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.

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。

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.

浓缩酪蛋白胶束成分的影响因素及其在奶酪生产中的应用

目前,采用膜过滤技术可从脱脂奶中分离酪蛋白,随后通过浓缩、杀菌、干燥等工艺获得浓缩酪蛋白胶束。对浓缩酪蛋白胶束成分的影响因素及其在奶酪生产中的应用进行综述,发现膜过滤期间的pH值、温度和洗滤条件均会影响浓缩酪蛋白胶束的成分,使其具有不同浓度的酪蛋白、乳清蛋白、乳糖以及钙。而且可以利用浓缩酪蛋白胶束标准化原奶,从而制备成分和品质一致的奶酪;也可以利用不同成分的浓缩酪蛋白胶束获得不同的原奶组合物,从而制备所需品质和功能的奶酪。总之,在奶酪生产过程中添加浓缩酪蛋白胶束能够影响奶酪的成分、质地以及风味等,但通过调整膜过滤和奶酪生产的工艺参数可以解决这些问题。未来还需获得一种经济有效的方式来保存浓缩酪蛋白胶束,赋予其更长的保质期,良好的凝乳酶凝乳特性,从而保证奶酪的品质和产量。

基于高通量定量蛋白组学技术的全脂牛乳和脱脂牛乳蛋白组差异分析研究

蛋白质是牛乳中重要的营养成分,然而关于脱脂处理对牛乳蛋白含量的影响尚不清楚。本文采用串联质谱标签(Tandem Mass Tags,TMT)标记定量蛋白组学方法,对全脂牛乳和脱脂牛乳中全蛋白组进行分析,探究脱脂对牛乳蛋白的影响。全脂和脱脂牛乳中共鉴定出1352个蛋白,筛选出199个差异表达蛋白。与全脂牛乳相比,脱脂后有67个蛋白上调,132个蛋白下调。牛乳主要活性蛋白中,κ-酪蛋白在脱脂后相对含量降低,而β-乳球蛋白和乳铁蛋白在脱脂后相对含量升高,α-乳白蛋白、αs1-酪蛋白、αs2-酪蛋白、β-酪蛋白、牛血清蛋白及乳过氧化物酶相对含量则无显著性差异。乳脂肪球膜蛋白中嗜乳脂蛋白和乳凝集素在脱脂后相对含量降低。脱脂对牛乳中骨架蛋白、代谢相关蛋白等也有影响,改变了牛乳品质及营养价值。通过对全脂和脱脂牛乳中蛋白质分析,明确了脱脂对牛乳蛋白的影响,可以为婴幼儿乳品开发及消费者购买不同脂肪含量的牛奶提供参考。

叶绿素与膜分离乳蛋白相互作用及复合物性质研究

叶绿素是一种广泛分布于高等植物中的天然色素,具有多种生物活性,但稳定性较差。该文采用荧光光谱法研究了酪蛋白胶束、乳清蛋白以及脱脂乳与叶绿素的结合常数;利用喷雾干燥技术制备了乳蛋白与叶绿素的复合物,采用傅里叶变换红外光谱、扫描电镜等手段表征了复合物结构;分析了复合物的溶解性、热稳定性和色度,并评价了复合物中叶绿素的胃肠模拟消化释放性。结果显示,不同类型乳蛋白与叶绿素的结合常数均高于10^(6)L/mol。结合叶绿素后,喷干乳清蛋白多数由葡萄干状转变为半球形中空结构,而酪蛋白胶束与脱脂乳形貌变化不大。叶绿素的结合改善了3种类型乳蛋白在pH 4.5~8的溶解性。乳蛋白负载改善了叶绿素在贮藏30 d内的颜色稳定性。模拟消化试验表明,消化24 h后,游离叶绿素累计释放率最低,酪蛋白胶束负载的叶绿素累计释放率最高,表明乳蛋白负载提高了叶绿素在胃肠道系统中的稳定性。该研究结果可为叶绿素以及乳蛋白在功能食品中的应用提供参考依据。

复合发酵剂发酵脱脂乳与豆乳的工艺优化

该研究以脱脂乳和豆乳为原料,马克斯克鲁维酵母、嗜热链球菌和保加利亚乳酸杆菌作为发酵剂,以滴定酸度和感官评分作为指标,通过单因素试验及响应面试验,优化脱脂乳豆乳体积比、白砂糖添加量、发酵剂接种量及发酵温度,并对优化后的复配发酵乳在贮藏期间的质构、持水力、滴定酸度、感官评分和活菌数进行测定。结果表明,最佳发酵条件为脱脂乳与豆乳体积比82∶18、白砂糖添加量7.25%、发酵剂接种量3.92%,在42℃发酵4.5 h,所得复配发酵乳的感官评分最高,为94.85;在该优化条件下制备的发酵乳后熟12 h,硬度为99.39 g,滴定酸度为97.52°T,活菌数为2.7×10^(9)CFU/mL,发酵乳颜色呈乳黄色、组织状态均匀、酸甜适口、口感柔和润滑、具有复配发酵乳独特的豆香味和浓郁的奶香味,在14 d贮藏期内具有良好的风味、质构及感官接受度。

芦荟提取物对脱脂发酵乳品质及感官的影响

为了提高脱脂发酵乳的品质,以芦荟提取物为脱脂发酵乳的添加剂,探究其对脱脂发酵乳理化特性及感官品质的影响。将芦荟提取物按照0.1%、0.2%和0.3%的比例加入到脱脂发酵乳中,通过测定脱脂发酵乳的pH值、滴定酸度、持水力、脱水收缩性、1,1-二苯基-2-三硝基苯肼自由基清除率、乳杆菌含量、色泽、黏度及进行感官评价,评价芦荟提取物在脱脂发酵乳中的作用。结果表明:在冷藏过程中,芦荟提取物的添加可以改善脱脂发酵乳酸度偏低的问题,增加脱脂发酵乳的黏度,使脱脂发酵乳呈现更好的质构,增加持水力和降低脱水收缩性,增加脱脂发酵乳的冷藏稳定性,增加脱脂发酵乳中乳杆菌的含量,提高脱脂发酵乳抗氧化能力;但添加芦荟提取物的脱脂发酵乳在气味和外观方面评分有所下降。

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。