干酪风味物质的生成

干酪发酵中物质的生成来自于糖酵解(牛奶中的乳糖)、脂解以及蛋白质水解和肽水解过程中所涉及到的一系列生化化学反应，尤其是由发酵荆引起蛋白质(酪蛋白)发生酶降解导致重要风味成分的形成，这有助于干酪制品的感官特性。酪蛋白继续降解成肽和氨基酸，而氨基酸是挥发性风味化合物的前体，本文就从氨基酸产生风味物质的主要途径尤其是蛋氨酸、芳香族氨基酸和枝链氨基酸的代谢过程进行了综述。转氨酶启动的反应在风味形成方面有重要的作用，而α—酮戊二酸是LAB氨基酸代谢的关键中间媒介。     

Key enzymes involved in methionine catabolism by cheese lactic acid bacteria

Cheese microbiota and their enzymatic conversion of l-methionine to volatile sulphur compounds (VSCs) play an important role in aroma formation during cheese ripening. Here, lactic acid bacteria (LAB) strains isolated from raw goats' milk cheeses were screened for the major enzymes critical to the formation of VSCs from l-methionine. A large natural biodiversity in enzyme capabilities and high inter- and intra-species variability was found among the LAB isolates investigated. From those isolates tested, lactococci displayed higher C–S lyase specificities towards the sulphur-containing compounds examined than did Lactobacillus and Leuconostoc, in some cases generating higher levels of VSCs than B. linens, known to be an efficient producer of methanethiol (MTL) and related VSCs. Moreover, these differences in C–S lyase activities (determined spectrophotometrically by measuring the formation of free thiol groups) were shown to correspond with the enzymatic potential of the isolates as determined by visualization of enzymatic activities. This technique could therefore prove valuable for the detection and preliminary characterization of C–S lyase activities among LAB isolates. Lactococci were also found to possess higher aminotransferase activities than lactobacilli and leuconostocs, while glutamate dehydrogenase activities were observed to be highest among Leuconostoc and Lactobacillus spp. Meanwhile, α-keto acid decarboxylase activities were highly variable and were measurable in only a limited number of isolates, mainly lactobacilli. From these data, combining indigenous isolates showing high VSCs-producing capabilities with those that facilitate the completion of the metabolic pathway responsible for degrading l-methionine into volatile compounds may provide an efficient approach to enhance cheese aroma development.     

干酪中非发酵剂乳酸菌

非发酵剂乳酸菌是干酪中主要的次生菌群，不属于发酵微生物，通常不能很好的在牛奶中生长，不能产酸，但对干酪的风味形成有很重要作用。本文概述了非发酵剂乳酸菌的一些特征，包括不同原料制成的干酪中非发酵剂乳酸菌的来源、生长能源的利用、自身存在的蛋白分解系统对干酪风味形成影响及作为一种提高干酪品质的附属发酵剂的应用展望。     

Improvement of sensory quality of lowfat kefalograviera-type cheese by using commercial special starter cultures

Two commercially available special starter culture systems, Alp DIP and a mixture of Alp DIP D and Joghurt V1, were compared with one commercial regular starter culture, CH-1, for their effects on the compositional, sensory and textural characteristics of lowfat (9.5%) high moisture (49.6%) Kefalograviera-type cheese during aging. A full-fat control Kefalograviera cheese (30.8% fat, 37.8% moisture) was also made with the regular starter culture. The results indicated that the type of starter did not affect the composition (moisture, fat, protein, salt and pH) of the lowfat cheese. Sensory analysis showed that the lowfat cheeses made with the special cultures received greater body and texture scores and significantly higher flavor scores than the lowfat control cheese after aging for 90 and 180 d. Moreover, the former cheeses received body and texture and flavor scores not significantly different from those of the full-fat cheese. Texture profile analysis by Instron showed that there were no significant differences in the textural characteristics (force and compression to fracture, cohesiveness, springiness, gumminess and chewiness) between lowfat cheeses made with the special cultures and that made with the regular starter, except for hardness which was significantly lower in the former cheeses.     

Short communication: empirical and mechanistic evidence for the role of pyridoxal-5'-phosphate in the generation of methanethiol from methionine

The catabolism of the sulfur-containing AA Met to form flavor-active volatile sulfur compounds (VSC) is an important mechanism in flavor development during cheese maturation. Numerous enzymes catalyzing AA catabolism require the presence of the cofactor pyri-doxal-5′-phosphate (PLP). In fact, reports have shown that some of these reactions can be catalyzed by PLP alone, albeit at a reduced rate. We hypothesized that, as a specific application in cheese flavor reactions, PLP can react directly with free Met to generate a specific VSC, methanethiol (MTH). In this study, the ability of PLP to catalyze MTH generation from Met was examined under “cheeselike” conditions of salt and pH. Methionine and varying concentrations of PLP were incubated in a buffer (pH 5.2 + 4.0% NaCl) analogous to the aqueous phase of aged Cheddar cheese. Samples were analyzed using headspace solid-phase microextraction, and relative concentrations of VSC were determined by gas chromatography-mass spectrometry. Results showed MTH, dimethyl disulfide, and dimethyl trisulfide production when Met and PLP were incubated together at 7°C (cheese-aging temperature). These results indicate that the production of VSC from Met can occur nonenzymatically as catalyzed by free PLP.     

干酪成熟过程发酵剂的作用及快速成熟的研究进展

干酪的成熟过程,是在添加酶及各种微生物的协同作用下完成的.干酪中的微生物作用也包括人为添加发酵剂的作用.本文综述了干酪生产过程中发酵剂的使用和成熟过程所起的作用,主要有酸化、改善质构、形成风味物质.此外,本文也讨论了干酪的成熟机制,综述了目前用于促进干酪成熟所使用的一些方法和技术以及对它们的最新研究进展.     

浓缩型乳酸菌发酵剂的工业化生产

对乳品发酵剂工业化生产中的重要技术，诸如高活力菌体的增殖培养、发酵过程控制与分析、细胞富集和生物保存等问题进行了探讨．以期为我国进一步发展浓缩型乳酸菌发酵剂提供理论依据。     

Genotypic identification of some lactic acid bacteria by amplified fragment length polymorphism analysis and investigation of their potential usage as starter culture combinations in Beyaz cheese manufacture

In this study, 2 different starter culture combinations were prepared for cheesemaking. Starter culture combinations were formed from 8 strains of lactic acid bacteria. They were identified as Lactococcus lactis ssp. lactis (2 strains), Lactobacillus plantarum (5 strains), and Lactobacillus paraplantarum (1 strain) by amplified fragment length polymorphism analysis. The effects of these combinations on the physicochemical and microbiological properties of Beyaz cheeses were investigated. These cheeses were compared with Beyaz cheeses that were produced with a commercial starter culture containing Lc. lactis ssp. lactis and Lc. lactis ssp. cremoris as control. All cheeses were ripened in brine at 4°C for 90 d. Dry matter, fat in dry matter, titratable acidity, pH, salt in dry matter, total N, water-soluble N, and ripening index were determined. Sodium dodecyl sulfate-PAGE patterns of cheeses showed that α_S-casein and β-casein degraded slightly during the ripening period. Lactic acid bacteria, total mesophilic aerobic bacteria, yeast, molds, and coliforms were also counted. All analyses were repeated twice during d 7, 30, 60, and 90. The starter culture combinations were found to be significantly different from the control group in pH, salt content, and lactobacilli, lactococci, and total mesophilic aerobic bacteria counts, whereas the cheeses were similar in fat, dry matter content, and coliform, yeast, and mold counts. The sensory analysis of cheeses indicated that textural properties of control cheeses presented somewhat lower scores than those of the test groups. The panelists preferred the tastes of treatment cheeses, whereas cheeses with starter culture combinations and control cheeses had similar scores for appearance and flavor. These results indicated that both starter culture combinations are suitable for Beyaz cheese production.     

浓缩乳酸菌发酵剂的浓缩培养的研究进展

随着国内消费者对酸奶需求量的增加以及对发酵剂品质要求不断提高,传统的液体发酵剂因为所含活菌数较少,正在被活菌含量更高、使用更方便的浓缩型乳酸发酵剂所取代。浓缩乳酸发酵剂生产的关键是对乳酸菌进行浓缩培养,以获得较高的活菌数。为进一步发展提供依据,总结了浓缩培养技术的现状和进展。     

基因工程改良乳酸菌发酵剂品质的研究进展

综述了近5年来利用基因工程技术改良乳酸菌发酵剂蛋白质水解能力、产生双乙酰能力、合成胞外多糖能力、抗杂菌病源菌污染能力4个方面研究的最新进展，并简要介绍了基因食品可能存在的不安全问题及解决方法。     

乳酸菌发酵剂高密度培养的研究

研究了乳酸菌生长繁殖的环境条件(温度、接种量、起始pH等)和培养基组成(氮源、碳源、缓冲盐等),优化确定了乳酸菌发酵剂的适宜培养条件为:起始pH值为6.5,培养温度为37℃,培养基配比为麦芽糖∶乳糖(1∶1)2%、牛肉膏1.0%、缓冲盐A0.5%、NaCl0.25%、MgSO40.1%,接种量4%,进一步探索了半连续法进行高密度培养,结合优化的培养条件,可使乳酸菌的液体发酵活菌密度至1.1×1012CFU/mL。     

乳酸菌发酵剂优良菌种的选育

系统论述了当前有关乳酸菌发酵剂优良菌种的筛选方法与标准，具体包括：抗性菌株筛选、适宜蛋白水解力、乳糖水解力菌株的筛选、产粘菌株的筛选、产Nisin类细菌素菌株的筛选、柠檬酸利用阳性菌株与半乳糖利用阳性菌发酵剂中优良菌株筛选及应用具有一定参考意义。     

Selection criteria for lactic acid bacteria to be used as functional starter cultures in dry sausage production: An update

Lactic acid bacteria (LAB) have long been used as starter cultures in the production of fermented dry sausages and other meat-derived commodities. These cultures are generally designed to meet food safety, shelf-life, technological effectiveness and economic feasibility criteria. Besides all these traditional properties, novel starter cultures should take into account the risks posed by the formation of biogenic amines in food, and the development and spreading of bacterial resistance to antibiotics. Further, 'functional starters' could protect consumers from harmful bacteria either by a rapid acidification or by the production of antimicrobials (bacteriocins). Specially-selected cultures may also provide probiotic benefits, and, if properly modified, they may even be endorsed with nutraceutical traits. The present review discusses the technological and new selection criteria that should be taken into account when selecting LAB starter cultures for the production of fermented dry sausages.     

适于大豆干酪加工的乳酸菌发酵剂研究

以豆浆为发酵底物,对8株乳酸菌的凝乳性能进行了研究,检测了凝乳的黏度、酸度、双乙酰、胞外多糖和pH值为4.6可溶性氮含量,采用混合权重法对结果进行统计分析,得出凝乳性能的排序结果为Lb2→La→St1→Lb1→Sl→St2→Sc→Lc。此外,对8株乳酸菌的α-低聚半乳糖代谢能力进行了研究,乳杆菌代谢能力显著高于乳球菌。最后,优选出适于大豆干酪加工的乳酸菌发酵剂为St1和La。     

功能性乳酸菌发酵剂在食品发酵工业中的应用

功能性乳酸茼发酵剂是具有一种或几种固有功能特性的发酵剂，其在感官、技术、营养或健康方面的各种优势使之在食品发酵工业中的应用潜力巨大：它可用于食品防腐、提高食品安全性，改善食品质构和风味，加速奶酪成熟，可产生功能因子、去除毒副因子从而使食品具有保健功效；某些抗噬菌体发酵剂还可解决乳品工业中的噬菌体污染问题。通过基因工程或菌株筛选可以获得所需的功能性发酵剂．它使人们可以更好地控制发酵过程，为消费者提供新型健康食品，此外还有利于食品企业的多样化生产。     

发酵乳风味物质乙醛、双乙酰的合成途径及其调控机制

乳酸菌在发酵过程中能够产生醇、醛、酮等多种风味物质,这些风味物质和乳酸菌在糖酵解过程中产生的乳酸、生醋酸、丙酸等有机酸相互作用,形成具有独特风味的发酵乳。在酸奶的风味物质中,乙醛、双乙酰是其主要成分。目前,关于酸奶风味物质研究的焦点主要集中在通过改变乙醛、双乙酰的代谢途径提高乙醛、双乙酰的产量上。这篇综述主要描述了乳酸菌在酸奶发酵过程中生产的风味物质、以及乙醛、双乙酰的合成途径和国内外学者在乙醛、双乙酰产量调控方面取得的研究成果。     

不同成熟期Camembert干酪挥发性风味物质的分析

应用顶空固相微萃取气质联用(HS-SPME-GC-MS)对成熟期分别为15、30和45 d的Camembert干酪中的挥发性风味物质进行了分析检测.结果表明,在成熟过程中Camembert干酪产生的主要挥发性风味物质是醇、醛、酮、酯、苯环物质、烯烃类物质和挥发性有机酸.醇类物质在成熟过程中基本消失;醛类物质有增加趋势,而更多的是参与了酸类物质与酯类物质的相互转化;酮类物质大量增加,主要产生甲基酮;酸类物质含量减少或消失;酯类物质大量生成.     

国外优质干酪中乳酸菌的分离鉴定与应用

对进口干酪中乳酸菌进行初步分离鉴定。同时利用其中优良菌株进行风味干酪生产小试，结果较好。     

乳酸菌发酵剂浓缩培养的研究

为制备高效浓缩乳酸菌发酵剂,通过综合运用复合生长培养基、缓冲盐法及化学中和法,对乳酸菌的浓缩培养进行了研究。试验结果表明,以 12％ NFM＋ 0.3％酵母膏＋ 0.1％ Tween 80作为复合生长培养基,加 0.5％ K2HPO4作为缓冲盐,接种量为 2％ (嗜热链球菌∶保加利亚乳杆菌 =1∶ 1),培养温度 42℃,培养过程用 30％ Na2CO3溶液作中和剂,将 pH值控制在 6.3,培养 6～ 7 h后,可使乳酸菌的活菌数达到 5.89× 109 mL－ 1。与普通的液体发酵剂相比,获得了显著的浓缩效果。     

Accumulation of short n-chain ethyl esters by esterases of lactic acid bacteria under conditions simulating ripening Parmesan cheese

EstA from Lactobacillus helveticus CNRZ32 (Lbh-EstA), EstB, and EstC from Lactobacillus casei LILA, and EstA from Lactococcus lactis MG1363 (Lcl-EstA) were evaluated for their ability to accumulate esters in a model system simulating Parmesan cheese ripening conditions (10 degrees C, 2 to 3% NaCl, pH 5.4 to 5.5, aw = 0.850 to 0.925) using Capalase K from kid goat as a positive control. All of the LAB esterases and Capalase K mediated the accumulation of esters in the model system in an enzyme specific manner, which was influenced by a, and selectivity for fatty acid chain-length. In general, enzyme mediated accumulation of ethyl esters was higher at aw values of 0.850 and 0.900 than at aw of 0.925, demonstrating that aw is a critical parameter influencing ester accumulation. The substrate selectivity of esterases, aw, and enzyme type may be important factors in the development of fruity flavors, as evidenced by results in this model system simulating Parmesan cheese ripening conditions.