澄清橫山梨果汁加熱後混濁生成機制之研究

Abstract

澄清果汁生成混濁沈澱是影響果汁品質之重大缺陷。澄清橫山梨果汁經加熱殺菌後立即有顯著混濁生成之現象發生，對果汁品質的影響甚鉅。本研究之目的在探討橫山梨果汁加熱後混濁生成之原因，以作為改善加工製程之理論依據。 研究結果顯示橫山梨果汁加熱後水溶性蛋白變性是造成混濁現象之主因。隨著加熱溫度的上升，果汁混濁度亦隨之升高。當果汁以100oC處理時，矽藻土過濾之澄清橫山梨果汁所生成之混濁度達到最高 (31.3NTU)，果汁中之水溶性蛋白含量則減少53.8%。以100k或10k MWCO超過濾薄膜作澄清加工皆能有效降低果汁中熱不穩定水溶性蛋白質含量，因而得以改善果汁熱處理（如殺菌）後混濁生成之現象。橫山梨果汁以3000mg/L皂土處理，可去除71.6%的水溶性蛋白質，加熱後果汁之混濁度也與加熱前相近。木瓜酵素處理果汁僅可分解大於45kDa以上之蛋白質，與PVPP處理之結果一樣，對改善橫山梨果汁遇熱生成混濁沈澱的現象沒有幫助。 橫山梨果汁所含水溶性蛋白質之分子量分佈主要在6.5kDa、14 kDa及30 kDa，其中熱不穩定性蛋白質分子之分子量約為30 kDa。橫山梨果汁所含蛋白質之胺基酸組成中含88~90%的aspartic acid及少量之serine、glutamic acid與histidine。aspartic acid與serine為熱不穩定蛋白質之重要胺基酸成分。橫山梨果汁所含之主要的酚類化合物為gallic acid、catechin、epicatechin。 橫山梨、新興梨與新世紀梨等東方梨之果汁皆有明顯的遇熱混濁生成之現象。起因亦是與蛋白質熱變性有關，其果汁中也都含高比率之aspartic acid及serine與glutamic acid。綠安琪兒西洋梨與富士蘋果之果汁遇熱生成混濁之現象極不顯著。

Haze formation and precipitation are critical quality defects for clear fruit juice. The objective of this study is to investigate the mechanism of haze formation of Hengshan pear juices during heating process. The results of this study demonstrated that heat-unstable soluble proteins in the clear Hengshan pear juice are the main reason to cause haze formation during heating process. The turbidity increases as the heating temperature increases. Thermal processing of the diatomaceous earth filtered Hengshan pear juices at 100oC resulted in the reduction of 53.8% of soluble protein and yielded highest juice turbidity of 31.3NTU. Clarification of Hengshan pear juices by 100k or 10k MWCO ultrafiltration membranes effectively removed heat-unstable soluble proteins of the juice, and help to improve the reduction of haze formation after heating. Hengshan pear juice treated with 3000mg/L bentonite removed 71.6% of soluble protein, and prevented haze formation by heating. Papain treatment on the Hengshan pear juice can only degrade proteins with molecular weight larger than 45kDa. PVPP treatment also had little effect to prevent haze formation of the juice after heating. The main molecular sizes of soluble proteins in Hengshan pear juices are 6.5kDa, 14kDa, and 30kDa. Soluble proteins with 30kDa molecular weight are identified as the major heat-unstable proteins in Hengshan pear juice. The amino acid analyses showed soluble proteins of Hengshan pear juices contain 88~90% of aspartic acid and some minor amino acids such as serine, glutamic acid, and histidine. Aspartic acid and serine has been reported as the major amino acid constituents in the heat-unstable soluble proteins of fruit juices and wines. The major phenolic compounds in Hengshan pear juices are gallic, catechin, and epicatechin. In addition to Hengshan pear juice, oriental pear juices such as Sinku pear juice and Sinseiki pear juice also tend to form haze due to the precipitation of heat-unstable soluble protein when the juice is heated. The soluble proteins of these juices also contain high percentage of aspartic acid. Green Anjou pear juice and Fuji apple juice are less prone to form haze after heating.