A simple procedure using trinitrobenzenesulphonic acid for monitoring proteolysis in cheese

Measurement of degree of hydrolysis and molecular weight distribution of protein hydrolysates by liquid chromatography-mass spectrometry

Enzymatic hydrolysis of milk protein is an effective way to improve protein digestibility, to reduce their allergenicity and to produce peptides with better functionalities. Among the process, the degree of hydrolysis (DH) and molecular weight distribution (MWD) of protein hydrolysates are two important parameters that need to be monitored. In this work, a new method based on liquid chromatography-mass spectrometry (LC-MS) was developed for the first time to accurately detect the DH and the MWD of proteolytic peptides. With LC-MS, the content of free amine groups released during hydrolysis was acquired by direct analysis of free trinitrobenzene sulfonic acid (TNBS) for DH assay, overcoming the disadvantage of TNBS-based spectral method. Based on this method, the DH% values of five protein hydrolysis samples were determined and consistent with file specification values. Compared to the size-exclusion high-performance liquid chromatography (SE-HPLC) method, LC-MS was capable of measuring MWD (similar results with file specification values) while additionally providing precise molecular weight and amino acid sequence information for each proteolytic peptide. This method was characterized by its simplicity, accuracy, and reproducibility, making it a valuable technology for monitoring proteolysis and producing peptides with better functionality.

Gallic and ferulic acids suppress proteolytic activities and volatile trimethylamine production in the food-borne spoiler Rahnella aquatilis KM05

BACKGROUND

Rahnella aquatilis is a recognised microbial threat that alters the sensory properties of seafood. The high frequency with which R. aquatilis is isolated from fish has prompted a search for alternative preservatives. In the present study, in vitro and fish-based ecosystem (raw salmon-based medium) approaches were used to validate the antimicrobial effects of gallic (GA) and ferulic (FA) acids against R. aquatilis KM05. The results were compared with data describing the response of KM05 to sodium benzoate. Bioinformatics data of the whole genome were used to analyse the potential for fish spoilage by KM05 in detail, and the results revealed the main physiological characteristics that underlie reduced seafood quality.

RESULTS

In the KM05 genome, the most abundantly enriched Gene Ontology terms were 'metabolic process', 'organic substance metabolic process' and 'cellular process'. Through an evaluation of the Pfam annotations, 15 annotations were found to be directly involved in the proteolytic activity of KM05. Peptidase_M20 was the most abundantly represented (abundance value of 14060). Proteins representing the CutC family (abundance value of 427) indicated the potential for KM05 degradation of trimethyl-amine-N-oxide. Subinhibitory concentrations of GA and FA suppressed the proteolytic activities of KM05 both in vitro and in RS medium by an average of 33-45%. These results were confirmed by quantitative real-time PCR experiments, which also showed that the expression levels of genes involved in proteolytic activities and volatile trimethylamine production were also decreased.

CONCLUSION

Phenolic compounds can be used as potential food additives for preventing quality deterioration of fish products. © 2023 Society of Chemical Industry.

In situ approaches show the limitation of the spoilage potential of Juniperus phoenicea L. essential oil against cold-tolerant Pseudomonas fluorescens KM24

Abstract

The present study aimed to elucidate the effect of subinhibitory concentrations (sub-MICs) of juniper essential oil (EO), α-pinene, and sabinene on the quorum-sensing (QS)–mediated proteolytic and lipolytic properties of Pseudomonas fluorescens KM24. These activities were verified under in situ conditions, in which sub-MICs of the agents altered the morphology of KM24 cells. RNA-Seq studies revealed key coding sequences (CDSs)/genes related to QS and the proteolytic/lipolytic activities of pseudomonads. In this work, all the examined agents decreased autoinducer synthesis and influenced the mRNA expression of the encoding acyltransferase genes lptA, lptD, and plsB. The highest reduction on the 3^rd and 5^th days of cultivation was observed for the genes lptD (−5.5 and −5.61, respectively) and lptA (−3.5 and −4.0, respectively) following treatment with EO. Inhibition of the lptA, lptD, and plsB genes by singular constituents of EO was on average, from −0.4 to −0.7. At 5 days of cultivation the profile of AHLs of the reference P. fluorescens KM24 strain consisted of 3-oxo-C14-HSL, 3-oxo-C6-HSL, C4-HSL, and N-[(RS)-3-hydroxybutyryl]-HSL, the concentrations of which were 0.570, 0.018, 3.744, and 0.554 μg ml⁻¹, respectively. Independent of the incubation time, EO, α-pinene, and sabinene also suppressed the protease genes prlC (−1.5, −0.5, and −0.5, respectively) and ctpB (−1.5, −0.7, and −0.4, respectively). Lipolysis and transcription of the lipA/lipB genes were downregulated by the agents on average from −0.3 to −0.6. α-Pinene- and sabinene-rich juniper EO acts as an anti-quorum-sensing agent and can repress the spoilage phenotype of pseudomonads.

Key points:

Juniper EO, α-pinene, sabinene exhibited anti-QS potential toward KM24.

RNA-Seq revealed key CDSs/genes related to QS/proteolytic/lipolytic activities of KM24.

Agents at sub-MIC levels influenced the mRNA expression of QS/lipase/protease genes.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-021-11338-3.

Tarragon essential oil as a source of bioactive compounds with anti-quorum sensing and anti-proteolytic activity against Pseudomonas spp. isolated from fish - in vitro, in silico and in situ approaches

The present study aimed to evaluate the anti-quorum sensing (anti-QS) and anti-proteolytic potentials of tarragon essential oil (TEO) and its major compounds against food-associated Pseudomonas spp. The activities were verified by in vitro, in silico and in situ approaches. In this work, methyl eugenol (ME)- and β-phellandrene (β-PH)-rich TEO was investigated. TEO at subMIC increased the percentage of saturated fatty acids in the bacterial membranes (from 7 to 22%) and exhibited anti-quorum sensing via decreasing the efficiency of QS autoinducer synthesis [3-oxo-C12-HSL (from 2.028 μg/mL to <LOD), C4-HSL (from 1.312 μg/mL to <LOD) and PQS (from 0.007625 μg/mL to <LOD)]. ME and β-PH were docked into LasR, RhlR and PqsR proteins, with docking scores comparable to native autoinductors. The subMICs of TEO, ME and β-PH decreased the proteolysis in the examined bacteria by 33, 29, and 21% (in TSB medium) and by 29, 26, and 19% (in fish juice medium), respectively. Almost all genes encoding proteases were downregulated by the applied agents. The ME- and β-PH-rich TEO acts as an anti-QS agent and significantly suppresses the proteolytic activity of food-associated pseudomonads. It might therefore increase the quality of fish-based products, where Pseudomonas spp. predominate.

Development of Reduced-Fat, Reduced-Sodium Semi-Hard Sheep Milk Cheese

This paper examines the effects of the incorporation of denatured whey proteins along with salting in NaCl/KCl brine on the characteristics and ripening of sheep milk reduced-fat (RF), semi-hard cheese. Incorporation of denatured whey proteins was carried out by: i. adding commercial microparticulated whey protein (MWP) in reduced-fat cheese milk (RFM), or ii. by ‘in situ’ heat-induced partial denaturation of whey proteins of reduced-fat cheese milk (RFD). The implemented cheesemaking conditions included curd washing, moderate clotting, scalding temperatures, and ripening of cheeses packed in plastic bags under vacuum at 10 °C. Full-fat cheeses (FF) were manufactured in parallel. Physicochemical composition, textural profile, and proteolysis were assessed throughout 60 days of ripening. The mean moisture, fat on dry matter (FDM), moisture on non-fat substances (MNFS), protein on dry matter (PDM), salt, and salt-in-moisture (S/M) content of the RF cheeses were 47.4%, 32.8%, 57.3%, 54.3%, 1.63%, and 3.36%, respectively; pH ≈ 5.0, a_w ≈ 0.977, Ca ≈ 1000 mg/100 g cheese. The MNFS of FF and RF cheeses were similar. Proteolysis indices were not affected by any of the treatments, and they were similar to the FF counterparts. The applied cheesemaking technology was adequate for the production of semi-hard reduced-fat and reduced-sodium cheeses. Ripening under packaging hindered moisture loss without impairing the evolution of proteolysis and textural parameters. The same holds true for salting in NaCl/KCl brine. The high pasteurization of cheese milk was more effective for the increase of moisture and MNFS than the addition of MWP, without exhibiting any adverse effects.

Para-κ-casein during the ripening and storage of low-pH, high-moisture Feta cheese

The hypothesis of this research paper was that the physicochemical conditions in a low-pH, high-moisture white brined cheese such as Feta would make para-κ-casein vulnerable to residual chymosin activity during ripening and storage. It was important to address this hypothesis, since cheese para-κ-casein could theoretically be used for the assessment of the origin of cheese milk by means of various analytical methods. Feta cheese was manufactured from sheep milk and from four different mixtures of sheep and goat milk in triplicate. The para-κ-casein of Feta samples taken during 120 d of ripening and storage was estimated by means of cation-exchange HPLC and proteolysis was determined in terms of free amino groups. Despite their similarity, sheep and goat para-κ-casein were efficiently separated and the changes of their chromatographic areas indicated that hydrolysis took place during the first stage of ripening. In accordance to the evolution of free amino groups, para-κ-caseins remained stable thereafter. The hydrolysis pattern was not affected by the composition of the cheese milk mixture and after 120 d at least two thirds of the initial quantity remained intact. Considering the efficient separation of sheep and goat para-κ-caseins and their stability during Feta storage, the same method was used for the evaluation of the percentage of goat milk in the cheese milk. The actual and the estimated percentage of goat milk within the range 0-40 were strongly correlated (R = 0·997, n = 60) and the standard error of estimation was 0·914.

Characterization of Pseudomonas spp. and Associated Proteolytic Properties in Raw Milk Stored at Low Temperatures

Milk spoilage is caused by the presence of proteolytic enzymes produced by Pseudomonas spp. during storage at low temperatures. The aim of this study was to identify Pseudomonas spp. in raw milk and investigate their associated proteolytic properties at low temperatures. Raw milk samples (n = 87) were collected from 87 bulk tanks in Shaanxi Province in China. Pseudomonas spp. were identified using Pseudomonas specific 16S, universal 16S rRNA sequencing, and rpoB gene sequencing. The proteolytic properties of Pseudomonas spp. were examined using milk agar, quantitative trinitrobenzenesulfonic acid assay, and by the presence of alkaline metallopeptidase gene (aprX). A total 143 isolates from all 87 samples were confirmed as Pseudomonas, and were identified as belonging to 14 Pseudomonas species. Of these, 40 (28.0%) isolates revealed proteolysis on milk agar at 2°C, 74 (51.8%) at 4°C, 104 (72.7%) at 7°C, and 102 (71.3%) at 10°C. However, proteolytic activity of 45 (31.5%) isolates exceeded 2 μmol of glycine equivalents per mL at 7°C, followed by 43 (30.1%) at 10°C, 18 (12.6%) at 4°C, and 7 (4.9%) at 2°C. The results reveal proteolytic activity of Pseudomonas spp. present in milk and their spoilage potential at different temperatures.

Microbiological and biochemical aspects of inland Pecorino Abruzzese cheese

Little is known on physicochemical and biochemical characteristics of “Pecorino” Abruzzese cheese in L’Aquila province, an artisanal cheese produced from ewe raw full-cream milk.

Three batches of inland “Pecorino” Abruzzese cheese were examined for microbiological, compositional, biochemical and sensory characteristics at the aim of isolating and storing in a bacterial collection, indigenous strain to preserve the microbial biodiversity present in this cheese, to a possible definition of a PDO. Cheese samples from three dairies, at different stages of production were collected and 148 colonies were characterized. Physicochemical assays, species-specific PCR and 16S rRNA gene sequencing revealed that the majority of the lactic acid bacteria (LAB) isolates were Enterococcus faecium and En. faecalis. They were highly prevalent, accounting for 48% of the isolates. The lactic microflora consisted of lactobacilli and lactococci from the species Lactobacillus plantarum (12.2%), Lactobacillus brevis (10.1%), Lactococcus lactis subsp. cremoris (11.5%), respectively.

Urea-PAGE electrophoresis showed extensive degradation of α_S1-casein (CN) and moderate hydrolysis of β-CN. Formation of γ-CNs from β-CN were highlighted. RP-HPLC profiles of the ethanol-soluble and ethanol-insoluble fractions of the pH 4.6-soluble nitrogen showed only minor differences between the three farms: lower proteolysis in the soluble fraction than the insoluble.

Leucine, glutamic acid, lysine, valine were the free amino acids present at the highest levels in all the cheeses. Flavour and texture profile were characterized through a sensory analysis.

The Biodiversity of the Microbiota Producing Heat-Resistant Enzymes Responsible for Spoilage in Processed Bovine Milk and Dairy Products

Raw bovine milk is highly nutritious as well as pH-neutral, providing the ideal conditions for microbial growth. The microbiota of raw milk is diverse and originates from several sources of contamination including the external udder surface, milking equipment, air, water, feed, grass, feces, and soil. Many bacterial and fungal species can be found in raw milk. The autochthonous microbiota of raw milk immediately after milking generally comprises lactic acid bacteria such as Lactococcus, Lactobacillus, Streptococcus, and Leuconostoc species, which are technologically important for the dairy industry, although they do occasionally cause spoilage of dairy products. Differences in milking practices and storage conditions on each continent, country and region result in variable microbial population structures in raw milk. Raw milk is usually stored at cold temperatures, e.g., about 4°C before processing to reduce the growth of most bacteria. However, psychrotrophic bacteria can proliferate and contribute to spoilage of ultra-high temperature (UHT) treated and sterilized milk and other dairy products with a long shelf life due to their ability to produce extracellular heat resistant enzymes such as peptidases and lipases. Worldwide, species of Pseudomonas, with the ability to produce these spoilage enzymes, are the most common contaminants isolated from cold raw milk although other genera such as Serratia are also reported as important milk spoilers, while for others more research is needed on the heat resistance of the spoilage enzymes produced. The residual activity of extracellular enzymes after high heat treatment may lead to technological problems (off flavors, physico-chemical instability) during the shelf life of milk and dairy products. This review covers the contamination patterns of cold raw milk in several parts of the world, the growth potential of psychrotrophic bacteria, their ability to produce extracellular heat-resistant enzymes and the consequences for dairy products with a long shelf life. This problem is of increasing importance because of the large worldwide trade in fluid milk and milk powder.

Ascorbate and Apple Phenolics Affect Protein Oxidation in Emulsion-Type Sausages during Storage and in Vitro Digestion

The effect of sodium ascorbate and apple phenolics on the oxidative stability of emulsion-type sausages during storage and digestion was investigated. Emulsion-type sausages containing 0.05% sodium ascorbate or 3% freeze-dried apple pomace were subjected to chilled illuminated storage and subsequent in vitro digestion. Lipid oxidation was assessed as TBARS, and protein oxidation was evaluated as thiol oxidation, total carbonyls, and γ-glutamic and α-amino adipic semialdehyde. Proteolysis was measured after digestion to evaluate protein digestibility. The results suggest the presence of protein-ascorbate and protein-phenol interactions, which may decrease protein digestibility and may interfere with spectrophotometric methods for measuring oxidation.

Identification and characterization of a heat-resistant protease from Serratia liquefaciens isolated from Brazilian cold raw milk

The cold storage of raw milk before heat treatment in dairy industry promotes the growth of psychrotrophic microorganisms, which are known for their ability to produce heat-resistant proteolytic enzymes. Although Pseudomonas is described as the main causative genus for high proteolytic spoilage potential in dairy products, Serratia liquefaciens secretes proteases and may be found in raw milk samples as well. However, at the present there is no information about the proteolytic spoilage potential of S. liquefaciens in milk after heat-treatment. The main aim of this research was to assess the proteolytic spoilage potential of S. liquefaciens isolated from Brazilian raw milk and to characterize the involved protease. S. liquefaciens was shown to secrete one heat-resistant spoilage metalloprotease of, approximately, 52 kDa encoded by the ser2 gene. The heat-resistance of Ser2 was similar to the aprX encoded metalloprotease produced by Pseudomonas. Although the ser2 gene was detected in all S. liquefaciens isolates tested in this study, the proteolytic activity of the isolates in milk was highly heterogeneous. Since nucleotide and deduced amino acid sequences of ser2 of all tested isolates are identical, this heterogeneity may be attributed to differences in enzyme expression levels or post-translational modifications.

Cyclic lipodepsipeptides produced by Pseudomonas spp. naturally present in raw milk induce inhibitory effects on microbiological inhibitor assays for antibiotic residue screening

Two Pseudomonas strains, identified as closely related to Pseudomonas tolaasii, were isolated from milk of a farm with frequent false-positive Delvotest results for screening putative antibiotic residues in raw milk executed as part of the regulatory quality programme. Growth at 5 to 7°C of these isolates in milk resulted in high lipolysis and the production of bacterial inhibitors. The two main bacterial inhibitors have a molecular weight of 1168.7 and 1140.7 Da respectively, are heat-tolerant and inhibit Geobacillus stearothermophilus var. calidolactis, the test strain of most of the commercially available microbiological inhibitor tests for screening of antibiotic residues in milk. Furthermore, these bacterial inhibitors show antimicrobial activity against Staphylococcus aureus, Bacillus cereus and B. subtilis and also interfere negatively with yoghurt production. Following their isolation and purification with RP-HPLC, the inhibitors were identified by NMR analysis as cyclic lipodepsipeptides of the viscosin group. Our findings bring to light a new challenge for quality control in the dairy industry. By prolonging the refrigerated storage of raw milk, the keeping quality of milk is influenced by growth and metabolic activities of psychrotrophic bacteria such as pseudomonads. Besides an increased risk of possible spoilage of long shelf-life milk, the production at low temperature of natural bacterial inhibitors may also result in false-positive results for antibiotic residue screening tests based on microbial inhibitor assays thus leading to undue production loss.

The effect of storage temperatures and packaging methods on properties of Motal cheese

The effects of storage temperature (+4 degrees C and -18 degrees C) and packaging method (nonvacuum and vacuum) on biogenic amines in Motal cheese during storage periods were investigated. In addition, dry matter, titratable acidity, total nitrogen, water-soluble nitrogen, trichloroacetic acid-soluble nitrogen, phosphotungstic acid-soluble nitrogen, free amino group (proteolysis), electrophoretic patterns of casein, and amounts of lactic acid bacteria and coliforms were determined. Storage period had a significant effect on all of the biogenic amines. When compared with vacuum packaging, normal packaging had higher amounts of putrescine, cadaverine, histamine, and tyramine. Coliforms were not found at detectable levels (<100cfu/g) in all cheese samples. Results of urea-PAGE analysis of cheese samples were in good agreement with biogenic amine results and other proteolysis parameters.

Toxinogenic and spoilage potential of aerobic spore-formers isolated from raw milk

The harmful effects on the quality and safety of dairy products caused by aerobic spore-forming isolates obtained from raw milk were characterized. Quantitative assessment showed strains of Bacillus subtilis, the Bacillus cereus group, Paenibacillus polymyxa and Bacillus amyloliquefaciens to be strongly proteolytic, along with Bacillus licheniformis, Bacillus pumilus and Lysinibacillus fusiformis to a lesser extent. Lipolytic activity could be demonstrated in strains of B. subtilis, B. pumilus and B. amyloliquefaciens. Qualitative screening for lecithinase activity also revealed that P. polymyxa strains produce this enzyme besides the B. cereus group that is well-known for causing a 'bitty cream' defect in pasteurized milk due to lecithinase activity. We found a strain of P. polymyxa to be capable of gas production during lactose fermentation. Strains belonging to the species B. amyloliquefaciens, Bacillus clausii, Lysinibacillus sphaericus, B. subtilis and P. polymyxa were able to reduce nitrate. A heat-stable cytotoxic component other than the emetic toxin was produced by strains of B. amyloliquefaciens and B. subtilis. Heat-labile cytotoxic substances were produced by strains identified as B. amyloliquefaciens, B. subtilis, B. pumilus and the B. cereus group. Variations in expression levels between strains from the same species were noticed for all tests. This study emphasizes the importance of aerobic spore-forming bacteria in raw milk as the species that are able to produce toxins and/or spoilage enzymes are all abundantly present in raw milk. Moreover, we demonstrated that some strains are capable of growing at room temperature and staying stable at refrigeration temperatures.

Measurement of proteolysis in milk and cheese using trinitrobenzene sulphonic acid and a new dissolving reagent

Milk contains indigenous plasmin-like and thrombin-like proteinases and also exogenous proteinases, mainly extracellular enzymes of psychrotrophic bacteria. Their activities lead to protein hydrolysis which may have important effects on milk quality and cheese yield. It is also useful to estimate proteolysis and its development during cheese ripening.

Milk opacity or turbidity due to calcium phosphocaseinate micelles and fat globules necessitates preliminary sample treatment, i.e. precipitation and filtration before spectrophotometric measurement. It is possible to eliminate these firs steps of sample treatment by dispersing the colloidal constituents. Some authors have already used different dissolving mixtures for sample analysis. Nakai & Anh Chi Le (1970) were the first to determine protein contents of milk and dairy products by absorbance measurement at 280 nm after clarification. Bosset & Blanc (1977) used the Biuret method. Owen & Andrews (1984) measured free amino groups with 2,4,6-trinitrobenzene sulphonic acid (TNBS). Humbert et al. (1982) and Stead (1987) used synthetic or specific substrates for proteinase determination.

In this work, we report the application of a new dissolving reagent to measure free amino groups in milk and cheese.

Measurement of proteolysis in cheese: relationship between phosphotungstic acid-soluble N fraction by Kjeldahl and 2,4,6- trinitrobenzenesulphonic acid-reactive groups in water-soluble N

Free amino groups produced during cheese ripening are used to indicate the extent of cheese proteolysis. Several studies have shown a high correlation between the level of free amino acids and the flavour of Gouda (Aston et al. 1983) or Comté (Grappin & Berdagué, 1989). Measurement of the level of free amino acids seems useful for the investigation of flavour chemistry in cheese (Lemieux et al. 1990). The determination of N fractions is often used to estimate the degree of proteolysis in cheese, but since this procedure is laborious and time consuming several attempts have been made to replace it by more rapid methods (Ardö & Meisel, 1991). Since its introduction by Satake et al. (1960), the 2,4,6-trinitrobenzenesulphonic acid (TNBS) method has been widely used for the determination of free amino groups. Because TNBS does not react with the imino groups of histidine and proline or the hydroxyl groups of tyrosine, serine or threonine, it has been accepted as a selective reagent for primary amino groups (Burger, 1974). Measurement of N by Kjeldahl in the phosphotungstic acid (PTA)–sulphuric acid extract (Gripon et al. 1975) estimates the N of free amino acids and low molecular mass peptides. The purpose of this study was to compare the TNBS and PTA-soluble N methods in order to find out whether the TNBS procedure can replace that of PTA-soluble N in the determination of a cheese proteolysis index.

Accelerated cheese ripening: use of Lac−mutants of lactococci

Lactose-negative mutants ofLactococcus lactissubsp.lactisandLacto-coccus lactissubsp.cremoriswith good autolytic properties were used at two different levels in addition to the normal starter in Gouda-type cheesemaking experiments. Increased numbers of bacteria were observed in fresh cheese, and the pH changes during ripening were as normal. A more rapid development of soluble N compounds was observed in cheeses with mutant addition, especially withLc. lactissubsp.lactisIMN-L2–3 andLc.lactissubsp.cremorisIMN-C12–1, compared with the control cheese. Cheese with addedLc. lactissubsp.lactisIMN-L2–3 andLc.lactissubsp.cremorisIMN-C12–1 showed the most extensive ripening and the highest product quality of the mutants tested. The experimental cheeses were graded higher than the control cheese. The ripening time was significantly reduced, and the quality was retained at an acceptable level during storage.

The effect of natural cheddar cheese ripening on the functional and textural properties of the processed cheese manufactured therefrom

Cheddar cheese ripened at 8 degrees C was sampled at 7, 14, 28, 56, 112, and 168 d and subsequently used for the manufacture of processed cheese. The cheddar cheese samples were analyzed throughout ripening for proteolysis while the textural and rheological properties of the processed cheeses (PCs) were studied. The rate of proteolysis was the greatest in the first 28 d of cheddar cheese ripening but began to slow down as ripening progressed from 28 to 168 d. A similar trend was observed in changes to the texture of the PC samples, with the greatest decrease in hardness and increase in flowability being in the first 28 d of ripening. Confocal scanning laser microscopy showed that the degree of emulsification in the PC samples increased as the maturity of the cheddar cheese ingredient increased from 7 to 168 d. This increased emulsification resulted in a reduction in the rate of softening in the PC in samples manufactured from cheddar cheese bases at later ripening times. Multivariate data analysis was performed to summarize the relationships between proteolysis in the cheddar cheese bases and textural properties of the PC made therefrom. The proportion of alpha(s)(1)-casein (CN) in the cheddar cheese base was strongly correlated with hardness, adhesiveness, fracturability, springiness, and storage modulus values for the corresponding PC. Degradation of alpha(s) (1)-CN was the proteolytic event with the strongest correlation to the softening of PC samples, particularly those manufactured from cheddar cheese in the first 28 d of ripening.

Chymosin-mediated proteolysis, calcium solubilization, and texture development during the ripening of cheddar cheese

Full fat, milled-curd Cheddar cheeses (2 kg) were manufactured with 0.0 (control), 0.1, 1.0, or 10.0 micromol of pepstatin (a potent competitive inhibitor of chymosin) added per liter of curds/whey mixture at the start of cooking to obtain residual chymosin levels that were 100, 89, 55, and 16% of the activity in the control cheese, respectively. The cheeses were ripened at 8 degrees C for 180 d. There were no significant differences in the pH values of the cheeses; however, the moisture content of the cheeses decreased with increasing level of pepstatin addition. The levels of pH 4.6-soluble nitrogen in the 3 cheeses with added pepstatin were significantly lower than that of the control cheese at 1 d and throughout ripening. Densitometric analysis of urea-PAGE electro-phoretograms of the pH 4.6-insoluble fractions of the cheese made with 10.0 micromol/L of pepstatin showed complete inhibition of hydrolysis of alpha(S1)-casein (CN) at Phe23-Phe24 at all stages of ripening. The level of insoluble calcium in each of 4 cheeses decreased significantly during the first 21 d of ripening, irrespective of the level of pepstatin addition. Concurrently, there was a significant reduction in hardness in each of the 4 cheeses during the first 21 d of ripening. The softening of texture was more highly correlated with the level of insoluble calcium than with the level of intact alpha(S1)-CN in each of the 4 cheeses early in ripening. It is concluded that hydrolysis of alpha(S1)-CN at Phe23-Phe24 is not a prerequisite for softening of Cheddar cheese during the early stages of ripening. We propose that this softening of texture is principally due to the partial solubilization of colloidal calcium phosphate associated with the para-CN matrix of the curd.

Effect of paracasein degradation on sensory properties of Gouda cheese

The relation between the sensory quality of Gouda cheese and the extent of paracasein degradation, i. e., the content of soluble N, peptide N, amino acid N and amine N, was studied. The above-mentioned parameters of paracasein degradation differently determined the sensory properties of Gouda cheese. The flavour of cheese after 6-week ripening depended to the largest extent on the content of amine N and soluble N. The effect of the content of peptide N on cheese flavour was smaller but statistically significant. Also the smell of Gouda cheese was to the largest extent correlated to the content of amine N. A dependence between smell and the content of peptide N was found only in the cheeses after 4-week ripening. None of the sensory quality parameters of the examined cheeses depended on the content of amino acid N.