Influence of sporulation awon heat resistance and germination ofPenicillium roquefortispores

Spores ofPenicillium roquefortiwere produced using potato dextrose agar which was solute agjusted using either NaCl, sucrose, or glycerol to water activity (a_w) levels of: 0·99 (control), 0·97, 0.·90 and 0·88. All spore crops, regardless of the sporulation a_won which they were produced, exhibited near total germination within 24 h at 22°C in sterile water. However, when spores were initially heated either in water (42°C, 2 h or 55°C, 10 min) or in air (60°C, 20 min) and then germinated, levels were observed to decrease particularly for spores produced on 0·88 a_wadjusted media. Germination levels were also observed to decrease with time when harvested spores were initially suspended in sterile distilled water (0 to 24 h, 2°C) and then heated. Control spores (0·99 a_w) showed no effects. Heat studies of spores suspended in water were used to calculate thermal decimal reduction times (D). Control spores and spores produced at 0·88 a_wwith glycerol, exhibitedD_55°Cvalues of 10·3and 2·3 min, respectively. Specific effects attributable to the nature of the solute appeared to influenceDvalues; however, no clear trend was observed. The results of this study indicated that spores that were produced at reduced a_wbecame heat-sensitized perhaps owing to injury and/or damage incurred during sporogenesis.     

Influence of water activity and storage conditions on survival and growth of proteolytic Clostridium botulinum in peanut spread

Outgrowth of Clostridium botulinum spores followed by toxin production in peanut spread at A_w0·98, 0·96, 0·94 and 0·92 stored at 30°C under anaerobic or aerobic conditions for 0, 3, 7 and 16 weeks or 0, 1, 9 and 16 weeks, respectively, was investigated. Botulinal toxin was not detected in peanut spreads stored under anaerobic conditions for 16 weeks. Peanut spreads at A_w0·98 and two of three samples at A_w0·96 stored aerobically became toxic after 9 and 16 weeks, respectively. Clostridium botulinum in peanut spread at A_w0·98 and 0·96 grew to populations of 10⁶and 10⁵cfu g⁻¹, respectively, within 16 weeks. Lactic acid bacteria grew within 3 days in peanut spread at A_w0·98 and 0·96 stored under aerobic or anaerobic conditions. Regardless of A_w, populations of aerobic and anaerobic micro-organisms decreased in peanut spread stored under anaerobic conditions. Only slight decreases occurred in samples stored under aerobic conditions. The pH of inoculated and uninoculated peanut spread at A_w0·98 and 0·96 increased from 4·8 to 7·0 within 16 weeks and was attributed to growth of Penicillium andMucor spp. Similarly, redox potential (Eh) of peanut spread stored under anaerobic conditions for 3 weeks, decreased as the A_wwas increased. Significantly lower Eh values in peanut spread samples at A_w0·98 or 0·96 stored under aerobic conditions occurred within 1 week and/or 9 weeks compared to peanut spread at A_w0·92 or 0·94. Peanut spreads were judged inedible due to growth of lactic acid bacteria and molds which resulted in ‘off’ aromas before toxicity developed, thus greatly minimizing the likelihood of consumption.     

Effect of modified atmosphere and vacuum packaging on some quality characteristics and the shelf-life of “morcilla”, a typical cooked blood sausage

The effect of modified atmosphere and vacuum packaging on the shelf-life of “morcilla”, a traditional cooked blood sausage, was investigated. A total of 99 “morcillas” were packaged under vacuum and in modified atmosphere using three different gas mixtures: 15:35:50/O₂:N₂:CO₂ (atmosphere 1), 60:40/N₂:CO₂ (atmosphere 2) and 40:60/N₂:CO₂ (atmosphere 3), and stored during 2, 4, 6 and 8 weeks at 4 °C. Shelf life evaluation was based on pH, water activity (a_w), colour (CIE L*, a*, b*, C* and h*), TBARS formation and microbial counts. The results indicated that, in general, storage time affected (P < 0.05) all parameters whereas no significant differences were observed (P > 0.05) among packaging conditions. Based on the microbial counts, the shelf-life of “morcilla” would be greater than 8 weeks for all packaging conditions. Samples packaged with high CO₂ concentrations (40:60/N₂:CO₂) showed the lowest values of TBARS at the end of storage.     

Influence of storage temperature on the quality of salted mackerel (Rastrelliger kangurta) and pink perch (Nemipterus japonicus)

Dry-salted mackerel and pink perch were stored at two temperatures: ambient (26·8 ± 3·3°C) and 2·5 ± 1°C. Changes in moisture content, salt content, water activity (a_w), peroxide value (PV), free fatty acid content (FFA), total volatile base nitrogen (TVBN) content, halophilic bacterial count and sensory scores for overall acceptability were studied. Loss of moisture and absorption of salt were considerably higher in the products stored at ambient temperature. The decrease in a_w was more pronounced at ambient temperature than at the lower temperature. Although the chemical indices of freshness (PV, FFA and TVBN) and the halophilic counts showed increasing trends, they were considerably lower in the products stored at the lower temperature. Sensory evaluation for overall acceptability indicated that storage at the lower temperature could considerably extend the shelf-life of salted fish.     

Effect of modified atmosphere packaging on the microbiological and sensory quality on a dry cured beef product: "Cecina de león"

The aim of this study was to investigate the shelf-life of commercial “Cecina de León” (CL) packaged in vacuum (VP) and in CO₂/N₂ atmospheres (20/80% CO₂/N₂ and 80/20% CO₂/N₂). Packaged product was stored at 6 °C, measuring microbiological, physico-chemical and sensory parameters during 210 days. The values obtained for mesophilic aerobic, anaerobic and psychrotrophic counts did not vary in both the vacuum and the gas packaged samples after 210 days of storage. Enterobacteria and enterococci were always below the detection limit, and in the gas-packaged CL, pseudomonad numbers were significantly inhibited. No changes were observed in the counts of the typical microflora of CL (Lactic Acid Bacteria, yeasts and moulds and Micrococcaceae) up to 210 days and no differences (p > 0.05) were found between the counts in the VP and in the gas-packaged samples. A pronounced initial fading was observed in redness (a^∗) within the first 15 days in the VP samples and within the second month in the gas-packaged samples attributed to the presence of white film on surface of CL portions. No differences were observed (p > 0.05) in texture parameters evaluated between packaging methods during storage, and the values found are within the range for CL not packaged. The sensory properties of CL stored in 20/80% CO₂/N₂ were slightly less acceptable than the samples packed under vacuum and under 80/20% CO₂/N₂ at 210 days of storage. It is concluded that no clear advantage of the gas packaging was observed compared to the VP of “Cecina de León”.     

Chitosan inhibits growth of spoilage micro-organisms in chilled pork products

The aim of this study was to develop novel preservation systems for chilled, comminuted pork products that are sold raw using the natural compound chitosan (polymeric ß -1,4- N -acetylglucosamine). In vitro testing showed that viable numbers of Saccharomycodes ludwigii were reduced by up to 4 log cfu ml⁻¹ on exposure to 0·05% chitosan in 0·9% saline at pH 6·2. Higher concentrations of chitosan (0·25 and 0·5%) were required to achieve similar levels of inactivation withLactobacillus viridescens, Lac. sake and Listeria innocua. Torulaspora delbrueckii and Salmonella enteritidis PT4 were resistant to chitosan at the concentrations tested in this study (up to 0·5%). Trials in real foods showed that dipping of standard and skinless pork sausages in chitosan solutions (1·0%) reduced the native microflora (total viable counts, yeasts and moulds, and lactic acid bacteria) by approximately 1–3 log cfu g⁻¹ for 18 days at 7°C. Chitosan treatment increased the shelf-life of chilled skinless sausages from 7 to 15 days. Addition of 0·3 and 0·6% chitosan to an unseasoned minced pork mixture reduced total viable counts, yeasts and moulds, and lactic acid bacteria by up to 3 log cfu g⁻¹ for 18 days at 4°C compared with the untreated control. The results indicated that chitosan was an effective inhibitor of microbial growth in chilled comminuted pork products.     

Kinetics of degradation of sorbic acid in aqueous glycerol solutions

Degradation of sorbic acid in aqueous glycerol solutions at pH 4·0 over the a_w range 0·71–1·00 and the temperature range 40°–60°C was found to follow first-order reaction kinetics and to conform to the Arrhenius equation. Activation energy values obtained were 5·8 kcal mol^?1 and 7·8 kcal mol^?1 for systems at 0·80 a_w with and without added Co⁺⁺, respectively. The rate of sorbic acid degradation was observed to increase with decreasing a_w (i.e. increasing glycerol concentration). The presence of added Co⁺⁺ decreased the rate of sorbic acid breakdown at any particular a_w or temperature. Browning of sorbate solutions during storage was markedly inhibited by Co⁺⁺.     

Greek dry-salted olives: Monitoring the dry-salting process and subsequent physico-chemical and microbiological profile during storage under different packing conditions at 4 and 20 °C

Naturally black olives cv. Thassos were processed in dry salt according to industrial procedure by uniformly dispersing 40 kg of coarse salt in 100 kg freshly harvested olives. Dry-salting process was monitored by measuring selected physico-chemical (weight loss, NaCl content, pH, a_w, reducing sugars) and microbiological parameters (total viable counts, lactic acid bacteria, enterobacteria, yeasts, pseudomonads). Total weight loss amounted to 21 g/100 g after 80 days of dry-salting. Salt content in the fruits increased to 7.4 g/100 g followed by a decrease in water activity from 0.98 to 0.76. The pH did not change significantly presenting a slight decrease from 5.25 to 4.92. The initial microflora of the fruits comprised of lactic acid bacteria (4.1 log₁₀ cfu/g), yeasts (5.7 log₁₀ cfu/g), enterobacteria (3.7 log₁₀ cfu/g ) and pseudomonads (4.0 log₁₀ cfu/g). In the end of dry-salting, no microbial groups were enumerated apart from yeasts, due to the low water activity of the product. After dry-salting, olives were packed in HDPE bags under air (control samples) and 100 ml/100 ml CO₂. Another lot of fruits was dipped in 1 g/100 ml solution of potassium sorbate for 10 min prior to packing in the same bags under aerobic conditions. All packages were stored at 4 and 20 °C for a period of 180 days. During storage, the microbial flora comprised of yeasts, while no lactic acid bacteria, enterobacteria, pseudomonads or Staphylococcus aureus were detected as the low water activity/high salt content does not favour their growth. At 4 °C, the population of yeasts declined steadily throughout storage, but to a different extend depending on the packing treatment. At 20 °C, only potassium sorbate was effective in suppressing yeast growth. All packing treatments prevented fungal growth at both storage temperatures, apart from the samples stored in air. The pH, a_w and salt content did not change significantly throughout storage.     

Physicochemical and microbiological changes of “Souvlaki” – A Greek delicacy lamb meat product: Evaluation of shelf-life using microbial,colour and lipid oxidation parameters

Fresh Souvlaki-type lamb meat was packaged under vacuum (VP) and modified atmospheres (MAs) and stored under refrigeration (4 °C) for a period of 13 days. The following gas mixtures were used: M1: 30%/70% (CO₂/N₂) and M2: 70%/30% (CO₂/N₂). Identical samples were aerobically-packaged and used as control samples. Quality evaluation of product stored under the above packaging conditions was conducted using physicochemical and microbiological analyses. Of the chemical parameters determined, pH values of product showed no significant differences for all packaging treatments as a function of storage time. Lipid oxidation of lamb meat was enhanced by aerobic storage and gas mixture M1, whereas VP and gas mixture M2 controlled lipid oxidation to a greater extent. Souvlaki colour stability (as determined by a^∗, b^∗ and L^∗ values) was not negatively affected by either VP or MA conditions during the 13 days of storage. Of the two MAs and VP used, gas mixture M2 and VP were the most effective treatments for the inhibition of total viable counts (TVC), Pseudomonas spp., yeasts and Brochothrix thermosphacta in Souvlaki meat. Lactic acid bacteria (LAB) and Enterobacteriaceae were also found in the microbial flora of Souvlaki and increased during storage under all packaging conditions used. Based on microbiological analysis data and on the proposed a^∗ values, the use of VP and MAP (M2: 70%CO₂/30N₂) extended the shelf-life of “Souvlaki” meat stored at 4 °C by approximately 4–5 days compared to aerobic packaging.     

Vapor pressure and water activity measurements of saturated salt solutions made with D2O at 20°C

The water activity (a_w) values of 10 saturated salt solutions made with D₂O were measured in this study at 20°C: MgCl₂, K₂CO₃, Mg(NO₃)₂, KI, NaCl, KCl, ZnSO₄, BaCl₂, KNO₃, and K₂SO₄. These salts were selected because they represent a wide range of water activity values, from approximately 0.33 to 0.98. The a_w values were measured using two methods: (1) a vapor pressure manometer, and (2) an electronic chilled-mirror (AquaLab) instrument. The a_w values obtained by the manometer method were slightly, but consistently, higher than the a_w values obtained by the electronic chilled-mirror instrument. The a_w values of the saturated salt solutions made with D₂O (as measured by both methods) were also slightly, but consistently, higher than the a_w values of the same saturated salt solutions made with H₂O at 20°C. ©     

Reactivity of sorbate and glycerol in some model intermediate moisture systems

Model systems were developed to study the rôle of selected humectants and antimycotics in non-enzymic browning, haemoprotein breakdown and collagen degradation reactions at a_w 0·85 and initial pH 5·5. a_w adjustment was made using sodium chloride and the solutions contained 0·5% glucose, 0·5% sorbate, 0·5% propionate, 30% glycerol or 30% glycerol plus 0·5% sorbate. During aerobic storage at 38°C or 65°C, 10% lysine or glutamate solutions all exhibited increased browning and 0·01% or 0·02% haemoglobin solutions increased loss of haemoprotein from solution in the presence of glycerol and/or sorbate. During anaerobic storage only the glucose-containing solutions exhibited any reactivity.The pH and concentration of reactive carbonyls in the systems were also monitored and the rôle of sorbate and glycerol oxidation products in non-enzymic browning is discussed in the light of these results.Breakdown of the collagen in heat-treated tendon during storage at 38°C under similar conditions was also studied. Although the results were not unequivocal, it was apparent that, at least initially, storage in 30% glycerol caused increased degradation.     

Influence of partial replacement of NaCl with KCl and CaCl2on microbiological evolution of dry fermented sausages

The survival of lactic acid bacteria, Micrococcaceae, and pathogens in dry fermented sausages produced with a mixture of 1·0% NaCl, 0·55% KCl and 0·74% CaCl₂compared to a control product (2·6%NaCl) was studied. Hygienic quality was guaranteed in the modified products by an adequate process of desiccation and acidification. These products showed a higher acidification than the control during the entire ripening process.Lactobacillus and Micrococcaceae counts were not affected by the NaCl reduction. Enterobacteriaceae disappeared by the fifteenth day in both types of products. Escherichia coli, Staphylococcus aureus, Salmonella and sulfite-reducing clostridia counts in final products were acceptable in both types of products. These results indicated that the applied mixture of salts did not affect the development of the starter culture and guaranteed the hygienic quality of the products.     

Consumer acceptance of roasted peanuts affected by storage temperature and humidity conditions

Consumer acceptance and intensity ratings of roasted peanuts stored at temperatures of 23, 30, 35, and 40 °C, and water activities (a_w) of 0.33, 0.44, 0.54, 0.67 and 0.75 were determined over time. Consumer acceptance ratings, including overall, appearance, color, and texture were affected by storage water activity and time, but not by storage temperature. Consumer intensity ratings of crunchiness were affected by storage water activity and time, but not storage temperature. Aroma acceptance, flavor acceptance, and roasted peanutty and stale/oxidized/rancid intensity ratings of roasted peanuts were dependent on storage temperature, water activity and time.Shelf-life of roasted peanuts was predicted by all consumer attributes (R²>0.60) and was best predicted by aroma acceptance (R²=0.75). Using contour plots with ratings >5.0 for all acceptance attributes, the shelf-life of roasted peanuts stored at 23 °C and between 0.33 and 0.75 a_w was limited by overall acceptance and decreased by approximately 50% with each 0.1 a_w increase. At accelerated temperatures of 30, 35 and 40 °C, shelf-life of roasted peanuts was predominantly limited by flavor acceptance (>5.0), and to a lesser extent by overall acceptance (>5.0). Shelf-life of roasted peanuts stored at accelerated temperatures decreased by 50% or more with each 0.1 a_w increase.     

Survival of Escherichia coli O157:H7 in dried beef powder as affected by water activity,sodium chloride content and temperature

A study was carried out to determine the rate of inactivation of Escherichia coli O157:H7 in beef powder as affected by a_w(0·34±0·06±0·01), sodium chloride content (0·5, 3·0 and 20%) and temperature (5 and 25° C) over an 8-week storage time. Retention of viability of acid-adapted, acid-shocked, and control cells was determined. Overall, there were no significant differences (P≤0·05) in survival among the three types of cells subjected to the same test parameters, suggesting that mechanisms associated with induction of acid adaptation or acid shock do not result in cross protection against dehydration or osmotic stresses. At each a_wand within cell type, an increase in sodium chloride concentration resulted in significant reductions in the number of viable cells after a given storage time. Regardless of cell type, survival was significantly higher in beef powder containing 0·5 or 3% sodium chloride compared to powder containing 20% salt. The rate of inactivation was enhanced at a_w0·34±0·06 compared to a_w0·68±0·01 and at 25° C compared to 5° C.     

Microbial Growth Estimation in Liquid Media Exposed to Temperature Fluctuations

Comparisons of linear, square root and Arrhenius specific growth rate-temperature models showed no clear overall preference. At low a_w and refrigeration temperature, the linear model was satisfactory for Brochothrix thermosphacta and Pseudomonas fluorescens. A linear relationship between (specific growth rate)^?1 and lag phase was observed for these two psychrotrophs. The R² for B. thermosphacta in 3% (a_w= 0.968), 6% (a_w= 0.950), and 9% NaCl (a_w= 0.943) media was 0.865, 0.946 and 0.994, respectively. In 8% (a_w= 0.973) and 20% (a_w= 0.948) glycerol media, the R² was 0.998 and 0.999, respectively. R² for P. fluorescens growth in NaCl (3 or 4%) or glycerol (10 or 20%) media was >0.99. A cumulative growth adaptation function successfully estimated microbial growth at fluctuating temperatures.     

Effect of Adjusting the Water Activity to 0.95 with Different Solutes on the Kinetics of Thiamin Loss in a Model System

The kinetics of thiamin degradation in a model system of pH 6.0 and water activity (a_w) 0.95 controlled with NaCl, KC1, glycerol or Na₂SO₄, was studied. The type of solute used to adjust a_w, had a dramatic influence on the rate of degradation of thiamin. The loss of vitamin increased in the following order: NaCl > KCl > glycerol > Na₂SO₄. Activation energies, however, were independent of the type of solute and ranged between24–28 kcallmole.     

Use of response surface methodology in shelf life extension studies of a bakery product

Effects of water activity, pH, storage temperature, concentration of potassium sorbate, CO₂ concentration in the package headspace and inoculum level on growth of Aspergillus niger, the major spoilage isolate of English style crumpets, was studied on an agar model system using an experimental design termed Response Surface Methodology. The initial screening design indicated that a_w, pH, levels of potassium sorbate, CO₂ and storage temperature were significant variables. When a central composite rotatable design was subsequently applied to four variables with pH held constant at pH 6.5 only a_w, storage temperature and CO₂ level were found to have a significant effect on mold growth. Using multiple regression analysis, a second order polynomial model was derived and used to predict the number of days to visible mold growth under various combinations of the remaining three variables. Generation of contour plots enabled selection of levels for each variable to give a product with a desired mold free shelf life. Typically, crumpets with an a_w of 0.96, packaged in 62% CO₂ had a mold free shelf life of 20 d when stored at 20°C. The observed shelf life results agreed well with predicted values on Potato Dextrose Agar and demonstrated RSM as a powerful and elegant research tool when several variables are to be evaluated simultaneously.     

Survival of hepatitis A virus on modified atmosphere-packaged (MAP) lettuce

Experiments were designed to study the effect of various modified atmospheres (MA) on the survival rate of hepatitis A virus (HAV) on lettuce. Pieces of lettuce inoculated with HAV were incubated at room temperature (RT) and 4°C for 12 days in ambient air and under various modified atmospheres (CO₂:N₂at 30:70, 50:50, 70:30 and 100% CO₂) inside plastic bags of low O₂permeability. Samples were removed on days 1, 3, 6, 9 and 12 and the virus was recovered and plaque-assayed to determine residual titer. Incubation for 12 days at 4°C showed that the lowest HAV survival rate (47·5%) was on lettuce stored in a petri-dish (atmospheric air), whereas the greatest survival rates (83·6%) was on lettuce stored under 70% CO₂. Statistical analysis of virus survival at 4°C indicated that HAV titers decreased for all packages, but without a significant (P>0·05) difference between the package types. At RT, however, a significantly (P<0·05) lower HAV survival rate (0·01%) was evident on lettuce stored in a petri dish, whereas survival rates as high as 42·8% were observed on lettuce stored under 70% CO₂; much lower survival rates (≤8·6%) were obtained on lettuce stored under other MAP environments at RT. Statistical analysis of the RT data indicated that there was a highly significant (P<0·05) decrease in HAV titre with increasing storage time and between package types, except for lettuce stored under 70% CO₂. These data indicate that MAP does not influence HAV survival when present on the surface of produce incubated at 4°C. A slight improvement in virus survival on lettuce was seen in the presence of high CO₂levels at RT. This may have been attributed to the inhibition of spoilage-causing enzymatic activities in the lettuce, which may have reduced exposure of the virus to potential toxic by-products.     

An attempt to minimize potassium sorbate concentration in sponge cakes by modified atmosphere packaging combination to prevent fungal spoilage

The combined effect of modified atmosphere packaging (MAP) and potassium sorbate (PS) on the preservation of a typical Spanish cake (sponge cake) was studied. A full factorial experiment design including other factors, such as water activity (a_w) (0.80, 0.85 and 0.90) and pH (6 and 7.5) was applied. Eurotium amstelodami, E. herbariorum, E. repens and E. rubrum colony radii were measured daily for 28 days. All different combinations of PS concentration and MAP were more effective at pH 6 than at 7.5. As a_w increased, the amount of PS or carbon dioxide (CO₂) necessary to prevent or delay fungal growth increased. The antifungal activity of PS could be improved by packaging in a suitable modified atmosphere (MA). At pH 6, 0.1% of PS effectively prevented mould spoilage upto 28 days in the entire a_w range, by combining with 50–70% CO₂ level. On the other hand, 0.2% of PS in 30% N₂:70% CO₂ MA packages prevented spoilage of neutral cake analogues at all a_w levels. Control air-packaged bags with 0.2% of PS showed visible growth after 6 days, at pH 6 and 0.90 a_w. If no MAP was used, low a_w was the main preservation factor.     

Temperature,water activity and gas composition effects on the growth and aflatoxin production by Aspergillus flavus on paddy

The main aim of this study was to evaluate the combined effects of temperature with water activity (a_w) and CO₂ with a_w on the growth and aflatoxin production by Aspergillus flavus Link on paddy. The effects of temperature (20–30 °C) and a_w (0.92–0.98) on the relationship between colony diameter and aflatoxin production, and the influence of a_w (0.92–0.98) and CO₂ (20–80%) on the growth and toxin production were studied using full factorial design. Colony diameters were regularly measured and aflatoxins were periodically analyzed using HPLC with fluorescence detector. The growth and aflatoxin formation increased with a_w at the temperatures studied, and toxin production was positively correlated with the incubation time and colony diameter. Except at 0.92 a_w, as much as 80% CO₂ failed to inhibit the growth of fungi completely. However, at all a_w levels studied the growth parameters as estimated by Baranyi function and aflatoxin were affected by the increment in CO₂ where growth rates and aflatoxin were negatively correlated with CO₂ while the lag phase durations were positively correlated with CO₂. Under 0.98 a_w, the atmosphere enriched with 20% and 80% CO₂ lead to at least 59% and 88% reduction in growth and 47% and 97% in the toxin production, respectively. At 0.95 a_w, the lag phases of both isolates in average increased by a factor of 1.7–12 when the CO₂ levels in the headspace were between 20 and 80% compared to the control. The growth rate and lag phase durations under the modified atmospheres were successfully described using a polynomial equation (R² > 0.97). The results of the study could form a basis of indicative guidelines on the possible control of A. flavus and aflatoxin in paddy during temporary storage prior to drying.