Effect of somatic cell count on Prato cheese composition

The objective of this research was to evaluate the effect of 2 levels of somatic cell counts (SCC) in raw milk on Prato cheese composition, protein and fat recovery, cheese yield, and ripening. A 2 × 6 factorial design with 3 replications was performed in this study: 2 levels of SCC and 6 levels of storage time. Initially, 2 groups of dairy cows were selected to obtain low (<200,000 cells/ mL) and high (>600,000 cells/mL) SCC in milks that were used to manufacture 2 vats of cheese: 1) low SCC and 2) high SCC. Milk, whey, and cheese compositions were evaluated; clotting time was measured; and cheese yield, protein recovery, and fat recovery were calculated. The cheeses were evaluated after 5, 12, 19, 26, 33, and 40 d of ripening according to pH, moisture, pH 4.6 soluble nitrogen, 12% trichloroacetic acid soluble nitrogen as a percentage of total nitrogen, and firmness. High-SCC milk presented significantly higher total protein and nonprotein nitrogen and lower true protein and casein concentrations than did low-SCC milk, indicating an increased whey protein content and a higher level of proteolysis. Although the pH of the milk was not affected by the somatic cell level, the cheese obtained from high-SCC milk presented significantly higher pH values during manufacture and a higher clotting time. No significant differences in cheese yield and protein recovery were observed for these levels of milk somatic cells. The cheese from high-SCC milk was higher in moisture and had a higher level of proteolysis during ripening, which could compromise the typical sensory quality of the product.     

相对湿度对牦牛毛霉霉菌奶酪成熟过程中品质的影响

为研究不同相对湿度对牦牛霉菌奶酪成熟过程中品质的影响,并确定其在成熟过程中的最佳相对湿度和成熟时间,本实验以牦牛霉菌奶酪为试验材料,将其分别置于相对湿度为30%、50%、70%条件下成熟,测定其在成熟过程中感官品质、质构、pH、营养成分、蛋白质和脂肪分解指标以及挥发性风味物质的变化。结果表明:相对湿度为50%,成熟6 d时,奶酪的感官评分最高且质构较佳。随着成熟时间的延长,三个实验组的水分、蛋白质和脂肪含量呈下降趋势,灰分逐渐上升;pH呈先下降后保持不变或上升。除成熟第0和8 d外,在同一成熟时间点,相对湿度为30%的水分含量均显著低于其他两组(P<0.05);灰分、蛋白质和脂肪含量变化不明显;相对湿度为50%的pH下降速度最快。pH4.6醋酸盐缓冲液可溶性氮含量、12%三氯乙酸可溶性氮含量、酸价和硫代巴比妥酸值均在奶酪成熟过程中不断升高,且相对湿度为50%时值最高。对牦牛毛霉霉菌奶酪挥发性风味物质检测发现,相对湿度为50%,成熟第6 d时,奶酪的风味物质含量较高。因此,牦牛霉菌奶酪在相对湿度为50%,成熟6 d时,其品质最佳。     

PRELIMINARY OBSERVATIONS ON EFFECTS OF FAT CONTENT AND DEGREE OF FAT EMULSIFICATION ON THE STRUCTURE-FUNCTIONAL RELATIONSHIP OF CHEDDAR-TYPE CHEESE

Cheddar type cheeses of different fat contents were produced and denoted: full-fat (FFC), 306g/kg; half-fat (HFC), 174 g/kg; and low fat (LFC, 13 g/kg). Full-fat Cheddar cheese (FFCH) was also prepared from milk which had been homogenized at first and second stage pressures of 25 and 5 MPa, respectively. The cheeses were held at 4C for 30 days and at 7C for the remainder of the 190-day ripening period. Reducing the fat level from 174 to 13 g/kg resulted in decreases in contents of moisture in nonfat substance and pH 4.6 soluble N as a percentage of total N (pH4.6SN), and increases in the contents of moisture, protein and intact casein. Homogenization of cheesemilk resulted in a slight increase in moisture content and an increase in pH4.6SN. Confocal laser scanning microscopy revealed that the extent of fat globule clumping and coalescence in both the unheated and heated (to 95C) cheeses decreased with homogenization of the cheesemilk and with fat reduction. Homogenization of the cheesemilk and reducing the fat content of the cheese resulted in a decrease in the flowability and stretchability of the melted cheese. Dynamic measurement of the viscoelastic changes on heating the cheese from 20 to 90C showed that reduction of fat content resulted in a decrease in the magnitude of the phase angle, δ, at temperatures >50C. At temperatures<∼60C, the storage modulus, G', increased on reducing the fat content from 306–174 g/kg to 13 g/kg. Homogenization resulted in a marked decrease in δ at temperatures>45–50C, with δ_max typically decreasing from ∼65–70° in the FFC to ∼35° in the FFCH.     

新疆特色酸凝干酪成熟期间理化特性研究

对新疆特色酸凝干酪成熟50 d内理化特性研究结果表明:干酪成熟过程中水分含量减少,变化差异极显著(P<0.01);pH值在前20 d内下降较快,达到最低值;脂肪含量减少;pH4.6可溶性氮和12%TCA可溶性氮含量有不同程度的上升,且pH4.6可溶性氮比12%TCA可溶性氮含量高;通过SDS-PAGE图谱分析,得知干酪在成熟30 d之后蛋白质降解程度较深;乳糖含量在成熟前期下降较快,后期较慢。     

Composition of Ragusano cheese during aging

Ragusano cheese is a brine-salted pasta filata cheese. Composition changes during 12 mo of aging were determined. Historically, Ragusano cheese has been aged in caves at 14 to 16 degrees C with about 80 to 90% relative humidity. Cheeses (n = 132) included in our study of block-to-block variation were produced by 20 farmhouse cheese makers in the Hyblean plain region of the Province of Ragusa in Sicily. Mean initial cheese block weight was about 14 kg. The freshly formed blocks of cheese before brine salting contained about 45.35% moisture, 25.3% protein, and 25.4% fat, with a pH of 5.25. As result of the brining and aging process, a natural rind forms. After 12 mo of aging, the cheese contained about 33.6% moisture, 29.2% protein, 30.0% fat, and 4.4% salt with a pH of 5.54, but block-to-block variation was large. Both soluble nitrogen content and free fatty acid (FFA) content increased with age. The pH 4.6 acetate buffer and 12% TCA-soluble nitrogen as a percentage of total nitrogen were 16 and 10.7%, respectively, whereas the FFA content was about 643 mg/100 g of cheese at 180 d. Five blocks of cheese were selected at 180 d for a study of variation within block. Composition variation within block was large; the center had higher moisture and lower salt in moisture content than did the outside. Composition variation within blocks favored more proteolysis and softer texture in the center.     

地衣芽孢杆菌凝乳酶对切达干酪成熟过程中蛋白水解的影响

利用地衣芽孢杆菌凝乳酶制作切达干酪和切达干酪类似物,分析干酪成熟过程中各蛋白水解指标的变化规律,以揭示地衣芽孢杆菌凝乳酶对切达干酪成熟过程中蛋白水解的影响。结果表明,CDF组（添加地衣芽孢杆菌D3.11凝乳酶所制切达干酪）、CD3组（添加地衣芽孢杆菌D3.11凝乳酶但未添加发酵剂制成的干酪类似物）和CCF组（添加商品凝乳酶所制切达干酪）干酪蛋白含量、pH 4.6-可溶性氮、12%三氯乙酸-可溶性氮、5%磷钨酸-可溶性氮、总游离氨基酸含量均随着成熟时间延长呈显著增加趋势,并且成熟期间CDF组干酪均显著高于CCF组干酪（P＜0.05）;十二烷基硫酸钠-聚丙烯酰氨凝胶电泳分析表明,CDF组干酪α-酪蛋白水解程度较大;pH 4.6-可溶性肽段分析表明,随着干酪的成熟,总肽含量呈先增加后下降趋势,但疏水性肽与亲水性肽的比值呈持续下降趋势,在成熟第6个月时,CDF组、CD3组和CCF组干酪疏水性肽与亲水性肽比值分别为2.668、2.822、3.788。主成分分析表明,3 组干酪的蛋白水解程度与成熟度呈正相关,与疏水性肽和亲水性肽的比值呈负相关。以上结果表明,利用地衣芽孢杆菌凝乳酶制作的干酪蛋白水解度更高,但其疏水性肽比例较小,研究结果可为地衣芽孢杆菌凝乳酶在干酪生产中的应用提供理论依据。     

姜黄素-牛血清白蛋白复合物对牦牛乳干酪品质的影响

本研究旨在探讨牛血清白蛋白构建包封姜黄素的纳米复合物作为提高牦牛乳干酪成熟品质的潜在策略,试验测定了牦牛乳干酪的水分、灰分、粗蛋白、粗脂肪、pH、pH4.6醋酸盐缓冲液可溶性氮、12%三氯乙酸溶液可溶性氮、硫代巴比妥酸反应物、酸价以及感官指标。结果表明:姜黄素-牛血清白蛋白纳米复合物的存在能够显著提升牦牛乳干酪的水分与蛋白质含量（P<0.05）,还能促进乳酸菌的增殖并抑制杂菌生长,有助于牦牛乳干酪中脂肪与蛋白质的分解,形成短链脂肪酸和多肽,呈现较好的独特风味;而复合物添加量过高会减弱益生效果,降低脂肪和蛋白分解效率,同时也引起干酪色泽发黄而降低感官评分。在成熟过程初期,感官评分显著提升（P<0.05）得益于蛋白质水解反应以及脂肪分解反应的发生,但成熟期过长会导致苦味小肽等物质的堆积,形成不良风味,影响感官品质。总体来讲,姜黄素-牛血清白蛋白复合物添加有助于牦牛乳干酪品质的提升,且复合物添加量为4%,成熟时间为60 d时,干酪产品品质最佳。     

Electrophoretic patterns and thin-layer chromatography of common cheeses in Egypt: Comparison and quantification

The extent of cheese ripening and the type of proteolysis and lipolysis of common cheeses in Egypt were measured by concentration of each of soluble tyrosine; soluble trypophan; amino N; soluble N/total N; total volatile fatty acids and free fatty acids, and by quantitative gel electrophoresis and thin-layer chromatography. The effects of concentration-related factors (e.g. moisture, salt and pH) on cheese protein and fat hydrolysis were also studied.

The results showed that, as a heterogeneous group of cheese, differences were marked in gross chemical composition and both the extent of cheese ripening and the relative proportions of protein, fat and their degradation products. Among the selected cheeses, ras cheese has higher values of ripening indices, while kariesh cheese has lower values. Increasing the salt content of mish cheese caused an inhibition in degradation of its protein and fat.

The principal protein regions in electrophoretic patterns and fractions of fat in TLC patterns were similar in number and relative mobility. In most of the cheeses, _s-casein was degraded more extensively than β-casein, while the whole of the γ-caseins were resistant to further hydrolysis. Also, there was close correlation between _s-casein and its degradation products. In spite of the absence of significant relationships between the soluble nitrogen and the relative amounts of unattached _s-, β-, and γ-caseins, the amino nitrogen and soluble tyrosine and tryptophan were in close correlation with _s- and β-caseins and their degradation products.

A positive relationship was noted between pH (from 4·40 to 5·85) and both protein and fat hydrolysis. The fat of roquefort cheese was more hydrolysed than other cheeses; however, the fat of the soft cheeses was less hydrolysed. Moreover, negative and highly significant correlations between triglycerides and their degradation by both TLC and chemical analysis were obtained.     

Proteolysis and microstructure of Piacentinu Ennese cheese made using different farm technologies

The aim of this study was to provide the biochemical and structural characterization of Piacentinu Ennese cheese and to evaluate the impact of different farm technologies on cheese proteolysis and microstructure. Fifteen cheeses were manufactured according to traditional technology, i.e., from raw milk and farmhouse rennet in the absence of starter culture. Pasteurized milk, commercial rennet, and starter were used for production of 20 nontraditional cheeses. Proteolysis in Piacentinu Ennese cheese was monitored during a 2- to 10-mo ripening time. Low rates of overall proteolysis were observed in cheese, as percentages of total N soluble at pH 4.6 and in 12% trichloroacetic acid were about 11.40 and 8.10%, respectively, after 10 mo of age. Patterns of primary proteolysis by urea-PAGE showed that alpha(s)-caseins were degraded to a larger extent than were beta-caseins, although a considerable amount of both caseins was still intact after 10 mo. Reversed phase-HPLC analysis of the cheese peptide fractions showed a slow decrease in the levels of hydrophobic peptides coupled to increasing levels of hydrophilic compounds as the cheese aged. The structural characteristics of Piacentinu Ennese cheese were evaluated by scanning electron microscopy after 2, 4, and 6 mo of age. The micrographs showed a sponge-like structural network with a well-distributed system of empty spaces, originally occupied by whey and fat. The microstructure changed during cheese ripening to become more compact with cavities of smaller size. Farm technology significantly affected cheese proteolysis and microstructure. Nontraditional cheeses had higher levels of pH 4.6-soluble N and showed a larger hydrolysis of alpha(s)-casein fractions by urea-PAGE analysis than did traditional cheeses. Large differences between cheese-types also concerned the patterns of secondary proteolysis. Nontraditional cheeses had higher levels of 12% trichloroacetic acid-soluble N and showed larger proportions of free amino acids and hydrophilic peptides in the HPLC profiles of the corresponding 70% ethanol-soluble N fraction than traditional cheeses. Nontraditional cheeses also had a more open structure with a coarser and less continuous appearance than did traditional cheeses. A large amount of variability in cheese proteolysis and structure within nontraditional treatment reflected farm-dependent changes in manufacturing conditions related to the use of various types of rennet and starter.     

雅致放射毛霉在干酪中的应用

雅致放射毛霉(Actinomucorelegans)是酿造腐乳的优良菌株,鉴于其优良发酵特性和蛋白酶解效力,将其引入到传统表面霉菌成熟干酪的发酵成熟过程中,研究该菌种对干酪成熟期间理化特性等各项指标的影响。理化特性研究表明,雅致放射毛霉表面成熟干酪在成熟期间水分含量由59%(W/W)下降到50%;外部和内部的pH都呈上升的趋势,最后达到pH7.0左右;乳酸含量由1.2%(W/W)下降到0.4%;pH4.6 SN(可溶性氮)和12%TCA(三氯乙酸)SN在成熟的30 d内分别由10.1%(W/W)和2.5%增长到了45.2%和27.1%;电泳结果表明,酪蛋白发生强烈水解;毛霉干酪游离脂肪酸含量在成熟末期达到6.54%(W/W);TPA硬度由450 g下降到150 g。毛霉干酪成品的主要成分指标为:水分50%,蛋白质20%(W/W),脂肪20.6%(W/W),盐含量2.1%(W/W),符合NY478-2002软质干酪农业部标准,与Camembert青霉干酪的理化指标相似。     

非发酵剂乳酸菌A-3和D-3对半硬质山羊奶干酪成熟的影响

目的获得加速半硬质山羊奶干酪成熟的非发酵剂乳酸菌菌株(non-starterlacticacidbacteria,NSLAB)。方法以前期分离自地中海地区山羊奶干酪中的2株优良NSLAB菌株为研究对象,测定其对干酪成熟过程中组成成分、微生物菌群、蛋白质水解和质构的影响。结果添加NSLAB菌株对干酪组成成分没有显著影响, NSLAB菌株没有影响乳球菌生长,在干酪成熟期间pH 4.6-SN和12%TCA-SN逐渐增加,且添加NSLAB的干酪在成熟30 d后显著增加了pH 4.6-SN和12%TCA-SN含量, 5%PTASN/TN的增加主要是由于乳酸菌中肽酶作用的结果, SDS-PAGE电泳结果说明添加NSLAB菌株的干酪中小分子多肽含量明显比对照干酪多,RP-HPLC分析得出干酪水溶性中肽的数量随着成熟时间增加。添加NSLAB菌株A-3没有改变干酪的硬度,使干酪的弹性增加。结论添加菌株A-3作为NSLAB的干酪样品中微生物自溶率高,蛋白水解程度强,质构性能良好,具有加速干酪成熟的潜力,是山羊奶干酪工业化生产的优良NSLAB。     

酶反应pH值对比萨干酪理化和功能特性的影响

研究了酶反应条件pH值对比萨干酪理化性质及功能特性的影响。选择在pH值为6.6与6.4时添加凝乳酶。通过对比萨干酪水分、脂肪及蛋白质质量分数的统计分析表明,酶反应条件pH值对比萨干酪的脂肪和蛋白质质量分数没有显著影响,对水分质量分数有显著影响。酶反应条件pH值为6.6比萨干酪水分质量分数为47.70%,pH值为6.4的水分质量分数为51.80%。通过测定干酪中pH值为4.6缓冲溶液及12%的TCA中可溶性氮质量分数,监测其水解情况。在贮藏过程中,可溶性氮质量分数不断增加。酶反应条件pH值影响比萨干酪的功能特性;酶反应条件pH值为6.6比萨干酪的硬度由最初为100.8N,50d后降低为88.9N,融化性增加了1.28cm,油析性增加了2.7%;pH值为6.4干酪功能特性的变化情况分别为:硬度降低为75.5N,融化性增加了2.16cm,油析性增加了3.1%。     

Texture,flavor, and sensory quality of buffalo milk Cheddar cheese as influenced by reducing sodium salt content

The adverse health effects of dietary sodium demand the production of cheese with reduced salt content. The study was aimed to assess the effect of reducing the level of sodium chloride on the texture, flavor, and sensory qualities of Cheddar cheese. Cheddar cheese was manufactured from buffalo milk standardized at 4% fat level by adding sodium chloride at 2.5, 2.0, 1.5, 1.0, and 0.5% (wt/wt of the curd obtained). Cheese samples were ripened at 6 to 8°C for 180 d and analyzed for chemical composition after 1 wk; for texture and proteolysis after 1, 60, 120, and 180 d; and for volatile flavor compounds and sensory quality after 180 d of ripening. Decreasing the salt level significantly reduced the salt-in-moisture and pH and increased the moisture-in-nonfat-substances and water activity. Cheese hardness, toughness, and crumbliness decreased but proteolysis increased considerably on reducing the sodium content and during cheese ripening. Lowering the salt levels appreciably enhanced the concentration of volatile compounds associated with flavor but negatively affected the sensory perception. We concluded that salt level in cheese can be successfully reduced to a great extent if proteolysis and development of off-flavors resulted by the growth of starter and nonstarter bacteria can be controlled.     

Evaluation of bitterness in Ragusano cheese

The appearance of undesirable bitter taste in Ragusano cheese was investigated by comparing the composition of 9 bitter cheeses with that of 9 reference cheeses of good quality by means of chemical, electrophoretic, and chromatographic analyses. Rates of proteolysis were significantly affected in cheeses of different quality. Primary proteolysis, as measured by pH 4.6-soluble N, was significantly greater in bitter cheeses compared with reference samples. Urea-PAGE profiles showed an almost complete breakdown of caseins in bitter cheeses and the further degradation of primary peptides into smaller compounds not detectable by this technique. Cheeses with defects had significantly lower levels of secondary proteolysis as reflected by the percentage of pH 4.6-soluble N soluble in 12% trichloroacetic acid and the amounts of total free amino acids. Peptides separated by reversed phase-HPLC revealed that the large and significant differences in peptide profiles of the soluble fractions between bitter and reference cheeses were mainly due to a much higher proportion of hydrophobic peptides in the former. The occurrence of bitterness in Ragusano cheese was therefore attributable to unbalanced levels of proteolysis and peptidolysis. Extensive degradation of caseins and primary peptides by activities of proteases produced large amounts of small- and medium-sized hydrophobic peptides that were not adequately removed by peptidases of microflora and therefore accumulated in cheese potentially contributing to its bitter taste. The presence of these compounds in bitter cheeses was related to high salt-in-moisture and low moisture contents that limited the enzymatic activities of microflora important in secondary proteolysis. Combining salt-in-moisture and the ratio of hydrophobic-to-hydrophilic soluble peptides resulted in the best logistic partial least squares regression model predicting cheese quality. Although bitterness is known to be rarely encountered in cheese at salt-in-moisture levels >5.0, all of the bitter cheeses analyzed in this study had salt-in-moisture levels much greater than this value. According to the logistic model, a risk of bitterness development may exist for cheeses with a midrange (5 to 10%) salt-in-moisture content but with an inadequate level of secondary proteolysis.     

Effect of pH and calcium concentration on proteolysis in mozzarella cheese

Low-moisture Mozzarella cheeses (LMMC), varying in calcium content and pH, were made using a starter culture (control; CL) or direct acidification (DA) with lactic acid or lactic acid and glucono-delta-lactone. The pH and calcium concentration significantly affected the type and extent of proteolysis in Mozzarella cheese during the 70-d storage period at 4 degrees C. For cheeses with a similar pH, reducing the calcium-to-casein ratio from -29 to 22 mg/g of protein resulted in marked increases in moisture content and in primary and secondary proteolysis, as indicated by polyacrylamide gel electrophoresis and higher levels of pH 4.6- and 5%-PTA-soluble N. Increasing the pH of DA cheeses of similar moisture content, from approximately 5.5 to 5.9, while maintaining the calcium-to-casein ratio almost constant at approximately 29 mg/g, resulted in a decrease in primary proteolysis but had no effect on secondary proteolysis. Comparison of CL and DA cheeses with a similar composition showed that the CL cheese had higher levels of alpha(s1)-CN degradation, pH 4.6- and 5%-PTA-soluble N. Analysis of pH 4.6-soluble N extracts by reverse-phase HPLC showed that the CL cheese had higher concentrations of compounds with low retention times, suggesting higher concentrations of low molecular mass peptides and free amino acids.     

Effect of fat reduction on chemical composition, proteolysis, functionality, and yield of Mozzarella cheese

Mozzarella cheese was made from skim milk standardized with cream (unhomogenized, 40% milk fat) to achieve four different target fat percentages in the cheese (ca. 5, 10, 15, and 25%). No statistically significant differences were detected for cheese manufacturing time, stretching time, concentration of salt in the moisture phase, pH, or calcium as a percentage of the protein in the cheese between treatments. As the fat percentage was reduced, there was an increase in the moisture and protein content of the cheese. However, because the moisture did not replace the fat on an equal basis, there was a significant decrease in the moisture in the nonfat substance in the cheese as the fat percentage was reduced. This decrease in total filler volume (fat plus moisture) was associated with an increase in the hardness of the unmelted cheese. Whiteness and opacity of the unmelted cheese decreased as the fat content decreased. Pizza baking performance, meltability, and free oil release significantly decreased as the fat percentage decreased. The minimum amount of free oil release necessary to obtain proper functionality during pizza baking was between 0.22 and 2.52 g of fat/100 g of cheese. Actual cheese yield was about 30% lower for cheese containing 5% fat than for cheese with 25% fat. Maximizing fat recovery in the cheese becomes less important to maintain high cheese yield, and moisture control and the retention of solids in the water phase become more important as the fat content of the cheese is reduced.     

Effect of proteolysis and calcium equilibrium on functional properties of natural cheddar cheese during ripening and the resultant processed cheese

The changes in proteolysis, calcium (Ca) equilibrium, and functional properties of natural Cheddar cheeses during ripening and the resultant processed cheeses were investigated. For natural Cheddar cheeses, the majority of the changes in pH 4.6 soluble nitrogen as a percentage of total nitrogen (pH 4.6 SN/TN) and the soluble Ca content occurred in the first 90 d of ripening, and subsequently, the changes were slight. During ripening, functional properties of natural Cheddar cheeses changed, that is, hardness decreased, meltability was improved, storage modulus at 70 °C (G'T=70) decreased, and the maximum tan delta (TDmax) increased. Both pH 4.6 SN/TN and the soluble Ca were correlated with changes in functional properties of natural Cheddar cheeses during ripening. Kendall's partial correlation analysis indicated that pH 4.6 SN/TN was more significantly correlated with changes in hardness and TDmax. For processed cheeses manufactured from natural Cheddar cheeses with different ripening times, the soluble Ca content did not show significant difference, and the trends of changes in hardness, meltability, G'T=70, and TDmax were similar to those of natural Cheddar cheeses. Kendall's partial correlation analysis suggested that only pH 4.6 SN/TN was significantly correlated with the changes in functional properties of processed cheeses.     

Rheological and calcium equilibrium changes during the ripening of Cheddar cheese

The small deformation rheological properties and the calcium (Ca) equilibrium of Cheddar cheese were investigated as a function of ripening time. The proportion of insoluble Ca as a percentage of the total Ca decreased from ∼72 to 57% between 3 days and 9 months; most changes occurred within the first 4 weeks. During ripening, the storage modulus (G′) of the cheese increased at low temperature, but decreased rapidly at high temperature. At temperatures >40 °C, the loss tangent increased to reach a maximum at a temperature of ∼70 °C in young cheese and there was a steady decline in this temperature during ripening. The maximum loss tangent values increased substantially during the first 4 weeks and then showed little change. Changes in the insoluble Ca content significantly correlated with pH 4.6 soluble nitrogen (pH 4.6 SN). Partial correlation analysis indicated that the insoluble Ca content was more significantly correlated with the rheological properties than was pH 4.6 SN.     

一种毛霉表面成熟干酪的蛋白质水解作用评价

从毛豆腐中分离出一株毛霉,并应用于表面成熟干酪,以研究干酪成熟过程中所发生的蛋白质水解作用。在90d 的成熟过程中,干酪的pH 值增加;蛋白质水解作用的评价指标,如干酪外层的水溶性氮- 总氮比、pH4.6水溶性氮- 总氮比、12g/100mL 三氯乙酸可溶性氮- 总氮比,在成熟90d 后分别增加至(23.68 ± 1.07)%、(19.38 ± 1.32)%和(8.61 ± 0.85)%,并且高于干酪的内部相应指标。SDS-PAGE 和毛细管电泳分析干酪的pH4.6 不溶性组分,结果表明酪蛋白在干酪成熟过程中被降解。对干酪成熟过程中分离出的水溶性组分进行RP-HPLC 分析,结果显示成熟过程中蛋白质被水解以及形成一些新肽分子。     

Primary proteolysis and textural changes during ripening in Cheddar cheeses manufactured to different fat contents

The effects of varying fat content in Cheddar cheese, from 6.3 to 32.5 g 100 g⁻¹, on changes in pH, primary proteolysis and texture were monitored over a 225 d ripening period. Reduction in the fat content resulted in significant (P<0.05) increases in pH, moisture and protein contents and decreases in the concentration of moisture in the non-fat substance. The increase in pH as the fat content increased was attributed to the concomitant decrease in the lactate-to-protein ratio. Polyacrylamide gel electrophoresis showed that the concentration of intact casein decreased in all cheeses during ripening and that the rate of decrease was not affected by the fat content. However, for a given concentration of casein, α_s1-casein was degraded more slowly, and β-casein more rapidly, as the fat content was reduced. The slower degradation of α_s1-casein with decreased fat content coincided with a decrease in the ratio of residual chymosin activity to protein in the cheese. At most ripening times, reduction in the fat content resulted in significant increases in the concentration of intact casein, fracture stress, fracture strain, and cheese firmness. The effects of fat reduction on proteolysis and rheology are probably due to the interactive effects of the concomitant changes in composition.