如下：
(1)随压力和处理时间增加,DPCD对鲜榨梨汁中菌落总数杀灭效果显著增强(p<0.05)。处理温度对杀菌效果有协同作用,随温度升高细菌残存率显著降低(p<0.05)。30 MPa-40℃-60 min下鲜榨梨汁中菌落总数残存率最大降低了2.664个对数。Weibull模型能较好的拟合DPCD处理后梨汁中菌落总数的失活曲线,且随处理压力和温度升高,比例因子a和形状因子b逐渐降低。
(2)一级反应动力学能较好分析DPCD对梨汁PPO的钝化效果。随温度和压力升高,D值下降,k值上升。30 MPa-40℃-60 min的DPCD处理鲜榨梨汁中PPO酶活降低了83.34%。40℃时,Eyring公式分析DPCD处理梨汁PPO钝化效果参数为ZP=37.88MPa, Va=-68.67 cm3/mol。30 MPa时,Arrhenius公式分析DPCD处理梨汁PPO钝化效果参数为ZT=86.21℃, Ea=20.19 KJ/mol。
(3) DPCD处理后鲜榨梨汁可溶性固形物Brix值和黏度没有显著变化(p>0.05),总色差△E<2；pH值和亮度(L值)比未处理样品显著降低(p<0.05),但不同条件(温度和时间)处理差异不显著(p>0.05)。随DPCD处理压力、温度和时间延长,梨汁透光率显著下降,褐变程度显著抑制(p<0.05)；粒径分布呈现低压低温时减小,高压高温时增大的变化规律。
(4)筛选30 MPa-40℃-60 min作为鲜榨梨汁DPCD处理参数。4℃贮藏期内,DPCD处理后鲜榨梨汁第5周起梨汁褐变度显著上升(p<0.05),第6周起菌落总数、pH值和透光率变化显著(p<0.05)；8周内可溶性固形物含量和L值变化不显著(p>0.05)。PPO酶活的活性有部分回复,但整体保持在低活性水平。一级反应动力学公式能较好地分析DPCD处理后贮藏期内鲜榨梨汁L值的变化。△E<1.5,没有发生可见变化,DPCD处理对梨汁色泽变化有较好的抑制效果。综上所述,DPCD处理后4℃下可以达到6周贮藏期。
Consumption of fresh pear juice has been increasing rapidly due to its flavor and nutritional value. At present fruit juice processing widely uses thermal treatment technology to carry on inactivation of microbe and enzyme. The traditional thermal pasteurization processes not only have effect on the fruit juice quality specially freshness, but also have destructive effect on some nutrition ingredient and functionality ingredient. While, non-thermal processing can keep freshness at high limit, and it's better than other methods. The objective of this article was to study effects of DPCD (Dense Phase Carbon Dioxide) on the processing of fresh pear juice, inactivation of microbe and enzyme, and effects on the quality of pear juice at 4℃.
(1) More and significantly inactivation of total bacteria counts was achieved at higher pressure, temperature and exposure time (p<0.05). The maximum reduction was 2.664-log at 30 MPa and 40℃for 60 min. The survival curves of total bacteria counts in pear juice against pressure or temperature were fitted by Weibull Model with high regression coefficients, and a value (scale factor) and b value (shape factor) decreased with increasing pressure or temperature.
(2) Analysis of first-order reaction kinetic data showed that D value of PPO in pear juice decreased and k value increased with increasing pressure and temperature of DPCD treatment.83.34% of PPO inactivation was achieved at 30 MPa and 40℃for 60min. The ZP and Va value of PPO was 37.88 MPa and -68.67 cm3/mol treated by DPCD with 40℃. The ZT and Ea value of PPO was 86.21℃and 20.19 KJ/mol treated by DPCD with 30 MPa.
(3) After DPCD treatment, Brix value and viscosity of fresh pear juice were not significantly changed (p>0.05) and the total color difference (ΔE) value was less than 2. But pH value and L value were significantly reduced (p<0.05). And they were not significantly changed with different treatment (temperature and time). T value decreased and browning degree was inhibited significantly with higher pressure, temperature and exposure time (p<0.05). The average particle size decreased under low temperature low pressure and increased under high temperature high pressure.
(4) 30 MPa and 40℃for 60 min was suitable treatment for fresh pear juice. After DPCD treatment, browning degree significantly rose from the fifth week (p<0.05), and the total bacteria counts, pH value, T value were significantly changed (p<0.05) form the sixth week during 8 weeks storage time at 4℃. Brix value and L value of pear juice were not significantly changed (p>0.05). The activity of PPO recorded partly during storage time, but it was at low level. The L data followed first-order reaction kinetics during 8 weeks storage time after DPCD treatment.ΔE value of pear juice treated with 30 MPa and 40℃for 60min was less than 1.5, and the color was not obviously changed during storage time. Incorporation of low temperature, the storage time could reach 6 weeks after DPCD treatment.
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