乳中蛋白酶与UHT乳贮存中的胶凝现象

乳中的蛋白酶有2个主要的来源途径，乳中天然存在的蛋白酶和由某些微生物产生的蛋白酶，其中纤维蛋白溶酶和嗜冷菌产生的耐热性蛋白酶是存在于UHT乳中的主要蛋白酶，这些蛋白酶非常耐热，经UHT灭菌处理仍可存活。耐热性蛋白酶在UHT乳贮存中水解乳蛋白质从而导致了UHT乳的胶凝．简要介绍了纤维蛋白溶酶、嗜冷菌耐热性蛋白酶的性质、活性测定方法，论述了这些酶引起的UHT乳的胶凝性质、形成原因及影响因素，并提出了一些防止措施。     

Rapid determination of sodium in milk and milk products by capillary zone electrophoresis

A capillary zone electrophoresis method for the determination of Na in milk and milk products was developed and compared with an International Organization for Standardization/International Dairy Federation standard method that is based on flame atomic absorption spectrometry. The adoption of a background electrolyte consisting of 10 mM imidazole adjusted to pH 3.75 by the addition of oxalic acid allowed baseline separation of Na from other milk cations and from Li ion, which was adopted as an internal standard. Method validation was performed and the results for linearity, precision, limit of detection, limit of quantification, and recovery are presented. The procedure was tested on commercial milk samples differing in fat content (whole, semiskimmed, and skimmed) and processing conditions (pasteurization, UHT sterilization, and microfiltration). The reliability of the method was confirmed for different varieties of cheese and other milk products. The method enables the routine measurement of Na content by a rapid and accurate capillary zone electrophoresis procedure.     

Evaluation of a rapid protein analyzer for determination of protein in milk and cream

Accurate and rapid measurement of the protein content of milk is important from both a product quality and an economic standpoint. The Sprint rapid protein analyzer (CEM Corporation, Matthews, NC) is a commercial system based on a dye-binding technique and can be used for rapid measurement of protein in foods. The objective of the present study was to compare the Sprint method with the reference method (Kjeldahl method). Milk and cream samples were analyzed in duplicate for true protein and crude protein (CP) using the reference method as well as the rapid method. Method comparison statistics (regression analysis, graphical representation, standard deviation of residuals, repeatability, and so on) were used to evaluate the agreement between the 2 methods. Regression coefficients and the intercepts were not significantly different from 1 and zero for CP measurement in milk and cream, respectively. The average coefficient of variance between the duplicate CP measurements for the Sprint method was found to be 0.40, 0.49, and 0.76 for milk, light cream, and heavy cream, respectively. True protein measurement in milk and cream also followed a similar trend. Overall, there exists a sufficient level of agreement between the Sprint rapid protein analyzer and Kjeldahl method for true protein and CP measurement of milk and cream samples.     

Analysis of calcium in milk using an embedded system

The objective of the present work is to design and implement a low cost PIC18F452 microcontroller based instrument for the measurement of calcium in milk samples using Light Emitting Diode (LED) as source and photodiode as detector. The developed instrument measures the absorbance, calculates the concentration and displays the results in Liquid Crystal Display (LCD). The principle of the measurement is based on the reaction between calcium and Ortho Cresolphthalein Complexone (OCPC) reagent in alkaline medium to form purple complex with maximum absorption at 570 nm. Algorithm is developed to monitor and control the process sequences and transmit data to PC via serial communication module using an RS232 protocol. Statistical analyses are carried out to evaluate the performance characteristics of the developed instrument and compared with the conventional instrument. Linear calibration curve is obtained between 0 and 5.0 mmol L^?1 and lower limit of detection is 0.05 mmol L^?1. The developed system shows the good performance and the results are in good agreement with the current clinical spectrophotometric method at 96% of confidence level.     

原子吸收法测定牛乳中钙含量的不确定度

目的 评定原子吸收法(atomic absorption spectrometry, AAS)测定牛乳中钙含量测量结果的不确定度。方法 从测量重复性、校准曲线等方面分析该方法的不确定度来源, 依据GB5009.92-2016《食品安全国家标准食品中钙的测定》建立数学模型, 根据JJF 1059.1-2012《测量不确定度的评定与表示》对测量结果的各不确定度来源进行分析和量化。分别计算各分量的不确定度, 最后计算检测结果的合成标准不确定度。 结果 当牛乳中钙含量为125 mg/100 g时, 其扩展不确定度为5 mg/100 g (k=2)。结论 不确定度主要来源于测量重复性、标准工作曲线、标准溶液配制3个方面, 本研究可为提高原子吸收法测定牛乳中钙含量的测量准确性提供参考。     

The measurement and significance of ionic calcium in milk – A review

The measurement of ionic calcium in milk and milk products is of vital importance in understanding the role of calcium in milk. This review compares the methods of measurement including ion equilibration, murexide and ion selective electrodes. Secondly the variations in milk from individual cows and goats, and in bulk milk samples are reviewed. The third section examines the differences in ionic calcium in relation to processing: addition and removal of calcium and other salts, rennet coagulation, miscellaneous influences, filtration and evaporation and drying are all reviewed. Finally the review considers why ionic calcium measurement is not more widely measured within the diary industry and argues that it could be beneficial to do so.     

Effect of pH at heating on the acid-induced aggregation of casein micelles in reconstituted skim milk

Reconstituted skim milk samples at pH between 6.5 and 7.1 (heating pH) were heated at 80°C, 90°C or 100°C for 30 min (heating temperature). The particle size of the casein micelles was measured at pH 4.75-7.1 (measurement pH) and at temperatures of 10°C, 20°C and 30°C (measurement temperature) using photon correlation spectroscopy. The particle size of the casein micelles, at a measurement pH of 6.7 and a measurement temperature of 20°C, was dependent on the heating pH and heating temperature to which the milk was subjected. The casein micelle size in unheated milk was about 215 nm. At a heating pH of 6.5, the casein micelle size increased by about 15, 30 and 40 nm when the milk was heated at 80°C, 90°C or 100°C, respectively. As the heating pH of the milk was increased, the size of the casein micelles decreased so that, at pH 7.1, the casein micelles were ∼20 nm smaller than those from unheated milk. Larger effects were observed as the heating temperature was increased from 80°C to 100°C. The size differences as a consequence of the heating pH were maintained at all measurement temperatures and at all measurement pH down to the pH at which aggregation of the micelles was observed. For all samples, size measurements at 10°C showed no aggregation at all measurement pH. Aggregation occurred at progressively higher pH as the measurement temperature was increased. Aggregation also occurred at a progressively higher measurement pH as the heating pH was increased. The particle size changes on heating and the aggregation on subsequent acidification may be related to the pH dependence of the association of whey proteins with, and the dissociation of κ-casein from the casein micelles as milk is heated.     

ELISA法测定牛奶中链霉素不确定度的评定

对酶联免疫吸附法(ELISA)测定牛奶中链霉素残留量的测量不确定度进行评定,分析了该方法的不确定度来源, 建立数学模型,并进行分析后找到主要影响因素。当牛奶中链霉素含量为7.7 ng/ml时,其扩展不确定度为1.52ng/ml (k=2)。其中,测量样品重复性和反应温度影响较大。采用酶联免疫吸附法(ELISA)测定牛奶中链霉素含量应保证实验具有良好的重复性,并且严格控制实验反应条件。     

The Biotrace method for estimating bacterial numbers in milk by bioluminescence

A study of the hygienic quality of samples of commercial raw milks was carried out using total plate count and a reagent kit for estimating bacterial adenosine triphosphate (ATP) in milk. The method based on ATP is of greater precision. A relationship between the two counting procedures is derived and the statistical properties of computed plate counts from measurement of ATP are discussed.     

Use of enzyme-linked immunosorbent assay for measurement of bovine serum and milk progesterone without extraction

An enzyme immunoassay for progesterone, using horseradish peroxidase as the label, was adapted for direct measurement of progesterone in serum or milk. Values obtained by direct measurement are highly correlated with values measured in extracts and are usable for convenient, rapid, accurate monitoring of the reproductive status of dairy cows. The assay is sensitive (ca. 1 pg), rapidly performed (3.5 h), and allows 92% accuracy in assessment of pregnancy status by direct measurement of progesterone in paired milk samples collected at breeding and 21 d later.     

Day-to-day variation in milk yield and milk composition at the udder-quarter level

Automatic in-line measurement of milk composition and milk yield could be a useful tool in management of the dairy herd. Data on milk components and milk yield provide information on milk quality alterations and cow health status but are also useful in planning feeding and breeding. In automatic milking systems, udder quarters are milked individually, enabling analysis and recording at the udder-quarter level. Frequent records of components require knowledge about day-to-day variations. A component with greater day-to-day variation needs more frequent sampling when used as a diagnostic tool and for management decisions. Earlier studies have described the day-to-day variations in milk components for cow composite milk, but with the quarter milking technique and the possible sampling at the udder-quarter level, knowledge about day-to-day variations at the udder-quarter level is needed. In this study, udder-quarter and cow composite milk samples were collected from 42 consecutive milkings of 10 cows during 21 d. Milk yield was recorded and the milk was analyzed for total protein, whey protein, casein, fat, lactose, and somatic cell count. The results showed that the day-to-day variations and mean values for 4 healthy udder quarters within a cow were similar. In addition, different milk components had different levels of day-to-day variation, the least variation being found in lactose (0.9%) and the greatest in fat (7.7%). This suggests that repeated milk sampling and analysis at the udder-quarter level can be used to detect alterations in composition and cow health and would, thus, be helpful in the management of the dairy herd.     

微生物法测定乳粉中叶酸的不确定度评定

目的评定微生物法测定乳粉中叶酸的不确定度。方法按照GB 5009.211—2014《食品安全国家标准食品中叶酸的测定》规定的微生物法对婴儿配方乳粉中叶酸含量进行测定,根据CNAS-CL01-G003:2019要求,依据GB/Z22553—2010和JJF1059.1—2012,分析测定过程中影响检验结果的因素主要包括测量重复性、试样前处理(称量、稀释)、标准物质、标准溶液配制(称量、稀释)、实验温度、拟合标准曲线、紫外分光光度计,通过对各个不确定度分量进行分析、评估,计算合成得出被测样品中叶酸含量测量结果的标准不确定度,得出扩展不确定度。结果经测定,婴儿配方乳粉中叶酸含量为(80.2±4.28)μg/100 g,k=2。结论微生物因素对测量不确定度具有显著影响,由标准曲线拟合和重复测量体现。     

Monitoring of acidified milk gel formation by ultrasonic shear wave measurements. High-frequency viscoelastic moduli of milk and acidified milk gel

In the present paper we describe the application of the thickness shear mode resonator technique for the measurement of viscoelastic parameters of milk and acidified milk gels in the megahertz frequency range. The technique provides information on the viscoelasticity of milk and milk gels in the time scale of 10(-8) to 10(-7) s. The length scale of the measurements, determined by the depth of penetration and the wavelength of the shear wave, falls between the submicron and micron range. In milk acidified by glucono-delta-lactone we observed an increase in the high- and low-frequency loss modulus, G", below pH 5.1, indicating aggregation of casein particles into clusters. There was a sharp rise in high- and low-frequency storage (G') and loss (G") moduli between pH 4.85 and 4.7, as a result of gel network formation in acidified milk. Both G" and G' of milk gel in the megahertz frequency range are several orders higher than those we obtained at low frequencies (0.02 to 10 Hz) using dynamic rheology. The high-frequency (5 to 25 MHz) viscosity of milk was found to be the same as at low frequencies. Overall, our results demonstrate the high sensitivity of the ultrasonic shear wave measurements to the changes in the rheological parameters in acidified milk during gelation.     

Refinement of the pressure assay for milk urea nitrogen

We report improvements in the application of a pressure-based assay for urea. The assay involved the enzymatic hydrolysis of urea and subsequent measurement of CO2 partial pressure. Effects of the preservative bronopol on the assay and their implications for laboratory applications are discussed. A method of remediating these effects with cysteine is described. A method is also described wherein these additives can be used to prepare standards of known urea concentration in milk. The improved assay can be used to measure urea N in unadulterated milk or in bronopol preserved milk with an accuracy of +/-0.7 mg/dl (0.25 mM) in the range from 0 to 30 mg/dl (0 to 10.7 mM).     

GC-MS测定牛奶中有机磷残留的不确定度评定研究

目的 采用气相色谱法-质谱法对牛奶中有机磷农药残留的不确定度进行评估。方法 通过气质联用法对测定牛奶中有机磷农药残留量测定过程进行分析,应用数学模型计算出测定过程中的不确定度,最后计算出相对合成标准不确定度和扩展不确定度。结果 当牛奶中敌敌畏含量为0.507 mg/kg时,其扩展不确定度为7.35×10-3 mg/kg(k=2);当牛奶中甲拌磷含量为0.509 mg/kg时,其扩展不确定度为5.45×10-3 mg/kg(k=2);当牛奶中乐果含量为0.450 mg/kg时,其扩展不确定度为6.35×10-3 mg/kg (k=2);当牛奶中毒死蜱含量为0.459 mg/kg时,其扩展不确定度为6.09×10-3 mg/kg (k=2);当牛奶中对硫磷含量为0.435 mg/kg时,其扩展不确定度为6.53×10-3 mg/kg (k=2)。结论 影响测量结果不确定度的主要来源为测量重复性和方法的准确度,其他因素的影响相对较小。     

An automated enzymic micromethod for the measurement of fat in human milk

Most (98%) of the fat in human milk is present as triglycerides. This paper describes the use of a clarification procedure that enables the level of human milk fat to be determined by measurement of glycerol released by enzymic hydrolysis of triglycerides. The method requires only 10-50 microliters milk, thus presenting a possible technique for work with small mammals, and is suitable for use with autoanalysers, permitting rapid sample throughput.     

分光光度法测定乳粉中亚硝酸盐含量的不确定度评定

目的 研究分光光度法测定乳粉中亚硝酸盐的不确定度评定。方法 以乳粉为例, 参考JJF 1059.1-2012《测量不确定度评定与表示》, 对GB 5009.33-2016《食品安全国家标准 食品中亚硝酸盐与硝酸盐的测定》第二法分光光度法测定亚硝酸盐的不确定度进行评定。结果 乳粉中亚硝酸盐的含量为1.0 mg/kg 时, 测定结果的合成标准不确定度为0.042 mg/kg, 取k=2, 扩展不确定度为0.084 mg/kg, 其结果可表示为(1.0±0.1) mg/kg。结论 测量结果的不确定度主要是标准曲线拟合、标准工作液配制、试样检测重复性和测定用样液体积。前两者引入的不确定度分量贡献较大。     

Correcting NIR spectra of dimethyl fumarate in milk measured for different brands and in different dates

The calibration transfer was used to correct near-infrared spectra (NIR) of dimethyl fumarate (DMF) for their variations resulting from differences of sample brand and measurement date. Because the concentration of DMF in milk is very small, an enrichment method called fluidized bed enrichment was used to concentrate the analyte before NIR measurement. For correcting spectra between different brands and measurement dates, the spectra of Bright dairy measured on 2/5/2011 were assigned as primary spectra. And the spectra of Yili dairy measured on 2/5/2011 and Bright dairy measured on 17/5/2011 were set as secondary spectra for changes in brand and measurement date, respectively. Two spectral pretreatment methods were applied to preprocess spectra: multiple scattering correction (MSC) and the combination of MSC and differentiation (Savitzky–Golay differentiation). After spectral pretreatment, two calibration transfer methods including canonical correlation analysis (CCA) and piecewise direct standardization were used to correct the secondary spectra. The results showed that although the variations in both brand and measurement date can cause secondary spectra to deviate from primary spectra, the CCA under the combination of MSC and differentiation can correct the secondary spectra to use the model of primary spectra with high accuracy.     

Processing of extended shelf life milk using microfiltration

Extended shelf life (ESL) milk was processed with integrated microfiltration (pore size 1.4 µm). The germ-enriched retentate was not used for the final whole milk. Microfiltration led only to a negligible change in the content of the main components of the ESL product compared with the source milk. The total protein was only slightly decreased (0.02–0.03%) and the ratio of the protein fractions was unchanged within the measurement accuracy. The furosine content of the isolated fat globuline membrane fraction could be used as a diagnostic to prove cream had been subjected to high-temperature treatment. The shelf life of the ESL milk was distinctly prolonged compared to HTST-pasteurized milk.     

微生物法测定婴幼儿乳粉中烟酸含量

目的建立微生物法测定婴幼儿乳粉中烟酸含量的分析方法。方法通过比较不同浓度标准曲线对同一标准物质样品的测定结果,确定准确最优的线性浓度范围,并在该标准范围内进行重复性及加标回收实验。结果当烟酸标曲线性浓度范围为5.0～50.0ng时,线性关系良好(r~2=0.996)。该方法的相对标准偏差(relative standard deviation, RSD)为1.71%(n=6),平均回收率为94.4%。结论该方法稳定准确,适合用于测定婴幼儿乳粉中烟酸的含量。