Influence of high-pressure processing (HPP) on physico-chemical properties of fresh cheese

Freshly prepared rennet-coagulated soft cheese was high-pressure (HP) treated at up to 291 MPa and 29 min and using a full 2-factor central composite design of experiment, its physico-chemical properties (colour, fat, lipid oxidation, moisture and protein content, pH, and texture) were examined. HP treatment influenced significantly (p < 0.05) the colour, fat, moisture, lipid oxidation, hardness and adhesiveness of the fresh cheese. Fat content increased apparently as moisture decreased significantly after HP treatment of above 100 MPa. Increased pressures reduced lipid oxidation but increased yellowness although the latter showed more effect over redness in the HP-treated fresh cheese. Also, increased pressures increased hardness, decreased acidity and adhesiveness in HP-treated fresh cheese although increased exposure was found to increase acidity.Industrial relevanceHigh isostatic pressure for processing fresh cheese is yet to be adopted on an industrial scale. There is a need for research to provide evidence that improved properties of fresh cheese can be realized. The effects of HPP on rennet-coagulated soft Scottish cheese are investigated and the data from this study have provided points where optimized characteristic properties of HPP fresh cheese can be attained, which can serve as a lead for HPP users on fresh cheese.     

Commercial application of high-pressure processing for increasing starter-free fresh cheese shelf-life

Different non-thermal technologies have been proposed to extend the shelf-life of solid food products, high-pressure processing (HPP) being one of the emerging technologies which has been most extensively studied. In this study, one of the first commercial industrial-scale applications of HPP on a starter-free fresh cheese, with the aim of increasing its shelf-life, is presented. The effect of 500 MPa (5 min, 16 °C) on physico-chemical, microbial, colour, microstructure, texture and sensorial characteristics of starter-free fresh cheeses during cold storage of 21 days was studied. The results showed that pressurised cheeses presented a shelf-life of about 19–21 days when stored at 4 °C, whereas control cheese became unsuitable for consumption on day 7–8. On the other hand, cheese treated at 500 MPa was firmer and more yellow than the untreated one. However, these changes, which were detected by instrumental and sensory analysis, did not affect the preference for pressurised cheese. These results may lead to practical applications of HPP in the food industry to produce microbiologically safe cheese with extended shelf-life and sensory quality.     

High-pressure processing decelerates lipolysis and formation of volatile compounds in ovine milk blue-veined cheese

Enzyme-rich cheeses are prone to over-ripening during refrigerated storage. Blue-veined cheeses fall within this category because of the profuse growth of Penicillium roqueforti in their interior, which results in the production of highly active proteinases, lipases, and other enzymes responsible for the formation of a great number of flavor compounds. To control the excessive formation of free fatty acids (FFA) and volatile compounds, blue-veined cheeses were submitted to high-pressure processing (HPP) at 400 or 600 MPa on d 21, 42, or 63 after manufacture. Cheeses were ripened for 30 d at 10°C and 93% relative humidity, followed by 60 d at 5°C, and then held at 3°C until d 360. High-pressure processing influenced the concentrations of acetic acid and short-chain, medium-chain, and long-chain FFA. The effect was dependent on treatment conditions (pressure level and cheese age at the time of treatment). The lowest concentrations of acetic acid and FFA were recorded for cheeses treated at 600 MPa on d 21; these cheeses showed the lowest esterase activity values. Acetic acid and all FFA groups increased during ripening and refrigerated storage. The 102 volatile compounds detected in cheese belonged to 10 chemical groups (5 aldehydes, 12 ketones, 17 alcohols, 12 acids, 35 esters, 9 hydrocarbons, 5 aromatic compounds, 3 nitrogen compounds, 3 terpenes, and 1 sulfur compound). High-pressure processing influenced the levels of 97 individual compounds, whereas 68 individual compounds varied during refrigerated storage. Total concentrations of all groups of volatile compounds were influenced by HPP, but only ketones, acids, esters, and sulfur compounds varied during refrigerated storage. The lowest total concentrations for most groups of volatile compounds were recorded for the cheese pressurized at 600 MPa on d 21. A principal component analysis combining total concentrations of groups of FFA and volatile compounds discriminated cheeses by age and by the pressure level applied to HPP cheeses.     

Effect of hydrostatic high-pressure processing on the chemical,functional, and rheological properties of starter-free Queso Fresco

Queso Fresco (QF), a popular high-moisture, high-pH Hispanic-style cheese sold in the United States, underwent high-pressure processing (HPP), which has the potential to improve the safety of cheese, to determine the effects of this process on quality traits of the cheese. Starter-free, rennet-set QF (manufactured from pasteurized, homogenized milk, milled before hooping, and not pressed) was cut into 4.5- × 4.5- × 15-cm blocks and double vacuum packaged. Phase 1 of the research examined the effects of hydrostatic HPP on the quality traits of fresh QF that had been warmed to a core temperature of 20 or 40°C; processed at 200, 400, or 600 MPa for 5, 10, or 20 min; and stored at 4°C for 6 to 8 d. Phase 2 examined the long-term effects of HPP on quality traits when QF was treated at 600 MPa for 3 or 10 min, and stored at 4 or 10°C for up to 12 wk. Warming the QF to 40°C before packaging and exposure to high pressure resulted in loss of free whey from the cheese into the package, lower moisture content, and harder cheese. In phase 2, the control QF, regardless of aging temperature, was significantly softer than HPP cheeses over the 12 wk of storage. Hardness, fracture stress, and fracture rigidity increased with length of exposure time and storage temperature, with minor changes in the other properties. Queso Fresco remained a bright white, weak-bodied cheese that crumbled and did not melt upon heating. Although high pressures or long processing times may be required for the elimination of pathogens, cheese producers must be aware that HPP altered the rheological properties of QF and caused wheying-off in cheeses not pressed before packaging.     

Volatile compounds and sensory changes after high pressure processing of mature “Torta del Casar” (raw ewe's milk cheese) during refrigerated storage

The main objective of this paper was to study the changes during refrigerated storage in the volatile compounds and the sensory characteristics of raw ewe's milk mature Torta del Casar cheese High Pressure Processing-treated (200 or 600 MPa for 5 or 20 min) on day 60 after manufacture. Most of the volatile compounds and sensory characteristics were significantly affected by the refrigerated storage and to a lesser extent by the HPP treatment. The 600 MPa HPP treatments caused a decrease in the levels of the most abundant volatile compounds (acids and esters) and in bitterness scores on days 120 and 180. A significant effect was also found for some compounds, the overall flavour, and saltiness on day 240 after manufacture.Industrial relevance textHigh pressure processing treatments at 600 MPa applied to mature raw ewe's milk cheese such as “Torta del Casar” cheese could be an interesting option for the cheese industry to prevent some of the changes that occur during refrigerated storage that are related to over-ripening and excessive bitterness development. In addition, compared to the application of the treatment before maturation is completed, treating mature cheeses is convenient for the cheese industry because it provides the advantage of not requiring any unpackaging steps, which allows the food industry the commercialization of the cheeses without any additional packaging.     

Effect of high pressure treatment on starter bacteria and spoilage yeasts in fresh lactic curd cheese of bovine milk

The effect of high pressure treatment on the inactivation of starter bacteria and spoilage yeasts in a commercially manufactured fresh lactic curd cheese was investigated. Fresh cheeses made from pasteurised bovine milk using a commercial Lactococcus starter preparation were vacuum-packaged and subjected to high pressure treatment within the range of 200 to 600 MPa for 5 min at ambient temperature (≤ 22 °C), and subsequently stored at 4 °C for up to 8 weeks. The number of viable starter bacteria and spoilage yeasts were enumerated immediately after treatment and at time intervals of 1, 2, 3, 4, 6, and 8 weeks during refrigerated storage. The viable count of Lactococcus in the cheeses treated at 200, 300, 400, and 600 MPa, showed approximate reductions of 2, 5, 6, and 7 log units under aerobic incubation conditions; and 3, 5, 6, and 7 log units under anaerobic incubation conditions. Treatment at 200 MPa did not significantly prevent the growth of yeasts, but in samples subjected to pressures ≥ 300 MPa, the growth of yeasts was effectively controlled for 6 to 8 weeks.Industrial relevanceAustralian specialty cheese manufacturers are interested in extending the shelf-life of selected products to extend domestic distribution and to take advantage of export opportunities. The potential domestic and export market of fresh lactic curd cheese can be hampered by relatively short shelf-life. High pressure processing (HPP), under optimised conditions, can be utilised as an effective tool to extend shelf-life while maintaining the quality attributes of this product.     

High-pressure effects on the microstructure, texture, and color of white-brined cheese

Abstract: White‐brined cheeses were subjected to high‐pressure processing (HPP) at 50, 100, 200, and 400 MPa at 22 °C for 5 and 15 min and ripened in brine for 60 d. The effects of pressure treatment on the chemical, textural, microstructural, and color were determined. HPP did not affect moisture, protein, and fat contents of cheeses. Similar microstructures were obtained for unpressurized cheese and pressurized cheeses at 50 and 100 MPa, whereas a denser and continuous structure was obtained for pressurized cheeses at 200 and 400 MPa. These microstructural changes exhibited a good correlation with textural changes. The 200 and 400 MPa treatments resulted in significantly softer, less springy, less gummy, and less chewy cheese. Finally, marked differences were obtained in a* and b* values at higher pressure levels for longer pressure‐holding time and were also supported by ΔE* values. The cheese became more greenish and yellowish with the increase in pressure level. Practical Application: The quality of cheese is the very important to the consumers. This study documented the pressure‐induced changes in selected quality attributes of semisoft and brine‐salted cheese. The results can help the food processors to have knowledge of the process parameters resulting in quality changes and to identify optimal process parameters for preserving pressure‐treated cheeses.     

High-Pressure Processing for the Control of Lipolysis,Volatile Compounds and Off-odours in Raw Milk Cheese

Build-up of flavour compounds throughout ripening of raw milk cheeses may result in strong over-ripening notes during refrigerated storage. In order to control the formation of free fatty acids (FFAs) and volatile compounds, and the appearance of off-odours, raw milk cheeses were high-pressure-processed (HPP) 21 or 35 days after manufacture at 400 or 600 MPa. Ripening proceeded at 8 °C until day 60 and, afterwards, cheeses were held at 4 °C until day 240. The effect of HPP on the formation of FFAs and volatile compounds was dependent on pressure level and cheese age at the time of treatment. On day 60, acetic and propionic acids, branched-chain FFAs and short-chain FFAs showed the lowest (p?<?0.05) concentrations in cheeses treated at 400 or 600 MPa on day 21, while medium- and long-chain FFAs were at similar levels in all cheeses. HPP influenced significantly (p?<?0.05) 84 out of the 94 volatile compounds found in cheese. On day 60, the lowest (p?<?0.05) concentrations of acids, alcohols and esters were recorded for cheeses treated at 400 or 600 MPa on day 21, and the lowest (p?<?0.05) concentrations of ketones for cheeses treated at 400 MPa on days 21 or 35. On day 240, all HPP cheeses showed lower (p?<?0.05) concentrations of aldehydes, esters and, particularly, sulphur compounds than control cheese, which exhibited putrid and rancid off-odours from day 120 onwards. Principal component analysis combining FFAs and volatile compounds discriminated 240-day control cheese from 120-day control cheese and both from the rest of cheeses.     

Effect of high pressure treatment on microbial populations of sliced vacuum-packed cooked ham

In this study, culture-dependent and culture-independent approaches were used to reveal the microbial diversity and dynamic changes occurring in sliced vacuum-packed cooked ham after high pressure processing (HPP, 400MPa or 600MPa for 10min at 22°C) during refrigerated storage over 90days. Direct extraction of genome DNA and total RNA from meat samples, followed by PCR-denaturing gradient gel electrophoresis (DGGE) and RT-PCR-DGGE on 16S rDNA V3 region, was performed to define the structure of the bacterial populations and active species in pressurized cooked ham. Results showed that HPP affected differently the various species detected. The predominant spoilage organisms of cooked ham, such as Lactobacillus sakei and Lactobacillus curvatus, were found to be very sensitive to pressure as they were unable to be detected in HPP samples at any time during refrigerated storage. Weissella viridescens and Leuconostoc mesenteroides survived HPP at 600MPa for 10min at 22°C and were responsible for the final spoilage. An RNA-based DGGE approach clearly has potential for the analysis of active species that have survived in pressurized cooked ham. High pressure processing at 400 or 600MPa for 10min at room temperature (22°C) has a powerful inhibitory effect on the major spoilage bacteria of sliced vacuum-packed cooked ham. High pressure treatment may lead to reduced microbial diversity and improve the products' safety.     

High-pressure processing (HPP) of raw and dry-cured ham from experimentally infected pigs as a potential tool for the risk control of Toxoplasma gondii

Raw and dry-cured meats have been identified as a potential source of Toxoplasma gondii infection for humans. The present study evaluated the efficacy of an alternative non-thermal food-processing treatment, high hydrostatic pressure, on the viability of T. gondii bradyzoites in raw and dry-cured ham. Meat of pigs experimentally exposed to 4000 oocysts of T. gondii VEG strain was vacuum-packaged and subjected to high pressure processing (HPP). Tap water (6 °C ± 1 °C) was used as the pressure-transmitting fluid, and its temperature during HPP increased 2.7 °C per 100 MPa. The effect was evaluated by bioassay in mice followed by qPCR. In raw ham, 100–400 MPa/1 min did not inactivate T. gondii, whereas 600 MPa/20 min was effective. In dry-cured ham, 600 MPa for 3 or 10 min were not effective and a 20-min treatment was required to render the bradyzoites non-infectious for mice. Our results point toward the potential use of HPP as a tool for risk control of T. gondii and as a food safety guarantee.Industrial relevance textUnder real production conditions, the usual HPP treatments applied by the food industry to control L. monocytogenes and other pathogenic bacteria are 600 MPa of pressure and holding times of 3–10 min. Our study demonstrated, however, that longer treatment times are required to inactivate the parasite and, thus, to guarantee the safety of raw and dry-cured meats in order to reduce the public health risk of toxoplasmosis. Further research is needed to evaluate other HPP conditions, including pulsed cycles, for the inactivation of T. gondii in foods of animal origin.     

超高压处理抑制低温烟熏火腿中的优势腐败菌

研究尝试应用微生物菌体总RNA提取代替DNA提取,进而通过反转录-PCR( RT-PCR),结合变性梯度凝胶电泳(denaturing gradient gel electrophoresis,DGGE)技术,以期揭示超高压处理后低温烟熏火腿中腐败微生物的存活情况,探索RNA-DGGE手段判定超高压处理后微生物存活状态的可行性.分别以400 MPa和600 MPa的压力在室温(22℃)条件下,对包装后的烟熏火腿进行10 min超高压处理,未经高压处理样品作对照,于4℃冷藏条件下,贮藏1、15、30、60、90 d,直接提取样品中微生物的总RNA,对其进行RT-PCR和DGGE指纹图谱分析.DGGE指纹图谱显示,超高压处理对烟熏火腿中的优势腐败菌具有较强的抑制作用,且随压力的升高抑菌效应增强;超高压处理后烟熏火腿微生物种群结构变得单一,Weissella viridescens和Leuconostoc mesenteroides是超高压处理后烟熏火腿中的优势腐败菌.基于菌体总RNA提取的DGGE手段能够有效检测超高压处理后微生物的存活状况,揭示超高压对低温烟熏火腿中优势腐败微生物的抑菌效果.     

Microbial inactivation after high-pressure processing at 600 MPa in commercial meat products over its shelf life

The behaviour of spoilage and pathogenic microorganisms was evaluated after high-pressure treatment (600 MPa 6 min, 31 °C) and during chilled storage at 4 °C for up to 120 days of commercial meat products. The objective was to determine if this pressure treatment is a valid process to reduce the safety risks associated with Salmonella and Listeria monocytogenes, and if it effectively avoids or delays the growth of spoilage microorganisms during the chilled storage time evaluated. The meat products covered by this study were cooked meat products (sliced cooked ham, pH 6.25, a_w 0.978), dry cured meat products (sliced dry cured ham, pH 5.81, a_w 0.890), and raw marinated meats (sliced marinated beef loin, pH 5.88, a_w 0.985). HPP at 600 MPa for 6 min was an efficient method for avoiding the growth of yeasts and Enterobacteriaceae with a potential to produce off-flavours and for delaying the growth of lactic acid bacteria as spoilage microorganisms. HPP reduced the safety risks associated with Salmonella and L. monocytogenes in sliced marinated beef loin.     

Effect of pressure and holding time on colour,protein and lipid oxidation of sliced dry-cured Iberian ham and loin during refrigerated storage

The effect of high pressure (HP) treatments (200 MPa 15 min, 200 MPa 30 min, 300 MPa 15 min, 300 MPa 30 min) on colour, lipid and protein oxidation in sliced vacuum-packed dry-cured Iberian ham and loin during refrigerated storage (90 days, + 4 °C) was evaluated. Pressure level and holding time increased the extent of lipid oxidation in both products. Dry-cured ham showed a higher susceptibility to lipid oxidation than dry-cured loin since HP treatment increased TBA-RS values in dry-cured ham samples while HP treatment decreased TBA-RS values in dry-cured loin samples. However, HP treatment did not affect protein oxidation in both meat products. On the other hand, HP treatment affected instrumental colour since non-pressurized dry-cured meat products showed higher redness than pressurized ones. Regarding changes under storage, after 90 days of refrigerated storage lipid and protein oxidation increased while redness decreased in both HP treated and non-treated dry-cured meat products. Changes induced by HP were only noticeable after HP treatment, as storage reduced the initial differences between HP treated and non-treated samples. Therefore, the lack of differences in long stored dry-cured ham and loin HP treated and non-treated indicates that the application of HP (200–300 MPa/15–30 min) could not affect the quality of dry-cured meat products.Industrial relevanceDry-cured meat products are the meat-based products with the highest sensory quality in Spain and have a high projection in exterior markets. High pressure processing is effective in controlling pathogen and spoilage microorganisms in meat and meat products although it can promote color and oxidation changes that modify sensory characteristics. The study aimed the evaluation of pressure and holding time on color changes and protein and lipid oxidation at vacuum packed slices of Iberian dry-cured ham and loin during subsequent extended chilled storage. High pressure treatment of dry-cured Iberian ham and loin induce changes after treatment although initial differences are not maintained along refrigerated storage.     

Effect of high-pressure-processing on the microbiology,proteolysis, texture and flavour of Brie cheese during ripening and refrigerated storage

Brie cheeses were high pressure (HP)-treated at 400 or 600 MPa on days 14 or 21 after manufacture to prevent over-ripening. Lactic acid bacteria and Penicillium camemberti numbers declined markedly after HP treatment. In control cheese pH increased 2.0 units from day 21 to day 60, but less than 0.3 units in HP-treated cheeses. Cheeses treated at 600 MPa showed the maximum concentrations of residual caseins during refrigerated storage and control cheese the minimum concentrations. A 7.6-fold increase in hydrophobic peptides was recorded from day 21 to day 60 in control cheese and 0.8–1.6-fold increases in HP-treated cheeses. The maximum aminopeptidase activity was detected in control cheese, the highest free amino acid concentrations in cheeses treated at 400 MPa. The firmest texture was recorded for cheeses treated on day 14 at 400 or 600 MPa. HP-treated cheeses showed higher flavour quality scores than control cheese from day 60 onwards.     

Residual polyphenol oxidase and peroxidase activities in high pressure processed bok choy (Brassica rapa subsp. chinensis) juice did not accelerate nutrient degradation during storage

Polyphenol oxidase (PPO) and peroxidase (POD) cannot be fully inactivated by commercial high pressure processing (HPP) operations, and their residual activities may accelerate nutrient degradation during storage. This study hence aimed to establish the effect of residual enzyme activity on nutrient preservation in bok choy (Brassica rapa subsp. chinensis) juice. Bok choy juice was treated at 600 MPa for up to 20 min and enzyme inactivation, nutrient retention immediately after treatment and nutrient preservation during storage were determined. High residual PPO (85.1 ± 2.6%) and POD (68.5 ± 1.0%) activities remained after 20 min of treatment. Increasing the pressure holding time to enhance enzyme inactivation did not compromise total antioxidant capacity, vitamin C, carotenoids, isothiocyanates and vitamin K levels. Neither did it significantly reduce the vitamin C degradation rate during refrigerated storage. Maximising enzyme inactivation may thus not be necessary for nutrient preservation during the storage of HPP-treated bok choy juice.Industrial relevance textWith HPP, an increase in pressure or holding time is required to achieve higher levels of enzyme inactivation. Without the need to maximize PPO and POD inactivation, juice processors may employ the minimum pressure and holding time required for microbial inactivation. As vegetative bacteria are typically less resistant to HPP inactivation than these enzymes, this could translate to reduced energy costs and increased throughput.     

Effects of Penicillium roqueforti and whey cheese on gross composition,microbiology and proteolysis of mould‐ripened Civil cheese during ripening

Four different types of mould‐ripened Civil cheese were manufactured. A defined (nontoxigenic) strain of a Penicillium roqueforti (SC 509) was used as the secondary starter with and without addition of the whey cheese (Lor); in parallel, secondary starter‐free counterparts were manufactured. Chemical composition, microbiology and proteolysis were studied during the ripening. The incorporation of whey cheese in the manufacture of mould‐ripened Civil cheese altered the gross composition and adversely affected proteolysis in the cheeses. The inoculated P. roqueforti moulds appeared to grow slowly on those cheeses, and little proteolysis was evident in all cheese treatments during the first 90 days of ripening. However, sharp increases in the soluble nitrogen fractions were observed in all cheeses after 90 days. Microbiological analysis showed that the microbial counts in the cheeses were at high levels at the beginning of ripening, while their counts decreased approximately 1–2 log cfu/g towards the end of ripening.     

Effect of high pressure treatment on the quality of rainbow trout (Oncorhynchus mykiss) and mahi mahi (Coryphaena hippurus)

ABSTRACT:  High pressure processing (HPP) is becoming a promising seafood preservation method. The objective was to investigate the effect of HPP on quality of rainbow trout and mahi mahi during cold storage. Skinless fillets treated with different pressures (150, 300, 450, and 600 MPa for 15 min) and stored at 4 °C were analyzed at 1, 3, and 6 d storage. Red muscle was analyzed for lipid oxidation products by measuring thiobarbituric reactive substances (TBARS) and whole muscle was analyzed for total aerobic count, texture profile analysis, and color. A pressure of 300 MPa effectively inactivated the initial microbial population in rainbow trout (6-log reduction). However, inactivation of the initial population on mahi mahi was only about 4-log reduction at the same pressure. Microbial growth was significantly retarded after HPP. Color results showed that redness ( a * value) of rainbow trout at 300 MPa and above was significantly ( P < 0.05) lower compared to mahi mahi. TBARS values for rainbow trout increased with increased pressure, whereas the same trend was not seen for mahi mahi where maximum oxidation was found at 300 MPa and then declined. This study demonstrates the usefulness of HPP in seafood processing and the influence of species variation on processing parameters. The optimum HPP conditions for influencing lipid oxidation, microbial load, and color changes were found to be 300 MPa for rainbow trout and 450 MPa for mahi mahi.     

Effect of high pressure processing on physicochemical and microbiological properties of marinated beef with reduced sodium content

High pressure processing (HPP) is one of the newer technologies that has shown great potential for manufacturing meat products with reduced sodium content. The aim of this study was to evaluate the effect of HPP applied at different levels in the inactivation of Listeria innocua and Enterococcus faecium inoculated in marinated beef (Longissimus lumborum) with reduced sodium content, as well as its influence on the physicochemical properties of the meat. Samples were inoculated with 10⁶ CFU/g of L. innocua and E. faecium, and marinated with solutions in different concentrations of NaCl (1 or 2%) and citric acid (1 or 2%) for 18 h and treated with high pressure (300, 450 or 600 MPa). Samples treated with 600 MPa were also evaluated regarding physicochemical stability after 14 days of refrigerated storage. Different marinating solutions were not sufficient to reduce initial microbial loads in the non-pressurized samples, but the combination with high pressure caused six log cycle reductions of both microorganisms. The treatment with 2% salt/2% citric acid was the most effective for each pressure considered for both bacteria. No significant changes were observed in the water activity of the samples, however samples with higher concentration of citric acid showed lower (p < 0.05) pH and lower (p < 0.05) lipid oxidation after 14 days of storage under refrigeration. Only samples treated with 600 MPa showed an increase (p < 0.05) in hardness. The results showed that HPP was able to process a safe meat product with reduced sodium concentration.Industrial relevanceThe result from this study shows the benefits of using HPP as an alternative meat processing technology to develop a meat product with reduce sodium content. Meat processed by this method have better nutritional quality and extended shelf life as compared to conventional processing. The step taken in this study will aid the meat processing industry for the development of microbiologically safe meat product with low sodium content by the application of high pressure processing.     

Effects of high pressure treatment on glycolytic enzymes of Lactococcus lactis subsp. lactis,Streptococcus thermophilus and Lactobacillus acidophilus

High pressure processing (HPP) reduces the glycolytic activity of lactic acid bacteria (LAB) and provides a means to control further production of acidic metabolites in fermented dairy products during storage. However, there is limited information on the effects of HPP on specific enzymes of dairy starter bacteria responsible for the metabolism of lactose. The aim of this study was to determine pressure-induced inactivation of glycolytic enzymes in Lactococcus lactis subsp. lactis C10, Streptococcus thermophilus TS1 and Lactobacillus acidophilus 2400. Cultures were grown for 16 h in M17 or MRS broth containing 5% (w/v) lactose at pH 6.5 (maintained by addition of 10 M NaOH). The cells were harvested by centrifugation, washed and resuspended in 100 mM phosphate buffer (pH 6.5) and pressure-treated at 300 and 600 MPa (≤ 22 °C, 5 min). The ability of pressure-treated resting cells of Lactococcus, incubated with 5% (w/v) lactose at 30 °C, to ferment lactose was evaluated by determining titratable acidity (TA) during incubation. The activities of phospho-β-galactosidase (P-β-gal), β-galactosidase (β-gal) and lactate dehydrogenase (LDH) were determined in cell-free extracts of untreated and pressure-treated cells. Resting cells of Lactococcus treated at 600 MPa had a substantially lower rate of acidification than the controls and those treated at 300 MPa. Both P-β-gal and β-gal were significantly inactivated (p < 0.01) in the starter cultures treated at 300 or 600 MPa. The LDH in Lactococcus and Lactobacillus was highly resistant to pressure treatment at 300 MPa. In contrast, the LDH in Streptococcus was almost completely inactivated at ≥ 300 MPa.Industrial relevanceContinuing production of acidic metabolites in fermented dairy products during storage can be a technological challenge that adversely affects product quality. The current study demonstrates that high pressure processing (HPP) offers the potential of controlling this problem by inactivation of glycolytic enzymes in various mesophilic and thermophilic starter cultures. The findings of this research will assist in establishing optimised operating parameters for HPP treatment of cultured products to extend shelf-life, by reducing acid production during storage.     

Proteolysis and biogenic amine buildup in high-pressure treated ovine milk blue-veined cheese

Penicillium roqueforti plays an important role in the ripening of blue-veined cheeses, mostly due to lactic acid consumption and to its extracellular enzymes. The strong activity of P. roqueforti proteinases may bring about cheese over-ripening. Also, free amino acids at high concentrations serve as substrates for biogenic amine formation. Both facts result in shorter product shelf-life. To prevent over-ripening and buildup of biogenic amines, blue-veined cheeses made from pasteurized ovine milk were high-pressure treated at 400 or 600 MPa after 3, 6, or 9 wk of ripening. Primary and secondary proteolysis, biogenic amines, and sensory characteristics of pressurized and control cheeses were monitored for a 90-d ripening period, followed by a 270-d refrigerated storage period. On d 90, treatments at 400 MPa had lowered counts of lactic acid bacteria and P. roqueforti by less than 2 log units, whereas treatments at 600 MPa had reduced lactic acid bacteria counts by more than 4 log units and P. roqueforti counts by more than 6 log units. No residual α-casein (CN) or κ-CN were detected in control cheese on d 90. Concentrations of β-CN, para-κ-CN, and γ-CN were generally higher in 600 MPa cheeses than in the rest. From d 90 onwards, hydrophilic peptides were at similar levels in pressurized and control cheeses, but hydrophobic peptides and the hydrophobic-to-hydrophilic peptide ratio were at higher levels in pressurized cheeses than in control cheese. Aminopeptidase activity, overall proteolysis, and free amino acid contents were generally higher in control cheese than in pressurized cheeses, particularly if treated at 600 MPa. Tyramine concentration was lower in pressurized cheeses, but tryptamine, phenylethylamine, and putrescine contents were higher in some of the pressurized cheeses than in control cheese. Differences in sensory characteristics between pressurized and control cheeses were generally negligible, with the only exception of treatment at high pressure level (600 MPa) at an early ripening stage (3 wk), which affected biochemical changes and sensory characteristics.