嗜热链球菌发酵改良纳豆工艺优化

采用嗜热链球菌为改良菌种,通过二次发酵改良纳豆。熟制灭菌后的大豆在纳豆菌接种量2%、发酵温度37℃、发酵时间24 h条件下进行一次发酵。以嗜热链球菌的发酵温度、发酵时间、接种量为因素,以感官评分、纳豆激酶酶活为指标,通过单因素和响应面实验,研究嗜热链球菌二次发酵改良纳豆的最佳工艺条件。结果表明:在发酵温度42℃、发酵时间18.6 h、接种量1.5%时改良效果最佳,得到的感官评分为8.0分,纳豆激酶酶活为1964 U/g。改良效果较好,纳豆臭味下降,纳豆激酶酶活提高。     

鼠李糖乳杆菌与嗜热链球菌协同发酵制备酸花生乳研究

本试验以嗜热链球菌(Streptococcus thermophilus,ST)和鼠李糖乳杆菌(Lactobacillus rhamnosus,LGG)协同发酵的酸花生乳为研究对象,以pH、酸度、粘度及活菌数为指标,确定最佳发酵用菌,并通过单因素实验与正交试验确定制备酸花生乳的最佳工艺条件。结果表明,鼠李糖乳杆菌在花生乳中的发酵性能优于保加利亚乳杆菌,并进一步研究鼠李糖乳杆菌与嗜热链球菌协同发酵花生乳的效果,得到最佳发酵工艺为:ST:LGG为2:1,接种量为5 mL/100 mL,发酵时间为4.5 h,蔗糖添加量为6 g/100 mL,此时,pH为4.36,酸度为76 °T,粘度为1532 mPa·s,活菌数为3.62×10¹¹ CFU/mL,感官得分为90,蛋白含量为4.78 g/100 g,可溶性固形物为9.5%,该酸花生乳的色泽为乳白色,香味浓郁,口感适中,组织状态均匀一致,质地细腻。     

具有良好风味嗜热链球菌的筛选及其产香特性分析

嗜热链球菌作为酸奶发酵剂的常用菌株之一,在牛乳发酵和贮藏过程中可以赋予产品优良的质地、丰富的营养价值和独特的风味。本实验在前期研究的基础上,以商业发酵剂为对照组,以分离自不同地区传统发酵乳中的具有良好发酵特性的7株嗜热链球菌为实验菌株,采用固相微萃取结合气相色谱-质谱联用技术对发酵终点,即pH4.5时发酵乳中的挥发性风味物质进行检测分析。结果表明,在所有实验菌株中,G80-2发酵乳中的挥发性风味化合物的组成及含量最接近对照组。在此基础上,对G80-2菌株在牛乳发酵和贮藏过程中所产生的挥发性风味物质进行动态分析,发现该菌株在发酵和贮藏期间检测出酸类（4种）、醇类（10种）、酮类（12种）、醛类（6种）、酯类（1种）等多种化合物,且一些主要特征风味物质如乙酸、乙醛、双乙酰、乙偶姻、2-庚酮、1-庚醇等相对含量较高,说明该菌株可作为发酵剂应用于乳制品生产中。     

保加利亚乳杆菌和嗜热链球菌单菌发酵与复配发酵对酸奶品质的影响

为研究保加利亚乳杆菌与嗜热链球菌单菌发酵与复配发酵对酸奶品质的影响,本实验采用保加利亚乳杆菌与嗜热链球菌单菌发酵和复配发酵两种方式,通过对比两种发酵方式制得酸奶发酵特性、流变特性、质构特性和感官特性等指标,系统全面揭示两种发酵方式对酸奶品质的影响。结果表明,复配发酵的酸奶凝乳时间为8 h,比单菌发酵酸奶的发酵时间更短,更先达到发酵终点70 °T。感官评价表明,复配发酵方式制得的酸奶感官评分优于单菌发酵制得的酸奶,且保加利亚乳杆菌与嗜热链球菌的最佳复配比为3:7。流变特性测定结果表明,复配发酵酸奶的表观黏度为0.4~0.5 Pa·s低于单菌发酵酸奶的1.6~1.9 Pa·s。质构数据显示,复配发酵酸奶的硬度、黏度和黏聚性绝对值均显著低于单菌发酵酸奶（P<0.05）。将单菌发酵酸奶和最佳配比复配发酵酸奶进行电子鼻和电子舌检测,结果表明复配发酵酸奶挥发性风味在电子鼻传感器上的气味响应值强于单菌发酵酸奶。在滋味方面,与单菌发酵酸奶相比,复配发酵酸奶在电子舌各个探头的响应值表现更为均衡。通过主成分分析发现,电子鼻和电子舌均能有效区分两种发酵方式之间的差异。综上,与单菌发酵方式相比,复配发酵方式能够有效缩短酸奶发酵时间,且发酵出的酸奶品质更好。     

不同益生菌与嗜热链球菌共培养发酵乳的理化、质构及感官特性研究

为探究不同发酵剂及不同发酵底物对发酵乳品质的影响,本文将动物双歧杆菌（Bifidobacterium animalis subsp. lactis,Ba）与传统酸奶发酵剂（SL）共培养,测定了发酵牛乳、牦牛乳及牛乳与牦牛乳混合乳（1:1）的酸化能力、活菌数、持水力、蛋白水解活力、流变特性、物理稳定性、微观结构、挥发性风味物质及感官特性。结果表明,对于相同的发酵底物,Ba联合SL发酵,可显著提高发酵乳的酸化能力和活菌数（P<0.05）,并使第1 d的发酵牛乳、牦牛乳、混合乳的蛋白水解活力比单一SL发酵分别提升了0.026、0.025、0.016,使发酵乳的凝胶网络结构更加致密,改善发酵乳的挥发性风味物质和感官品质,但联合发酵剂对发酵乳的物理稳定性无显著影响（P>0.05）;SL和Ba共发酵能显著增加发酵牛乳和发酵混合乳的持水力（P<0.05）,使发酵混合乳在整个冷藏期间持水力高于其余各组,其中第7 d最高,为45.68%;此外,联合发酵提高了发酵牦牛乳的粘弹性,但降低了发酵牛乳和发酵混合乳的粘弹性。对于相同的发酵剂,发酵牛乳的酸化能力、活菌数、物理稳定性、感官评分显著高于其余两种发酵乳（P<0.05）;发酵牦牛乳的粘弹性最强;发酵混合乳持水力在整个冷藏期间都显著高于其余两种发酵乳（P<0.05）,凝胶网络结构最致密,关键风味物质种类较多,蛋白水解活力介于其余两种发酵乳之间。本研究表明,Ba联合SL共培养可以提高发酵乳的品质;相比于牦牛乳,牛乳和混合乳更适合作为发酵底物。     

Streptococcus thermophilus ND03在牛乳发酵过程及贮藏期间挥发性风味物质分析

为揭示Streptococcus thermophilus ND03在牛乳发酵过程和4℃贮藏期间产生的挥发性风味物质及其变化情况,采用3种不同纤维涂层的萃取头,运用固相微萃取-气相色谱-质谱联用(SPME-GC-MS)技术对各阶段发酵牛乳样品进行检测分析,利用主成分分析法分析各阶段样品的特征性风味物质并对各阶段样品进行感官评定。结果表明:利用50/30μm DVB/CAR/PDMS、65μm PDMS/DVB和100μm PDMS 3种萃取头分别检测出86、66和62种挥发性物质。在发酵及贮藏期间,挥发性风味物质的数量与含量存在动态变化。由主成分分析结果可知,样品的特征性风味物质在发酵期间为醛类、酮类及杂环化合物,在贮藏0 d(发酵终点)为醇类及含氮化合物,在贮藏1、3 d为酯类及芳香族化合物,在贮藏7、14 d为烃类及酸类化合物。由感官评定结果可知,贮藏1 d的样品在色泽、滋味、气味及组织状态方面具有良好表现。菌株Streptococcus thermophilus ND03作为发酵剂具有潜在的工业应用前景。     

Inhibition of Clostridium tyrobutyricum by Streptococcus macedonicus ACA-DC 198 under conditions mimicking Kasseri cheese production and ripening

Three fermentations in skim milk were used to study the effectiveness of the bacteriocin-producing Streptococcus macedonicus ACA-DC 198 strain to inhibit Clostridium tyrobutyricum LMG 1285T spore outgrowth under conditions prevailing during Kasseri cheese production and ripening. In fermentation A, Clostridium spores were used solely; in fermentation B, S. macedonicus ACA-DC 198 and Clostridium spores were used; in fermentation C, a commercial starter culture and Clostridium spores were used. The temperature program applied was similar to that of Kasseri cheese production and ripening. The presence of macedocin, the bacteriocin produced by S. macedonicus ACA-DC 198, was confirmed in fermentation B. The results showed that macedocin was able to inhibit the outgrowth of Clostridium spores, since significantly higher inhibition in spore outgrowth was detected in fermentation B than in fermentation C.     

A new phage on the ‘Mozzarella’ block: Bacteriophage 5093 shares a low level of homology with other Streptococcus thermophilus phages

Streptococcus thermophilus bacteriophage 5093 is a virulent phage that infects the industrial Mozzarella starter CSK939. The genome of phage 5093 is 37,184 base pairs (bps) containing 50 open reading frames (orfs). Genetic analysis revealed that the genome of phage 5093 is highly mosaic when compared with other sequenced S. thermophilus phages. This is particularly apparent in the late gene cluster with regions displaying high homology to prophage sequences of non-dairy streptococci and limited homology to either pac or cos-type S. thermophilus phages. In addition, a definitive antireceptor gene was not observed – suggesting that phage 5093 may have developed a different system for host recognition. Interestingly, the phage does contain a methylase domain that probably evolved as a phage counter-defence mechanism. These findings suggest that phage 5093 may represent a third group of S. thermophilus phage and provide the link between phages that infect S. thermophilus and its non-dairy ancestors.     

Determination of the effect of dairy powders on adherence of Streptococcus sobrinus and Streptococcus salivarius to hydroxylapatite and growth of these bacteria

Dental caries is a highly prevalent disease caused by colonisation of tooth surfaces by cariogenic bacteria, such as Streptococcus sobrinus and Streptococcus salivarius. Reducing initial adherence of such bacteria to teeth may delay onset of caries. Many foods, such as milk, can inhibit microbial adherence. In this investigation, the effect of untreated (UT) and enzyme-treated (ET) dairy powders on adherence of S. sobrinus and S. salivarius to hydroxylapatite (HA), an analogue of tooth enamel, was examined. Untreated (UT) acid whey protein concentrate (AWPC) 80 inhibited streptococcal adherence to phosphate-buffered saline-coated HA (PBS-HA) and saliva-coated HA (S-HA) by >80% at ?31.25 μg mL⁻¹. UT sweet WPC80, buttermilk powder and cream powder also significantly reduced adherence (P < 0.05). Enzyme-treatment of all dairy powders reduced their anti-adhesion activity. However, ET sweet WPC80 significantly inhibited growth of these streptococci (P < 0.05) at ?0.6 mg mL⁻¹. Therefore, dairy powders may reduce progression of dental caries by their anti-adhesion and/or antibacterial activity.     

Synbiotic potential of fresh cream cheese supplemented with inulin and Lactobacillus paracasei in co-culture with Streptococcus thermophilus

The influence of the addition of Lactobacillus paracasei and Streptococcus thermophilus on the fructan content at the beginning and at the end of storage at 4 ± 1 °C of a potentially synbiotic fresh cream cheese manufactured with inulin was investigated. Three cheese-making trials were prepared, all supplemented with a lactic culture of S. thermophilus (T1, T2 and T3). L. paracasei subsp. paracasei was added in T1 and T2. Inulin was added in T2 and the fructan content was measured after 1 and 21 days of storage. Samples of T2 possessed similar mean concentrations of fructans after 1 and 21 days of storage, 7.32% and 7.27%, respectively, and no significant difference was observed. These results indicated that the metabolism of starter and probiotic bacteria did not degrade the fructans present in those cheeses. Additionally, synbiotic cheeses possessed a fructan content higher than 7 g per 100 g, sufficient to confer prebiotic potential during the entire storage period of these products.     

不同条件下Camembert奶酪中无毒害李斯特菌空间生长分布

研究了在Camembert奶酪成熟过程中无毒害李斯特菌存活和生长的能力。用含有5lgCFU/mL无毒害李斯特菌的巴氏灭菌全脂牛奶精心制备Camembert奶酪。所有Camembert奶酪在以下三种情况下成熟:室温(约20℃)和相对湿度60%(36h);12℃、相对湿度93%(2周);7℃、相对湿度85%(3周)。在成熟过程中,分别在1、5、10、15、20、25、30、35d时,利用选择性培养基对奶酪表面和内部的无毒害李斯特菌进行计数。结果显示,在成熟第1d时,无毒害李斯特菌数量为7.16lgCFU/g,比初始值的4.76lgCFU/g增加了2.40lgCFU/g;而在第20d时,无毒害李斯特菌数量降低至6.54lgCFU/g;在第35d时,李斯特菌数量增加至7.38lgCFU/g。总体而言,无毒害李斯特菌在奶酪表面的生长快于中心位置。      

Response surface model for prediction of growth parameters from spores of Clostridium sporogenes under different experimental conditions

Clostridium sporogenes is considered to be a non-toxingenic equivalent of proteolytic Clostridium botulinum, and it also causes food spoilage. The effects of temperature (16.6-33.4 degrees C), pH value (5.2-6.8) and concentration of sodium chloride (0.6-7.4%) on the growth parameters of C. sporogenes spores were investigated. The growth curves generated within different conditions were fitted using Baranyi function. Two growth parameters (growth rate, GR; lag-time, LT) of the growth curves under combined effects of temperature, pH and sodium chloride were modeled using a quadratic polynomial equation of response surface (RS) model. Mathematical evaluation demonstrated that the standard error of prediction (%SEP) obtained by RS model was 1.033% for GR and was 0.166% for LT for model establishing. The %SEP for model validation were 43.717% and 5.895% for GR and LT, respectively. The root-mean-squares error (RMSE) was in acceptable range which was less than 0.1 for GR and was less than 8.0 for LT. Both the bias factor (B(f)) and accuracy factor (A(f)) approached 1.0, which were within acceptable range. Therefore, RS model provides a useful and accurate method for predicting the growth parameters of C. sporogenes spores, and could be applied to ensure food safety with respect to proteolytic C. botulinum control.     

无乳链球菌产CMP-唾液酸合成酶发酵培养基优化

为获得低成本高酶活的产CMP-唾液酸合成酶(Cytidine 5'-monophosphate N-acetylneuraminic acid synthetase,NmCSS)培养基,对无乳链球菌(Streptococcus agalactiae)产CMP-唾液酸合成酶发酵生产培养基进行优化。通过单因素试验筛选发酵温度、碳源、氮源、pH,确定对NmCSS酶活具有明显影响的因子及水平值;采用响应面法(Box-Behnken)确定以氮源浓度、碳源浓度、pH为三个主要影响因子的最适条件。结果表明,当培养温度为34℃时,蔗糖浓度为0.10%,氮源浓度为2.50%,pH为7.5、Na₂HPO₄ 2.5 g/L、NaCl 5.0 g/L时,NmCSS酶活为(5.895±0.005)×10⁷ U/mol,较基础发酵培养基的酶活(0.507±0.002)×10⁷ U/mol提高了10.627倍,显示出了良好的优化效果。响应面方法优化得到的低成本高酶活培养基为无乳链球菌产NmCSS及其应用奠定了基础。     

Survival and growth of Salmonella and Listeria in the chicken breast patties subjected to time and temperature abuse under varying conditions

Chicken breast patties were inoculated with a mixture of Salmonella Senftenberg, Salmonella Typhimurium, Salmonella Heidelberg, Salmonella Mission, Salmonella Montevideo, Salmonella California, and Listeria innocua. The initial inoculation of bacteria was approximately 10(7) log10 CFU/g. The inoculated patties were processed in a pilot-scale air convection oven at an air temperature of 177 degrees C, an air velocity of 9.9 m3/min, and a low (a wet bulb temperature of 48 degrees C) or high (a wet bulb temperature of 93 degrees C) humidity condition. The patties were processed to a final center temperature of 65 to 75 degrees C. The survivors of Salmonella and Listeria in the processed patties were evaluated. Processing humidity affected the survivors of bacteria. More survivors of Salmonella and Listeria (>2 logs) were obtained for the patties cooked at low humidity than at high humidity. After thermal processing, the patties were stored under air, vacuum, or CO2 at refrigerated (4 degrees C) or thermally abused (8 to 15 degrees C) temperatures. Storage temperature, time, and gas environment affected the bacteria growth. Higher storage temperature and longer storage time correlated to an increased growth of bacteria in the cooked chicken patties. Less Salmonella (2 logs) and Listeria (0.5 to 1 log) cells were obtained in the patties stored under vacuum than in air. Storing the patties in 30% CO2 reduced the growth of Salmonella more than 2 log10 CFU/g. At a CO2 level of 15%, 1 log10 CFU/g of reduction was obtained for Listeria in cooked chicken patties.     

The arginine deiminase pathway of Lactobacillus fermentum IMDO 130101 responds to growth under stress conditions of both temperature and salt

The arginine deiminase (ADI) pathway is a means by which certain sourdough lactic acid bacteria (LAB) convert arginine into ornithine via citrulline while producing ammonia and ATP, thereby coping with acid stress and gaining an energetic advantage. Lactobacillus fermentum IMDO 130101, an isolate from a spontaneous laboratory rye sourdough, possesses an ADI pathway which is modulated by environmental pH. In the present study, a broader view of the activity of the ADI pathway in response to growth under two other commonly encountered stress factors, temperature and added salt, was obtained. In both cases, an increase in ornithine production was observed as a response to growth under both temperature and salt stress conditions. Biokinetic parameters were obtained to describe the kinetics of the ADI pathway as a function of temperature and added salt. The arginine conversion rate increased as a function of added NaCl concentrations but was hardly affected by temperature. In addition, arginine-into-citrulline conversion rate was not affected by temperature but increased with increasing NaCl concentrations. Citrulline-into-ornithine conversion rate increased with increasing temperature, while it dropped to zero with added salt. These findings suggest a more pronounced adaptation of the strain through the ADI pathway to added salt, as compared with different constant temperatures. Furthermore, these results suggest that the ADI pathway in L. fermentum IMDO 130101 is active in adapting to non-optimal growth conditions.     

Use of an exopolysaccharide-producing strain of Streptococcus thermophilus in the manufacture of Mexican Panela cheese

An exopolysaccharide (EPS) producing strain of Streptococcus thermophilus was evaluated for the production of Panela cheese using two total solids milk (TSM) concentrations (12.5 and 17.5 g/100 mL). This ropy strain increased cheese yield; nevertheless, with 12.5 TSM the increment was higher than with 17.5 TSM. Analysis of cheese composition showed that with 12.5 TSM, the ropy strain increased moisture, but did not change the fat or non fat solids on dry weight basis (dwb), suggesting that the increment of the yield is only due to water retention. In 17.5 TSM cheeses the ropy strain caused an increase in the moisture and fat (dwb), suggesting that besides water retention, fat also contributed to the yield. The difference in yield increment could be explained by cheese composition: higher fat content creates a more hydrophobic environment, which would expel more water than the cheese with lower fat content. Electron microscopy showed EPS attached to the protein matrix of the cheeses. In 17.5 TSM cheeses EPS was observed around the milk fat globules (MFG), confirming that higher TSM causes EPS to bind the MFG besides binding the protein matrix, retaining fat within the cheese. Sensory evaluation demonstrated that ropy cheeses were softer and creamier.     

Formation of methional by Lactococcus lactis IFPL730 under cheese model conditions

The present work describes the capacity of Lactococcus lactis to produce methional and other sulphur compounds derived from methionine (Met) in a cheese model system. Cheese slurries were prepared from pasteurized ewes' skimmed milk, chemically acidified with glucono-'-lactone and homogenized aseptically adding Met, !-ketoglutarate, pyridoxal 5'-phosphate, thiamine pyrophosphate and NaCl. Slurries were incubated at 12 °C for 14 days with cellular suspensions of L. lactis IFPL359, L. lactis IFPL730 and L. lactis NCDO763 in different combinations. Slurries added with resting cells and the intracellular fraction from L. lactis IFPL730 showed the highest production of methional at the outset of incubation, which decreased during incubation along with a concomitant increase in 3-methylthiopropanol. The sensorial analysis of slurries indicated a characteristic methional aroma (cooked potato-like) in samples containing 4-methylthio-2-ketobutyrate and the intracellular fraction from L. lactis IFPL730. As incubation proceeded, the intensity of methional aroma decreased but samples were judged by the panel tasters as developing a cheese-like flavour.     

Growth characteristics of Listeria monocytogenes as affected by a native microflora in cooked ham under refrigerated and temperature abuse conditions

This study examined the growth characteristics of Listeria monocytogenes as affected by a native microflora in cooked ham at refrigerated and abuse temperatures. A five-strain mixture of L. monocytogenes and a native microflora, consisting of Brochothrix spp., isolated from cooked meat were inoculated alone (monocultured) or co-inoculated (co-cultured) onto cooked ham slices. The growth characteristics, lag phase duration (LPD, h), growth rate (GR, log₁₀ cfu/h), and maximum population density (MPD, log₁₀ cfu/g), of L. monocytogenes and the native microflora in vacuum-packed ham slices stored at 4, 6, 8, 10, and 12 °C for up to 5 weeks were determined. At 4-12 °C, the LPDs of co-cultured L. monocytogenes were not significantly different from those of monocultured L. monocytogenes in ham, indicating the LPDs of L. monocytogenes at 4-12 °C were not influenced by the presence of the native microflora. At 4-8 °C, the GRs of co-cultured L. monocytogenes (0.0114-0.0130 log₁₀ cfu/h) were statistically but marginally lower than those of monocultured L. monocytogenes (0.0132-0.0145 log₁₀ cfu/h), indicating the GRs of L. monocytogenes at 4-8 °C were reduced by the presence of the native microflora. The GRs of L. monocytogenes were reduced by 8-7% with the presence of the native microflora at 4-8 °C, whereas there was less influence of the native microflora on the GRs of L. monocytogenes at 10 and 12 °C. The MPDs of L. monocytogenes at 4-8 °C were also reduced by the presence of the native microflora. Data from this study provide additional information regarding the growth suppression of L. monocytogenes by the native microflora for assessing the survival and growth of L. monocytogenes in ready-to-eat meat products.     

Inhibition of Clostridium tyrobutyricum in cheese by Lactobacillus gasseri

A semi-hard cheese produced from milk artificially contaminated with Clostridium tyrobutyricum spores (2.5×10³ mL⁻¹) was used as a model for studying the ability of bacteriocin-producing Lactobacillus gasseri K7 (Rif^r) to inhibit clostridia. The added lactobacilli did not inhibit the primary starter culture (Streptococcus thermophilus), but inhibited non-starter mesophilic lactobacilli. Late blowing as a result of Cl. tyrobutyricum outgrowth and butyric acid fermentation occurred in all cheeses however it was reduced in cheeses with added Lb. gasseri. After 6 weeks, the average amount of butyric acid was significantly higher in cheeses without added lactobacilli (1.43 vs. 0.70 g kg⁻¹). At the end of 8-weeks ripening, 2.8×10⁷ cfu g⁻¹ of K7 (Rif^r) viable cells were detected. Using the total DNA from cheeses with added K7 (Rif^r) strain, PCR products were amplified with primers specific for Lactobacillus, Lb. gasseri and K7 bacteriocin gene.     

Development of a predictive program for microbial growth under various temperature conditions

A predictive program for microbial growth under various temperature conditions was developed with a mathematical model. The model was a new logistic model recently developed by us. The program predicts Escherichia coli growth in broth, Staphylococcus aureus growth and its enterotoxin production in milk, and Vibrio parahaemolyticus growth in broth at various temperature patterns. The program, which was built with Microsoft Excel (Visual Basic Application), is user-friendly; users can easily input the temperature history of a test food and obtain the prediction instantly on the computer screen. The predicted growth and toxin production can be important indices to determine whether a food is microbiologically safe or not. This program should be a useful tool to confirm the microbial safety of commercial foods.