Proteolysis,protein distribution and stability of UHT milk during storage at room temperature

Proteolytic degradation and distribution of caseins and whey proteins between the soluble and colloidal phases were studied in six batches of commercial UHT milk (three skim and three whole milks) during storage at 25 ± 2 °C. For that purpose, at 30 day intervals, milk samples were ultracentrifuged and the pellets and supernatants analysed by capillary electrophoresis and SDS‐PAGE. Samples were also visually examined for signs of gelation. Extensive proteolytic degradation of the micellar fractions and severe changes in the electrophoretic pattern of the proteins present in the serum fractions were observed in all the batches. A higher proportion of denatured whey proteins not attached to the micelle surface was found in the skim milk samples as compared with the whole milk samples that could provide less resistance against gelation. In addition to β‐Lg, para‐κ‐casein was also found in the serum fraction. A high proteolytic activity against κ‐casein could be responsible for the hydrolysis of serum‐liberated κ‐casein or could have enhanced the liberation of β‐Lg–para‐κ‐casein complexes through proteolysis of micellar κ‐casein. © 1999 Society of Chemical Industry     

Proteolysis and storage stability of UHT milk produced in Turkey

The effect of raw milk quality (total and psychrotrophic bacterial and somatic cell counts, proteinase and plasmin activity) and UHT temperature (145 or 150 °C for 4 s) on proteolysis in UHT milk processed by a direct (steam-injection) system was investigated during storage at 25 °C for 180 d. High proteinase activity was measured in low-quality raw milk, which had high somatic cell count, bacterial count and plasmin activity. The levels of 12% trichloroacetic acid–soluble and pH 4.6-soluble nitrogen in all milk samples increased during storage, and samples produced from low-quality milk at the lower UHT temperature (145 °C) showed the highest values. Bitterness in UHT milk processed from low-quality milk at 145 °C increased during storage; gelation occurred in that milk after 150 d. The RP-HPLC profiles of pH 4.6-soluble fraction of the UHT milk samples produced at 150 °C showed quite small number of peaks after 180 d of storage. Sterilization at 150 °C extended the shelf-life of the UHT milk by reducing proteolysis, gelation and bitterness.     

Changes in physicochemical characteristics of rice during storage at different temperatures

This study investigated the changes in the physicochemical properties of rice during storage at different temperatures. Milled rice stored at high temperatures showed higher fat acidity than rice stored at low temperatures. Although the moisture content of milled rice stored at 30 °C and 40 °C decreased below 15.5% (15.33% and 15.22%, respectively) after 1 month, adequate values were maintained with storage at 4 °C for 3 months (15.50%) and at 20 °C for 2 months (15.53%). Rice stored at low temperatures retained its white coloration, whereas low color retention values were obtained at higher storage temperatures. Peak viscosity increased during 4 months of storage and larger changes were found at higher storage temperatures. Breakdown decreased and setback increased with storage, regardless of storage temperatures. Storage at higher temperatures increased cohesiveness and hardness in compared with storage at lower temperatures. High temperatures also led to a decrease in adhesiveness with age. High temperatures (30 °C and 40 °C) significantly decreased all sensory values even after 1 month of storage. These results are similar to those obtained in an analysis of cooked rice texture. The results of this study indicate that storage temperature is an important factor affecting the physicochemical properties of rice. Short storage periods below room temperatures are recommended to maintain rice quality.     

Changes in flavour and volatile components during storage of whole and skimmed UHT milk

Six batches of commercial UHT milk, submitted to direct treatment, three whole and the other three skimmed milk, were stored at 25±2°C for 4 months. Non-casein nitrogen (NCN) and sensorial analysis were carried out on packs opened every month. Volatile composition was analysed every 15 days, using a purge-and-trap concentrator coupled on-line to a GC–MS instrument. NCN increased during storage; the increase was greater in skimmed milk samples. Sensory characteristics were slightly better in the whole samples, although the scores decreased for both groups in the third month. Quantification of about 40 volatile components in whole milks showed no changes until 90 days (the legal shelf-life in Spain); the main change was the increase of methyl ketones. New components appeared in skimmed samples after 65 days storage; they could be related to both proteolysis and Maillard reaction. This is consistent with the poorer sensory quality found in skimmed milk samples.     

Effect of green tea catechins on physical stability and sensory quality of lactose-reduced UHT milk during storage for one year

Ultra-high temperature (UHT) processed lactose-reduced milk containing added green tea extract (GTE) at two concentrations (0.1% and 0.25%) was stored at 22 ± 2 °C for one year. The effect of GTE addition on physical stability, protein binding, and sensory quality was evaluated. Sedimentation in skim milk and creaming of full fat milk were inhibited by addition of GTE. The formation of Maillard-related flavour compounds was inhibited during storage as determined by dynamic headspace GC–MS. Using Western blot analysis, milk proteins were found to be highly conjugated to polyphenols. Addition of GTE before UHT treatment resulted in increased bitterness and astringency in UHT milk and this remained during storage. Even though GTE addition improved the physical stability and inhibited Maillard reactions in the milk, the taste and flavour contribution from GTE was dominating throughout storage, and alternative sources of polyphenols should be explored for increasing shelf-life stability of long-life milk.     

UHT乳贮存中常见问题及其原因分析

UHT乳作为“营养全价”食品倍受消费者喜爱。但是UHT乳在贮藏中容易出现一些质量问题,常见的有：脂肪上浮、形成沉淀和凝胶、乳清析出、酸包和胀包及色香味的改变等。针对以上问题进行了论述,并探讨了产生的原因及相应的预防措施。     

不同均质压力处理对UHT灭菌乳在储藏期内脂肪上浮的影响

3组分别经过10,20和30MPa均质处理的超高温(Ultra Head Treated,UHT)灭菌乳,在25℃条件下储藏6个月(180 d);通过测定均质后粒径的大小,以及在储藏期内UHT奶上、中、下不同部位的脂肪质量分数及脂肪上浮高度.结果表明,均质压力越高,脂肪球直径越小,上层脂肪质量分数越低、脂肪上浮高度越低;均质压力越高,UHT奶稳定性越好.     

评估超高温灭菌奶安全和质量的2个重要参数

为探究初始蛋白质与脂肪含量、贮藏温度对UHT纯牛奶包装货架期的影响,以三种UHT纯牛奶为研究对象,试验测定23、30和37 ℃贮藏过程中样品褐变指数、蛋白水解度指标。将数据集整合,根据其在预测集上的表现确定具体的输入参数,开展基于BP神经网络的UHT纯牛奶包装货架期预测。结果表明,BP神经网络模型对UHT牛奶褐变指数、蛋白水解度指标的拟合度为0.9412、0.9527,相较于传统多元线性回归模型的0.8799和0.9211,经优化隐含层神经元数的BP神经网络模型对UHT纯牛奶贮藏期间的特征指标变化预测精度更高,为不同配方UHT纯牛奶货架期的快速准确预测提供技术支持。

     

Release of galactose and N-acetylglucosamine during the storage of UHT milk

The changes in the concentration of monosaccharides and the influence of dephosphorylation of sugar phosphates thereon were studied in four batches of UHT milk, (two commercial and two experimental batches produced in a pilot plant) during storage at 10–30°C for 120 days. An increase in galactose and N-acetylglucosamine, together with a decrease in N-acetylglucosamine-1P and galactose-1P, were found, which were larger as the temperature of storage increased. The effects of microbial content and thermal treatment on these processes were evaluated. The contribution of dephosphorylation of the phosphorylated sugars to the total increment in N-acetylglucosamine and galactose was examined. While, in most cases, the monosaccharide increases found were consistent with the disappearance of phosphorylated sugars, changes in galactose during storage at 30°C seemed to be due to other processes, probably involving glycosidases, that may have potential detrimental consequences to milk stability.     

Clay facial masks: physicochemical stability at different storage temperatures

Clay facial masks--formulations that contain a high percentage of solids dispersed in a liquid vehicle--have become of special interest due to specific properties presented by clays, such as particle size, cooling index, high adsorption capacity, and plasticity. Although most of the physicochemical properties of clay dispersions have been studied, specific aspects concerning the physicochemical stability of clay mask products remain unclear. This work aimed at investigating the accelerated physicochemical stability of clay mask formulations stored at different temperatures. Formulations were subjected to centrifuge testing and to thermal treatment for 15 days, during which temperature was varied from −5.0° to 45.0°C. The apparent viscosity and visual aspect (homogeneity) of all formulations were affected by temperature variation, whereas color, odor, and pH value remained unaltered. These results, besides the estimation of physicochemical stability under aging, can be useful in determining the best storage conditions for clay-based formulations.     

Biochemical changes in green sweet peppers during storage at different temperatures

The effect of temperature on the levels of moisture, ascorbic acid, proteins and phenolic compounds in green sweet peppers (cvs. Sumo and Quadrato d'Asti) was investigated after seven days' storage at a relative humidity of 85–90%. Chemical composition did not differ appreciably after storage at 10°C, whereas changes were found after storage at 20°C and 30°C. Therefore, 10°C was the most favourable storage temperature for peppers although, at 20°C, retention of quality was adequate in terms of texture and appearance.     

UHT乳贮存过程中V_A和V_C的变化

选择市场上3种UHT乳通过25、37℃和45℃恒温进行贮存温度加速实验比较VA和Vc含量的变化。结果表明,对于VA在3个不同的贮存温度下随贮存时间延长均不断下降,下降速率最高为45℃贮存,最高下降率达到82%,且随贮存温度升高下降越快;贮存中Vc含量变化在3种产品间无差异,且随贮存时间延长略下降,不同贮存温度间变化无差异。对于贮存中UHT乳的品质变化可以选择VA作为指标之一。     

不同体细胞数原料乳对UHT贮存期间蛋白和脂肪水解的影响

不同体细胞数(21.4×104mL-1,75.8×104mL-1,118.1×104mL-1和216.2×104mL-1)原料乳生产的4组UHT乳在37℃贮存84d,对其贮存期间的蛋白水解及脂肪水解进行研究。结果表明,4组UHT乳贮存期间的蛋白水解速率无显著性差异(P>0.05),原料乳体细胞数并未对蛋白水解造成影响;4组UHT乳贮存期间的脂肪水解速率具有显著性差异(P<0.005),原料乳体细胞数与脂肪水解速率间存在极明显的正相关(R=0.9886,P<0.05)。     

不同温度贮藏凌枣V_C含量的动态变化模型

实验试图寻找能够定量描述凌枣（Zizyphus jujube Mill.cv.Lingzao）分别在0、4、10、20℃贮藏期间VC含量变化规律的方法。实验首先选取零级动力学模型、一级动力学模型和一个基于Weibull分布的模型分别来对不同温度下所测得的实验数据进行拟合,得到不同温度下不同模型的模型参数以及拟合效果统计学信息。根据R2adj、RMSE、χ2、AIC这些拟合效果统计学指标对不同温度下,上述3种模型的拟合优度进行比较,并用速率参数-温度关系模型Y=ln（1+exp[c（T-Tc）]）来拟合速率参数随温度的变化规律。结果表明:0、4、10及20℃贮藏条件下,均是Weibull模型的R2adj值最接近于1,且RMSE、χ2值最小,拟合度最高。赤池信息量准则筛选出凌枣0、4、10℃下VC含量预测最佳模型为Weibull模型,20℃下为一级动力学模型。Weibull模型速率参数随温度变化规律的拟合结果为R2adj值等于0.9894,表明上述速率参数-温度关系模型拟合效果较好,可以用来估计某特定温度下Weibull模型的反应速率参数。      

不同温度贮藏凌枣V_C含量的动态变化模型

实验试图寻找能够定量描述凌枣(Zizyphus jujube Mill.cv.Lingzao)分别在0、4、10、20℃贮藏期间VC含量变化规律的方法。实验首先选取零级动力学模型、一级动力学模型和一个基于Weibull分布的模型分别来对不同温度下所测得的实验数据进行拟合,得到不同温度下不同模型的模型参数以及拟合效果统计学信息。根据R2adj、RMSE、χ2、AIC这些拟合效果统计学指标对不同温度下,上述3种模型的拟合优度进行比较,并用速率参数-温度关系模型Y=ln(1+exp[c(T-Tc)])来拟合速率参数随温度的变化规律。结果表明:0、4、10及20℃贮藏条件下,均是Weibull模型的R2adj值最接近于1,且RMSE、χ2值最小,拟合度最高。赤池信息量准则筛选出凌枣0、4、10℃下VC含量预测最佳模型为Weibull模型,20℃下为一级动力学模型。Weibull模型速率参数随温度变化规律的拟合结果为R2adj值等于0.9894,表明上述速率参数-温度关系模型拟合效果较好,可以用来估计某特定温度下Weibull模型的反应速率参数。     

不同温度储藏条件下鲅鱼生物胺变化的研究

为研究鲭鱼中毒的组胺来源以及温度对鲅鱼生物胺的影响,通过HPLC-柱后衍生-FLD检测技术分析了鲅鱼体内7种生物胺(酪胺、腐胺、尸胺、组胺、胍丁胺、亚精胺、精胺)的含量以及在不同储藏温度的变化情况。结果表明,新鲜鲅鱼肌肉中没有检出组胺。鲅鱼体内生物胺的变化随温度的升高而加快,0℃(冰藏)、4℃、20℃储藏8d的生物胺总量变化范围分别为3.09～6.77、16.28～274.65、154.96～1846.29mg/kg。因此,细菌是引起组胺中毒的主要来源,低温储藏是控制鲅鱼产生生物胺的有效措施。     

不同贮藏温度下鲜切甘薯的生理生化变化

研究了鲜切甘薯在18、10、4℃下贮藏期间生理生化的变化。测定了呼吸强度、总酚含量、MDA、相对电导率、颜色饱和度及亮度,以及PPO、POD、PAL、CAT、SOD的活性。结果表明:切割伤害导致鲜切甘薯呼吸强度增大,引发膜脂过氧化程度加深,MDA含量和相对电导率增加。随着贮藏时间的延长,PAL活性呈现上升趋势,其催化底物总酚也随之增加,与果蔬组织褐变相关的PPO、POD等酶活性也相应增大。抗氧化体系的两种主要酶SOD、CAT变化趋势相近。低温贮藏能够有效地保持鲜切甘薯的感官品质,并能在一定程度上控制生理生化的变化。     

Survival of undesirable micro-organisms in fruit yoghurts during storage at different temperatures

The survival of undesirable micro-organismsYersinia enterocolitica,Escherichia coli,Rhodotorula mucilaginosa,Kluyveromyces marxianusin fruit yoghurts after inoculation at two different levels (10²–10³cfu/ml and 10⁴–10⁶cfu/ml) was investigated during storage at 4 and 8°C. When stored at 4°C, the survival ofY. enterocoliticawas not significant (3 days) except when the size of the initial inoculum was larger than 10⁵cfu/ml: in this case, viable cells of the pathogen were still recovered after 17 days of storage. Cells ofE. coliwere detected in the yoghurt samples after 24–48 days at 4°C, regardless of the inoculum size. The growth of yeasts easily occurred at 4°C;R. mucilaginosashowed an increase of 10–100 fold andK. marxianus) was able to grow from 10²to 10⁵–10⁶cfu/ml during the 30 days of storage. Similar trends were noted with 8°C storage. The survival of both contaminants was longer than 8–10 days and yeasts were able to grow remarkably well, with microbial values of 10⁶cfu/mlR. mucilaginosaand 10⁸cfu/ml (K. marxianusin the case of the large inoculum. Throughout the whole experiment, the counts for lactic streptococci were around 10⁸–10⁹cfu/ml whereas lactobacilli levels were 10⁶cfu/ml, and viability declined after 15 days of storage.The competitionin vitrobetween bothLactobacillusandStreptococcusstrains isolated from yoghurt andE. coliwas also investigated. WithLactobacillus, the inhibition ofE. coliwas very effective; no viableE. colicells were detected after 24 h incubation. TheStreptococcusstrain was also able to inhibitE. colibut not so rapidly; slight growth ofE. coliwas observed during the first 3 h, followed by a marked decrease in the number of cells.     

Use of reconstituted concentrated nonfat milk for the production of UHT milk: physical and chemical effects and stability

The objective of this work was to evaluate the use of reconstituted concentrated nonfat milk for the production of UHT milk and to evaluate its physical and chemical effects and stability. The study considered three treatments related to the raw material and processing parameters. Increasing the intensity of thermal treatments resulted in increased hydroxymethylfurfural (HMF) content as well as the denaturation of whey protein. During storage, the milk that was produced from reconstituted concentrate displayed a lesser degree of proteolysis and higher pH value, sedimentation index and HMF, compared to natural milk. Under higher storage temperatures, the degree of proteolysis, HMF and pH decreased and the sedimentation index increased.