酵母菌對醬醪發酵期間風味與品質之影響

Abstract

醬油為東方傳統發酵食品之一，除亞洲地區外，世界各地的使用量有逐漸增加的趨勢。醬油製程可分為製麴(koji-making)、醬醪發酵(fermentation)以及精製(refining)。發酵過程則對醬油品質最為重要，工廠會先測定酵母菌液的吸光值(OD600)，確保在特定的時間內能夠添加合適的酵母菌數到醬醪中，但在實務操作中發現，吸光值並不是一個能夠反應菌數量的良好指標，造成品管上的困難。因此，本試驗主要分為兩個目的。首先擬探討四株酵母菌(Saccharomyces rouxii、Candida etchellsii、Candida versatilis NBRC 1908及Candida versatilis NBRC 10664)於培養期間吸光值(OD600)和菌數生長間的相關性。接者將三株Candida菌屬之酵母菌分別接種於醬醪中，分析不同菌株在醬醪發酵6個月期間理化性質特性與風味化合物的不同。結果得知，S. rouxii和C. etchellsii兩株菌種生長速率較快，而C. versatilis NBRC 1908和NBRC 10664則生長較慢。將菌數和吸光值取對數作圖，並以線性迴歸進行分析時，分別可知四種酵母菌之R2 介於 0.819-0.9613之間，代表相關性高，因此，可依照相關方程式，將測量的OD值轉換成相對菌數，以推測加入到發酵醬醪內的酵母菌數量。接著，在6個月醬醪發酵過程中，醬醪的pH值隨發酵時間的延長而降低，總氮、總酸、及總固形物含量則增加，色度提高，而還原糖含量則先增加後減少。風味物質上，各類化合物含量與種類皆因發酵時間而增加，醛類、酮類和雜環類因加熱而大幅增加，醇類和酯類則是有下降的趨勢。比較三組各別加入不同Candida菌株的醬醪，其風味化合物的含量沒有一個明顯的趨勢。最後將第25週加熱前後的醬醪樣品以主成分分析(Principal component analysis, PCA)的方法進行分析，結果得知，試驗樣品大致可被區分為四個群組，第一、二組成分可解釋77.1%的總變異數，而大部分醛類、酮類和雜環類與加熱的組別有相關，顯示加熱步驟會影響風味物質的形成。

Soy sauce is one of the traditional fermented foods and the most used condiment in the East. Except Asian, soy sauce is becoming more and more popular around the world. The processes of soy sauce production can be divided into koji-making, fermentation, maturation and refining. The fermentation is the most important process to the quality of soy sauce. Practically, the factory will inoculate a desirable amount of yeasts into soy sauce mash at specific time based on the measured absorbance value (OD600) of the incubation broth of microorganisms. However, it is found that the OD600 may not a good indicator due to the quality variations among batches in actual production. This may be one of the causes resulting in the difficulties of quality control. Thus, there are two major objectives of this study. First, the correlations between the OD600 values and the numbers of four yeasts (Saccharomyces rouxii, Candida etchellsii, Candida versatilis NBRC 1908 and Candida versatilis NBRC 10664) from the total plate counts were established. Next, the three Candida species were inoculated into the soy sauce mash, and the effects of each Candida specie on the phyicochemcial properties and the produced flavor compounds during 6-month fermentation were monitored, determined and analyzed. The results show that S. rouxii and C. etchellsii grew faster than the C. versatilis NBRC 1908 and NBRC 10664. High correlation coefficients (R2=0.819-0.9613) of the growth of four yeasts by taking the logarithm of number of yeast and the logarithm of OD600 during incubation were obtained and the correlation equations were established. These equations can provide an easy and suitable conversion method for yeast inoculation during operation practically. During 6-month fermentation, the pH values of the fermented soy sauce mash decreased as the fermentation time increased. Total nitrogen, total acid, total solid content, and color increased as the fermentation time increased. The reducing sugar content increased first then decreased as the fermentation time increased. The amounts and varieties of flavor compounds increased with increasing the fermentation time. Aldehydes, ketones and heterocyclic compounds in soy sauce significantly increased after heating. In contrast, alcohols and esters decreased after heating. Comparing the contents of flavor compounds in soy sauces making of three Candida species, there is no obvious trend. The amounts of flavor compounds determined from the 6-month fermented soy sauces before and after heating were taken for Principal component analysis (PCA). The results show that there are four groups can be divided, and the first two principal components explained 77.1% of the total variance of samples. Most aldehyde, ketones and heterocyclic compounds are related to the heated group, showing that the heating step will significantly affect the formation of flavor compounds.