Lactic acid and pH as indicators of spoilage for vacuum-packed cooked ring sausages

Lactic acid production and pH changes of 206 vacuum-packed cooked ring sausages stored at 2, 4 and 12°C from 21 different production runs were monitored as a function of time and of microbial growth. The total lactic acid concentrations and pH values were first at a constant level, starting to increase sharply after the lactobacilli count reached about 5 × 10⁷ or 6 × 10⁷ cfu/g, respectively. The lactic acid and pH changes as a function of the lactobacilli count were similar at 4 and 12 ° C. The sharp increase at high lactobacilli counts was observed in both -lactic acid and -lactic acid. The variation was lesser and the increase greater in -lactic acid formation than in -lactic acid. Above a level of 3–4 mg lactic acid/g most of the samples were deemed unfit. The pH started to decrease from a level of approx. 6.3; below 5.8–5.9 the samples were deemed unfit. The lowest pH value observed was 4.58. Both a high lactic acid content and a low pH indicated that the sausage was spoiled. These changes, however, took place at later stages of storage, and do not give information about the early phase of spoilage.     

Microbiological, physicochemical, and sensory characteristics of kefir during storage

Changes in certain microbiological, physicochemical, and sensory parameters of kefir were studied during refrigerated storage. Kefir batches were prepared using 1% and 5% added kefir grains, and samples for analysis were taken 24 h after inoculation and then after 2, 7, 14, 21, and 28 days of storage at 5 ± 1 °C. After fermentation for 24 h after inoculation, lactobacilli and lactococci were present at levels of 10⁸ cfu/ml, and yeasts and acetic acid bacteria were present at levels of 10⁵ and 10⁶ cfu/ml, respectively. The lactic acid flora decreased by about 1.5 log units between days 7 and 14 and then stabilized at that level. Yeast and acetic acid bacterial counts, lactose, and pH all remained constant over the storage period, while the total fat content and dry matter decreased. The percentage inoculate did exert an influence, and the sample batches made using 1% added kefir grains had higher lactic acid bacterial counts, lactose, and pH, while the sample batches made using 5% added kefir grains had higher yeast and acetic acid bacterial counts and viscosity. The total fat and dry matter contents were similar in both sample batches. Sensory analysis of the kefir samples revealed maximum acceptability levels in the first 2 days of storage.     

The effect of vacuum packaging and gaseous atmosphere on microbial growth in tripe

Changes in bacterial flora of tripe samples, stored at 4°C in air, vacuum packaged or in a CO₂-enriched atmosphere were studied. Aerobic plate counts showed a rapid increase in samples stored in air reaching a level of 1·6 × 10⁹/g from an initial level of 9·0 × 10³/g. The aerobic bacterial population inhibited in both vacuum packed and CO₂-enriched atmosphere storage. The shelf lives of samples stored in air, under vacuum packaging or in gas mixtures, were 4, 8 and 9 days, respectively. Enterobacteriaceae and anaerobic counts tended to be higher under vacuum storage than in a CO₂-enriched atmosphere. The numbers of lactic acid-producing bacteria were generally found to be lower under vacuum storage than in gas mixtures.     

Potential role of Proteus-Providencia-Morganella species in quality deterioration of beef

Proteeae Isolation Medium of Hawkey et al. (1986) was effective for isolation and enumeration of Proteus-Providencia-Morganella spp. from beef and beef products. On this medium colonies were brown, surrounded by a zone of clearing. In an examination of retail beef samples packaged and stored in polyvinyl chloride (PVC) film, 5% contained these bacteria in a concentration of 10¹–10² cells per gram or cm². Isolates from retail beef samples included P. vulgaris, P. alcalifaciens, P. rettgeri, and M. morganii. None of the retail samples packaged and stored in high oxygen barrier (HOB) film contained detectable Proteus-Providencia-Morganella spp. Steaks inoculated with Proteus-Providencia-Morganella spp. and packaged and stored at 4°C in PVC film (15 days) or in HOB film (42 days) usually had a reduced count of Proteus-Providencia-Morganella spp. during storage. Slight increases in counts occurred on steaks inculated with P. vulgaris and P. rettgeri and stored at 4°C for 15 days in PVC film.     

Microbiological quality of ayib, a traditional Ethiopian cottage cheese

One-hundred samples of ayib, a traditional Ethiopian cottage cheese, were purchased from Awassa market and analysed for their aerobic mesophilic counts, psychrotropic counts, yeasts and molds, coliforms, spore-formers, enterococci, lactic acid bacteria, Listeria monocytogenes, staphylococci and Bacillus cereus. The majority of the samples showed counts of mesophilic aerobic bacteria, yeasts and enterococci of 10⁸, 10⁷ and 10⁷ cfu/g. About 55% of the samples were positive for coliforms and faecal coliforms. Listeria spp. were not detected in any of the samples. B. cereus and S. aureus were isolated at varying frequencies but at low numbers (10²–10³). The pH value of the samples varied between 3.3 and 4.6 with about 40% having pH lower than 3.7.     

Bacteriological quality assurance (BQA) of mechanically deboned meat (MDM)

Adequacy of bacteriological quality assurance during the commercial production of mechanically deboned meat (MDM) was assessed. Lax standards of hygiene during production were observed, resulting in high numbers of Staphylococcus aureus, viz. 10⁴ to 10⁵ cfu g⁻¹, and severe contamination with Enterobacteriaceae: 10⁵ to 10⁶ cfu g⁻¹. These data indicate that measures of hygiene observed during boning of carcasses and during collection, storage and transport of bones or poultry parts should be markedly tightened, while conditions of refrigerated storage of raw materials and MDM should be improved. Use of bones of poor sensory quality (discoloration, abnormal smell) generally resulted in MDM of inferior bacteriological quality.

Phage typing, biotyping and assessment of enterotoxin production was carried out with 136 St. aureus cultures, isolates from mechanically deboned pork produced at one plant. Fifty-five per cent of the isolates was not typable, 28% was typable with human phages, 8% with bovine phages. The majority of the strains could not be explicitly assigned to any Meyer and/or Hájek and Mar álek types. Applying the simplified system of Devriese to eighteen strains isolated in our investigation, ten were found to belong to the poultry ecovar, one to the bovine ecovar, while seven strains were non-host specific. None of the isolates produced enterotoxins A–E.

Microbiological inspection of end products is recommended as part of an integrated quality assurance system. The following reference values for the final product (maximal colony counts to be expected under GMP conditions expressed as 95th percentile) were calculated: Pig MDM: log₁₀ mesophilic colony count 6·8 and log₁₀ cfu mesophilic Enterobacteriaceae g⁻¹ 4·8; Poultry MDM: log₁₀ mesophilic colony count 6·6 and log₁₀ cfu mesophilic Enterobacteriaceae g⁻¹ 4·7.     

Selective enumeration and survival of bifidobacteria in fresh cheese

Five species of bifidobacteria (15 strains), two strains of Lactococcus lactis ssp. lactis, two strains of L. lactis ssp. cremoris, and one strain of L. lactis ssp. lactis var diacetylactis were included in a study to develop a selective medium for enumeration of bifidobacteria from fresh cheese. Viable counts of bifidobacteria or lactococci on modified Columbia agar base (CAB with 0.05% cysteine-HCl) plus raffinose (0.5%) containing various selective agents were compared with non-selective media. The mCAB plus raffinose with lithium chloride (2 g L⁻¹) and sodium propionate (3 g L⁻¹) with pH adjusted to 5.1 was used successfully as a selective medium for the enumeration of bifidobacteria from fresh cheese. Using this medium, it was determined that bifidobacteria could survive up to 15 days at a level higher than 10⁶ cfu g⁻¹ in a fresh cheese stored at 4 or 12°C. The decrease in the viable counts of bifidobacteria was faster during storage at 4°C than at 12°C.     

Synergistic inhibition of Listeria monocytogenes on cold-smoked rainbow trout by nisin and sodium lactate

The inhibition of Listeria monocytogenes and mesophilic aerobic bacteria in cold-smoked rainbow trout by nisin, sodium lactate or their combination was studied. Nisin (4000-6000 IU/ml), sodium lactate (60%) or their combination (1:1) were injected into rainbow trout at an industrial scale before the smoking process, or injected into the finished smoked product. Both types of fish samples were smoked, sliced and vacuum-packed according to normal practice in the plant. Packages were opened and L. monocytogenes was inoculated (10(3)-10(4) log colony forming units (cfu)/g) onto the fish samples, which were then vacuum packed again. Samples were stored at 8 degrees C for 17 days or at 3 degrees C for 29 days. Listeria and mesophilic aerobic bacteria counts were measured once a week. The effects of treatments on sensory characteristics and storage stability were also analyzed. Both nisin and lactate inhibited the growth of L. monocytogenes in smoked fish, but the combination of the two compounds was even more effective. The combination of nisin and sodium lactate injected into smoked fish decreased the count of L. monocytogenes from 3.26 to 1.8 log cfu/g over 16 days of storage at 8 degrees C. The level of L. monocytogenes remained almost constant (4.66-4.92 log cfu/g) for 29 days at 3 degrees C in the samples injected before smoking and which contained both nisin and sodium lactate. The treatments did not affect the sensory characteristics of cold-smoked rainbow trout. Based on a triangle test, the sensory quality of all test samples remained unchanged for 23 days of storage at 3 degrees C, whereas the control fish prepared without additives or additional salt remained unchanged only for 16 days.     

Shelf-life of vacuum-packed cooked ring sausages at different chill temperatures

Microbiological and sensory changes in 313 vacuum-packed cooked ring sausages from 28 different production runs and stored at 2, 4, 8 or 12 degrees C were monitored as a function of time. The sensory scores started to decrease at a level of approx. 10(7) lactobacilli/g. The judges began considering the samples unfit for human consumption when the lactobacilli counts were between 10(7) and 10(8) cfu/g; above a level of 10(8) cfu/g most of the samples were deemed unfit. At 2 degrees C, however, spoilage did not always seem to be microbiological, and four out of six different production runs were deemed unfit without any marked increase in microbial counts. In such cases, the judges described the sensory defects as a 'musty' rather than a sour aroma and taste. The sausages were deemed unfit when the lactobacilli were in a stationary growth phase which was considerably later than the point when the bacterial counts exceeded 10(7) cfu/g. The mean length of this delay was 30, 19, 16 and 7 days at 2, 4, 8 and 12 degrees C, respectively. The average shelf-lives were 55, 43, 29 and 17 days at 2, 4, 8 and 12 degrees C, respectively. The dependence of shelf-life on temperature can be formulated as follows: Shelf-life = 10(1.835 - 0.048 X temperature) The maximal shelf-life of this product, including nonmicrobiological spoilage, is assessed as approx. 10-11 weeks. A lactobacilli count greater than 10(7) cfu/g indicates that either the spoilage process has started or the product is already spoiled. When the lactobacilli count exceeds 10(8) cfu/g it is highly probable that the sausage sample is unacceptable.     

Listeria innocua and aerobic mesophiles during chill storage of inoculated mechanically recovered poultry meat treated with high hydrostatic pressure

Mechanically recovered poultry meat (MRPM) was inoculated with Listeria innocua 910 CECT at a level of approximately 10⁸ CFU g⁻¹. Vacuum-packaged samples were treated by combinations of pressure (350, 400, 450 and 500 MPa), time (5, 10, 15 and 30 min) and temperature (2, 10 and 20°C) and later stored at 2°C for 2 months. Counts of L. innocua and aerobic mesophilic bacteria were determined 1, 4, 7, 15, 30 and 60 days after pressurisation. For mesophiles, in most treatments, pressurization at 2°C gave the significantly best results. High pressure caused a marked bactericidal effect on L. innocua: reductions higher than 7.5 log units were achieved in several cases. Some cells were just sublethally injured by pressure. Samples treated at 500 MPa for 30 min at 2°C had counts of only 2.3 log units after 60 days of chill storage. Noninoculated pressurised MRPM did not show Listeria growth throughout storage. These results suggest that high pressure processing can enhance the microbiological quality of MRPM.     

Pre-selection of protective colloids for enhanced viability of Bifidobacterium bifidum following spray-drying and storage, and evaluation of aguamiel as thermoprotective prebiotic

The objective of this work was to demonstrate that Bifidobacterium bifidum viability to spray-drying and with storage time could be substantially enhanced by pre-selecting protective colloids offering resistance to oxygen diffusion (high activation energy) and adding aguamiel as a thermoprotector prebiotic. Three different protective colloids blends exhibiting relatively high, medium and low activation energies were mixed with B. bifidum harvested in the late log phase, with or without aguamiel, spray-dried at 130, 140 and 155 °C, and stored at 4 °C at a water activity of 0.32. Viability was determined by cell total count in MRS agar. Best viability was achieved when microencapsulating the microorganism in the protective colloids blend exhibiting highest activation energy (40.7 kJ mol⁻¹), with aguamiel, and dried at 140 °C. Viability ranged from 1.26 × 10⁸ cfu g⁻¹ immediately after drying to 6.0 × 10⁶ cfu g⁻¹ after 5 weeks storage time at 4 °C.     

Behaviour of Listeria monocytogenes in raw sausages (merguez) in presence of a bacteriocin-producing lactococcal strain as a protective culture

The effectiveness of a bacteriocin produced by Lactococcus lactis subsp. lactis M in reducing population level and growth of Listeria monocytogenes ATCC 7644 in fermented merguez sausage was examined. Two different formulas (with or without added nitrites) were assayed and predetermined numbers of Listeria (ca 10⁶ cfu g⁻¹) were added to sausage mixture. The effect of in situ production of the bacteriocin by Lactococcus lactis M on Listeria monocytogenes ATCC 7644 during fermentation and storage of merguez sausages at room (ca 22 °C) or at refrigeration (ca 7 °C) temperature was tested. Results indicated that counts of Listeria monocytogenes were decreased during fermentation of merguez samples fermented with either the bacteriocin-producing Lactococcus lactis M (Bac⁺) or a nonbacteriocin-producing Lactococcus lactis J (Bac⁻). However, reduction in Listeria cfu's was greater in samples fermented with the Bac⁺ than in those fermented with the Bac⁻ starter. In merguez sausage made without nitrites addition, the Bac⁺ starter induced further decrease in Listeria counts by 1.5 log cycles compared with that induced by the Bac⁻ starter. While in merguez samples with added nitrites (0.4%), the effect of the bacteriocin produced in situ was less important than in those made without nitrites addition.     

Inhibition of Non-proteolytic Clostridium botulinum with Lactic Acid Bacteria in Extended Shelf-Life Cook-Chill Soups

The possibility of protecting cook-chill foods with microbial cultures against the risk of botulism was demonstrated. Three commercial soups were incubated with Clostridium botulinum 17B (10³ spores/g) and protective cultures (PCs) during 10-15 days at 10°C. The PCs populations were enumerated on M17, MRS and maltose tryptic soy agar, C. botulinum—on sorbitol tryptic soy agar, botulinal toxin was detected by the immunoassay, bacteriocins—by well diffusion assay. C. botulinum did not grow in two soups with low pH (5.2-5.5) and was unaffected by the PCs. In seafood chowder (pH 6.2) C. botulinum populations reached 10⁸ cfu/g. The co-incubation with the PCs, nisin-producing Lactococcus lactis (10⁷ cfu/g) or pediocin-producing Pediococcus pentosaceus (3×10⁸ cfu/g) singularly and as a mixture, prevented toxigenesis as well as reduced the product pH to 4.8-5.0 and C. botulinum populations to undetectable levels. Color, mouth-feel, texture, flavor and the overall acceptability of seafood chowder was not affected by the presence of the PCs.     

Chilled storage life of hot-boned, pre-rigor, salted minced beef

Pre-rigor beef mince with 2% added salt was stored under CO₂ at −1.5 °C (A). The same mince with 100 ppm sodium nitrite was stored under CO₂ at 5 °C (B) and −1.5 °C (C), and under vacuum at −1.5 °C (D). Microbiological and sensory analyses were carried out for up to 21 weeks. Indicative storage life was taken as the time for microbial numbers to reach 10⁷ colony forming units per g. Mince stored under regimes B or D attained these numbers by 6 and 14 weeks, respectively; mince stored under regimes A and C had not attained these numbers by the end of the storage trial. Mince stored at 5 °C developed storage flavours of sufficient intensity to be detectable by consumers by 9 weeks storage. In general, the other minces did not develop unacceptable levels of storage or off flavours. Over 90% of the added sodium nitrite had disappeared after 10 weeks of storage, partly through conversion to sodium nitrate. Mince pH was not affected by the storage conditions and remained at about 6.0. The water holding capacity of the pre-rigor mince deteriorated during prolonged storage.     

The occurrence of Listeria monocytogenes in cheese from a manufacturer associated with a case of listeriosis

A case of listeriosis was associated with the consumption of a soft cheese produced in England. Goats cheese and other products from the same food manufacturer were examined for the presence of Listeria over the following 11 months. Listeria monocytogenes was isolated from 16 of 25 cheese samples on retail sale, 12 of 24 cheese samples obtained directly from the factory, and from shelving within the plant. Phage-typing of 68 isolates of L. monocytogenes from cheese samples and the factory showed that 66 (97%) were indistinguishable from the strain isolated from the patient's cerebrospinal fluid and stool. L. monocytogenes was not isolated from seven goats milk or two yoghurt samples. Listeria innocua was isolated from 10 cheese samples, two of which contained no other species of Listeria. Levels of L. monocytogenes shortly after production were low (<10/g), but were higher (10⁵–10⁷ cfu/g) in six of the 16 cheese samples obtained from retail outlets. Multiplication of L. monocytogenes was demonstrated in cheeses contaminated at the factory and held at 4°C in the laboratory.     

Growth characteristics of motile Aeromonas spp. isolated from different environments

The growth of 80 strains of motile Aeromonas spp. derived from environments with temperatures above 25°C and below 15°C, respectively, were examined at five temperatures (5°C, 10°C, 25°C, 37°C and 44°C) and four salt levels (0.05%, 2%, 4% and 6% NaCl). Sixty-one strains were further examined at two pH levels (pH 7.3 and pH 5.3). All strains grew at 25°C and 10°C with the majority of the isolates proliferating from approx. 10² to approx. 10⁷ cfu/ml within 1 and 3 days, respectively. In contrast, there were significant differences in the proportion of isolates able to grow at 5°C and 37°C depending on the temperature of their source of isolation. The ecological background of the organisms thus influences their thermal growth range and their ability to proliferate at body temperature, a highly significant factor in infective disease. At 25°C and pH 7.3, all strains grew in 0.05% NaCl, 96% grew in 2% NaCl, 96% grew in 2% NaCl while few grew in broth containing 4% or 6% NaCl. Lowering the pH to 5.3 with lactic acid caused a marked increase in the lag phase at 25°C and prevented growth of a large number of isolates at suboptimal conditions. Thus, none of the isolates from warm environments and only 8% of the isolates from cold environments grew at this pH at 5°C. The observed differences in growth optima between strains from different environments are discussed in relation to food- or waterborne infection.     

Production of cured meat color in nitrite-free Harbin red sausage by Lactobacillus fermentum fermentation

Lactobacillus fermentum was substituted for nitrite to produce cured pink color in a Chinese-style sausage. Treatments included inoculations (10⁴, 10⁶, and 10⁸ CFU/g meat) followed by fermentation at 30 °C for 8 h and then at 4 °C for 16 h. Control sausage (with sodium nitrite, 60 mg/kg meat) was cured at 4 °C for 24 h without L. fermentum. The UV–Vis spectra of pigment extract from L. fermentum-treated sausage were identical to that of nitrosylmyoglobin (NO-Mb) formed in nitrite-treated control. The NO-Mb concentration and the colorimetric a^* value of sausage treated with 10⁸ CFU/g meat of L. fermentum essentially replicated those in nitrite-cured meat. Free amino acid content in sausage treated with L. fermentum was greater and the pH slightly lower compared with the nitrite-cured control sample. This study showed that L. fermentum has the potential to substitute for nitrite in the sausage production.     

The microbial flora of vacuum packed smoked herring fillets

The quantitative and qualitative changes in the microbial flora of vacuum packed smoked herring fillets, held at 10°C over a period of 18 weeks, were examined. Samples were periodically tested by a sensory panel for taste and smell. Initial total counts were in the order of 10⁴/g. After 7 weeks' storage the counts reached 10⁸/g. No significant changes were recorded thereafter. Initially, the incidence of yeasts was high (64%). After 9 weeks of storage these were no longer detected. Homofermentative Lactobacillus sp(p). increased during the keeping period, reaching 84% of the total flora after 12 weeks' storage. Sensory data indicated a keeping quality of at least 3 months at 10°C. No correlation was observed between sensory scores and total microbial counts. It appears that total plate counts have a minor significance in quality assessment of vacuum packed smoked herring fillets.     

Migration of a UV stabilizer from polyethylene terephthalate (PET) into food simulants

The migration characteristics of the UV stabilizer Tinuvin 234 (2-(2H-benzotriazol-2-yl)-4,6-bis (1-methyl-1-phenylethyl)phenol) into food simulants has been measured from polyethylene terephthalate (PET) using HPLC with UV detection. Ethanol/water, isooctane and a fractionated coconut oil simulant (Miglyol®) were used as food simulating solvents. The migration characteristics were measured at temperatures in the range of 40-70°C. Diffusion coefficients were determined to be in the range of 1 × 10^-14 cm² s^-1 to 1 × 10^-18 cm² s^-1. At 40°C, the amount of migration into 95% ethanol after 10 days was 2 μg dm^-2. Isooctane is determined to be a good fatty food simulant that provides similar results for PET to those of fatty foods.     

The bacteriological quality of British beef 3. Frozen primal joints

The bacteriological status of 286 primal cuts stored frozen in intervention stores in England, Scotland and Northern Ireland for between 18 and 216 weeks was assessed in two surveys carried out during 1993 (120 cuts) and 1994 (166 cuts). Overall the aerobic plate count at 25°C and the presumptive pseudomonad counts were <10⁵ cm² on 269 (94%) and 273 (95·5%) of the cuts, respectively. Similarly the coliform and enterococcal counts were <10³ cm² on 98·3% and 97·9% of the cuts, respectively. These findings suggest that the quality of dressing and butchery of the carcasses was of a generally satisfactory standard although on occasions there may have been suboptimum hygiene control during slaughter and butchery or some delay before freezing. The bacterial numbers were higher on average on the cuts obtained from the lower part of the carcass while there was a tendency for the number of aerobic spoilage organism to decrease slightly with increasing storage time. Evidence was obtained in the second survey which indicated differences between microbiological quality of meat coming from different boning plants although it was not possible to make a detailed evaluation of this point as the number of cuts available for sampling from each plant differed in each year.