Use of fluorometry for determination of skim milk powder adulteration in fresh milk

A FAST (fluorescence of advanced Maillard products and Soluble Tryptophan) method for identification of recon- stituted milk made from skim milk powder in the fresh milk was developed. Considering milk and skim milk powders variations from different seasons and countries, milk was collected from different dairy farms in different seasons and skim milk powders were collected from different countries to measure the Tryptophan (Trp), advanced Maillard products (AMP) fluorescence values. The results showed that there were differences (P<0.01) between raw and reconstituted milk. The plot of values in each mixed level of raw and reconstituted milk had a correlation coefficient >0.97. The FAST method is a simple, rapid, low-cost and sensitive method enabling the detection of 5% reconstituted milk in fresh milk. The measurement of the Trp, AMP fluorescence values and calculation of the FAST index is a suitable method for large-scale monitoring of fresh milk samples.

A Study on the Tryptic Hydrolysis of Proteins in Skim Milk and the Characteristics of the Hydrolysate

Limited enzymatic hydrolysis of skim milk by trypsin was investigated in this paper, and it was found that the degree of hydrolysis (DH) of milk proteins had a relationship of DH -1 =a+bt -1 with the time(t) of hydrolysis at low enzyme concentrations, some properties of the protein such as NSI in PI or TCA were changed obviously. It was suggested that most milk proteins would be degraded into peptides by trypsin .

Application of Renneted Skim Milk as a Fat Mimetics in Nonfat Yoghurt

For a long time, fermented dairy products have been consumed for human’s nutrition and health. Recently, consumption of whole dairy products, such as full fat yoghurt, has declined due to the awareness of probable harmful effects of fat on consumers’ health, resulting change of market interest in favor of low or non fat dairy products. It is a challenge for food scientists to produce a suitable fat substitute providing the functionality of the missing fat. The present investigation was carried out to examine the effect of 0 to 50% renneted skim milk (RSM) as a fat mimetic upon the physico-chemical, physical, rheological and sensory characteristics of nonfat yoghurt in comparison to control samples (full fat and skim yoghurt). By increasing RSM content, the chemical characteristics (titratable acidity, pH as well as the content of fat and total protein) of yoghurts did not alter except decrease of total solid in sample with 50% RSM. The yoghurts with more content of RSM exhibited higher b*and a* values, while the highest L* value, viscosity, water holding capacity (WHC), firmness and lowest syneresis were found in sample containing 30% RSM. It seems that new interaction in gel network of yoghurts containing RSM, exhibited greater ability to bind water than control skim yoghurt. In addition, probably new bridges between milk proteins (interacting partially hydrolyzed casein with casein) can increase the number of bounds between protein particles leading to lower syneresis. However, excessive RSM content (higher than 30%) did not increase the L* value, viscosity and WHC of samples probably because of too much aggregation of casein. Sensory results revealed that sample containing 30% RSM was exactly alike to full fat yoghurt in terms of overall sensory attributes. New formulation of palatable skim yoghurt was developed using 3% RSM successfully.

Biocontamination Risk of <i>Staphylococcus aureus</i>in Dairy Manufacturing Presence of Two Types of UHT Milk

The biofilm formation on equipment surfaces in dairy manufacturing is a major concern for industry and consumers alike, which may affect the safety and quality of dairy products. In order to identify the bio-contamination risk of materials commonly used in dairy manufacturing, adhesion of Staphylococcus aureus on glass coated by two kinds of UHT milk (whole milk and skimmed milk) was investigated. It is known that adhesion is mainly governed by surface physicochemical properties, for that, the effect of milk components on physicochemical properties of glass and bacterial surfaces were exanimated through contact angle measurements. MATLAB software was used to evaluate the ability of S. aureus adhesion on glass. The hydrophobic quantitative and electron acceptor characteristics of the glass appear to increase with the presence of fat in milk, while its electron donor property decreases with this component. The percentage of occupied surface of untreated glass was more important than in treated surfaces for 50% of the strains studied. As well, the percentage of occupied surface by bacterial strains in untreated glass by skimmed milk is generally more important compared to the whole milk. Therefore, the risk of bio-contamination of untreated glass is more favorable for bacteria cultured in skimmed milk compared to these in whole milk but the bio-contamination risk on covered glass by milk is not milk dependent and is strain dependent.

基于快速气相色谱电子鼻研究超高压射流处理脱脂乳的感官品质

以新鲜脱脂乳为原料,探究超高压射流(ultra-high pressure jet,UHPJ)处理对脱脂乳感官品质的影响。脱脂乳经100~300 MPa UHPJ处理后,采用电子眼、电子鼻、感官评价分别检测脱脂乳颜色、气味及综合风味变化。结果表明:脱脂乳的主要色号以4095白色为主,占比46.80%~53.83%;电子鼻共检测到酯类、酮类、醛类等42种挥发性风味成分,大部分风味成分呈乳香、果香、烘烤香、青草等气味,主成分分析结果显示,300 MPa处理脱脂乳样品和其他样品风味相差较大;感官评价结果显示,UHPJ处理脱脂乳的乳香和甜味较为突出,不同样品之间在甜味、蛋白胨味、铁锈味、烧焦味和寡淡味方面差异明显,250、300 MPa处理样品生成的二甲基硫等硫化物能够显著增强脱脂乳的烧焦味、蛋白胨味。综上,200 MPa UHPJ处理能较完整地保留脱脂乳中的风味成分,进而实现原料乳风味的保持。

碱性及复合蛋白酶复配水解对脱脂牛乳致敏性的影响

牛乳中致敏蛋白在一定程度上限制了其在婴幼儿配方奶粉中的使用。以脱脂牛乳为原料,以水解度、脱脂牛乳IgG抑制率、过敏患者血清IgE抑制率为指标,研究碱性、复合蛋白酶复配处理对其致敏性的影响。结果表明,复配酶的酶活与底物比(E/S)为6000 U/g,碱性蛋白酶与复合蛋白酶质量比为3∶1,酶解时间60 min时,脱脂牛乳的水解度为9.21%,对脱脂牛乳IgG及过敏患者血清IgE抗体抑制率分别为86.94%与43.46%,显著高于单一蛋白酶(P<0.05)的处理效果。研究结果为以脱脂牛乳为婴幼儿奶粉基料的开发提供了重要参考。

4℃诱导解离对山羊乳酪蛋白胶束结构的影响

山羊脱脂乳于25℃超离心,分为乳清和胶束,后者复溶于脱脂乳超滤液,于4℃超离心,分为乳清和胶束,探究4℃诱导解离对胶束结构的影响。4℃下超离心和超滤液复溶2次后,胶束态酪蛋白的解离达平衡,总酪蛋白的解离达36.5%,其中β-、κ-、αs-酪蛋白的解离达60.4%、31.4%、11.8%;25℃下游离β-酪蛋白以2P、3P和4P的低磷酸化形式存在,4℃下胶束钙的解离导致5P和6P的高磷酸化β-酪蛋白的解离;4℃诱导解离后,胶束钙相对含量从72.0%减少至50.2%,胶束中钙与酪蛋白物质的量之比从21.8减少至19.4,胶束仍保持完整的球状形貌,胶束均方回转半径R_(g)、流体力学半径R h、重均分子质量均减小,胶束形状因子R_(g)/R h、水合率和内源荧光强度均增加,胶束内部结构中酪蛋白空间分布的非均一性降低,表明低温诱导解离后胶束内部组分发生了重排,骨架结构变得更加松散。该研究为羊乳酪蛋白配料的分离提供参考。

脱脂奶粉-水混合体系热致粘度变化特性的测定研究

本文应用快速粘度测试仪建立了测定脱脂奶粉-水混合体系在升温-保持温度-降温三阶段测试程序中粘度随温度变化的方法,通过曲线中是否存在峰及峰值粘度的大小,以及运行结束时最终粘度的大小,评价奶粉在生产加工时经历的热处理变性程度。本文研究了脱脂奶粉-水混合体系中奶粉比例、温度程序中保持温度的高低及保持时间的长短对测定结果的影响。实验结果表明:实验条件明显影响粘度随温度的变化曲线。为使奶粉中蛋白质在水中充分凝胶,同时保证凝胶强度及粘度落在一定范围,最适宜实验条件分别为:奶粉比例为44.5%(m/m),保持温度为90℃、保持时间为5min。在此最优条件下,选择了在粘度曲线中分别出现两个峰与一个峰的两批脱脂奶粉,在同一天内每隔一小时以及连续七天每隔一天分别进行奶粉-水混合体系的热致凝胶特性测定,实验结果具有较好的重复性,各个峰及最终粘度的相对标准偏差均小于5%。

脱脂乳饼加工工艺研究

乳饼作为云南地区具有民族特色的乳制品,深受彝族、白族人民的喜爱。采用脱脂牛奶为原料制作乳饼,以感官评价为指标,开展凝乳酶添加量、凝乳温度以及脱脂奶粉的添加量单因素实验以及Box-Behnken响应面优化实验,确定脱脂乳饼的最佳工艺配方,并进行产品感官、理化指标以及菌落总数的测定。结果表明:脱脂乳饼的最佳工艺参数为:凝乳酶添加量2%、凝乳温度40℃、脱脂奶粉添加量14%、白砂糖添加量6%。在此工艺参数条件下,脱脂乳饼产品具有一定的奶香味,高蛋白、低脂肪,同时含有丰富的钙、VD等营养物质。本研究对云南民族特色乳制品的开发具有一定意义。

黄瓜霜霉病菌保存方法

Pseudoperonospora cubensis(Berk.& M.A.Curtis)Rostovzev,the causal agent of cucumber downy mildew,is an obligate parasitic fungus.Up to now it can not be preserved on culture medium.In this study,the sporangia of P.cubensis were preserved in protective substances of 10% dimethyl sulfoxide plus 5% skim milk,10% dimethyl sulfoxide plus 10% skim milk,sterilized water and leaves in vitro,and stored at-20℃,-70℃,and preliminary freezing at-20℃ for 24 h prior to-70℃ respectively.The results showed that the sporangia preserved in 10% dimethyl sulfoxide plus 5% skim milk,preliminary freezing at-20℃ for 24 h prior to-70℃ were still highly pathogenic after 12 months preservation.The percentage of germination of sporangia,incidence and disease index were 46%,50.0% and 40.0 respectively 9 d after inoculation.This method solved the problem of P.cubensis preservation.

乳酸菌基础培养基比较研究

研究了脱脂乳中性蛋白酶的最适酶解条件,并以保加利亚乳杆菌和嗜热链球菌为混合发酵菌种,对MRS、脱脂乳和脱脂乳酶解液3种基础培养基进行了比较研究。结果表明:(1)脱脂乳中性蛋白酶的适宜酶解条件为:加酶量5000U/g蛋白质,在pH=7.0,50℃下酶解2h;(2)脱脂乳酶解液是保加利亚乳杆菌和嗜热链球菌混合菌种较好的基础培养基,其冻干发酵剂的质量和数量均优于脱脂乳培养基。以脱脂乳酶解液为基础培养基制备的冻干发酵剂的活菌数达到3.5×1011cfu/g,冻干发酵剂的重量是脱脂乳的1.93倍。

植物乳杆菌对提高发酵牛乳中共轭亚油酸含量的研究

利用植物乳杆菌(Lactobacillusplantarum)LT2-6发酵添加亚油酸(LA)的脱脂牛乳,转化LA生成共轭亚油酸(CLA),以期提高发酵牛乳中的CLA含量。结果表明:LA诱导可提高发酵牛乳中的CLA产量,发酵温度37℃,CLA生成量和LA转化率为最高。加入葡萄糖、酪蛋白酶水解物,有助于植物乳杆菌LT2-6的生长和提高CLA的生成量。添加0.075%的LA,发酵24h,43%的LA转化为CLA,发酵后牛乳中的CLA含量达到320μg/mL,为普通发酵牛乳的5～6倍。经脂肪酸萃取和甲酯化,气相色谱检测显示生成的CLA主要为:cis-9,trans-11-CLA。

脱脂奶粉分散稳定性的实验研究

粉体的分散稳定性对粉体的加工、贮存及品质有重要影响。以脱脂奶粉及其添加剂(黄原胶和β-环糊精)为模型材料,研究了非晶态粉体分散稳定性的影响因素及评价方法。应用流变仪测定了脱脂奶粉及其与添加剂混合物的结块性能,考察了贮存时间和组成对结块性能的影响。结果表明:脱脂奶粉的结块倾向随时间的延长而增加;添加黄原胶可明显改善脱脂奶粉的分散稳定性,但添加β-环糊精的效果不明显,而黄原胶和β-环糊精复配型添加剂对提高脱脂奶粉的分散稳定性效果最好。

牦牛脱脂乳中酪蛋白受热处理温度及时间影响的研究

研究了随着加热温度(30～90℃)的上升及时间的延长(5～25min),牦牛脱脂乳的热稳定性、乳化性、浊度的变化,以及脱脂乳中存在的大量酪蛋白表面疏水性及平均粒径的改变。结果表明,加热温度的上升及加热时间的延长,导致牦牛脱脂乳的热稳定性下降、乳化性降低、浊度增大,酪蛋白表面疏水性及平均粒径总体呈增大的趋势,温度在70℃以上、加热时间大于15min时尤为明显。这一研究结果为牦牛乳酪蛋白的生产加工提供了一定的理论基础。

发酵温度对冻干乳酸菌发酵剂活菌数的影响

以脱脂乳为基础培养基，选用保加利亚乳杆菌和嗜热链球菌混合菌种在30℃、34℃、37℃、42℃发酵，用灭菌30%Na2CO3控制发酵液pH值6．0～6．3，制成冻干发酵剂，研究发酵温度对冻干酸奶发酵剂活菌数的影响。结果表明：不同发酵温度下制得的冻干发酵剂活菌数有显著差异，尤其是在30℃发酵制得的冻干发酵剂其活菌数显著高于其它发酵温度，其冻干发酵剂活菌数高达4．2×10^11cfu／g。

嗜水气单胞菌不同分离株生化特性及胞外蛋白酶的检测

嗜水气单胞菌可引起多种宿主患病,该菌致病性与其产生的多种毒力因子特别是胞外蛋白酶有关。本研究检测了26株嗜水气单胞菌不同分离株的生化指标,采用脱脂乳平板法检测了胞外蛋白酶的产生情况,并利用PCR技术检测了2种胞外蛋白酶基因ahp-A(编码丝氨酸胞外蛋白酶)和eprCAI(编码温敏胞外蛋白酶)。结果表明,大部分菌株生化指标表现为蔗糖(+)、阿拉伯糖(+)、葡萄糖(+)、甘露醇(+)、鸟氨酸脱羧酶(-)、肌醇(-)、七叶苷(+);脱脂乳平板法检测发现,91.7%(22/24)的致病菌株均能产生胞外蛋白酶,而2株无毒株则不能产生,对2个胞外蛋白酶基因的检测也得到相似的结果。本研究将为嗜水气单胞菌的流行病学调查和致病机制的深入研究奠定基础。

不同热分级脱脂奶粉中乳清蛋白热变性聚合物分析

为了解不同受热程度脱脂乳粉中乳蛋白各组分以及各组分之间的变化规律,利用还原型和非还原型聚丙烯酰胺凝胶电泳(SDS-PAGE)对乳清蛋白氮指数(WPNI)分别为7.08、4.24、1.45的不同受热程度脱脂乳粉进行蛋白组分分析。结果表明,不同受热程度乳粉形成的大分子聚合物组成和含量差异显著,低温加热脱脂乳粉中乳清蛋白变性程度最低,形成的大分子聚合物较少而高温加热脱脂乳粉中乳清蛋白变性程度较大,形成的大分子聚合物较多。利用还原型SDS-PAGE对不同受热程度脱脂乳粉的电泳胶片中大分子聚合物进行分析可知,不同受热程度脱脂乳粉形成的聚合物组成大致相同,但含量差异显著。β-Lg和κ-酪蛋白参与了几乎所有的聚合反应。形成的大分子聚合物分子量≥200 ku。

感官评价结合电子鼻分析脱脂风味乳饮料

通过感官评价的可接受性结果对脱脂风味乳饮料的工艺进行优化,关键优化参数包括水与脱脂乳的体积比、白砂糖与食用盐的质量比、柠檬酸钠添加量、稳定剂添加量和小苏打添加量。同时结合电子鼻综合评价乳饮料的风味,并根据电子鼻传感器对不同样品的风味响应情况进行PCA分析。结果表明,传感器对不同参数调整后的样品的区分效果明显,并结合感官评价确定纯净水与脱脂乳的体积比为25∶1,白砂糖与食用盐的质量比为100∶1,柠檬酸钠的添加量为0.89 g/L,稳定剂的添加量为0.44 g/L,小苏打的添加量为0.18 g/L(均为质量浓度,下同)为最佳优化参数。

嗜热链球菌和乳杆菌最佳促生长剂的选择

本文研究了七种不同添加剂对嗜酸乳杆菌、嗜热链球菌和保加利亚乳杆菌产酸及凝乳的影响。结果表明：７％番茄汁可作为这三种菌的促生长剂；嗜酸乳杆菌的最佳促生长剂为７％番茄汁或１０％胡萝卜汁；７％五星啤酒是嗜热链球菌的最佳促生长剂，１０％的信阳毛尖茶次之；７％番茄汁是保加利亚乳杆菌的最佳促生长剂。

葡萄糖对脱脂乳酪蛋白胶束稳定性的影响研究

研究了葡萄糖添加量、体系pH、热处理温度、热处理时间对脱脂牛乳酪蛋白胶束稳定性的影响。研究表明,在接近中性pH条件下,少量葡萄糖分子可降低脱脂乳酪蛋白胶束的稳定性,大量葡萄糖分子可增强其稳定性。添加葡萄糖后,在高温热处理的诱导下,酪蛋白沉淀的pH升高。随着热处理时间的延长,脱脂乳中酪蛋白胶束表现出聚集行为,致使粒径增大,浊度和沉淀率总体呈现上升趋势。该研究结论可为乳品加工提供参考依据。