Short communication: Volatiles in microfluidized raw and heat-treated milk

As innovative processing equipment is introduced to milk processing, it is essential to determine its effect on milk aroma, a critical factor in consumer acceptance of the final dairy product. Microfluidization is known to cause severe high-pressure homogenization of milk fat and, although severe processing is known to release undesired aromas, no information is available on the levels of the volatile compounds in milk immediately after microfluidization. We hypothesized that microfluidization would alter levels of volatile compounds in milk that may affect aroma. The concentration of 11 selected volatile compounds in raw, thermized, pasteurized, and UHT 3.0% fat milk samples were compared before and after microfluidization at 170 MPa and common 2-stage homogenization at 15 MPa. Overall, the different milk samples had similar trends in response to homogenization, although UHT milk started with lower values of nonanoic acid, and acetone and higher levels of hexanal and heptanol. In many cases, microfluidization did not significantly alter volatile levels compared with the starting milk. Heptanal was the only compound observed to increase in thermized and UHT milk, whereas nonanoic acid and acetone decreased in raw, thermized, and pasteurized milks and octanoic acid decreased in thermized and UHT milks. The highest levels of almost all of the volatiles were found in the 2-stage homogenized milk. Overall, microfluidization had minimal effect on the volatile compound profiles of milk, although sensory evaluation is needed to confirm effects on aroma and flavor.     

The effects of microfluidization on the physical,microbial, chemical,and coagulation properties of milk

This work examines the use of mild heat treatments in conjunction with 2-pass microfluidization to generate cheese milk for potential use in soft cheeses, such as Queso Fresco. Raw, thermized, and high temperature, short time pasteurized milk samples, standardized to the 3% (wt/wt) fat content used in cheesemaking, were processed at 4 inlet temperature and pressure conditions: 42°C/75 MPa, 42°C/125 MPa, 54°C/125 MPa, and 54°C/170 MPa. Processing-induced changes in the physical, chemical, and microbial properties resulting from the intense pressure, shear, and cavitation that milk experiences as it is microfluidized were compared with nonmicrofluidized controls. A pressure-dependent increase in exit temperature was observed for all microfluidized samples, with inactivation of alkaline phosphatase in raw and thermized samples at 125 and 170 MPa. Microfluidization of all samples under the 4 inlet temperature and processing pressure conditions resulted in a stable emulsion of fat droplets ranging from 0.390 to 0.501 μm, compared with 7.921 (control) and 4.127 (homogenized control) μm. Confocal imaging showed coalescence of scattered fat agglomerates 1 to 3 μm in size during the first 24 h. We found no changes in fat, lactose, ash content or pH, indicating the major components of milk remained unaffected by microfluidization. However, the apparent protein content was reduced from 3.1 to 2.2%, likely a result of near infrared spectroscopy improperly identifying the micellar fragments embedded into the fat droplets. Microbiology results indicated a decrease in mesophilic aerobic and psychrophilic milk microflora with increasing temperature and pressure, suggesting that microfluidization may eliminate bacteria. The viscosities of milk samples were similar but tended to be higher after treatment at 54°C and 125 or 170 MPa. These samples exhibited the longest coagulation times and the weakest gel firmness, indicating that formation of the casein matrix, a critical step in the production of cheese, was affected. Low temperature and pressure (42°C/75 MPa) exhibited similar coagulation properties to controls. The results suggest that microfluidization at lower pressures may be used to manufacture high-moisture cheese with altered texture whereas higher pressures may result in novel dairy ingredients.     

粒度分析在牛奶均质化过程中的应用

目的为确保均质化效果,采用一种快速有效的方法对牛奶均质化前后的粒度进行监控。方法利用HORIBA LA-950激光粒度分析仪对牛奶的均质化过程进行监控。结果通过激光粒度法,可以快速得到均质化前后牛奶的粒度分布图,并可以看出均质化后的牛奶粒度分布明显小于均质化前。结论在牛奶的均质化的过程中,激光粒度法是一种快速有效的方法,它可以实现对均质化过程中牛奶粒度的实时监控。     

Effect of moderate inlet temperatures in ultra-high-pressure homogenization treatments on physicochemical and sensory characteristics of milk

The effect of ultra-high-pressure homogenization (UHPH) on raw whole milk (3.5% fat) was evaluated to obtain processing conditions for the sterilization of milk. Ultra-high-pressure homogenization treatments of 200 and 300 MPa at inlet temperatures (Ti) of 55, 65, 75, and 85°C were compared with a UHT treatment (138°C for 4 s) in terms of microbial inactivation, particle size and microstructure, viscosity, color, buffering capacity, ethanol stability, propensity to proteolysis, and sensory evaluation. The UHPH-treated milks presented a high level of microbial reduction, under the detection limit, for treatments at 300 MPa with Ti of 55, 65, 75, and 85°C, and at 200 MPa with Ti = 85°C, and few survivors in milks treated at 200 MPa with Ti of 55, 65, and 75°C. Furthermore, UHPH treatments performed at 300 MPa with Ti = 75 and 85°C produced sterile milk after sample incubation (30 and 45°C), obtaining similar or better characteristics than UHT milk in color, particle size, viscosity, buffer capacity, ethanol stability, propensity to protein hydrolysis, and lower scores in sensory evaluation for cooked flavor.     

Effect of homogenization and heat treatment on the behavior of protein and fat globules during gastric digestion of milk

The effects of homogenization and heat treatment on the formation and the breakdown of clots during gastric digestion of whole milk were investigated using a human gastric simulator. Homogenization and heat treatment led to formation of coagula with fragmented and crumbled structures compared with the coagulum formed from raw whole milk, but a larger fraction of the protein and more fat globules were incorporated into the coagula induced by action of the milk-clotting enzyme pepsin. The fat globules in the whole milk appeared to be embedded in the clots as they formed. After formation of the clot, the greater numbers of pores in the structures of the clots formed with homogenized milk and heated whole milk led to greater rates of protein hydrolysis by pepsin, which resulted in faster release of fat globules from the clots into the digesta. Coalescence of fat globules occurred both in the digesta and within the protein clots no matter whether they were in homogenized or heated milk samples. The formation of clots with different structures and hence the changes in the rates of protein hydrolysis and the release of milk fat into the digesta in the stomach provide important information for understanding the gastric emptying of milk and the potential to use this knowledge to manipulate the bioavailability of fat and other fat-soluble nutrients in dairy products.     

Effects of ultra-high pressure homogenization on the cheese-making properties of milk

The effects of single- or 2-stage ultra-high pressure homogenization (UHPH; 100 to 330 MPa) at an inlet temperature of 30°C on the cheese-making properties of bovine milk were investigated. Effects were compared with those from raw, heat-pasteurized (72°C for 15 s), and conventional homogenized-pasteurized (15 + 3 MPa, 72°C for 15 s) treatments. Rennet coagulation time, rate of curd firming, curd firmness, wet yield, and moisture content of curds were assessed. Results of particle size and distribution of milk, whey composition, and gel microstructure observed by confocal laser scanning microscopy were analyzed to understand the effect of UHPH. Single-stage UHPH at 200 and 300 MPa enhanced rennet coagulation properties. However, these properties were negatively affected by the use of the UHPH secondary stage. Increasing the pressure led to higher yields and moisture content of curds. The improvement in the cheese-making properties of milk by UHPH could be explained by changes to the protein-fat structures due to the combined effect of heat and homogenization.     

加工工艺对脱脂发酵乳饮料酪蛋白颗粒粒径和稳定性的影响

介绍了脱脂发酵乳饮料生产工艺中发酵条件（温度）、菌种、发酵奶均质、稳定剂对产品酪蛋白颗粒粒径大小和稳定性的影响。结果表明，MA14菌种、低温35℃长时间发酵、发酵奶均质、选择高甲氧基果胶做稳定剂均能够降低发酵乳饮料的酪蛋白颗粒粒径和离心沉淀率．提高产品的稳定性。     

均质及杀菌条件对花生奶稳定性的影响

通过分析产品的稳定系数R，油脂析出率，离心沉淀率及粒度分布，研究均质及杀菌条件对花生奶稳定性的影响。结果表明，花生奶在温度65～70℃，压力25MPa下均质2次，再经121℃杀菌15min后产品的稳定性最佳。     

Effects of ultra-high pressure homogenization on microbial and physicochemical shelf life of milk

The effect of ultra-high pressure homogenization (UHPH) on microbial and physicochemical shelf life of milk during storage at 4°C was studied and compared with a conventional heat preservation technology used in industry. Milk was standardized at 3.5% fat and was processed using a Stansted high-pressure homogenizer. High-pressure treatments applied were 100, 200, and 300 MPa (single stage) with a milk inlet temperature of 40°C, and 200 and 300 MPa (single stage) with a milk inlet temperature of 30°C. The UHPH-treated milks were compared with high-pasteurized milk (PA; 90°C for 15 s). The microbiological quality was studied by enumerating total counts, psychrotropic bacteria, lactococci, lactobacilli, enterococci, coliforms, spores, and Pseudomonas. Physicochemical parameters assessed in milks were viscosity, color, pH, acidity, rate of creaming, particle size, and residual peroxidase and phosphatase activities. Immediately after treatment, UHPH was as efficient (99.99%) in reducing psychrotrophic, lactococci, and total bacteria as was the PA treatment, reaching reductions of 3.5 log cfu/mL. Coliforms, lactobacilli, and enterococci were eliminated. Microbial results of treated milks during storage at 4°C showed that UHPH treatment produced milk with a microbial shelf life between 14 and 18 d, similar to that achieved for PA milk. The UHPH treatments reduced the L* value of treated milks and induced a reduction in viscosity values of milks treated at 200 MPa compared with PA milks; however, these differences would not be appreciated by consumers. In spite of the fat aggregates detected in milks treated at 300 MPa, no creaming was observed in any UHPH-treated milk. Hence, alternative methods such as UHPH may give new opportunities to develop fluid milk with an equivalent shelf life to that of PA milk in terms of microbial and physicochemical characteristics.     

Effect of dietary supplementation or cessation of magnesium-based alkalizers on milk fat output in dairy cows under milk fat depression conditions

We aimed to evaluate the effects of dietary supplementation with magnesium oxide and calcium-magnesium dolomite on milk fat synthesis and milk fatty acid profile or persistency in milk fat synthesis after their cessation in dairy cows under milk fat depression conditions. Twenty-four multiparous dairy cows in early lactation (mean ± standard deviation; 112 ± 14 d in milk) were used in a randomized complete block design. Milk fat depression was induced in all cows for 10 d by feeding a diet containing 35.2% starch, 28.7% neutral detergent fiber, and 4.8% total fatty acid (dry matter). The experiment was conducted in 2 periods. During the Mg-supplementation period (d 1–20), cows were randomly assigned to (1) the milk fat depression diet used during the induction phase (control; n = 8), (2) the control diet plus 0.4% magnesium oxide (MG; n = 8), or (3) the control diet plus 0.8% calcium-magnesium dolomite (CMC; n = 8). Compared with the control group, feeding the magnesium-supplemented diets increased milk fat concentration and yield by 12% within 4 d. During the 20-d Mg-supplementation period, both the MG and CMC diets increased milk fat concentration and yield, as well as 3.5% fat-corrected milk and energy-corrected milk yield, without affecting dry matter intake, milk yield, and milk protein and lactose concentrations. In the Mg-cessation period (d 21–30), all cows received the control diet, which resulted in a greater milk fat concentration and yield in the cows that had already received the MG and CMC diets in the Mg-supplementation period. Whereas, milk fat concentration and yield remained high after discontinuation of the magnesium-containing alkalizer until d 27. The difference in milk fat synthesis was associated with lower trans-10 C18:1 (?22%) and higher trans-11 C18:1 (+12.5%) concentrations in milk during the Mg-supplementation period. Furthermore, it was evident that within 2 d of supplementation, the trans-10:trans-11 ratio was lower in MG and CMC cows compared with cows receiving the control. This suggested that the effect of magnesium-based alkalizers on milk fat synthesis was mediated via a shift in ruminal biohydrogenation of cis-9,cis-12 C18:2 in the rumen. In conclusion, abrupt addition of magnesium oxide and calcium-magnesium dolomite increased milk fat synthesis, which persisted for 7 d after cessation of magnesium-based alkalizers. A similar ability to recover milk fat synthesis and normal fatty acid biohydrogenation pathways was observed for magnesium oxide and calcium-magnesium dolomite.     

碱水解法测定乳及乳制品中脂肪的含量

目的建立简单、快速的碱水解法测定乳及乳制品中脂肪的含量。方法取0.5 g样品于离心管中,用10 mL纯水复溶后加入2 mL氨水(25%)水解并65℃水浴20 min。加入乙醇沉淀蛋白质并使用乙醚、石油醚提取样品中脂肪。将上清液提取至收集瓶中,蒸干提取液,称量收集瓶前后质量,计算样品中脂肪含量。结果此法测得样品脂肪含量为26.53 g/100 g,结果相对偏差为0.37%。测定值在质控样品值满意区间内,且符合国标规定的两次独立测定结果之差≤0.3 g/100 g的要求。结论本法测定乳及乳制品中脂肪的含量具有简便、快速、重现性好的优点,适合实验室日常样品测定及考核使用。     

微射流均质处理对碎米淀粉性质影响的研究

采用高压微射流均质处理碎米制备低蛋白含量的大米淀粉产品,并对淀粉颗粒形貌学、大小、淀粉破损率以及热学性质进行观察与测定。研究表明:高压微射流均质压力达到100MPa,均质次数为2次的情况下,可以获得最低蛋白回收率与纯度的淀粉样品,分别为15.30%和1.29%。淀粉颗粒表面积平均粒径(d32)与体积平均粒径(d43)分别从19.70μm与121.04μm降低到9.30μm与65.13μm。破损淀粉的比例从46.72%显著提高到64.66%,且对大米淀粉的热性质影响并不显著。该处理能有效解聚大米淀粉-蛋白质复合体结构,制备出结构和性质更接近天然大米淀粉的产品,有利于其在食品工业中的应用。     

基于人乳脂肪组成的酶法人乳脂肪替代物的构建

为了开发新型高相似性人乳脂肪替代物,依据中国人乳脂肪的脂肪酸组成和甘油三酯结构,以巴沙鱼油、樟树籽仁油、亚麻籽油、微生物油(富含ARA)及DHA藻油(质量比为153.81∶15.16∶62.33∶7.89∶10.81)为原料油脂,以固定化脂肪酶Lipozyme RM为催化剂,在60℃下酯交换反应8 h制备人乳脂肪替代物,测定其脂肪酸、sn-2位脂肪酸及甘油三酯组成及含量,分析其红外光谱图、熔融性质、结晶性质及流变学性能。结果表明,构建的人乳脂肪替代物中链脂肪酸含量为12%、亚油酸和亚麻酸总含量为19.53%且比例约为1∶1.5,ARA含量为1.17%,DHA含量为1.11%,其sn-2位富含棕榈酸(43.10%),结构甘油三酯含量高(58.75%),无反式脂肪酸生成,熔融温度低于人体温度且在20℃下仍能保持18.7%的固体脂肪含量,结晶形态细腻且流变性能优异。制备的人乳脂肪替代物具有应用于婴幼儿配方奶粉、婴幼儿米糊和婴幼儿饼干等各种婴幼儿食品的潜力。     

Transfer of dietary zinc and fat to milk--evaluation of milk fat quality, milk fat precursors, and mastitis indicators

The present study demonstrated that the zinc concentration in bovine milk and blood plasma is significantly affected by the intake of saturated fat supplements. Sixteen Holstein cows were used in a 4 × 4 Latin square design with 4 periods of 12 d, and 4 dietary treatments were conducted. A total mixed ration based on corn silage, grass-clover silages, and pelleted sugar beet pulp was used on all treatments. A high de novo milk fat diet was formulated by adding rapeseed meal and molasses in the total mixed ration [39 mg of Zn/kg of dry matter (DM)], and a low de novo diet by adding saturated fat, fat-rich rapeseed cake, and corn (34 mg of Zn/kg of DM). Dietary Zn levels were increased by addition of ZnO to 83 and 80 mg of Zn/kg of DM. Treatments did not affect daily DM intake, or yield of energy-corrected milk, milk fat, or milk protein. The high de novo diet significantly increased milk fat percentage and milk content of fatty acids with chain length from C6 to C16, and decreased content of C18 and C18:1. Treatments did not influence milk free fatty acids at 4°C at 0 or 28 h after milking. The average diameter of milk fat globules was significantly greater in milk from cows offered low de novo diets. Furthermore, the low de novo diet significantly increased the concentration of nonesterified fatty acids and d-β-hydroxybutyrate in blood plasma, the latter was also increased in milk. Treatments did not affect the enzyme activity of lactate dehydrogenase and N-acetyl-β-d-glucosaminidase in milk or the activity of isocitrate dehydrogenase and malate dehydrogenase in blood plasma. The low de novo diet significantly increased plasma Zn and milk Zn content, whereas dietary Zn level did not in itself influence these parameters. This indicates that the transfer of fat from diet to milk might facilitate transfer of Zn from diet to milk.     

Effect of weaning system on milk composition and distribution of milk fat within the udder of East Friesian dairy ewes

We investigated whether the inhibition of milk ejection during and/or between machine milkings is responsible for the low milk fat observed in commercial milk obtained from dairy ewes managed with a mixed system (MIX) of partial daily suckling (10 h) and once daily machine milking (after 14 h of udder filling). East Friesian crossbred dairy ewes were randomly allocated postpartum to the MIX system (n = 9) or to exclusive twice-daily machine milking (DY1, n = 8). Following wk 4, MIX ewes were permanently weaned from their lambs and milked twice daily. All ewes were injected with saline, oxytocin, or an oxytocin-receptor antagonist prior to three morning milkings during wk 2,4, and 6 of lactation to study cisternal and alveolar milk distribution. Overall milk yield (cisternal + alveolar) for MIX ewes was 42% greater than for DY1 ewes during wk 2 and 4, which demonstrates the beneficial effect of lamb suckling on milk production of dairy ewes. However, during normal machine milking, only the cisternal fraction was obtained from MIX ewes, confirming that milk ejection did not occur for as long as these ewes remained in partial daily contact with their lambs. Although the volume of milk stored within the cistern, and its concentration of milk protein was similar for the two weaning systems, milk of MIX ewes was significantly inferior in cisternal milk fat concentration and yield compared to DY1 ewes. This provides evidence that not only is there inhibition of milk ejection during machine milking of MIX ewes, there is additional inhibition of transfer of milk fat, but not milk protein, from the alveoli to the cistern during the evening when MIX ewes a reseparated from their lambs. Following weaning of MIX ewes, the majority of lactation traits studied were similar compared to DY1 ewes.     

Differences in particle characteristics and oxidized flavor as affected by heat-related processes of milk powder

Understanding the formation of oxidized flavor will be highly useful in the improvement of milk powder quality. Effects of preheating, concentration and spray-drying on the particle characteristics and the oxidized flavor stability of milk powder were investigated. The surface composition and free radicals were analyzed using x-ray photoelectron spectroscopy and electron spin resonance spectrometry, respectively. The concentrations of selected oxidized volatiles hexanal and 2-heptanone were determined using solid-phase microextraction gas chromatography-mass spectrometry. Levels of hexanal and 2-heptanone in fresh milk powder were higher than those in raw milk and heated milk, which drastically increased with increasing time of storage. Differences in the morphological observations, free fat, and surface composition of fresh milk powder were found among different heat-related processes. During storage, a radical (g value, a characteristic constant whose value serves to identify any given free radical, was 2.0054) was detected in milk powder. The specific population of the radical increased from 2.99 × 10⁷ at 3 mo to 1.23 × 10⁸ at 6 mo of storage. Addition of ascorbic acid in milk powder changed the type of radicals and reduced the oxidation off-flavor. According to the Pearson correlations, not the surface compositions but the morphological characteristics of milk powder particles should be considered in maintaining the stability of oxidized flavor in storage.