ULTRAVIOLET SPECTROPHOTOMETRIC DETERMINATION OF PROTEIN IN SOME FOOD PRODUCTS

SUMMARY— The spectrophotometric method of estimating the protein content of whole milk at wavelength 280 nm (Nakai and Le, 1970) was modifed for corned beef, flour, bean and egg yolk. A clear solution was obtained by adding 50% sulfuric acid or 2N sodium hydroxide with and without 7–8M urea depending on solubility of the food products. The protein content was calculated from the absorbance at 280, 243 or 215 nm on o spectrophotometer. The correlation coefficient between the absorbance and the protein by Kjeldahl method was 0.99, 0.99, 0.99 and 0.99 with the coefficient of variability of Kjeldahl protein for absorbance of 1.8, 4.1, 3.8 and 4.1% for bean, corned beef, egg yolk and flour, respectively.     

Nisin及其在果蔬制品中的应用

Nisin是一种新型天然食品防腐、保鲜剂，它是目前世界上唯一允许使用在食品防腐方面的抗菌素。本文综述了Nisin的结构、性质、作用机理等，并论述其在蔬菜制品上的应用。     

生活饮用水和肉制品中亚硝酸盐的快速测定

实验建立了一种简便、快速测定亚硝酸盐含量的新方法。在这种方法中，NO2^-与对氨基苯磺酸发生重氮化反应，将反应液缓慢倒入由吸附1-萘酚的D290树脂填充的吸附柱中生成红色的偶氮染料。亚硝酸盐测定的线性范围为0．1μg／mL～10.0μg／mL，检出限为0．1μg／mL。将显色结果与标准比色卡比对可进行生活饮用水和肉制品中亚硝酸盐含量的定性和半定量分析，结果满意。     

EFFECT OF SUGARS AND MAILLARD REACTION PRODUCTS ON POLYPHENOL OXIDASE AND PEROXIDASE ACTIVITY IN FOOD

The effect of glucose, fructose and sucrose aqueous solutions on polyphenol oxidase (PPO) and peroxidase (POD) activity was studied. Solutions with high concentrations of sugars showed a slight inhibiting effect on PPO while an activation was observed for POD. Glucose solutions, tested after sufficient heat treatments, showed an inhibiting action only on PPO activity. Maillard reaction products, obtained by heating a glucose/glycine solution, caused an unexpectedly strong inhibiting effect on both enzymes. The inhibition became more evident with increasing heating times of the glucose/glycine solution, reducing the activity of PPO and POD nearly to zero. Thus, the Maillard reaction products, already known for their antioxidant properties, were also shown to have a strong inhibiting effect on enzymatic browning.     

DETERMINATION OF FERMENTABLE SUGARS AND NITROGENOUS COMPOUNDS IN WORT BY NEAR- AND MID-INFRARED SPECTROSCOPY

Near-infrared (NIR) and mid-infrared (MIR) spectroscopy techniques were evaluated for the analysis of brewery worts. Good calibrations were obtained for maltose (SEP_NIR 2.6 g/l, SEP_MIR 2.0 g/l), glucose SEP_NIR 0.6 g/l, SEP_MIR 0.4 g/l) and the sum of fermentable sugars (SEP_NIR 2.8 g/l, SEP_MIR 2.6 g/l) with both methods. Satisfactory calibration was obtained for maltotriose (SEP_NIR 1.4 g/l, SEP_MIR 1.5 g/l), nitrogen (SEP_NIR 122 mg/l, SEP_MIR 92 mg/l) and free alpha-amino nitrogen (SEP_NIR 29 mg/l, SEP_MIR 23 mg/l). MIR spectroscopy gave slightly better results, but sample handling was more difficult than in the NIR region. The NIR measurement is simple and rapid and could be used for on-line process control during mashing. No sample preparation other than filtration is needed.     

EXTRACTION, PURIFICATION AND DETERMINATION OF PECTIN IN TROPICAL FRUITS

The optimum pH, time and acid solution-fruit ratio for the extraction and purification of pectin was determined utilizing grapefruit rinds as a pectin source. The determination of pectin content utilizing the ethanol yield method and the calcium pectate method was conducted in 13 and 7 tropical fruits, respectively. The variation in pectin content from the green to the mature and to the ripe stages was determined in 6 fruits. Finally, estimation of the percentage of methoxylation, as an indication of the gel forming potential of the pectin, was measured in 3 of the most promising pectin source fruits.     

DETERMINATION OF CARBOXYMETHYLCELLULOSE IN FOOD PRODUCTS

SUMMARY –A method was developed to determine sodium carboxymethylcellulose (CMC) in food products containing three or more hydrocolloids after prior removal of interfering substances. Papain was used to digest the proteins; calcium chloride was added to precipitate algin and pectic acid and sulfated hydrocolloids were then precipitated by cetylpyridinium chloride (CPC) in the presence of 0.5M NaCl. Celite 535 or Hyflo Super Cel was added, the mixture filtered over Reeve angel No. 202 paper and the residue washed with 0.01% CPC-0.01M NaCl. The filtrate and washings were collected and diluted with distilled water to attain a final concentration of 0.2M NaCl at which concentration the CMC was precipitated selectively by CPC. The mixture was filtered over a Celite 535 or Hyflo Super Cel column and the residue washed with 0.5M Na₂SO₄ and then with 0.01% CPC-0.01M NaCl until the washings were negative to the phenol-H₂SO₄ reagent. Finally, the residue on the column was washed with hot 30% H₂SO₄ and the hydrolyzed CMC in the eluate determined by the 2,7-naphthalenediol test. Based on the amount added to mixtures, recoveries of 75–86% were obtained from milk and other highly complex and proteinaceous products, with a standard deviation of ± 0.58 mg, when 10 mg of CMC were added. The determination is critically dependent upon the degree of substitution (DS) of the CMC. As a consequence, the method cannot be applied with absolute certainty to unknown samples.     

IDENTIFICATION OF PROTEOLYTIC PRODUCTS AS INDICATORS OF QUALITY IN GROUND AND WHOLE MEAT

This study was devoted to the identification of specific peptides and proteins which can serve as indicators of spoilage in meat. Samples of ground and whole meat were subjected to storage at 4C; at intervals of 0, 2, 4, 8, 12 and 16 days, samples were analyzed for pH and microbial populations and subjected to extraction and separation of individual sarcoplasmic and myofibrillar peptides and proteins by SDS (sodium dodecyl sulfate) and native electrophoresis and by RP-HPLC. Sarcoplasmic protein and peptide fractions from RP-HPLC were collected and identified by (electrospray ionization mass spectrometry) ESI-MS. The results demonstrated substantial differences in microbial population and pH between ground and whole meat during storage. Separation by SDS-electrophoresis showed substantial changes in myofibrillar protein of ground meat after 12 days and of whole meat after 16 days of storage. Separation and identification of sarcoplasmic proteins by SDS-electrophoresis and by RP-HPLC followed by ESI-MS revealed the disappearance of a protein fraction band of MW 36 kDa after 8 days of storage in ground and whole meat.     

鱼腥草叶总黄酮含量的测定及鉴别

以芦丁为标样，选择测定波长为502nm，用分光光度法测定鱼腥草叶总黄酮含量，结果表明：芦丁浓度在8．12μg/mL～48．72μg/mL范围内，吸光度和浓度呈良好的线性关系（R。=0．9998），测定的平均回收率为99．4％，精密度试验RSD：0．49％，重现性试验RSD：2．07％。该方法精密度高，结果稳定可靠。此外用纸色谱和显色反应对鱼腥草叶黄酮作了初步鉴别。     

槟榔果及其制品中总酚和单宁含量的测定

以分光光度法对槟榔果果实及其加工后制品中的总酚、缩合单宁和原花青素进行了测定.结果表明,不同槟榔青果间酚类物质含量差异明显,榔玉的酚类物质含量远高于槟榔青果和槟榔外壳.经加工后的槟榔制品中,酚类物质含量明显减少.     

盐酸萘乙二胺分光光度法测定肉制品中的亚硝酸盐

用盐酸萘乙二胺分光光度法对市售的9种肉制品中的亚硝酸盐含量进行了测定,并选取其中的3种进行了精密度试验和回收率试验,RSD在1.3～1.6之间,回收率在95%～103%之间.所测9种肉制品样品中亚硝酸盐含量在0.64 mg/kg～12.31 mg/kg之间,均符合我国食品添加剂使用卫生标准.散装腊肉腊肠的亚硝酸盐含量高于真空包装的腊肉腊肠中亚硝酸盐含量;在同一种包装制品中,腊肉的肥肉部分亚硝酸盐含量高于瘦肉部分的亚硝酸盐含量.