Alicyclobacillus acidoterrestris in fruit juices and its control by nisin

Summary The acid-tolerant and heat-resistant bacterium Alicyclobacillus acidoterrestris is a spoilage problem in pasteurized and heat-treated fruit juices. In this study it was shown to grow in orange juice, grapefruit juice and apple juice to produce detectable taint at levels of about 10⁴–10⁵ c.f.u. ml⁻¹. Decimal reduction times were determined at 80 °, 90 ° and 95 °C in each juice and confirmed the heat-resistant nature of the spores under normal juice pasteurization conditions. They also confirmed that heat sensitivity increased with decreasing pH but that this effect was less pronounced at higher temperatures. The organism was, however, sensitive to the bacteriocin food preservative nisin. The presence of nisin during heating decreased the D value by up to 40% and the MIC for nisin against spores at 25 °C was only 5 International Units (IU) ml⁻¹. The results indicate that use of nisin is a potentially useful way of controlling this organism in fruit juices and fruit juice-containing products.     

Comparison of high pressure treatment and thermal pasteurization effects on the quality and shelf life of guava puree

The effects of high pressures and thermal pasteurization on the survival of microorganisms, enzyme inactivation and quality changes of guava puree during storage at 4°C were investigated and compared with untreated samples. After treatment at a pressure of 600MPa and 25°C for 15 min, the microorganisms in guava puree were inactivated to less than 10 cfu mL⁻¹ and the product exhibited no change in colour, pectin, cloud and ascorbic acid content as compared with fresh samples. The inactivation of enzymes in guava puree by thermal pasteurization was greater than by high pressures. The microbial count in guava puree reduced to 200 cfu mL⁻¹ and the product showed marked changes in viscosity, turbidity and colour when heated at 88–90°C for 24s. The content of pectin, cloud and ascorbic acid as well as colour in untreated and high pressurized (400MPa) guava puree gradually decreased, whereas these changes were not observed in pasteurized (88–90°C) and high pressurized (6000MPa) puree during storage at 4°C for 60 days. The guava puree treated at 600MPa and 25°C for 15 min retained good quality similar to the freshly extracted puree after storage at 4°C for 40 days.     

Effect of technological parameters on the characteristics of kasseri cheese made from raw or pasteurized ewes' milk

Two groups of kasseri cheese (pasta filata type) were manufactured from raw or pasteurized ewes' milk, without starter cultures. Cheeses of each group were divided into two subgroups: the first was ripened and stored at 4°C and packaged in plastic film; the second ripened and stored at 15°C and coated with paraffin wax. Milk pasteurization and technological parameters had a significant effect on the pH ( P  < 0.05), while only technological parameters had an effect on the total solids content. At day 120, the range of mean cfu/g counts for the mesophilic aerobic flora was 9.5 × 10⁷−1.4 × 10⁸; for the thermophilic streptococci, the range was 2.6 × 10⁷−7.6 × 10⁷; and for the thermophilic bacilli, 9.8 × 10⁶−1.7 × 10⁷. Changes in the N fractions became significant after 30 days of ripening. For mature 120-day-old cheeses, the percentage of total N soluble at pH 4.6 was 22.7%–22.9% in raw milk cheeses and 19.0%–21.7% in pasteurized milk cheeses. The percentage of total N soluble at 12% TCA was 10.1%–12.2% in raw milk cheeses and 7.3%–11.5% in pasteurized milk cheeses; the percentages of total N soluble at 5% PTA were 3.1%–4.0% and 2.6%–3.6%, respectively. The residual α_s-casein percentages at day 120 ranged between 63% and 78% of the respective area at day 1; the residual β-casein ranged between 67% and 75%. There were some characteristic differences in the reverse phase-HPLC peptide profiles of the four cheeses. In general, the effect of the different ripening conditions was more pronounced in cheeses made from pasteurized milk.     

Viscosity changes during rennet coagulation of Murciano-Granadina goat milk

The viscosity changes during coagulation of raw and pasteurized Murciano-Granadina goat milk with 10 rennets and coagulant enzymes were studied using a cylindrical rotational viscometer. The effect of three shear rates (7.32, 14.68, 73.42 s^-1) was analysed in reconstituted milk using 60% chymosin rennet and different enzyme concentrations. The shear rate selected (14.68 s^-1) corresponded to that in which the coagulation time was not modified. At a sheer rate of 73.42 s^-1 there was a significant difference (p < .05) in the milk clotting time determined compared with the shear rates of 7.32 and 14.68 s^-1. At the beginning of the coagulation process there was a decrease in the viscosity value, which was higher in raw milk (22%) than in the two heat treated samples (17% in milk pasteurized at 65°C for 30 min, low temperature long time, and 19% in milk pasteurized at 75°C for 15 s, high temperature short time). After this period the viscosity rose rapidly and reached a value 2.89 times greater than at time 0. A highly significant Pearson correlation (0.952-0.994) was found between the coagulation time determined by the Berridge method and viscometric measurement in raw and pasteurized milk.     

Effect of heat treatment of milk on activation of Bacillus spores

The quality and shelf life of fluid milk products are dependent on the amount and type of microorganisms present following pasteurization. This study evaluated the effects of different pasteurization processes on the microbial populations in fluid milk. The objective was to determine whether certain pasteurization processes lead to an increase in the amount of bacteria present in pasteurized milk by activating Bacillus spores. Samples of raw milk were collected on the day of arrival at the dairy plant. The samples were pasteurized at 63 degrees C for 30 min (low temperature, long time), 72 degrees C for 15 s (high temperature, short time), 76 degrees C for 15 s, and 82 degrees C for 30 min. The pasteurized samples were then stored at 6 and 10 degrees C for 14 days. The samples were analyzed for standard plate count and Bacillus count immediately after pasteurization and after 14 days of storage. Pasteurization of milk at 72 and 76 degrees C significantly (P < 0.05) increased the amount of Bacillus spore activation over that of 63 degrees C. There was no detection of Bacillus in initial samples pasteurized at 82 degrees C for 30 min, but Bacillus was present in samples after storage for 14 days, indicating that injury and recovery time preceded growth. The majority of isolates were characterized as Bacillus mycoides and not Bacillus cereus, suggesting that this organism might be more a cause of sweet curdling of fluid milk than previously reported.     

Incidence, source and some properties of psychrotrophic Bacillus spp found in raw and pasteurized milk

Spores of psychrotrophic Bacillus spp were isolated from 58% of farm bulk tank milks and about 69% of pasteurized milks. Counts of Bacillus spp in about 10% of raw milk samples reached 1 × 10⁵ cfu/ml and above within seven days at 6°C. Psychrotrophic spore counts in pasteurized milks ranged from <0.5 to 170 spores/litre with an average of about 17/1. There was little correlation between the total bacterial count of the raw milk and presence of psychrotrophic Bacillus spores. There was some evidence that the bulk tank itself may be a source of contamination. The spores in pasteurized milk probably were not the result of postpasteurization contamination. The optimum germination temperature for psychrotrophic Bacillus spores was lower than that for spores of mesophilic strains. About 50% of the psychrotrophic Bacillus strains isolated from milk were capable of growth at 2°C.     

The influence of pasteurization temperature on the physicochemical composition and colour of Murciano-Granadina goat milk

An evaluation of the physicochemical characteristics and colour in Murciano-Granadina goat milk was carried out on several samples subjected to different pasteurization treatments (72°C/15 s, 76°C/15 s and 80°C/10 s). An apparent increase in the level of caseins was observed along with a decrease in the serum protein contents at pasteurization temperatures of 76 and 80°C. This change can be partly justified by the interactions between β-lactoglobulin and K-casein in pasteurized milk. The fat content, viscosity and density of the milk showed minimal sensitivity to the thermal treatments, but the heat treatment did affect the parameters which defined the colour of the milk, as shown by a decrease in the Yellow Index and an increase in the difference in colour.     

Factors affecting the antilisterial effects of nisin in milk

The ability of Listeria monocytogenes to proliferate in milk and the antilisterial activities of nisin are well documented. Although milk fat was reported to reduce the antimicrobial activities of nisin, there is little information on the influence of milk fat on the antilisterial activities of nisin in refrigerated milk, and whether pasteurization and homogenization influence these activities. Fresh, pasteurized, and homogenized milk samples (0.1%, 2.0%, and 3.5% fat) were treated with nisin (0-500 IU/ml) and challenged with 10(4) CFU/ml L. monocytogenes strain Scott A. The organism was most sensitive to nisin in skim milk, showing rapid decline in cell numbers to <10 CFU/ml after 12 days at 5 degrees C following treatment with 250 IU/ml. An initial decline in cell numbers in 2% and whole milk was followed by regrowth of the organism. Loss of the antilisterial effects of nisin was confirmed in homogenized whole milk, whether raw or pasteurized, but not in raw or pasteurized whole milk that was not homogenized. Tween 80, a nonionic emulsifier, partially counteracted the loss of the antilisterial activity of nisin, whereas lecithin, an anionic emulsifier, had no effect. These results demonstrate that the chemical composition and treatment of foods may play an important role in the antilisterial effects of nisin.     

Subject: Flavour problems in stored milk and cream:Chemical causes of flavour deterioration in pasteurized milk

In a study related to domestic practice, milk was laboratory pasteurized to give the time-temperature conditions of HTST pasteurization and stored in full glass bottles at 7°C in the dark for up to 14 days. The consumption of O₂ during storage was monitored, and the factors responsible for its consumption were quantitatively assessed. The principal factors were: loss through septum, 7.I per cent; ascorbic acid oxidation, 34 per cent; bacterial growth, 36.1 per cent: sulphydryl oxidation. 3.4 per cent. Thus, some 80.6 per cent of the O₂ consumed was accounted for. Lipid peroxidation used only 0.07 per cent. Lipolysis was also determined. Nevertheless, preliminary organoleptic findings indicated that oxidized flavour was the predominant element in 'stale' milk.     

Microbiological Shelf Life of Pasteurized Milk in Bottle and Pouch

ABSTRACT:  Shelf life of pasteurized milk in Brazil ranges from 3 to 8 d, mainly due to poor cold chain conditions that prevail throughout the country and subject the product to repeated and/or severe temperature abuse. This study evaluated the influence of storage temperature on the microbiological stability of homogenized whole pasteurized milk (75 °C/15 s) packaged in high-density polyethylene (HDPE) bottle and low-density polyethylene (LDPE) pouch, both monolayer materials pigmented with titanium dioxide (TiO₂). The storage temperatures investigated were 2, 4, 9, 14, and 16 °C. Microbiological evaluation was based on mesophilic and psychrotrophic counts with 7 log CFU/mL and 6 log CFU/mL, respectively, set as upper limits of acceptability for maintaining the quality of milk. The microbiological stability for pasteurized milk packaged in HDPE bottle and stored at 2, 4, 9, 14, and 16 °C was estimated at 43, 36, 8, 5, and 3 d, respectively. For milk samples packaged in LDPE pouch, shelf life was estimated at 37, 35, 7, 3, and 2 d, respectively. The determination of Q₁₀ and z values demonstrated that storage temperature has a greater influence on microbiological shelf life of pasteurized milk packaged in LDPE pouch compared to HDPE bottle. Based on the results of this study, HDPE bottle was better for storing pasteurized milk as compared to LDPE pouch.     

QUALITY OF ORANGE JUICE DRINK SUBJECTED TO A PREDICTIVE MODEL-BASED PASTEURIZATION PROCESS

The utility of a previously developed predictive model for the decimal reduction time ( D _72C) of Salmonella Typhimurium (ATCC 13311) in citrus systems was validated by establishing a pasteurization process ( P _72C) for Philippine native orange juice drink (POD). Quality characteristics of the pasteurized POD were evaluated and used as bases for assessing the utility of the developed model. The ^p P _72C for POD was calculated by multiplying the model-generated D _72C of the reference strain with the recommended order of lethality ( m ) of 5.00 log₁₀ cycles. A ^a P _72C was likewise calculated based on D _72C that resulted from actual thermal inactivation study of the reference strain on POD. Pasteurization resulted to PODs with significantly ( P <  0.05) altered color, microbial quality and sensory profile. Except for the overall quality, differences in the consumer acceptance of various unpasteurized and pasteurized PODs quality characteristics were found to be not significant.

PRACTICAL APPLICATIONS

A mathematical model was developed to estimate the thermal death times of the pathogen Salmonella Typhimurium (ATCC 13311) from the pH, titratable acidity (% citric acid) and soluble solid (°Brix) values of suspending citrus systems. In this study, the utility of this model in establishing a pasteurization process for a test orange juice drink was determined. As the establishment of specific thermal process schedule for individual food products can be difficult, utilization of the validated model shall be a convenient alternative to the traditional but rigorous process.     

Microbial growth and colour of minimally processed shiitake mushroom stored at different temperatures

The effect of storage temperature on the microbiological and sensory characteristics of minimally processed shiitake was investigated. Shiitake mushrooms were washed, sanitised, packed in polystyrene trays, overwrapped with plastic film and stored at 7, 10 or 15 °C. Microbial counts, polyphenoloxidase activity, external lightness ( L* ) and acceptance tests were conducted. Mesophilic, aerobic psychrotrophic bacteria, yeasts and moulds predominated during storage. Pseudomonas was detected after 10 days' storage showing an increase of, approximately, 7 log cycles at 15 °C. Mushroom rejection occurred when the microbial population was lower than 10⁶ CFU g⁻¹, suggesting that, other factors, such as browning, affect mushroom spoilage more than microbial activity. Mushrooms treated with citric or ascorbic acids maintained high L * value during 10 days. The shelf life of minimally processed shiitake mushrooms was 10 days at 7 °C, but less than 5 days at 10 °C, and approximately 3 days at 15 °C.     

Use of microfiltration to improve fluid milk quality

The objectives of the research were to determine the growth characteristics of bacteria in commercially pasteurized skim milk as a function of storage temperature; to determine the efficacy of a microfiltration and pasteurization process in reducing the number of total bacteria, spores, and coliforms in skim milk; and to estimate the shelf life of pasteurized microfiltered skim milk as a function of storage temperature. For the first objective, commercially pasteurized skim milk was stored at 0.1, 2.0, 4.2, and 6.1 degrees C. A total bacterial count >20,000 cfu/mL was considered the end of shelf life. Shelf life ranged from 16 d at 6.1 degrees C to 66 d at 0.1 degrees C. Decreasing storage temperature increased lag time and reduced logarithmic growth rate of a mixed microbial population. The increased lag time for the mixed microbial population at a lower storage temperature was the biggest contributor to longer shelf life. For the second objective, raw skim milk was microfiltered at 50 degrees C using a Tetra Alcross M7 Pilot Plant equipped with a ceramic Membralox membrane (pore diameter of 1.4 microm). The 50 degrees C permeate was pasteurized at 72 degrees C for 15 s, and cooled to 6 degrees C. Bacterial counts of raw skim milk were determined by standard plate count. Bacterial counts of microfiltered and pasteurized microfiltered skim milk were determined using a most probable number method. Across 3 trials, bacterial counts of the raw milk were reduced from 2,400, 3,600, and 1,475 cfu/mL to 0.240, 0.918, and 0.240 cfu/mL, respectively, by microfiltration. Bacterial counts in the pasteurized microfiltered skim milk for the 3 trials were 0.005, 0.008, and 0.005 cfu/mL, respectively, demonstrating an average 5.6 log reduction from the raw count due to the combination of microfiltration and pasteurization. For the third objective, pasteurized microfiltered skim milk was stored at each of 4 temperatures (0.1, 2.0, 4.2, and 6.1 degrees C) and the total bacterial count was determined weekly over a 92-d period. At 6 time points in the study, samples were also analyzed for noncasein nitrogen and the decrease in casein as a percentage of true protein was calculated. After 92 d, 50% of samples stored at 6.1 degrees C and 12% of samples stored at 4.2 degrees C exceeded a total bacterial count of 20,000 cfu/mL. No samples stored at 0.1 or 2.0 degrees C reached a detectable bacterial level during the study. When the bacterial count was <1,000 cfu/mL, shelf life was limited because sufficient proteolysis had occurred at 32 d at 6.1 degrees C, 46 d at 4.2 degrees C, 78 d at 2.0 degrees C, and >92 d at 0.1 degrees C to produce a detectable off-flavor in skim milk produced from a raw milk with a 240,000 somatic cell count.     

The influence of milk pasteurization temperature and pH at curd milling on the composition, texture and maturation of reduced fat cheddar cheese

Reduced fat milks were pasteurized, for 15 s, at temperatures ranging from 72 to 88°C to give levels of whey protein denaturation varying from ˜ 3 to 35%. The milks were converted into reduced fat cheddar cheese (16–18% fat) in 500 litre cheese vats; the resultant cheese curds were milled at pH values of 5.75 and 5.35. Raising the milk pasteurization temperature resulted in impaired rennet coagulation properties, longer set-to-cut times during cheese manufacture, higher cheese moisture and moisture in the non-fat cheese substance, lower levels of protein and calcium and lower cheese firmness. Increasing the pH at curd milling from 5.35 to 5.75 affected cheese composition and firmness, during ripening, in a manner similar to that of increasing milk pasteurization temperature. Despite their effects on cheese composition and rheology, pasteurization temperature and pH at curd milling had little influence on proteolysis or on the grading scores awarded by commercial graders during ripening over 303 days .     

INHIBITION OF STAPHYLOCOCCUS AUREUS AND BACILLUS CEREUS ON A VEGETARIAN FOOD TREATED WITH NISIN COMBINED WITH EITHER POTASSIUM SORBATE OR SODIUM BENZOATE

Various amounts of nisin (0, 10³ and 5 × 10³ IU/g) in combination with either potassium sorbate (0, 2, and 3%) or sodium benzoate (0, 0.06 and 0.12%) were tested for effectiveness in inhibiting growth of Staphylococcus aureus C10 and Bacillus cereus B7 inoculated on a vegetarian food. The strains used were isolated from vegetarian foods obtained commercially in Taiwan, and the test food, spice and dried bean curd, was selected for the study based on ability to support the growth of these organisms. After treatment with a preservative combination, the surfaces of sterilized food samples were inoculated, samples were stored in vacuum or nonvacuum packages at either 4C or 30C, and at appropriate times, tested for microbial growth. Growth of both isolates was unaffected by vacuum-packaging treatment; however, a bacteriostatic effect was found at 4C. Data indicated that during the 14-day storage at 4C, vacuum-packaged samples treated with 5 × 10³ IU/g nisin and 0.12% sodium benzoate significantly (p < 0.05) decreased the counts of S. aureus C10 and B. cereus B7 by 2.61 and 3.02 log₁₀ CFU/g, respectively. In the vacuum-packaged samples treated with 5 × 10³ IU/g nisin and 3% potassium sorbate, counts for C10 and B7 were decreased by 2.35 and 2.64 log₁₀ CFU/g, respectively. Thus, the combined treatment extended the shelf-life of the vegetarian food .     

Fate of Aflatoxin M1 during Manufacture and Storage of Feta Cheese

ABSTRACT:  The effect of feta cheese manufacture on aflatoxin M₁ (AFM₁) content was studied using an enzyme immunoassay technique. Feta cheese was made from milk spiked with 1 and 2 μg AFM₁ per kilogram milk. Pasteurization at 63 °C for 30 min caused <10% destruction of AFM₁. During cheese making, the remaining AFM₁ in milk was partitioned between curd and whey with two-thirds retained in the curd and one-third going into the whey. Cheeses were then stored for 2 mo in 8%, 10%, and 12% brine solutions at 6 and 18 °C. There was a 22% to 27% reduction of AFM₁ during the first 10 d of storage, with slightly more loss as salt concentration increased and when the cheese was stored at 18 °C. Further storage caused only slight decrease in AFM₁ and after 30 d of brining there was no difference in AFM₁ content of the cheese based upon salt concentration of the brine. At 18 °C, no further losses of AFM₁ occurred after 30 d, and at 6 °C, there was continued slight decrease in AFM₁ levels until 50 d. After 60 d of brining, there was a total loss of 25% and 29% of the AFM₁ originally present for cheese brined at 6 and 18 °C, respectively. Thus, the combination of pasteurization, conversion of milk into feta cheese, and at least 50 d storage of cheese in brine caused a total loss of about 50% of the AFM₁ originally present in the raw milk.     

Effect of carbon dioxide on improving the keeping quality of raw milk

Fresh cows' milk samples were treated with CO₂ to give a calculated CO₂ content of 77mM. The treated samples as well as the control were stored at 7° and 20 ± 5°C for three days and analysed periodically. The increase in the total count as well as the counts of psychrotrophic, lactic acid and coliform bacteria during storage at 7°C was found to be inhibited by the addition of CO₂. The treated samples had higher titratable acidity and lower pH values than the controls but satisfied the clot-on-boiling test. The keeping quality of cooled milk can therefore be improved by the addition of CO₂.     

KINETIC PARAMETERS FOR THERMAL INACTIVATION OF PLASMIN

Plasmin (EC 3.4.21.7) is the principal indigenous protease in bovine milk. Kinetic parameters for thermal degradation of plasmin were determined using a miniature continuous flow pasteurizer designed for heat treatment of small quantities of liquid. Plasmin activity was measured using a pH-stat titration, which measures the release of H ⁺ from a caseinate substrate.
Heating milk at various temperatures resulted in significant difference (P > 0.01) in residual plasmin activity and linear decrease in activity at each temperature. The calculated D-values were: 105, 90, 76.5, 62, and 47 s at 72, 78, 85, 92, and 100°C, respectively. The Z-value for plasmin inactivation was estimated to be 77.5°C, and the Arrhenius activation energy was 7.04 kcal/mole.
UHT milk containing plasmin was heated for an equivalent of 5 D inactivation of the enzyme at 100°C. After storage for 30 days at 4°C, no enzyme regeneration was observed.     

The effects of different incubation temperatures on the acetaldehyde content and viable bacteria counts of bio-yogurt made from ewe's milk

Yogurt and bio-yogurt were manufactured from ewe's milk using a starter culture and a probiotic culture. Incubation was carried out at 37°C and 42°C until pH 4.6 was reached and the yogurts were stored at 4 +1°C for 14 days. Analysis after 1, 7 and 14 days showed that incubation temperature and storage time significantly influenced overall properties of the samples. During the storage, whey separation and pH decreased, but titratable acidity, lactic acid and volatile fatty acid contents increased. Viable bacterial counts in all bio-yogurts were above 10⁷ cfu g⁻¹ at the end of storage.     

Modelling the relation between bacterial growth and storage temperature in pasteurized milks of varying hygienic quality

The shelf-life of pasteurized milk was mainly determined by the level of contamination with Gram-negative psychrotrophic bacteria. The length of lag phase of the bacteria was also important, although the generation times of the naturally contaminating flora seemed to be of little relevance except for milks where the shelf-life exceeded 10 days at 6°C. The effect of temperature on growth of the contaminants could be accurately determined by the 'square root' plot but the conceptual minimum temperature for growth (T_o) varied. The variation was related to the quality of the pasteurized milk. Effects of temperature on generation time, length of lag phase and shelf-life were most marked at temperatures below 15°C.
The microflora of pasteurized milk varied significantly with storage temperature. At refrigeration temperatures, spoilage was mainly due to the growth of Pseudomonas spp. Enterobacteriaceae and Gram-positive bacteria assumed greater importance in the spoilage of milks stored at temperatures above 10°C. Milks of good quality also contained Bacillus spp and this group of bacteria were not detectable in milks with short shelf-lives.