Determination of nitrogen and protein content of meat and meat products

Chemical and instrumental methods for determination of nitrogen and protein are reviewed for their mode of action and utility in analysis of meat proteins and products. Although the Kjeldahl digestion method is satisfactory for determining total nitrogen, it is imprecise for determining total protein content. Presence of variable amounts of nonprotein nitrogenous components and of connective tissue proteins such as collagen and elastin produces error if the formula (N X 6.25) is used to calculate crude protein. Such fibrous proteins have higher nitrogen levels (over 18%) than other muscle proteins (about 16%), and a higher than actual protein value will be determined unless a lower conversion factor is used to correct for their content. To determine meat protein content more accurately, a combination of Kjeldahl determination with one or more additional tests to correct for nonprotein and fibrous protein content is recommended. The choice of the additional method(s) is based on the user's requirement for protein characterization, available time, type of meat product, and sample size.     

代谢组学在肉及肉制品品质监测中的应用

代谢组学是通过研究机体受外界干扰前后小分子代谢物(分子量1 500 Da)的变化,进而探究其代谢机制的新兴科学。近年来,代谢组学在肉品科学研究领域受到广泛关注。但目前基于该技术监测宰前因素(遗传因素、肌肉部位及饲喂方式)及宰后成熟(时间、方式)对肉及肉制品品质影响的相关研究仍缺乏系统总结。同时,代谢组学技术的引入,也为肉品货架期预测、肉制品加工工艺优选、产地溯源及真伪鉴别提供了新的思路。因此,该研究概述了近年来代谢组学常用的分析检测技术(核磁共振技术、气相色谱质谱联用技术、液相色谱质谱联用技术)及数理统计方法(主成分分析、偏最小二乘判别分析等),重点对代谢组学在肉品生产诸多环节(动物饲喂、屠宰、加工等)中的最新研究进展进行综述,最后总结了目前肉品代谢组学研究中存在的代谢产物检测有限、试验重复性差等问题并认为多组学联合分析是监测肉品品质的未来发展方向,以期为其在肉品科学领域的应用提供参考。     

Determination of total lipid and lipid subclasses in meat and meat products

Current interest in physiological and nutritional activities of the sterol, polyunsaturated fatty acid, and polar lipid fractions of meats and other foods indicates that analytical methods for lipids should be evaluated on their ability to recover and quantitate these classes. Current methods of lipid isolation furnish an extract that is dependent on the solvent(s) used, the type of food material, the temperature of extraction, and the relative proportions of the lipid classes present. Extraction with ethers or other relatively nonpolar solvents removes principally the neutral fats and nonpolar lipids. For an approximation of the crude fat content, such extraction is often sufficient, because the nonpolar fraction generally constitutes over 90% of the total lipids present. The polar lipids include the biochemically important (omega-3) and (omega-6) polyunsaturated fatty acid classes; thus, the method of lipid extraction of food products becomes relevant for a more complete and valuable characterization of their nutritional value. The various methods of lipid determination for meat products are examined for their total recovery of these important lipid groups. A sequential extraction in conjunction with subsequent analytical methods is recommended.     

Chemical analysis of meat products

Meat, particularly muscle tissue, is basically a 3-component system of protein, moisture, and fat. Although this seems a simple analytical system in which to monitor product composition for regulatory compliance, the simplicity quickly erodes when meat is formulated into the broad variety of products commercially available today. Alternative protein sources, as well as preservatives, binders, extenders, emulsifiers, spices, and other flavoring ingredients, add to the analytes of concern and highlight the need for analytical methods suitable to support inspection and labeling requirements to ensure product compliance. Some key issues are noted which involve protein quality analysis, rapid compositional analysis, isolated soy protein analysis, and minced fish in meat products.     

Determination of soy in meat

A number of methods may be used for determining soy flour in meat products. Highly purified soy products are more difficult to determine because the nonprotein components used to quantify the flour are reduced. Immunoassays have been used to directly measure protein content of soy products. Immunological methods for determination of soy proteins in meat are complicated by changes in the structure of the soy proteins during processing. These changes alter the available epitopes, changing the immunoreactivity of soy proteins. The epitopes available are dictated by the details of the processing. Other workers circumvented this problem by denaturing the soy protein with urea and mercaptoethanol, and then removing these agents by dialysis; whatever the initial protein conformation, all soy samples came to the same final conformation after the denaturing agents were removed. The assay used antibody made against the "renatured protein." These steps made the assay long and laborious. Attempts to develop a rapid assay were complicated by the same protein denaturation problems. Sodium dodecylsulfate gel electrophoresis coupled with immunoblotting may be the best quantitative approach.     

Determination of fluoride in deboned meat

A rapid analytical procedure was investigated for measuring the fluoride content of deboned meat, using a specific ion electrode. The method involves a defatting procedure followed by chelation with EDTA before final solubilization and measurement. Fluoride content is determined by comparison to a standard curve on 2-cycle semi-logarithmic graph paper of fluoride ion vs. millivolt readings. The method is applicable to hand and mechanically deboned pork, beef, and poultry within a range of from 0.7 to 25.0 ppm fluoride. Higher concentrations may be measured by analyzing additional standards and extending the curve into a third cycle. The curve is not linear below 0.7 ppm. Samples less than 0.7 ppm may be estimated by extrapolation. Studies with fortified samples indicate average recoveries of 92 +/- 4.2%. The standard deviation of duplicates within the laboratory is 0.5 ppm in the range stated above.     

Impact of disaggregation of composite foods on estimates of intakes of meat and meat products in Irish adults

OBJECTIVE: To evaluate the impact of the disaggregation of composite foods on intake estimates of meat and individual meat categories and on the contribution of meat to nutrient intakes in Irish adults. DESIGN: Data were analysed from the North/South Ireland Food Consumption Survey, which used a 7-day food diary to estimate food intake. Of 742 food codes that contained meat, 320 were codes for meat consumed as an individual portion and 422 were composite foods and were disaggregated to estimate the meat content. SUBJECTS: A nationally representative sample of 475 men and 483 women (not pregnant or lactating) from the Republic of Ireland aged 18-64 years. RESULTS: The mean intake of meat was 134 g day(-1) in consumers (98.5%) and men (168 g day(-1)) consumed significantly more (P<0.001) than women (102 g day(-1)). Mean intakes of meat were higher in subjects with manual skilled occupations (P<0.01) and lower in those with third-level educational qualifications (P<0.05). Without disaggregating meat from composite foods, meat intake was overestimated by 43% (57 g day(-1)) and varied widely by meat category. Meat disaggregated from composite foods contributed 25% of meat intake. The contribution meat made to nutrient intakes ranged from 29% for protein, vitamin B12, zinc and niacin to 20% for vitamin D, 16% for vitamin B6, 15% for thiamine and 14% for iron. CONCLUSIONS: Failure to disaggregate meat from composite foods substantially overestimates meat intake, with a large variation between meat categories. This has important implications for estimates of meat intakes in nutritional epidemiological studies and for food safety purposes.     

Colorimetric determination of hydroxyproline as measure of collagen content in meat and meat products: NMKL collaborative study

A colorimetric method for the determination of hydroxyproline as a measure of collagen in meat and meat products has been collaboratively studied in 18 laboratories. The method includes hydrolysis with sulfuric acid, oxidation with chloramine-T, and formation of a reddish purple complex with 4-dimethylaminobenzaldehyde. Five frozen and 3 freeze-dried samples were tested, ranging in content from 0.11 to 0.88% and from 0.39 to 4.0% hydroxyproline, respectively. The mean values of 2 identical samples were 0.245 and 0.251%. The average recovery from a spiked sample was 96.1%. The hydroxyproline content of a known sample (a mixture of 2 samples in the ratio 5:2) was calculated to 1.42%, which agrees well with the analytical result, 1.40%. In comparison with other collaborative studies, based on the ISO analytical method, the repeatability and reproducibility of this method agree well with the other results. This method was accepted as an official NMKL method by all national Committees, and has been adopted official first action by AOAC as an NMKL-AOAC method.     

Rapid methods for determination of meat composition

Rapid analytical procedures are needed to determine the total protein, moisture, and fat content of meat and poultry products. During the past 5 years, the U.S. Department of Agriculture (USDA) has been studying various methods involving instrumentation that test for these constituents either in combination or separately. The studies are initiated on request of the Department or the instrumentation manufacturer and are conducted at the manufacturers' facilities using 2 sets of samples preanalyzed by conventional means. One set, with values, is used for calibration purposes, the other set is tested as unknown samples. The resultant data are evaluated statistically vs the conventional test results. Most studies show the usefulness of these rapid tests for product quality control, but some fall short of regulatory requirements because of unacceptable bias or variability. Typical within-product standard deviations obtained in rapid methods instrumentation tests have ranged from 0.47 to 0.67 for percent total protein, 0.73 to 1.71 for moisture, and 0.41 to 1.14 for fat. For conventional methods, the acceptable USDA performance criteria for repeatability are standard deviations of less than 0.24 for protein, 0.46 for moisture, and 0.63 for fat. Improvements in instrumentation are being made and studies continue.     

Direct and highly species-specific detection of pork meat and fat in meat products by PCR amplification of mitochondrial DNA

Highly species-specific primers for pork D-loop mtDNA have been designed. Use of these and restrictive PCR amplification conditions has improved a reliable and rapid method for detecting a PCR-amplified 531 bp band from pork. It has been proved useful for detecting both pork meat and fat in meat mixtures, including those dry-cured and heated by cooking. Absence of response in PCR-amplified samples or mixtures from bovine, ovine, chicken, and human was also demonstrated. Furthermore, wild boar and pork samples can be also easily distinguished by a simple AvaII restriction analysis.     

熟肉真空冷却过程的数值模拟

该文主要针对熟肉的真空冷却进行了研究,并模拟了真空冷却过程中的系统的压力变化、产品温度变化以及质量损失。模拟结果与试验数据相比较,结果发现数值模拟的结果与试验数据吻合得很好,模拟的压力与试验测定的压力的最大偏差在200 Pa以内;模拟温度与试验温度的差值在5℃以下;模拟与试验的质量损失的偏差在5％左右。数据模拟结果表明,文中的数学模型可以被用于预测真空冷却过程中的压力、温度和质量损失的变化,来分析熟肉的真空冷却过程。     

肉鸭粪便排放特征的季节性变化

本研究旨在探讨肉鸭粪便的特性,并对其不同季节的污染物排放特征进行评价,为肉鸭养殖场废弃物处理和资源化利用提供依据。分4个季节进行了北京Z型肉鸭饲养试验(饲养期37 d),记录采食量、产粪量,并定期测定饲料和粪便中水分和有机质含量及总氮(TN)、P、Cu、Zn含量。结果表明:肉鸭粪便中Zn含量秋季最高, TN、P、Cu含量均为冬季最高;冬季粪便中TN含量极显著高于夏、秋两季(P0.01); P、Cu含量在冬季均极显著高于其余季节(P0.01);粪便中Cu、Zn含量均为春季最低。肉鸭粪便平均含水率为84.61%,夏季最高,春季最低;平均有机质含量为83.38%,表现为冬春秋夏。春夏秋冬四季的粪便产生量分别为338.3g?d~(-1)?只~(-1)、275.9 g?d~(-1)?只~(-1)、317.6 g?d~(-1)?只~(-1)和327.0 g?d~(-1)?只~(-1),夏季最低。TN、P、Cu、Zn的排泄系数分别为2.13 g?d~(-1)?只~(-1)、2.48g?d~(-1)?只~(-1)、2.56 mg?d~(-1)?只~(-1)、21.10 mg?d~(-1)?只~(-1);春冬两季的TN日排泄量显著高于夏秋两季; P的日排泄量表现为冬季极显著高于其余3个季节;夏季Cu的日排泄量与春季差异不显著,极显著低于秋、冬两季(P0.01)。TN在秋季排泄占比最低,春季最高;而P排泄占比为秋季最高,春季最低。肉鸭在饲养期TN、P、Cu、Zn的排泄量与对应元素的摄入量具有极显著的正相关关系(P0.01)。研究表明:季节因素能对肉鸭粪便中含水率、有机质、TN、P、Cu和Zn含量产生显著影响,同时各季节各元素的排泄量与相应元素的摄入量显著相关。     

适宜宰后成熟时间提高牦牛肉品质

为了研究宰后成熟时间对牦牛肉品质的影响,该研究采集牦牛背最长肌,真空包装后进行成熟排酸,成熟期间取样测定pH值、剪切力、持水能力、色度,并通过标准化和动力学模型分析研究其变化规律。结果发现,除了黄色度b*以外,其余品质指标均在21d的成熟过程中有显著变化(P0.05)。相比于7 d成熟,延长至21 d的长时间成熟可将牦牛肉剪切力显著地降低47%(P0.05),并将蒸煮损失和红色度a*分别显著地提高28%和32%(P0.05)。通过对比线性函数、指数函数、二次函数对牦牛肉品质变化的预测效果,结果发现二次函数预测模型与牦牛肉宰后品质变化具有相对最好的拟合度(决定系数R2范围为0.90~0.98),其中pH值、剪切力、加压损失、亮度L*值随时间的变化函数呈现出凸函数特征,而蒸煮损失和a*值则呈现出凹函数特征。结果表明,相比于牛肉品质宰后预测中常用的线性函数和指数函数而言,二次函数更适用于牦牛肉品质变化的预测,而将牦牛肉宰后成熟时间延长至21 d可有效降低牦牛肉剪切力。研究可为牦牛肉品质控制提供参考。     

亚麻籽胶在低脂午餐肉中的应用

为研究亚麻籽胶在低脂午餐肉中的应用,采用全质构分析、感官鉴评、扫描电子显微镜、动态流变学等方法,研究了添加单一亚麻籽胶和添加亚麻籽胶与卡拉胶的复配胶对低脂午餐肉的质构的影响。结果表明,添加单一亚麻籽胶的午餐肉的持油持水性较好,但在质构方面,比添加复配胶CK(卡拉胶与魔芋胶以1∶1复配)的对照样品差。亚麻籽胶与卡拉胶按1∶1(w/w) 复配加入午餐肉中,其效果明显优于添加单一亚麻籽胶的样品,显著提高了午餐肉的品质,质构也与添加复配胶CK的对照样品接近,因此可将亚麻籽胶与卡拉胶以1∶1复配添加于低脂午餐肉中。     

Public health implications of meat production and consumption

The high level of meat and saturated fat consumption in the USA and other high-income countries exceeds nutritional needs and contributes to high rates of chronic diseases such as cardiovascular disease, diabetes mellitus and some cancers. Affluent citizens in middle- and low-income countries are adopting similar high-meat diets and experiencing increased rates of these same chronic diseases. The industrial agricultural system, now the predominant form of agriculture in the USA and increasingly world-wide, has consequences for public health owing to its extensive use of fertilisers and pesticides, unsustainable use of resources and environmental pollution. In industrial animal production there are public health concerns surrounding feed formulations that include animal tissues, arsenic and antibiotics as well as occupational health risks and risks for nearby communities. It is of paramount importance for public health professionals to become aware of and involved in how our food is produced.     

Amino acid analysis for meat protein evaluation

The Food Safety and Inspection Service procedure for determination of essential amino acid content of mechanically processed products from red meat animals and poultry is based on hydrolysis of a powder prepared by blending samples in acetone-chloroform. The hydrolysis procedure incorporates thioglycolic acid to prevent loss of tryptophan. Aliquots of prepared hydrolysates are injected into a liquid chromatographic system, using gradient elution on an ion-exchange column for separation. The system also uses post-column hypochlorite oxidation coupled with orthophthalaldehyde reagent and fluorescence detection. Modification of the elution program allows concurrent determination of tryptophan with minimal added cost. Chromatograms from beef, pork, and poultry products show adequate separation and quantitation of beta-alanine, 1-methyl-histidine, and 3-methyl-histidine, indicating that the procedure could be used to estimate muscle content of products. A colorimetric procedure for assay of hydroxyproline was introduced and validated as an adjunct method for protein quality estimation.