Effect of high pressure homogenization on microbial and chemico-physical characteristics of goat cheeses

FT-MIR Analysis of Water-Soluble Extracts during the Ripening of Sheep Milk Cheese with Different Phospholipid Content

The purpose of this work was to study the suitability of the water-soluble extracts (WSE) of semi-hard sheep milk cheese for analysis by diffuse reflectance Fourier transform mid-infrared spectroscopy (FT-MIR) and the development of classification models using discriminant analysis and based on cheese age or phospholipid content. WSE was extracted from three types of sheep milk cheeses (full-fat, reduced-fat and reduced-fat fortified with lyophilized sweet sheep buttermilk) at various stages of ripening from six to 168 days and lyophilized. The first model used 1854–1381 and 1192–760 cm−1 regions of the first-derivative spectra and successfully differentiated samples of different age, based on changes in the water-soluble products of ripening biochemical events. The second model used the phospholipid absorbance spectral regions (3012–2851, 1854–1611 and 1192–909 cm−1) to successfully discriminate cheeses of markedly different phospholipid content. Cheese WSE was found suitable for FT-MIR analysis. According to the results, a fast and simple method to monitor cheese ripening based on water-soluble substances has been developed. Additionally, the results indicated that a considerable amount of phospholipids migrates to the cheese WSE and that FT-MIR can be a useful tool for their assessment.

Effect of ripening time on bacteriological and physicochemical goat milk cheese characteristics

Cheese ripening involves lactose metabolism, lipolysis and proteolysis, which are affected by many factors. The aim of this study was to assess changes due to ripening (90 days) of goat milk cheese through bacteriological and physicochemical analysis in order to verify if, at the end of ripening period, this cheese could be considered "lactose-free". Three batches of the goat milk cheese were manufactured and ripened at 10 °C and 80% relative humidity for 90 days. Titratable acidity increased by about 59 °D due to carbohydrate degradation and organic acid production. However, pH (5.31-5.25) remained constant. Lactococcus was the dominant cheese microbiota, acting in the fermentation of lactose (1.17-0.06 mg/g) and lactic acid production (5.49-s10.01 mg/g). Thus, ripening time was decisive for bacteriological and physicochemical goat milk cheese characteristics.

Lactobacillus paracasei A13 and High-Pressure Homogenization Stress Response

Sub-lethal high-pressure homogenization treatments applied to Lactobacillus paracasei A13 demonstrated to be a useful strategy to enhance technological and functional properties without detrimental effects on the viability of this strain. Modification of membrane fatty acid composition is reported to be the main regulatory mechanisms adopted by probiotic lactobacilli to counteract high-pressure stress. This work is aimed to clarify and understand the relationship between the modification of membrane fatty acid composition and the expression of genes involved in fatty acid biosynthesis in Lactobacillus paracasei A13, before and after the application of different sub-lethal hyperbaric treatments. Our results showed that Lactobacillus paracasei A13 activated a series of reactions aimed to control and stabilize membrane fluidity in response to high-pressure homogenization treatments. In fact, the production of cyclic fatty acids was counterbalanced by the unsaturation and elongation of fatty acids. The gene expression data indicate an up-regulation of the genes accA, accC, fabD, fabH and fabZ after high-pressure homogenization treatment at 150 and 200 MPa, and of fabK and fabZ after a treatment at 200 MPa suggesting this regulation of the genes involved in fatty acids biosynthesis as an immediate response mechanism adopted by Lactobacillus paracasei A13 to high-pressure homogenization treatments to balance the membrane fluidity. Although further studies should be performed to clarify the modulation of phospholipids and glycoproteins biosynthesis since they play a crucial role in the functional properties of the probiotic strains, this study represents an important step towards understanding the response mechanisms of Lactobacillus paracasei A13 to sub-lethal high-pressure homogenization treatments.

Effect of processing on nutritive values of milk protein

Milk is an essential source of nutritionally excellent quality protein in human, particularly in vegan diet. Before consumption, milk is invariably processed depending upon final product requirement. This processing may alter the nutritive value of protein in a significant manner. The processing operations like thermal treatment, chemical treatment, biochemical processing, physical treatments, nonconventional treatments, etc. may exert positive or negative influence on nutritional quality of milk proteins. On one side, processing enhances the nutritive and therapeutic values of protein while on other side intermediate or end products generated during protein reactions may cause toxicity and/or antigenicity upon consumption at elevated level. The review discusses the changes occurring in nutritive quality of milk proteins under the influence of various processing operations.

Applications of High and Ultra High Pressure Homogenization for Food Safety

Traditionally, the shelf-life and safety of foods have been achieved by thermal processing. Low temperature long time and high temperature short time treatments are the most commonly used hurdles for the pasteurization of fluid foods and raw materials. However, the thermal treatments can reduce the product quality and freshness. Consequently, some non-thermal pasteurization process have been proposed during the last decades, including high hydrostatic pressure, pulsed electric field, ultrasound (US), and high pressure homogenization (HPH). This last technique has been demonstrated to have a great potential to provide “fresh-like” products with prolonged shelf-life. Moreover, the recent developments in high-pressure-homogenization technology and the design of new homogenization valves able to withstand pressures up to 350–400 MPa have opened new opportunities to homogenization processing in the food industries and, consequently, permitted the development of new products differentiated from traditional ones by sensory and structural characteristics or functional properties. For this, this review deals with the principal mechanisms of action of HPH against microorganisms of food concern in relation to the adopted homogenizer and process parameters. In addition, the effects of homogenization on foodborne pathogenic species inactivation in relation to the food matrix and food chemico-physical and process variables will be reviewed. Also the combined use of this alternative technology with other non-thermal technologies will be considered.

High pressure homogenization vs heat treatment: safety and functional properties of liquid whole egg

This research investigated the potential of multi-pass homogenization treatment for the inactivation of Salmonella enterica serovar Enteritidis inoculated at different levels in liquid whole egg (LWE) comparing the efficacy of this treatment with a traditional thermal one performed at 65 °C. Moreover, the effects of high pressure treatment (HPH) on structural and functional properties such as viscosity, microstructure and foaming abilities of LWE were investigated. The data obtained suggested that the multi-pass high pressure treatment at 100 MPa of S. enterica serovar Enteritidis inoculated in LWE at 7 and 4 log CFU/ml resulted in a first order inactivation kinetic, while the thermal inactivation curves of S. enterica serovar Enteritidis inoculated at 8 and 4 log CFU/ml presented a non-linear behaviour, with a marked tail after 3 min of treatment at 65 °C. Additionally, HPH treatment caused an increase in foaming capacity of LWE, with respect to the untreated samples, passing from values of 26% of the control to 50% of pressure treated samples.

High pressure homogenization versus heat treatment: effect on survival, growth, and metabolism of dairy Leuconostoc strains

The effect of high pressure homogenization (HPH) with respect to a traditional heat treatment on the inactivation, growth at 8°C after treatments, and volatile profile of adventitious Leuconostoc strains isolated from Cremoso Argentino spoiled cheeses and ingredients used for their manufacture was evaluated. Most Leuconostoc strains revealed elevated resistance to HPH (eight passes, 100 MPa), especially when resuspended in skim milk. Heat treatment was more efficient than HPH in inactivating Leuconostoc cells at the three initial levels tested. The levels of alcohols and sulfur compounds increased during incubation at 8°C in HPH-treated samples, while the highest amounts of aldehydes and ketones characterized were in heated samples. Leuconostoc cells resuspended in skim milk and subjected to one single-pass HPH treatment using an industrial-scale machine showed remarkable reductions in viable cell counts only when 300 and 400 MPa were applied. However, the cell counts of treated samples rose rapidly after only 5 days of storage at 8°C. The Leuconostoc strains tested in this work were highly resistant to the inactivation treatments applied. Neither HPH nor heat treatment assured their total destruction, even though they were more sensitive to the thermal treatment. To enhance the inhibitory effect on Leuconostoc cells, HPH should be combined with a mild heat treatment, which in addition to efficient microbial inactivation, could allow maximal retention of the physicochemical properties of the product.

The effect of high pressure homogenization on the activity of a commercial β-galactosidase

High pressure homogenization (HPH) has been proposed as a promising method for changing the activity and stability of enzymes. Therefore, this research studied the activity of β-galactosidase before and after HPH. The enzyme solution at pH values of 6.4, 7.0, and 8.0 was processed at pressures of up to 150 MPa, and the effects of HPH were determined from the residual enzyme activity measured at 5, 30, and 45 °C immediately after homogenization and after 1 day of refrigerated storage. The results indicated that at neutral pH the enzyme remained active at 30 °C (optimum temperature) even after homogenization at pressures of up to 150 MPa. On the contrary, when the β-galactosidase was homogenized at pH 6.4 and 8.0, a gradual loss of activity was observed, reaching a minimum activity (around 30 %) after HPH at 150 MPa and pH 8.0. After storage, only β-galactosidase that underwent HPH at pH 7.0 retained similar activity to the native sample. Thus, HPH did not affect the activity and stability of β-galactosidase only when the process was carried out at neutral pH; for the other conditions, HPH resulted in partial inactivation of the enzyme. Considering the use of β-galactosidase to produce low lactose milk, it was concluded that HPH can be applied with no deleterious effects on enzyme activity.

Review: efficiency of physical and chemical treatments on the inactivation of dairy bacteriophages

Bacteriophages can cause great economic losses due to fermentation failure in dairy plants. Hence, physical and chemical treatments of raw material and/or equipment are mandatory to maintain phage levels as low as possible. Regarding thermal treatments used to kill pathogenic bacteria or achieve longer shelf-life of dairy products, neither low temperature long time nor high temperature short time pasteurization were able to inactivate most lactic acid bacteria (LAB) phages. Even though most phages did not survive 90°C for 2 min, there were some that resisted 90°C for more than 15 min (conditions suggested by the International Dairy Federation, for complete phage destruction). Among biocides tested, ethanol showed variable effectiveness in phage inactivation, since only phages infecting dairy cocci and Lactobacillus helveticus were reasonably inactivated by this alcohol, whereas isopropanol was in all cases highly ineffective. In turn, peracetic acid has consistently proved to be very fast and efficient to inactivate dairy phages, whereas efficiency of sodium hypochlorite was variable, even among different phages infecting the same LAB species. Both alkaline chloride foam and ethoxylated non-ylphenol with phosphoric acid were remarkably efficient, trait probably related to their highly alkaline or acidic pH values in solution, respectively. Photocatalysis using UV light and TiO₂ has been recently reported as a feasible option to industrially inactivate phages infecting diverse LAB species. Processes involving high pressure were barely used for phage inactivation, but until now most studied phages revealed high resistance to these treatments. To conclude, and given the great phage diversity found on dairies, it is always advisable to combine different anti-phage treatments (biocides, heat, high pressure, photocatalysis), rather than using them separately at extreme conditions.

High-pressure homogenization of raw and pasteurized milk modifies the yield, composition, and texture of queso fresco cheese

High-pressure homogenization (HPH) of milk was studied as an alternative processing operation in the manufacturing of queso fresco cheese. Raw and pasteurized (65°C for 30 min) milks were subjected to HPH at 0, 100, 200, and 300 MPa and then used to manufacture queso fresco. The cheeses were evaluated for yield, moisture content, titratable acidity, nitrogen content, whey protein content, yield force, yield strain, and tactile texture by instrumental or trained panel analyses. The combination of HPH and thermal processing of milk resulted in cheeses with increased yield and moisture content. The net amount of protein transferred to the cheese per kilogram of milk remained constant for all treatments except raw milk processed at 300 MPa. The highest cheese yield, moisture content, and crumbliness were obtained for thermally processed milk subjected to HPH at 300 MPa. The principal component analysis of all measured variables showed that the variables yield, moisture content, and crumbliness were strongly correlated to each other and negatively correlated to the variables yield strain, protein content (wet basis), and sensory cohesiveness. It is suggested that the combination of thermal processing and HPH promotes thermally induced denaturation of whey protein, together with homogenization-induced dissociation of casein micelles. The combined effect results in queso fresco containing a thin casein-whey matrix that is able to better retain sweet whey. These results indicate that HPH has a strong potential for the manufacture of queso fresco with excellent yield and textural properties.

Suitability of high pressure-homogenized milk for the production of probiotic fermented milk containing Lactobacillus paracasei and Lactobacillus acidophilus

High pressure homogenization (HPH) is one of the most promising alternatives to traditional thermal treatment for food preservation and diversification. In order to evaluate its potential for the production of fermented milks carrying probiotic bacteria, four types of fermented milks were manufactured from HPH treated and heat treated (HT) milk with and without added probiotics. Microbiological, physicochemical and organoleptic analyses were carried out during the refrigerated period (35 d at 4 degrees C). HPH application to milk did not modify the viability of the probiotic cultures but did increase the cell loads of the starter cultures (ca. 1 log order) compared with traditional products. The coagula from HPH-milk was significantly more compacted (P<0.05) (higher firmness) than that obtained with HT-milk, and it had the highest values of consistency, cohesiveness and viscosity indexes compared with fermented milks produced without HPH treatment. All the samples received high sensory analysis scores for each descriptor considered. HPH treatment of milk can potentially diversify the market for probiotic fermented milks, especially in terms of texture parameters.

Inactivation of Salmonella enterica serovar Senftenberg 775W in liquid whole egg by ultrahigh pressure homogenization

Two batches of samples of liquid whole egg were inoculated with a load of approximately 3 and 7 log CFU/ml, respectively, of Salmonella enterica serovar Senftenberg 775W and submitted to different ultrahigh pressure homogenization (UHPH) treatments at 150, 200, and 250 MPa. The inlet temperature of the samples was 6 degrees C. Counts of viable and injured Salmonella cells were obtained 2 h after the UHPH treatments and after 5, 10, 15, and 20 days of storage at 4 degrees C. The level of pressure applied influenced the lethality attained significantly (P < 0.05). In the samples with an initial load of approximately 7 log CFU/ml, the highest lethality value of 3.2 log CFU/ml was obtained at 250 MPa, and it is similar to those values reported in other surveys for thermal pasteurization with this same Salmonella strain. When the initial load was approximately 3 log CFU/ml, total inactivation was apparently obtained after the 250-MPa treatment (2.7 log CFU/ml). After 10 days of storage at 4 degrees C, Salmonella counts decreased in UHPH-treated samples, and colonies were not observed in tryptone soy agar and yeast extract medium. Nevertheless, presence of viable Salmonella cells was detected with the VIDAS Salmonella immunoassay method during the entire storage period. These results encourage further investigation of UHPH processing of liquid whole egg, assaying the possibility of using higher pressures and fluid inlet temperatures.

High-Pressure Homogenization as a Non-Thermal Technique for the Inactivation of Microorganisms

In the pharmaceutical, cosmetic, chemical, and food industries high-pressure homogenization is used for the preparation or stabilization of emulsions and suspensions, or for creating physical changes, such as viscosity changes, in products. Another well-known application is cell disruption of yeasts or bacteria in order to release intracellular products such as recombinant proteins. The development over the last few years of homogenizing equipment that operates at increasingly higher pressures has also stimulated research into the possible application of high-pressure homogenization as a unit process for microbial load reduction of liquid products. Several studies have indicated that gram-negative bacteria are more sensitive to high-pressure homogenization than gram-positive bacteria supporting the widely held belief that high-pressure homogenization kills vegetative bacteria mainly through mechanical disruption. However, controversy exists in the literature regarding the exact cause(s) of cell disruption by high-pressure homogenization. The causes that have been proposed include spatial pressure and velocity gradients, turbulence, cavitation, impact with solid surfaces, and extensional stress. The purpose of this review is to give an overview of the existing literature about microbial inactivation by high-pressure homogenization. Particular attention will be devoted to the different proposed microbial inactivation mechanisms. Further, the different parameters that influence the microbial inactivation by high-pressure homogenization will be scrutinized.

Effect of a pre-treatment of milk with high pressure homogenization on yield as well as on microbiological, lipolytic and proteolytic patterns of "Pecorino" cheese

The principal aim of this work was to compare Pecorino cheeses obtained from ewes' milk previously subjected to high pressure homogenization (HPH) at 100 MPa with those produced from raw and heat treated ewes' milk. The HPH milk treatment induced a significant increase of the cheese yield and caused a reduction of enterococci, lactococci and yeasts in the curds. Enterococci cell loads remained at lower levels in cheeses obtained from HPH milk over the ripening period. Analyses of free fatty acids, Sodium Dodecil Sulphate (SDS)-PAGE profiles, Gas Chromatography-Mass Spectrometry-Solid Phase Microextraction (GC-MS-SPME) measurements of volatile compounds and sensory traits evidenced that the pressure treatment can be regarded also as a useful tool to differentiate products obtained from the same raw material. In fact such a milk treatment induced a marked lipolysis, an early proteolysis, a relevant modification of the volatile molecule profiles and sensory properties of Pecorino cheese.

Microbiological and Sensory Effects of Musts Treated by High-pressure Homogenization

The winemaking sector needs to ensure the microbiological quality of its products to guarantee acceptance and wide commercialization. There are treatments such as high pressure homogenization (HPH) that, apparently, do not affect the final nutritional and sensory food properties. This study is presented to evaluate possibilities of utilizing HPH treatments to reduce the indigenous flora accompanying wine musts, as well as their effects on the fermentative process, oenological parameters, color, aroma, and taste properties. Two different must varieties were used: a white must (Parellada variety) and a red one (Trepat variety). Results showed that the use of HPH at 200 MPa is capable of reducing the microbial load of musts. Residual populations of total bacteria were detected, but neither fungi, nor yeasts, nor lactic acid bacteria were detected after the treatment in either musts. Furthermore, as a result of the decrease of the wild microbiota of the musts, the implantation of the selected yeast for alcoholic fermentation was improved. Sensory assessments of the must and wines showed that there were no significant changes caused by the treatment.

Effects of high-pressure homogenization on the survival of Alicyclobacillus acidoterrestris in a laboratory medium

AIMS

This study was aimed to investigate the effectiveness of high-pressure homogenization (HPH) against Alicyclobacillus acidoterrestris.

METHODS AND RESULTS

The susceptibility of three different strains of A. acidoterrestris (DSMZ 2498, Gamma4 and c8) to HPH (500-1700 bar) was studied. The experiments were performed in a laboratory medium (malt extract broth) on cells and spores. HPH caused a significant reduction of the initial cell number (1-2 log CFU ml(-1) at the highest pressures) in Gamma4 and DSMZ 2498 strains, whereas the effect on the spores was less significant.

CONCLUSIONS

This study showed that the susceptibility of A. acidoterrestris to HPH was strain-dependent: DSMZ 2498 seemed the most susceptible strain, whereas c8 was the most resistant one.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results of our study will provide useful information on the sensitivity of an emerging spoilage micro-organism, such as A. acidoterrrestris, to HPH.

Effect of High-Pressure Homogenization, Nonfat Milk Solids, and Milkfat on the Technological Performance of a Functional Strain for the Production of Probiotic Fermented Milks

The aim of this research was the evaluation of the effects of milkfat content, nonfat milk solids content, and high-pressure homogenization on 1) fermentation rates of the probiotic strain Lactobacillus paracasei BFE 5264 inoculated in milk; 2) viability loss of this strain during refrigerated storage; and 3) texture parameters, volatile compounds, and sensorial properties of the coagula obtained. The data achieved suggested a very strong effect of the independent variables on the measured attributes of fermented milks. In fact, the coagulation times were significantly affected by pressure and added milkfat, and the rheological parameters of the fermented milk increased with the pressure applied to the milk for added nonfat milk solids concentrations lower than 3%. Moreover, the polynomial models and the relative response surfaces obtained permitted us to identify the levels of the 3 independent variables that minimized the viability loss of the probiotic strain used during refrigerated storage.

Use of Human Lysozyme Transgenic Goat Milk in Cheese Making: Effects on Lactic Acid Bacteria Performance

Genetically engineered goats expressing elevated levels of the antimicrobial enzyme lysozyme in their milk were developed to improve udder health, product shelf life, and consumer well-being. The purpose of this study was to evaluate the effect of lysozyme on the development of lactic acid bacteria (LAB) throughout the cheese-making process. Raw and pasteurized milk from 7 lysozyme transgenic goats and 7 breed-, age-, and parity-matched nontransgenic controls was transformed into cheeses by using industry methods, and their microbiological load was evaluated. The numbers of colony-forming units of LAB were determined for raw and pasteurized goat milk, whey, and curd at d 2 and at d 6 or 7 of production. Selective plating media were used to enumerate lactococcal species separately from total LAB. Although differences in the mean number of colony-forming units between transgenic and control samples in raw milk, whey, and cheese curd were non-significant for both total LAB and lactococcal species from d 2 of production, a significant decrease was observed in both types of LAB among d 6 transgenic raw milk cheese samples. In pasteurized milk trials, a significant decrease in LAB was observed only in the raw milk of transgenic animals. These results indicate that lysozyme transgenic goat milk is not detrimental to LAB growth during the cheese-making process.

Inactivation by ultrahigh-pressure homogenization of Escherichia coli strains inoculated into orange juice

The aim of this work was to evaluate the efficacy of ultrahigh-pressure homogenization (UHPH) for inactivation and/or sublethal injury of two strains of Escherichia coli (O58:H21 ATCC 10536 and O157:H7 CCUG 44857) inoculated into orange juice (pH 3.6). The effects of orange juice inlet temperature (6 and 20 degrees C) on the lethality values and the capacity of these strains for survival, repair, and growth during refrigerated storage after UHPH treatment also was evaluated. Samples of orange juice that had been treated with ultrahigh temperatures were inoculated with E. coli in the stationary phase of growth until a final concentration of approximately 7.0 log CFU/ml was reached. These samples were then treated for one cycle with a double-valve UHPH machine, with 300 MPa at the primary homogenizing valve and 30 MPa at the secondary valve. Counts of viable and injured bacterial cells were obtained for samples taken 2 h after UHPH treatment and after 3, 6, 9, 12, 15, 18, 21, 27, and 33 days of storage at 4 degrees C. The inlet temperature and the strain type both influenced significantly (P < 0.05) the lethality effect on E. coli, which was higher when the inlet temperature was 20 degrees C. No sublethal injuries were detected after any treatment. The changes in viable counts over time for both strains in pressurized and control samples were similar. The viable counts remained high from day 0 to day 18 and then tended to decrease. After 27 days of storage at 4 degrees C, E. coli O157: H7 was more resistant in orange juice samples pressurized at inlet temperatures of 6 and 20 degrees C, with viable counts of 3.41 and 3.20 log CFU/ml, respectively.

Application of exopolysaccharide-producing cultures in reduced-fat Cheddar cheese: cryo-scanning electron microscopy observations

The microstructure of reduced- and full-fat Cheddar cheeses made with exopolysaccharide (EPS)-producing and nonproducing cultures was observed using cryo-scanning electron microscopy. Fully hydrated cheese samples were rapidly frozen in liquid nitrogen slush (-207 degrees C) and observed in their frozen hydrated state without the need for fat extraction. Different EPS-producing cultures were used in making reduced-fat Cheddar cheese. Full-fat cheese was made with a commercial EPS-nonproducing starter culture. The cryo-scanning electron micrographs showed that fat globules in the fully hydrated cheese were surrounded by cavities. Serum channels and pores in the protein network were clearly observed. Young (1-wk-old) full-fat cheese contained wide and long fat serum channels, which were formed because of fat coalescence. Such channels were not observed in the reduced-fat cheese. Young reduced-fat cheese made with EPS-nonproducing cultures contained fewer and larger pores than did reduced-fat cheese made with a ropy strain of Lactococcus lactis ssp. cremoris (JFR1), which had higher moisture levels. A 3-dimensional network of EPS was observed in large pores in cheese made with JFR1. Major changes in the size and distribution of pores within the structure of the protein network were observed in all reduced-fat cheeses, except that made with JFR1, as they aged. Changes in porosity were less pronounced in both the full-fat and the reduced-fat cheeses made with JFR1.

Inactivation of Listeria innocua in milk and orange juice by ultrahigh-pressure homogenization

The aim of this work was to evaluate the bactericidal efficacy of ultrahigh-pressure homogenization (UHPH) against Listeria innocua ATCC 33090 inoculated into milk and orange juice. We also intended to study the effect of inlet temperature on the lethality and production of sublethal injuries in this microorganism and its ability to survive, repair, and grow in refrigerated storage after UHPH treatment. Samples of ultrahigh-temperature whole milk and ultrahigh-temperature orange juice inoculated at a concentration of approximately 7.0 log (CFU per milliliter) were immediately pressurized at 300 MPa on the primary homogenizing valve and at 30 MPa on the secondary valve, with inlet temperatures of 6.0 +/- 1.0 degrees C and 20 +/- 1.0 degrees C. L. innocua viable counts and injured cells were measured 2 h after UHPH treatment and after 3, 6, and 9 days of storage at 4 degrees C for milk and after 3, 6, 9, 12, 15, 18, and 21 days of storage at 4 degrees C for orange juice. Both the inlet temperature and the food matrix influenced significantly (P < 0.05) the inactivation of L. innocua, which was higher in whole milk at the 20 degrees C inlet temperature. The UHPH treatment caused few or no sublethal injuries in L. innocua. During storage at 4 degrees C after treatments, counts increased by approximately 2 logarithmic units from day 0 to 9 in whole milk, whereas in orange juice counts diminished by approximately 2.5 logarithmic units from day 0 to 18.

Inactivation of Escherichia coli by high-pressure homogenisation is influenced by fluid viscosity but not by water activity and product composition

The inactivation of Escherichia coli MG1655 by high-pressure homogenisation (HPH) at pressures ranging from 100 to 300 MPa was studied in buffered suspensions adjusted to different relative viscosities (1.0, 1.3, 1.7, 2.7 and 4.9) with polyethylene glycol 6000 (PEG 6000). The water activity of these suspensions was not significantly affected by this high molecular weight solute. Bacterial inactivation was found to decrease with increasing viscosity of the suspensions, an effect that was more pronounced at higher pressures. To study the effect of water activity, series of E. coli suspensions having a different water activity (0.953-1.000) but the same relative viscosity (1.3, 1.7, 2.7 and 4.9) were made using PEG of different molecular weights (400, 600, 1000 and 6000), and subjected to HPH treatment. The results indicated that water activity does not influence inactivation. Finally, inactivation of E. coli MG1655 by HPH in skim milk, soy milk and strawberry-raspberry milk drink was found to be the same as in PEG containing buffer of the corresponding viscosity. These results identify fluid viscosity as a major environmental parameter affecting bacterial inactivation by HPH, as opposed to water activity and product composition, and should contribute to the development of HPH applications for the purpose of bacterial inactivation.

Impact of Autolytic, Proteolytic, and Nisin-Producing Adjunct Cultures on Biochemical and Textural Properties of Cheddar Cheese

The effect of incorporating a highly autolytic strain (Lactobacillus delbrueckii subsp. bulgaricus UL12) a proteolytic strain (Lactobacillus casei subsp. casei L2A), or a nisin Z-producing strain (Lactococcus lactis, subsp. lactis biovar diacetylactis UL719) into Cheddar cheese starter culture (Lactococcus lactis KB and Lactococcus cremoris KB) on physicochemical and rheological properties of the resultant cheeses was examined. Cheeses were ripened at 7 degrees C and analyzed over a 6-mo period for viable lactococcal and lactobacilli counts, pH, titratable acidity (TA), lipolysis, proteolysis, and textural characteristics. The combination of the nisin-producing strain and autolytic adjuncts significantly increased the production of water-soluble nitrogen, free amino acids, and free fatty acids. The effect of Lc. diacetylactis UL719 alone or of Lb. casei L2A on water-soluble nitrogen and free amino acid contents were also significant, whereas their effect on free fatty acids was not. Viable counts of Lb. bulgaricus UL12 were significantly reduced in the presence of Lc. diacetylactis UL719. Lactobacilli-containing cheeses showed significantly lower values for hardness, fracturability, and springiness. It could be concluded that the addition of Lb. bulgaricus UL12 together with a nisin-producing strain produces a greater increase in cheese proteolysis and an improvement in Cheddar cheese texture.