高场强超声波对草莓浆流变特性的酶促调控机制

Fruit puree is processed from edible parts of fresh raw fruits and it is popular with the consumers who demand a quality product that is convenient and has fresh-like characteristics. The traditional thermal processing could inactivate natural micro-flora and enzymes in fruit puree, leading to a longer shelf life. However, some compounds (eg. VC and polyphenols) in fruit will be destroyed by heat, causing a loss in physicochemical and sensory properties. Among the properties, rheological property is important since it relates to mouth-feel, stability and processing characteristics of the product. As a novel non-thermal technology for pasteurization, high power ultrasound (HPU) can effectively inactivate natural micro-flora and enzymes in fruit puree while maintain the quality attributes since it works at relatively lower temperature (< 50 ℃). The most commonly used range of HPU in food technology is high intensity-low frequency (20–100 kHz) with a power of 10–1000 W/cm2. The inactivation of micro-flora and enzymes could be mainly due to the mechanical and chemical effects of ultrasound cavitation, which involves the formation, growth and implosion of bubbles. There are a few works in the literatures regarding the quality changes in fruit puree due to HPU, however, the mechanism is not fully understood. Therefore, the objective of this work is to study the effect of HPU on pectin methylesterase (PME), pectin and their reaction, to reveal the possible enzymatic mechanism of HPU on the rheological property changes. The study will provide information which will benefit the non-thermal processing fruit puree industry.

果浆香气浓郁、甘美可口、营养丰富，是重要的水果加工产品之一。传统的热力杀菌虽能有效的杀灭产品中的微生物和内源酶，并延长产品的货架期，但水果中的热敏成分会被高温破坏，从而导致营养和感官品质的损失。流变特性是果浆的一个重要品质特性，与产品的口感、稳定性以及操作单元的设计有关。本项目以草莓浆为对象，采用非热加工技术中的高场强超声波（HPU）取代热杀菌技术，以解决热力杀菌过程中流变特性易被破坏的问题。开展HPU对PME活性和分子构象的研究，解析HPU对酶的钝化效应和提取效应在流变特性变化中的作用；开展HPU对PME-果胶酶促反应分子接触以及反应产物空间交联的研究，解析HPU下PME-果胶酶促反应对流变特性的影响；开展HPU对草莓浆PME酶活、果浆颗粒、果胶等的影响研究，解析HPU空穴效应在流变特性变化中的作用。本研究旨在揭示HPU对草莓浆流变特性的酶学调控机制，为非热加工技术在果浆的应用提供依据。

草莓(Fragaia ananassa)是世界小浆果生产中的主要品种，因色泽鲜艳、香气浓郁、酸甜适口，含有丰富的抗氧化物质而广受欢迎。目前我国的草莓主要以鲜食为主，加工产品主要是草莓酱等初级加工产品，存在深加工程度不高、产品附加值低、产品形式单一等问题。传统的热加工技术会破坏草莓中的功能性成分，从而导致营养和感官品质的损失。因此，越来越多的学者将非热加工技术运用于草莓的加工中。高场强超声波(High-power ultrasound，HPU)是食品加工中最为常用的非热加工技术，具有良好的应用前景。.针对HPU加工过程中草莓浆的流变特性发生变化这一现象，研究不同HPU处理条件(0-45 ℃, 242-968 W/cm2, 2-16 min)对草莓多聚半乳糖醛酸酶（polygalacturonase，PG）、果胶甲基酯酶（pectin methylesterase，PME）活性和结构、水溶性果胶（WSP）分子结构、游离Ca2+含量及三者之间相互作用的影响，提出HPU影响草莓浆流变特性的可能机制，为HPU技术应用于果浆加工提供依据。.主要研究结果：（1）HPU提高了草莓浆粘度，增强其弹性和粘性性质；（2）HPU可以有效钝化PG，从而抑制果胶的降解，但不能有效钝化PME，残留的PME可能会使果胶发生低酯化；（3）HPU可以改变草莓WSP的分子结构，从而降低草莓果胶的粘度；（4）HPU降低了草莓浆中游离Ca2+的含量。以上结果表明HPU提高草莓浆粘度的可能机制为：HPU处理后残留的高活性PME诱导水溶性果胶低酯化，同时HPU促进了游离Ca2+ 和低甲基化果胶之间的相互作用，形成 Ca2+-低甲基化果胶的网络结构，从而增加复杂草莓体系中的粘度。.本研究为进一步评价非热加工技术在果浆中的应用提供基础数据与理论依据。