Light-Induced Changes in Taste,Appearance, Odor,and Riboflavin Content of Cheese

Light-induced changes in taste, appearance, odor, and riboflavin content of common cheeses stored under controlled laboratory conditions and in the dairy cases of four local grocery stores were investigated. Sensory evaluation was performed by 25 untrained panelists, and the riboflavin content was determined by HPLC method. The data obtained from controlled storage indicated that light intensity, type of packaging paper, and piece thickness affected riboflavin retention and color in stored cheese. However, under the display conditions used in the four local stores, retail cheese retained its riboflavin after a display time of 14 d with no significant change in taste, appearance, or odor.     

超高压处理对脱脂乳感官特性的影响

以新鲜脱脂乳为原料,研究高压处理对脱脂乳感官特性的影响。脱脂乳在室温下经不同压力（0.1～700 MPa）和时间（10～30 min）处理后测定其透光率和平均粒径的变化,利用电子眼、电子鼻、电子舌分别检测色泽、气味以及滋味的变化。结果表明：当压力超过200 MPa时脱脂乳透光率增加,而平均粒径不同程度减小;高压处理导致脱脂乳感官特性的改变,在0.1～700 MPa范围内分别处理10～30 min,脱脂乳的主要色号（3002和3018）随着压力的升高逐渐消失,同时出现一些新的色号（如2183和2200）;在处理10 min后脱脂乳中丙酮相对含量的变化不大,处理20～30 min时整体呈减小趋势;乙醛在700 MPa处理10 min、200 MPa处理20、30 min后消失,且出现2-甲基噻吩和乙醇等气味成分,但整体无明显差异（识别指数均小于80）;在处理20～30 min后脱脂乳酸味、咸味与鲜味整体减弱,甜味和苦味增强。因此,超高压处理对脱脂乳气味的影响不大,但会引起色泽、滋味、透光率与粒径产生变化。     

Effect of Fluorescent Light on Repartition of Riboflavin in Homogenized Milk

This study was designed to determine the effect of fluorescent light on riboflavin in homogenized milk and in the resulting fat, casein, and acid whey isolated therefrom. The major proportion (82%) of riboflavin in homogenized milk was associated with the acid whey fraction, about 15% with the casein phase and about 3% with the fat phase. Riboflavin loss due to light exposure (2150 lx for 48 h at 7 C) was greatest in the whey fraction. Further separation of the whey proteins by gel filtration on Sephadex G-75 superfine indicated that the riboflavin associated with the high molecular weight fraction decreased following exposure. In the whey fraction, 95% of the riboflavin was in the free form and not associated with the major whey proteins. The free riboflavin was most labile to light exposure.     

Effect of Fluorescent Light on the Isomerization of Retinyl Palmitate in Skim Milk

Skim milk fortified with retinyl palmitate was exposed to fluorescent light and analyzed for the isomers of retinyl palmitate. High performance liquid chromatography was used to separate and identify the all-trans and some cis forms. Besides all-trans retinyl palmitate, small amounts of 13-cis retinyl palmitate were present in unexposed skim milk. The level of all-trans decreased over the 48-hr period of light exposure. The 13-cis levels also decreased but not as rapidly. The level of the 9-cis isomer increased over the 48-hr period of light exposure.     

Effect of Fouling on Flux and on Energy Requirements in Reverse Osmosis of Skim Milk

The effects of fouling on the permeate flux and on the power and energy required in the process of concentrating skim milk by a plate and frame type reverse osmosis unit with 990 type cellulose-acetate membranes were investigated. The permeate flow characteristics indicate that the fouling layer is rapidly formed and that its resistance is nearly constant over reasonable processing intervals. These fouling properties permit the correlation of permeate flow, overall mass transfer coefficient, and osmotic pressure difference in terms of the energy required per permeate volume versus time. This correlation shows a decrease in energy required per permeate volume with increasing mass transfer coefficient and time. These flow characteristics permit the estimation with confidence of the permeate flow energy requirements and the capacity of a full scale plant. Mass transfer coefficients were estimated for the membrane, the polarized layer, and the fouling layer. The overall mass transfer coefficient was then calculated. The polarized and fouling layers are films of components from the skim milk. Fouling was the controlling resistance.     

Soluble, Dialyzable and Ionic Calcium in Raw and Processed Skim Milk, Whole Milk and Spinach

Chemical availability of calcium was measured with and without simulated digestion to determine processing effects. Available calcium was low in spinach and significantly higher in skim milk and whole milk, consistent with their known bioavailabilities. In nondigested processed milk it was significantly higher for whole milk than for skim milk. However, available calcium in digested whole milk was lower than in digested skim milk. Digestion increased available calcium in skim milk, dialyzable and ionic dialyzable calcium in whole milk and ionic calcium in spinach. Soluble and ionic soluble calcium were lowered in whole milk during digestion due to soap formation. Effects on % calcium were slight, but available calcium in nondigested skim milk was significantly lowered by processing.     

全脂乳与脱脂乳挥发性风味成分对比分析

HS-SPME结合GC/MS对比分析全脂乳(WM)和脱脂乳(SM)的挥发性风味成分。结果表明,50/30μm CAR/PDMS/DVB萃取纤维萃取得到的全脂乳、脱脂乳的挥发性风味物质的种类与65μm PDMS/DVB相当,即:全脂乳11种,脱脂乳8种,只是在萃取含量上有差异。因此,相对于65μm PDMS/DVB而言,50/30μm CAR/PDMS/DVB较适合乳风味挥发性物质的萃取。HS-SPME结合GC-O对比分析全脂乳和脱脂乳的呈香组分结果显示,脱脂乳中未发现香气强度较高的呈香成分。而在全脂乳中辛酸(清香)、癸酸(油脂味)香气强度较高。这些结果能为全脂乳和脱脂乳的风味物质组成提供有用信息。     

蛋白酶处理对脱脂乳感官品质及游离氨基酸的影响

以新鲜脱脂牛乳为原料,采用分光测色仪、电子舌及氨基酸自动分析仪等分析酶解处理对脱脂牛乳感官品质、游离氨基酸含量及组成的影响.结果 表明:经风味蛋白酶处理的脱脂牛乳水解度高,达到24.61％;蛋白酶处理会导致其感官性状的改变,与脱脂牛乳相比,3种酶解产物L*值均显著(P＜0.05)下降,a*(负值)显著上升(P＜0.05...     

Spontaneous Lipolysis in Bovine Milk: Combined Effects of Cream,Skim Milk,and Lipoprotein Lipase Activity

Total lipoprotein lipase activity has been studied in bovine milk. Lipase activity per milliliter milk decreased with decreasing yield in late lactation. Individual variation between cows was high, even at given milk yield. Evening samples were always higher in lipoprotein lipase activity than morning samples. This also applied to the degree of cold storage lipolysis (spontaneous lipolysis). However, the amount of liberated FFA per unit lipase was similar in morning and evening samples. Lipolysis in evening milk may be higher due to increased lipase. Methods to quantitate the influence of the skim milk fraction on lipolysis and the susceptibility of milk fat globules to lipolysis have been described previously. A linear multiple regression model that included these two parameters in addition to the total amount of lipoprotein lipase could explain 80 to 87% of cold storage lipolysis. Analysis including only one parameter resulted in low and nonsignificant coefficients. This demonstrated that cold storage lipolysis is a complex problem and that the properties of the fat globules and the skin milk, in addition to the total activity of lipoprotein lipase, are important.     

脱脂乳粉非酶褐变影响因素的实验研究

在食品、饮料、营养品、医药等行业广泛涉及非晶态粉体产品,这些粉体产品的非酶褐变,会严重影响产品质量。本文对脱脂乳粉的非酶褐变特性进行了实验研究;考查了水分活度、贮藏温度、储藏时间和玻璃化转变温度等参数对脱脂乳粉非酶褐变的影响。结果表明:随着水分活度、贮藏温度、贮藏时间以及贮藏温度T与脱脂乳粉的玻璃化转变温度Tg间的差值(T-Tg)的增加,脱脂乳粉非酶褐变指数显著增大;脱脂乳粉应在较低水分活度(例如0.3以下)和较低温度(例如40℃以下)下贮藏,以延长其货架寿命。     

Effect of Heat Treatment on Bovine Lactoperoxidase Activity in Skim Milk: Kinetic and Thermodynamic Analysis

ABSTRACT: The effect of heat on lactoperoxidase activity in bovine milk was studied over a range of 68 to 76 °C. Values of residual enzymatic activity after different treatments were studied by kinetic analysis, obtaining D‐values and the Z‐value (3.1 °C). Denaturation of lactoperoxidase, measured by loss in activity, can be described as a 1st‐order reaction. Rate constants were calculated, as was the energy of activation, which was 737.69 kJ/mol. Thermodynamic parameters were also calculated. The high value obtained for the variation in enthalpy of activation indicates that a high amount of energy is required to initiate denaturation, probably due to the molecular conformation of lactoperoxidase.     

脱脂奶粉勾兑牛奶酒工艺中pH值的影响

探讨了以脱脂奶粉为原料勾兑奶酒的工艺条件,主要研究了牛奶蛋白在乙醇存在下影响其稳定性的一个重要因素——pH值.实验表明,在以脱脂奶粉为原料勾兑奶酒时,酒精应该先进行适量的稀释再加入到复配奶中;随着复配奶pH值的升高,其牛奶蛋白的乙醇稳定性也随之升高.复配奶的pH值决定着勾兑奶酒的pH值,其最适pH值为6.5～7.3,但是酒精的pH对于勾兑奶酒的pH并没有决定性的影响.实验还研究了勾兑奶酒在储存过程中其体系的pH值的变化.     

Comparison of Electrochemical and Chemical Acidification of Skim Milk

The purpose of this study was to compare chemical and electrochemical acidification, in order to identify differences between the two acidification procedures. The results reveal differences in the acidification profiles obtained with the two methods. Whereas chemical acidification showed a salting‐in effect from addition of salts, bipolar‐membrane electroacidification (BMEA) removes salts through electrochemical demineralization, favoring protein precipitation. Hence, at pH 4.6, all the caseins were precipitated by BMEA, while some were not yet precipitated by chemical acidification. Nevertheless, the chemical composition and the casein fraction composition of the isolates were the same for both chemical and electrochemical acidification.     

An Assessment of Light-Induced Off-Flavors in Retail Milk

Samples of whole and skim milk from two local dairy plants that were packaged in 3.8-L plastic containers (nonreturnable high density polyethylene) and 1.9, .95, and .48-L fiberboard containers and displayed in the dairy cases of four local grocery stores under normal fluorescent light were analyzed for light-induced changes in flavor and riboflavin content. Milk samples were analyzed daily for flavor, odor, appearance, and riboflavin content. Sensory evaluation was by 25 untrained panelists, and riboflavin was determined by HPLC method. Milk samples were analyzed first in summer, then repeated in winter. Data indicated that under typical production and storage conditions in the area, type and size of container, fat content, and season of production had no major damaging effects on flavor, odor, appearance, or riboflavin content of retail milk.     

Concentration of Skim Milk by a Cascade Comprised of Ultrafiltration and Nanofiltration: Investigation of the Nanofiltration of Skim Milk Ultrafiltration Permeate

In order to reach a high volume reduction ratio (VRR) prior to drying of skim milk, a membrane cascade comprising of an ultrafiltration (UF) coupled with a nanofiltration (NF) can be applied. The present study investigated the impact of processing (filtration temperature, transmembrane pressure (TMP)) and product (feed pH) parameters on the NF of skim milk UF permeate. It could be shown that a low filtration temperature of 10 °C is more advantageous in terms of flux stability and rejection of the solute fraction as compared to higher filtration temperatures up to 45 °C. The solution pH did not affect permeate flux and lactose retention. However, in order to avoid calcium losses, it is more favorable to conduct the concentration at a pH of 6.8 instead of at a lower pH of 5. The application of a higher TMP (up to 4 MPa) enhances permeate flux and VRR as well as solute rejection during concentration of UF permeate. It was also shown that the retention of solutes decreases towards the end of the concentration process. As a consequence, the achievement of high final VRR must be weighed against increased product losses at the end.     

Growth, Viability and Activity of Bifidobacterium spp. in Skim Milk Containing Prebiotics

Growth, activity and mean doubling time (T_d) of five Bifidobacterium species in the presence of four types of prebiotics, and concentrations of acetic and lactic acids were determinted during fermentation and after 4 weeks of refrigerated storage. The T_d was lowest for B. animalis with raftilose and inulin. Retention of viability of bifidobacteria was greatest with hi‐amylose corn starch (hi‐maize). The average pH of skim milk at the end of 4 wks storage averaged 4.34 (for B. animalis with raftilose) to 4.07 (for B. longum with inulin). The highest levels of acetic acid and lactic acid were produced by B. pseudolongum with lactulose and B. infantis with lactulose, respectively.     

超高压结合温热处理对脱脂乳透光率和粒径及蛋白溶解性的影响

目的：研究高压结合温热（≤50 ℃）处理对脱脂乳粒径、透光率及蛋白溶解性的影响。方法：采用不同温度（常温、30、40、50 ℃）和压力（0.1～700 MPa）分别处理脱脂乳10～30 min,利用激光纳米粒度仪检测脱脂乳粒径变化,分光光度法检测透光率变化,考马斯亮蓝法测定可溶性蛋白质量浓度变化。结果表明,脱脂乳透光率在压力不高于100 MPa范围内不受温度、压力和处理时间的影响;在200～700 MPa范围内,常温条件下处理的脱脂乳透光率随压力的升高和处理时间的延长而增大,在700 MPa下处理20 min时透光率最大,增幅为1 011%;在30～50 ℃范围内,透光率随压力增大（200～700 MPa）呈现先升高后降低的趋势,在40 ℃、500 MPa下处理10 min时透光率增幅最大（537%）,透光率受温度和保压时间的影响,且超高压结合温热处理脱脂乳透光率均高于未处理脱脂乳。在常温、0.1～400 MPa范围内,脱脂乳的中位径（Dx(50)）随压力的增大而总体降低,在400～700 MPa范围内变化趋势平稳,但均小于未处理脱脂乳的Dx(50);30、40、50 ℃下,脱脂乳Dx(50)随压力增大呈现先升高后降低的趋势,分别在400～700、300～500、200～400 MPa范围内变化趋势平稳,且受时间影响较小。经高压处理的脱脂乳中可溶性乳蛋白（soluble protein,S-Pro）质量浓度总体呈增加趋势,且受压力、时间和温度的影响,在30 ℃、500 MPa下处理30 min,S-Pro质量浓度增幅最大（83.55%）。pH 4.6下可溶性蛋白（S-Pro-pH 4.6）质量浓度在压力不高于100 MPa时不受温度、压力和时间的影响;在200～700 MPa范围内,不同温度下,随压力升高和处理时间的延长,S-Pro-pH 4.6质量浓度呈现下降趋势。对各指标间相关性进行分析发现,透光率与Dx(50)间的相关性随温度升高减弱;透光率与S-Pro质量浓度呈正相关,与S-Pro-pH 4.6质量浓度呈负相关。S-Pro与S-Pro-pH 4.6之间呈负相关。结论：经超高压结合温热处理,能够引发脱脂乳透光率、粒径及蛋白溶解性的变化,且这些变化存在一定的相关性。     

Survival and Activity of Probiotic Lactobacilli in Skim Milk Containing Prebiotics

ABSTRACT: Seven strains were cultured anaerobically at 37 °C for 48 h in 12% (w/v) reconstituted skim milk containing 5% (w/v) Hi-maize, lactulose, inulin, or raftilose. Their viability was determined before and after 4 wk of storage at 4 °C. Doubling time (Td) was also determined. Concentrations of acetic and lactic acids produced during fermentation and storage were determined. The Td ranged from 301 to 751 min. In general, the viability of lactobacilli after storage was greatest with inulin. The pH after storage in skim milk ranged from 4.34 (for ASCC 1520 with raftilose) to 4.10 (for ATCC 15820 with inulin). Survival of lactobacilli in prebiotic was strainspecific but in general their survival was enhanced.     

Gelation and Water Binding Properties of Transglutaminase-treated Skim Milk Powder

Transglutaminase (TGase)-treated skim milk powder (TG-SMP) was prepared by freeze-drying skim milk after TGase treatment (10 U/g milk protein, 40°C for 3 h), followed by TGase inactivation at 85°C for 5 min. TGase modification resulted in significant increases in hardness and water holding capacity (WHC) of heat-induced gels (10% protein, w/v). A marked increase in storage modulus (G') of TG-SMP upon heating suggests that TG-SMP has a greater gelling ability than control-SMP (C-SMP) prepared with predenatured TGase. Acid gels prepared from TG-SMP had a significantly higher WHC at all solid levels (12%, 14%, and 16%) tested and formed a more elastic network than C-SMP.