Redefining the effect of salt on thermophilic starter cell viability, culturability and metabolic activity in cheese

peer-reviewedThis study investigated the differential effect of salt concentration in the outside and inside layers of brine salted cheeses on viability, culturability and enzyme activity of starter bacteria. The high-salt environment of the outside layer caused a sharp decrease in L. helveticus viability as measured by traditional plate counts. Remarkably, this was associated with lower release of intracellular enzymes (LDH), reduced levels of proteolysis and larger membrane integrity as measured by flow cytometry (FC) following classical Live/Dead staining. FC analysis of light scattering properties highlighted a significant reduction in size and granularity of the microbiota located in the cheese surface, suggestive of cell shrinkage and condensation of internal macromolecules probably due to hyperosmotic stress. The microbiota of the cheese surface were found to experience greater oxidative stress, as measured by FC analysis of the total levels of reactive oxygen species, compared to that of the interior layer. These results lead us to postulate that the physiology and health status of the microbiota were significantly different in the outer and inner layers of the cheese. The hyperosmotic environment of the outer layer resulted in reduced cell lysis, as measurable by assays based upon membrane integrity, but rather triggered cell death via mechanisms involving cell shrinkage and ROS-mediated damage of vital intracellular components. This study challenges the current thinking on how salt controls microbial activity in ripening cheese, especially in cheeses which are brine salted as local variations in biochemical ripening indices can differ significantly from the outside to the inside of a ripening cheese

Microstructure and Fracture Properties of Semi-Hard Cheese: Differentiating the Effects of Primary Proteolysis and Calcium Solubilization

The individual roles of hydrolysis of αS1- and β-caseins, and calcium solubilization on the fracture properties of semi-hard cheeses, such as Maasdam and other eye-type cheeses, remain unclear. In this study, the hydrolysis patterns of casein were selectively altered by adding a chymosin inhibitor to the curd/whey mixture during cheese manufacture, by substituting fermentation-produced bovine chymosin (FPBC) with fermentation-produced camel chymosin (FPCC), or by modulating ripening temperature. Moreover, the level of insoluble calcium during ripening was quantified in all cheeses. Addition of a chymosin inhibitor, substitution of FPBC with FPCC, or ripening of cheeses at a consistent low temperature (8 °C) decreased the hydrolysis of αS1-casein by ~95%, ~45%, or ~30%, respectively, after 90 d of ripening, whereas ~35% of β-casein was hydrolysed in that time for all cheeses, except for those ripened at a lower temperature (~17%). The proportion of insoluble calcium as a percentage of total calcium decreased significantly from ~75% to ~60% between 1 and 90 d. The rigidity or strength of the cheese matrix was found to be higher (as indicated by higher fracture stress) in cheeses with lower levels of proteolysis or higher levels of intact caseins, primarily αS1-casein. However, contrary to the expectation that shortness of cheese texture is associated with αS1-casein hydrolysis, fracture strain was significantly positively correlated with the level of intact β-casein and insoluble calcium content, indicating that the cheeses with low levels of intact β-casein or insoluble calcium content were more likely to be shorter in texture (i.e., lower fracture strain). Overall, this study suggests that the fracture properties of cheese can be modified by selective hydrolysis of caseins, altering the level of insoluble calcium or both. Such approaches could be applied to design cheese with specific properties

Influence of addition of plasmin or mastitic milk to cheesemilk on quality of smear-ripened cheese

peer-reviewedSmear-ripened cheese varieties are characterised by the growth of a smear culture, containing predominantly Brevibacterium linens, on the cheese surface during ripening. In such cheese, considerable zonal differences in biochemistry of ripening exist, due to moisture loss from, and growth and metabolic activity of smear microflora at, the cheese surface. In this study, the effects of adding exogenous plasmin or small amounts of mastitic milk to good quality milk on the quality of smear-ripened cheese made subsequently was examined. Addition of plasmin did not influence cheese composition immediately after manufacture, but slightly decreased the rate of moisture loss during cheese ripening. Plasmin activity decreased during the early stages of ripening, but subsequently increased towards the end of ripening, perhaps due to changing pH conditions in the cheese. Addition of plasmin increased rates of primary proteolysis in cheese, as measured by levels of pH 4.6- soluble N and urea-PAGE, although production of later products of proteolysis appeared less affected. Addition ofmastitic milk had largely similar effects to addition of exogenous plasmin, which may reflect a high content of plasmin or plasminogen activators in such milk. Overall, changes in milk quality and enzymology appear to influence the quality of smear-ripened cheese

Measurement of syneretic properties of rennet-induced curds and impact of factors such as concentration of milk: A review

peer-reviewedBackground The rate or extent of whey expulsion or syneresis from cheese curds during stirring in-vat determines curd moisture levels, which subsequently influences cheese moisture content. The outward migration of whey depends on curd contraction and on the structure of the pores permitting whey movement. Curd syneretic properties are one of the least understood areas of cheese science, particularly when milk of varying composition is used. Scope and approach This review provides an insight into the mechanisms of curd formation and curd syneresis, and factors influencing syneretic properties in unconcentrated and concentrated milk and appraises syneresis measurement methods in terms of their relative strengths and weaknesses. Key findings and conclusions Direct measurement of moisture content of curds is recommended as a simple and reliable method for measurement of syneresis of industrial relevance and, although inline measurement for curd moisture prediction has been a significant development in the last decade, its application to commercial production is still limited. A review of previous studies found that experimental conditions and methodologies used to measure syneresis vary widely, making it difficult to compare data between studies. Overall, interactions between process variables employed determines whether syneresis is accentuated or inhibited, and this can be exploited by cheese producers to attain target curd moisture contents by varying process parameters, particularly when milk is concentrated prior to cheese-making. Furthermore, further studies should be focused on endogenous syneresis and casein network rearrangement to clearly elucidate this mechanism and its influence on macrosyneresis under dynamic conditions

Solubility of carbon dioxide in renneted casein matrices: effect of pH, salt, temperature, partial pressure, and moisture to protein ratio

The solubility of carbon dioxide (CO2) in the moisture and protein components of cheese matrices and the influence of changing pH, salt and temperature levels remains unclear. In this study, model casein matrices were prepared, by renneting of micellar casein concentrate (MCC), with modulation of salt and pH levels by adding salt and glucono delta-lactone, respectively, to the MCC solutions prior to renneting. Different moisture-to-protein levels were achieved by freeze-drying, incubation of samples at different relative humidities, or by applying varying pressures during gel manufacture. The CO2 solubility of samples decreased linearly with both increasing temperature and salt-in-moisture content, whereas solubility of CO2 increased with increasing pH. A non-linear relationship was observed between CO2 solubility and the moisture-to-protein ratio of experimental samples. Overall, such knowledge may be applied to improve the quality and consistency of eye-type cheese, and in particular to avoid development of undesirable slits and cracks

Understanding preferences for and consumer behavior toward cheese among a cohort of young, educated, internationally mobile Chinese consumers

peer-reviewedThis study explores the experiences of a cohort of young, educated, internationally mobile Chinese consumers with cheese and other dairy products, and how these experiences shape their behavior toward cheese products. In total, 41 Chinese students studying at an Irish university participated in 5 focus groups (n = 41, n = 7–10). Thematic analysis identified important factors that influence consumer behaviors regarding cheese products. Individuals' expectations toward cheese were embedded in their knowledge structures, which were constructed from previous experience. Participants had general positive expectations toward cheese due to associations with western-style foods and nostalgia; however, direct eating experience determined long-term behavior. When making a purchase decision, choice motives were weighed and negotiated to establish a fundamental driving factor for purchase. Perceived probability of choice motive fulfillment was important in determining purchase decisions, with many participants having low perceived ability to select cheese and limited motivation to engage with cheese due to limited perceived relevance of cheese to their daily food life. Individuals' innovativeness was an important factor that influences their openness to cheese products when moving beyond familiar foods. Opportunities exist such as using nostalgic cues as marketing tools to increase consumers' interest in cheese or combining cheese with Chinese food to increase perceived relevance of cheese to their daily food life. Providing information at point of purchase could reduce the disconnect between expectation and actual experience, and innovative cheese products may be developed to better fulfill important choice motives

Effect of thermal treatment on serum protein reduced micellar casein concentrate: An evaluation of rennet coagulability, cheese composition and yield

Microfiltration at 0.10 μm removed ～70.29% of serum proteins from milk and the resultant micellar casein concentrates (MCC) were subjected to no heat treatment (control), pasteurisation (72 °C × 15 s) and high heat treatment (HHT; 90 °C × 15s) before formulation of cheese milk for Cheddar cheese manufacture. MCC showed good heat stability due to low serum protein content. For cheese milk of typical casein content, both pasteurisation and HHT did not significantly influence pH, calcium distribution and rennet coagulability, or subsequent cheese composition and yield; although HHT elongated cheese make time significantly. On increasing casein content from 3.09% to 4.31%, there was no significant difference for rennet to cut time between cheeses made from milk with different thermal histories and casein contents. Overall, HHT of MCC had no significant impact on cheese make properties, cheese composition and yield of Cheddar cheese

Effect of pasture versus indoor feeding systems on quality characteristics, nutritional composition, and sensory and volatile properties of full-fat Cheddar cheese

peer-reviewedThe purpose of this study was to investigate the effects of pasture-based versus indoor total mixed ration (TMR) feeding systems on the chemical composition, quality characteristics, and sensory properties of full-fat Cheddar cheeses. Fifty-four multiparous and primiparous Friesian cows were divided into 3 groups (n = 18) for an entire lactation. Group 1 was housed indoors and fed a TMR diet of grass silage, maize silage, and concentrates; group 2 was maintained outdoors on perennial ryegrass only pasture (GRS); and group 3 was maintained outdoors on perennial ryegrass/white clover pasture (CLV). Full-fat Cheddar cheeses were manufactured in triplicate at pilot scale from each feeding system in September 2015 and were examined over a 270-d ripening period at 8°C. Pasture-derived feeding systems were shown to produce Cheddar cheeses yellower in color than that of TMR, which was positively correlated with increased cheese β-carotene content. Feeding system had a significant effect on the fatty acid composition of the cheeses. The nutritional composition of Cheddar cheese was improved through pasture-based feeding systems, with significantly lower thrombogenicity index scores and a greater than 2-fold increase in the concentration of vaccenic acid and the bioactive conjugated linoleic acid C18:2 cis-9,trans-11, whereas TMR-derived cheeses had significantly higher palmitic acid content. Fatty acid profiling of cheeses coupled with multivariate analysis showed clear separation of Cheddar cheeses derived from pasture-based diets (GRS or CLV) from that of a TMR system. Such alterations in the fatty acid profile resulted in pasture-derived cheeses having reduced hardness scores at room temperature. Feeding system and ripening time had a significant effect on the volatile profile of the Cheddar cheeses. Pasture-derived Cheddar cheeses had significantly higher concentrations of the hydrocarbon toluene, whereas TMR-derived cheese had significantly higher concentration of 2,3-butanediol. Ripening period resulted in significant alterations to cheese volatile profiles, with increases in acid-, alcohol-, aldehyde-, ester-, and terpene-based volatile compounds. This study has demonstrated the benefits of pasture-derived feeding systems for production of Cheddar cheeses with enhanced nutritional and rheological quality compared with a TMR feeding system

Thermus and the Pink Discoloration Defect in Cheese

peer-reviewedA DNA sequencing-based strategy was applied to study the microbiology of Continental-type cheeses with a pink discoloration defect. The basis for this phenomenon has remained elusive, despite decades of research. The bacterial composition of cheese containing the defect was compared to that of control cheese using 16S rRNA gene and shotgun metagenomic sequencing as well as quantitative PCR (qPCR). Throughout, it was apparent that Thermus, a carotenoid-producing genus, was present at higher levels in defect-associated cheeses than in control cheeses. Prompted by this finding and data confirming the pink discoloration to be associated with the presence of a carotenoid, a culture-based approach was employed, and Thermus thermophilus was successfully cultured from defect-containing cheeses. The link between Thermus and the pinking phenomenon was then established through the cheese defect equivalent of Koch’s postulates when the defect was recreated by the reintroduction of a T. thermophilus isolate to a test cheese during the manufacturing process. IMPORTANCE Pink discoloration in cheese is a defect affecting many cheeses throughout the world, leading to significant financial loss for the dairy industry. Despite decades of research, the cause of this defect has remained elusive. The advent of high-throughput, next-generation sequencing has revolutionized the field of food microbiology and, with respect to this study, provided a means of testing a possible microbial basis for this defect. In this study, a combined 16S rRNA, whole-genome sequencing, and quantitative PCR approach was taken. This resulted in the identification of Thermus, a carotenoid-producing thermophile, in defect-associated cheeses and the recreation of the problem in cheeses to which Thermus was added. This finding has the potential to lead to new strategies to eliminate this defect, and our method represents an approach that can be employed to investigate the role of microbes in other food defects of unknown origin.Teagasc Walsh Fellowship Programm