Modeling rennet coagulation time and curd firmness of milk

Milk coagulation properties (MCP) are traditionally expressed using rennet coagulation time (RCT), time to curd firmness (CF) of 20 mm (k₂₀), and CF 30 min after enzyme addition (a₃₀) values, all of which are single-point measures taken from the output of computerized renneting meters, such as the Formagraph. Thus, traditional MCP use only some of the available information. Moreover, because of the worldwide spreading of breeds such as the Holstein-Friesian, characterized by late-coagulating milk, it happens often that some samples do not coagulate at all, that a₃₀ is strongly and negatively related to RCT, and that k₂₀ is not measurable. The aim of the present work was to model CF as a function of time (CF_t, mm) over a 30-min interval. The model tested was CFt=CFP×(1−e−kCF×(t−RCT)), where CF_P (mm) is the potential asymptotical CF at an infinite time, k_CF (min⁻¹) is the curd firming rate constant, and RCT is measured in minutes. The CF_t model was initially applied to data of milk of each of 105 Brown Swiss cows from 7 herds, each sampled once (trial 1). Four samples did not coagulate within 30 min. Eighty-seven of the 101 individual equations obtained fit the CF data of milk samples very well, even though the samples differed in composition, and were produced by cows of different ages and days in milk, reared on different farms (coefficient of determination >0.99; average residual standard deviation = 0.21 mm). Samples with a very late RCT (slowly coagulating samples) yielded so few observational data points that curve parameters could not be precisely estimated. The repeatability of CF_t equation parameters was estimated using data obtained from 5 replicates of each of 2 samples of bulk milk from 5 Holstein-Friesian cows analyzed every day for 5 consecutive days (trial 2). Repeatability of RCT was better than that of the other 2 parameters. Moreover, traditional MCP values (RCT, a₃₀, and k₂₀) can be obtained from the individual CF_t equations, using all available information. The MCP estimated from equations were very similar to the single-point measures yielded by the computerized renneting meter (coefficient of determination >0.97), but repeatability was slightly better. The model allowed the estimation of k₂₀ for samples with a very late coagulation or with very slow curd firming. Finally, the 3 novel parameters used to assess different milk samples were less interdependent than are the traditional measures, and their practical and scientific utility requires further study.     

A method for the inline measurement of milk gel firmness using an optical sensor

At present, selection of cutting time during cheesemaking is made based on subjective methods, which has effects on product homogeneity and has prevented complete automation of cheesemaking. In this work, a new method for inline monitoring of curd firmness is presented. The method consisted of developing a model that correlates the backscatter ratio of near infrared light during milk coagulation with the rheological storage modulus. The model was developed through a factorial design with 2 factors: protein concentration (3.4 and 5.1%) and coagulation temperature (30 and 40°C). Each treatment was replicated 3 times; the model was calibrated with the first replicate and validated using the remaining 2 replicates. The coagulation process was simultaneously monitored using an optical sensor and small-amplitude oscillatory rheology. The model was calibrated and successfully validated at the different protein concentrations and coagulation temperatures studied, predicting the evolution of storage modulus during milk coagulation with coefficient of determination values >0.998 and standard error of prediction values <3.4 Pa. The results demonstrated that the proposed method allows inline monitoring of curd firming in cheesemaking and cutting the curd at a proper firmness to each type of cheese.     

Measurement of the rennet clotting time of milk by digital image sequences (2D + t) processing

The aim of the work presented in this paper was to determine the rennet clotting time of milk by image sequences (2D + t) processing. A computer vision system (CVS) consisting of a computer coupled with a transmitted light microscope equipped with a digital camera was developed. Each image was decomposed into Red, Green, Blue (RGB) color components and luminance histograms, through algorithms implemented with Matlab 7.7 software. These algorithms computed and analyzed the R, G, B color components and luminance histogram peak in each image. It was shown that during milk coagulation, this histogram peak varies according to sigmoidal law Peak(t) = Peakmin + (Peakmax − Peakmin)/[1 + (t/TC)^α], where TC is the rennet coagulation time.     

Effect of artisanal kid rennet paste on lipolysis in semi-hard goat cheese

This study examined the use of hygienised kid rennet pastes in model cheese systems and also in goat milk semi-hard cheeses to promote lipolysis. The results obtained indicated that the use of rennet paste caused greater lipolysis and increased, mostly, the short-chain free fatty acid (FFA) content. The model systems made with whole goat’s milk using rennet paste and commercial rennet mixture exhibited a higher FFA content than did the rennet paste-free controls (31,600 vs. 25,600 μmol/kg cheese). For the pilot cheeses made with bovine rennet and rennet paste mixture, the increase in FFA level after 45 days of ripening compared with the cheeses prepared only with commercial rennet was as much as 6600 (μmol/kg cheese) and the increase in the butyric acid content was also 1650 (μmol/kg cheese). Moreover, after 15 days of ripening, industrially prepared cheeses made with rennet paste exhibited greater levels of FFA than did the cheeses made with commercial rennet (11,500 μmol/kg at 45 days of ripening). Their flavour was stronger and the organoleptic characteristics were better accepted, which implies less ripening time for commercial cheese manufacture.     

Characterization of curd formation during the rennet coagulation of milk by an optical microscopic method

Optical microscopic observation of the coagulation of rennet-treated milk was performed using a conventional light microscope under standardized conditions. The microscopic images, i.e., number and average area of milk aggregates, were analyzed using Optiscan software. The evolution of the latter parameters was followed as a function of the rennet coagulation time of milk. The curves presented two inflection points which correlated with the rennet flocculation time and the rennet clotting time values, as determined by reference methods. These relationships were validated on milk supplemented with different concentrations of calcium (5.0–7.5 m ) and subjected to varying temperatures for rennet coagulation (22.5–40°C). This optical microscopic method with associated image analysis software allowed (i) the observation of structural changes occurring during gel formation, even at low magnification (200X) and (ii) the determination of the rennet flocculation and clotting times.     

Rennet coagulation of sheep milk processed by ultrafiltration at low concentration factors

The ultrafiltration (UF) of sheep milk is carried out in concentration mode in order to evaluate the variation of the rennet clotting properties of the concentrates as a function of the volumetric concentration factors up to a value of 2.0. The UF unit is equipped with 25.5 cm² of membrane surface area. The cellulose acetate laboratory – made membrane has a molecular weight cut-off of 7000 Da. The evolution pattern of the rennet clotting properties of the skimmed sheep milk feed and UF retentates was assessed by the Optigraph. A significant increase on curd firmness and rate of curd firming was detected with the increase of the protein concentration, while RCT show a tendency to increase with milk protein content. The variation of curd firmness and rate of curd firming with the protein concentration was linear and correlations were established.     

Influence of an artisanal lamb rennet paste on proteolysis and textural properties of Murcia al Vino cheese

The aim of this study was to evaluate the influence of lamb rennet paste on the proteolysis and textural properties of Murcia al Vino cheese, compared with calf rennet. The enzyme concentration was adjusted according to its milk-clotting activity. The use of rennet paste led to higher values of all nitrogen fractions studied. Significant increases were observed in the water-soluble nitrogen fraction as a result of the lower pH of rennet paste cheeses; although the rennet paste is not characterised, three proteases are reported in the references consulted which can justify the greater proteolysis compared with calf rennet. The use of natural rennet paste produces a cheese with a more hydrolysed protein matrix, which is associated with significant changes in texture. The greater firmness determined in the rennet paste cheese was associated with higher fracture stress, lower fracture strain and lower moisture content.     

Effect of homogenization, microfiltration and pH on curd firmness and syneresis of curd grains

The impact of the independent variables, homogenization pressure (p₁), concentration factor of microfiltration (i) and pH on curd firmness (CF) and syneresis of curd grains was studied. Texture analysis was used to characterize CF of the rennet-induced gels. The analysis of a two-level factorial design revealed that i, p₁, pH and the interaction of i and pH had the most important influence on CF. Cutting time was therefore individually determined for each milk system using small amplitude oscillatory rheometry for generating comparable conditions for the syneresis experiments. Syneresis of curd grains with a diameter of 11 mm was followed at 35 °C close to semi-hard cheesemaking conditions. The permeate release during microfiltration was taken into consideration, allowing an evaluation of syneresis of grains made from concentrated and unconcentrated milk. It was shown that with increasing milk concentration less curd treatment time was needed to reach a certain syneresis value. Hence, total processing time in cheesemaking is decreased. Analysis of variance revealed that syneresis was affected by the individual variables. Kinetic parameters were satisfactorily estimated through regression (R²>0.98) and it was shown that milk composition and concentration due to microfiltration markedly influenced the endpoint of syneresis, RWR_max. The experiments demonstrate that microfiltration and homogenization can be combined to reach CF and syneresis comparable to untreated milk used in conventional cheesemaking. This meets one claim of the cheese industry when implementing both technologies in the manufacture process, since consistency and quality of the ripened cheese are expected to be unchanged.     

酸度和钙质量分数对复原乳酶凝胶质地特性的影响

考察了不同酸度、钙质量分数调节后复原乳酶凝胶的质地特性,并对其质地变量进行了主成分分析。结果表明,不同工艺处理样品间的差异可以清晰地表现在两个新变量中。酸度、钙质量分数调整可对凝胶的表观黏度和持水力产生很大影响。此外,通过对照分析可以发现,不同前处理的复原乳样品具有与原料乳相近的凝胶特性。     

Pathway-based genome-wide association analysis of milk coagulation properties,curd firmness,cheese yield,and curd nutrient recovery in dairy cattle

It is becoming common to complement genome-wide association studies (GWAS) with gene-set enrichment analysis to deepen the understanding of the biological pathways affecting quantitative traits. Our objective was to conduct a gene ontology and pathway-based analysis to identify possible biological mechanisms involved in the regulation of bovine milk technological traits: coagulation properties, curd firmness modeling, individual cheese yield (CY), and milk nutrient recovery into the curd (REC) or whey loss traits. Results from 2 previous GWAS studies using 1,011 cows genotyped for 50k single nucleotide polymorphisms were used. Overall, the phenotypes analyzed consisted of 3 traditional milk coagulation property measures [RCT: rennet coagulation time defined as the time (min) from addition of enzyme to the beginning of coagulation; k₂₀: the interval (min) from RCT to the time at which a curd firmness of 20 mm is attained; a₃₀: a measure of the extent of curd firmness (mm) 30 min after coagulant addition], 6 curd firmness modeling traits [RCT_eq: RCT estimated through the CF equation (min); CF_P: potential asymptotic curd firmness (mm); k_CF: curd-firming rate constant (% × min^?1); k_SR: syneresis rate constant (% × min^?1); CF_max: maximum curd firmness (mm); and t_max: time to CF_max (min)], 3 individual CY-related traits expressing the weight of fresh curd (%CY_CURD), curd solids (%CY_SOLIDS), and curd moisture (%CY_WATER) as a percentage of weight of milk processed and 4 milk nutrient and energy recoveries in the curd (REC_FAT, REC_PROTEIN, REC_SOLIDS, and REC_ENERGY calculated as the % ratio between the nutrient in curd and the corresponding nutrient in processed milk), milk pH, and protein percentage. Each trait was analyzed separately. In total, 13,269 annotated genes were used in the analysis. The Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway databases were queried for enrichment analyses. Overall, 21 Gene Ontology and 17 Kyoto Encyclopedia of Genes and Genomes categories were significantly associated (false discovery rate at 5%) with 7 traits (RCT, RCT_eq, k_CF, %CY_SOLIDS, REC_FAT, REC_SOLIDS, and REC_ENERGY), with some being in common between traits. The significantly enriched categories included calcium signaling pathway, salivary secretion, metabolic pathways, carbohydrate digestion and absorption, the tight junction and the phosphatidylinositol pathways, as well as pathways related to the bovine mammary gland health status, and contained a total of 150 genes spanning all chromosomes but 9, 20, and 27. This study provided new insights into the regulation of bovine milk coagulation and cheese ability that were not captured by the GWAS.     

Effect of pH on the gel properties and secondary structure of fish myosin

The relationships between gel properties and the secondary structures of silver carp myosin were investigated at pH 5.5–9.0 using dynamic rheological measurement, circular dichroism and scanning electron microscopy. The gel properties of fish myosin were strongly pH and temperature dependent. During heating at 1 °C/min, myosin formed gels in the pH range 5.5–7.5, but not at pH 8.0–9.0. α-Helix was the predominant structure at pH 7.0. The α-helix fraction declined with increasing temperature and the pH away from 7.0, whilst the other secondary structure fractions increased. The α-helix structure of myosin was more susceptive to acid-treatment than alkali-treatment. As pH increased, the gelation rate and gel strength decreased, and the water-holding capacity (WHC) showed an increasing trend followed by a plateau. High β-sheet and β-turn fractions prior to heating could improve G′ at 90 °C, but they depressed the WHC. A compact and uniform gel of fish myosin was obtained at pH 7.0.     

Rheology, microstructure, and functionality of low-fat Iranian white cheese made with different concentrations of rennet

A batch of full-fat (23% target fat) and 3 batches of low-fat (6% target fat) Iranian white cheese with different rennet concentrations (1-, 2-, and 3-fold the normal usage) were produced to study the effect of fat content reduction and promoted proteolysis on the textural and functional properties of the product. Cheese samples were analyzed with respect to their rheological parameters (uniaxial compression and small amplitude oscillatory shear), meltability, microstructure, and sensory characteristics. Reduction of fat content from 23 to 6% had adverse effects on the texture, functionality, cheese-making yield, and sensory characteristics of Iranian white cheese. Fat reduction increased the instrumental hardness parameters (storage modulus, stress at fracture, and Young's modulus of elasticity), decreased the cheese meltability and yield, and made the microstructure more compact. Doubling the rennet concentration reduced values of instrumental hardness parameters, increased the meltability, and improved the sensory impression of texture. Although increasing the rennet concentration to 2-fold the normal usage resembled somewhat the low-fat cheese to its full-fat counterpart, it appeared to cause more reduction in yield. Increasing the rennet concentration 3-fold the normal usage produced a product slightly more elastic than the low-fat cheese with normal concentration of rennet. Increasing the rennet concentration to 2-fold the normal usage was useful for improving the textural, functional, and sensory properties of low-fat Iranian white cheese.     

凝乳酶对低脂干酪微观结构和功能特性的影响

研究了低脂干酪成熟过程中蛋白质水解程度对干酪本身的微观结构和功能特性的影响.以低脂乳为原料,添加不同剂量的凝乳酶制备低脂干酪,测定干酪不同成熟期的熔度及质量分数为12%的TCA-SN并观察干酪的微观结构.结果表明,在低脂干酪中添加双倍的凝乳酶时可以减少干酪的硬度、增加熔度和改善其感官状态;当添加3倍凝乳酶时将会导致产品比添加正常凝乳酶量时更有胶弹性.因此,添加双倍凝乳酶时能有效改善低脂干酪的质构、功能特性及感官状态.     

Effects of reverse CO2 acidification cycles,calcium supplementation,pH adjustment and chilled storage on physico-chemical and rennet coagulation properties of reconstituted low- and medium-heat skim milk powders

The reversibility extent of one and two reverse CO₂ acidification cycles on the physico-chemical and rennet coagulation properties of milks reconstituted from low- (LH) or medium- (MH) heat skim powder, enriched or not with calcium and pH adjusted or not was investigated. The ionized calcium concentration, buffering properties and average casein micelle size of untreated and CO₂-treated milks were evaluated before and after a chilled storage for 2 days. The ionized calcium concentration and buffering properties have been modified by the CO₂-treatment, particularly after a second CO₂-cycle. These modifications were highly dependent on the initial milk properties and chilled storage. Inversely, the average casein micelle size was not significantly changed. In addition, the rennet-clotting behaviour checked by near infrared spectroscopy (NIR-S) and rheology (SAOR) indicated the main factors responsible for changes in the casein micelles environment and dynamic casein micellar calcium phosphate reorganization, especially after two CO₂-cycles. A single CO₂-cycle induced a better rennetability for non Ca-enriched milk reconstituted from MH-powder. A second CO₂-cycle was particularly efficient to improve Ca-enriched pH-adjusted milks.     

Effect of insoluble calcium concentration on rennet coagulation properties of milk

Rennet-induced gels were made from milk acidified to various pH values or milk at pH 6.0 that had added EDTA. The objective was to examine the effect of removing insoluble Ca (INS Ca) from casein micelles (CM) on rennet gelation properties. For the pH trial, diluted lactic acid was added to reconstituted skim milk to decrease the pH to 6.4, 6.0, 5.8, 5.6, and 5.4. For the EDTA trial, EDTA was slowly added (0, 2, 4, and 6 mM) to reconstituted skim milk, and the final pH values were subsequently adjusted to pH 6.0. Dynamic low amplitude oscillatory rheology was used to monitor gel development. The Ca content of CM and rennet wheys made from these milks was measured using inductively coupled plasma spectroscopy. The INS Ca content of milk was altered by the acidification pH values or level of EDTA added. In all samples, the storage modulus (G′) exhibited a maximum (GM), with a decrease in G′ during longer aging times. Gels made at pH 6.4 had higher GM compared with gels made at pH 6.7 probably due to the reduction in electrostatic repulsion, whereas the INS Ca content only slightly decreased. The highest GM value of gels was observed at pH 6.4 and the GM value decreased with decreasing pH from 6.4 to 5.4. This was due to an excessive loss of INS Ca from CM. There was a decrease in GM with the increase in the concentration of added EDTA, which was probably due to the loss of colloidal calcium phosphate, which weakens the integrity of CM. Loss tangent (LT) values at GM increased with a reduction in milk pH and the addition of EDTA to milk. Rennet gels at the point of the GM were subjected to constant low shearing to fracture the gels. With a reduction in INS Ca content, the yield stress decreased, whereas LT values increased indicating a weaker, more flexible casein network. Microstructure of rennet-induced gels near the GM point and 2 to 10 h after this point was studied using fluorescence microscopy. At GM, gels made from milk acidified to pH 6.4 exhibited more branched, interconnected networks, whereas strands and clusters became larger with a reduction in milk pH to 5.4. Gels made from milk with EDTA added had more finely dispersed protein clusters compared with gels made from milk with no EDTA added. These microscopic observations supported the effect of loss of INS Ca on GM and LT. There was a decrease in apparent interconnectivity between strands in gel microstructure during aging, which agreed with the decrease in G′ after GM. It can be concluded that low levels of solubilization of INS Ca and the decrease in milk pH resulted in an increase in GM. With greater losses of INS Ca there was excessive reduction in cross-linking within CM, which resulted in weaker, more flexible rennet gels. This complex behavior cannot be explained by adhesive hard sphere models for CM or rennet gels made from these CM.     

Manufacture of low-sodium Minas fresh cheese: effect of the partial replacement of sodium chloride with potassium chloride

We investigated the effect of sodium reduction by partial substitution of sodium chloride (NaCl) with potassium chloride (KCl) on the manufacture of Minas fresh cheese during 21 d of refrigerated storage. Four treatments of low-sodium Minas fresh cheese were manufactured, with partial replacement of NaCl by KCl at 0, 25, 50, and 75% (wt/wt), respectively. The cheeses showed differences in the content of moisture, ash, protein, salt, and lipid contents, as well as on the extent of proteolysis and hardness throughout the storage period. However, no difference was observed among treatments within each storage day tested. The partial substitution of NaCl by KCl decreased up to 51.8% the sodium concentration of the cheeses produced. The consumer test indicated that it is possible to manufacture a low-sodium Minas fresh cheese that is acceptable to consumers by partial substitution of NaCl by KCl at 25% (wt/wt) in the salting step.     

Effect of solutes and pH on the structure and firmness of cooked carrot

Summary. Xylary segments of carrot root were cooked in solutions of different hydrogenion concentration (pH 3-8) and different solute composition. 'Firmness'was measured as resistance of the cooked segments to compression, and histological sections were prepared. the segments were softer, the higher the pH of the solution. This softening was attended by increasing cellular separation and eventual cellular collapse at higher values of pH. Firmness increased (over that in water) in sucrose solutions and solutions with divalent cations, and it decreased in solutions with monovalent cations. Explanations are considered for these effects and for the relatively minor increase in firmness in solutions of A1Cl₃.     

腌制条件对大叶麻竹笋硬度的影响

研究大叶麻竹笋腌制过程中硬度与食盐浓度、温度、氯化钙和焦亚硫酸钠添加量、样品厚度的关系。研究结果表明:各腌制条件对样品硬度的变化都有较大影响,具体表现为:食盐浓度越高、环境温度越低、氯化钙和焦亚硫酸钠添加量越大、样品厚度越厚,腌制大叶麻竹笋的硬度降低的越少。     

The effect of sodium chloride substitution with potassium chloride on texture profile and microstructure of Halloumi cheese

The effect of partial substitution of NaCl with KCl on texture profile and microstructure of Halloumi cheese was investigated. Four batches of Halloumi cheese were made and kept in 4 different brine solutions (18%, wt/wt), including A) NaCl only, B) 3NaCl:1KCl, C) 1NaCl:1KCl, and D) 1NaCl:3KCl and then stored at 4°C for 56 d. The texture profile was analyzed using an Instron universal machine, whereas an environmental scanning electron microscope was used to investigate the effect of NaCl substitution on the microstructure of cheeses. No significant difference was found in hardness, cohesiveness, adhesiveness, and gumminess among experimental cheeses at the same storage day. Hardness, cohesiveness, and gumminess decreased significantly during storage period with the same salt treatment, whereas adhesiveness significantly increased. Environmental scanning electron microscope micrographs showed a compact and closed texture for cheeses at the same storage period. The microstructure of all cheeses became more closed and compact with storage period. Calcium content negatively correlated with hardness and Na and K contents during storage with the same salt treatment.     

Influence of precooking on the firmness and pectic substances of three stem vegetables

Edible portions of the stems of sprouting broccoli, asparagus lettuce, and large-stem mustard were compared for total pectin contents, amounts of different pectin fractions, pectinesterase activities and changes during cooking to investigate effects on the textural changes during cooking. Slices precooked for 30min at temperatures below 60°C (broccoli) or 70°C (lettuce, mustard) were firmer after 15 min recooking in boiling water than those directly cooked without precooking. Optimum temperatures for this firming effect of precooking were 50, 60 and 60°C, respectively, and coincided with the optimum temperatures of activity of pectinesterases extracted from the fresh tissues. Analysis of pectin fractions revealed that the firming effect of precooking is related to the shift from the cold water-soluble fraction to sodium hexametaphosphate-soluble and hot water-soluble fractions of pectins.