Effect of ionic strength on inactivation kinetics of Escherichia coli O157:H7 by electron beam in ground beef, chicken breast meat, and trout fillets

The ionic strengths (IS) of ground beef, chicken meat, and trout fillets were modified to intermediate and highest with 3.5% and 7.0% NaCl, respectively. The samples with modified and unaltered (native) IS were inoculated with Escherichia coli O157:H7 and subjected to electron beam (e‐beam). The water activity (a_w) of the meat samples were 1.0, 0.96–0.97, and 0.94 for native, intermediate, and highest IS. The D₁₀‐values were calculated from survivor curves. The D₁₀‐values for E. coli ranged from 0.19 kGy in trout (highest IS) to 0.31 kGy in beef and chicken (highest IS). Regardless of the meat type, the increased resistance of E. coli to e‐beam was only observed at the highest IS. The difference of the D₁₀‐value for E. coli in samples at intermediate and native IS was insignificant. Regardless of the IS, the E. coli in trout was most sensitive, while the difference of E. coli resistance to e‐beam in ground beef and chicken meat was insignificant.     

Influence of gamma irradiation on growth and survival of Escherichia coli O157:H7 and quality of cig kofte, a traditional raw meat product

Cig kofte is a traditional Turkish food containing raw ground meat. Samples inoculated with Escherichia coli O157:H7 were irradiated at 0.5–6 kGy with a ⁶⁰Co source and stored at 4 and 25 °C. Total aerobic mesophilic count decreased with increasing irradiation doses, D₁₀ value was 0.83 kGy. Escherichia coli O157:H7 count decreased from 5.1 log₁₀ CFU g^?1 to an undetectable level (<1 log₁₀ CFU g^?1) after 1‐day storage at 4 °C following irradiation at 2 kGy, D₁₀‐value was 0.29 kGy. Irradiation doses up to 2 kGy did not affect sensory quality after 1 day. There was colour loss in samples irradiated at 2 kGy or above and stored for longer periods. Storage of the irradiated products at abused temperature must be avoided for safety assurance. Irradiation at 2 kGy has a great potential for extending the shelf‐life of cig kofte and assuring safety by decreasing the number of E. coli O157:H7 and other bacteria, but further studies with suitable package designs are needed to decrease quality degradation during extended storage.     

Heat-Resistance of Escherichia Coli O157:H7 in Meat and Poultry as Affected by Product Composition

The effects of fat level and low fat formulation on survival of Escherichia coli O157:H7 isolate 204P heated in ground beef [7%, 10% and 20% fat], pork sausage [7%, 10%, and 30% fat], chicken (3% and 11% fat), and turkey (3% and 11% fat) were determined by D- and z-values. D-values for E. coli 0157:H7 in lowest fat products were lower than in traditional beef and pork products (P < 0.05). Overall, higher fat levels in all products resulted in higher D-values. D₆₀ values (min) ranged from 0.45–0.47 in beef, 0.37–0.55 in pork sausage, 0.38–0.55 in chicken and 0.55–0.58 in turkey. D₅₅ and D₅₀ values were respectively longer. Z-values ranged from 4.4–4.8°C. Product composition affected lethality of heat to E. coli O157:H7.     

Temperature Effect on Inactivation Kinetics of Escherichia coli O157:H7 by Electron Beam in Ground Beef, Chicken Breast Meat, and Trout Fillets

ABSTRACT:  Ground beef, boneless skinless chicken breast meat, and boneless skinless trout fillets were inoculated with Escherichia coli O157:H7 and incubated to approximately 10⁹ colony forming units per gram (CFU/g). Following incubation, temperature of the meat samples was equilibrated to −20, 4, and 22 °C. The meat samples at different temperatures were subjected to one-sided electron beam (e-beam) with fixed energy at 10 million electron volts (MeV) and doses at 0 (control), 0.5, 1.0, 2.0, 2.5, and 3.0 kGy. The survivors were enumerated using a standard spread-plating method. The survivor curves were plotted on logarithmic scale as a function of e-beam dose for each meat sample subjected to e-beam at different temperatures. The D -values were calculated as a negative reciprocal of the slope of the survivor curves. The D -values for E. coli ranged from 0.22 to 0.35 kGy in trout at 4 °C and chicken at −20 °C, respectively. The D -values were different between meat types. Regardless of temperature, E. coli in chicken had highest D -value followed by beef and trout. The D -values for E. coli in frozen samples were higher than D -values in samples irradiated at 4 and 22 °C regardless of species. Although there were numerical differences between D -values for samples subjected to e-beam while chilled (4 °C) or frozen (−20 °C), they were statistically insignificant. Water radiolysis is considered as an indirect mechanism for microbial inactivation. Therefore, while the physical state of water (frozen or unfrozen) in foods seems the major contributor to microbial inactivation by e-beam due to water radiolysis, product temperature most likely plays a minor role.     

Factors affecting radiation D-values (D₁₀) of an Escherichia coli cocktail and Salmonella Typhimurium LT2 inoculated in fresh produce

Abstract: This study evaluated the effect of produce type, resuspension medium, dose uniformity ratio (DUR), and sample preparation conditions (tissue exposure, MAP, anoxia) on the D₁₀‐value of an Escherichia coli cocktail (BAA‐1427, BAA‐1428, and BAA‐1430) and Salmonella Typhimurium LT2 inoculated on the surfaces of tomato, cantaloupe, romaine lettuce, and baby spinach. Produce at room temperature were irradiated using a 1.35 MeV Van de Graaf electron beam accelerator at 0.2 to 0.9 kGy. The D₁₀‐values for E. coli and Salmonella were 0.20 ± 0.01 kGy and 0.14 ± 0.01 kGy, respectively. Bacterial inactivation was not affected by produce type as long as the samples were irradiated in unsealed bags, the bacteria were suspended in broth, and the sample tissue was exposed. Sample location in front of the e‐beam source during exposure is crucial. A 20% increase in DUR yielded a 53% change in the D_10‐values. Variations in sample preparation, microbiological methods and irradiation set‐up, result in variable D₁₀‐values for different microorganisms on fresh produce. Practical Applications: Most irradiation studies disregard the effect of sample handling and processing parameters on the determination of the D₁₀‐value of different microorganisms in fresh and fresh‐cut produce. This study shows the importance of exposure of sample, resuspension medium, available oxygen, and dose uniformity ratio. D₁₀‐values can differ by 35% to 53% based on these factors, leading to considerable under‐ or over‐estimation of the irradiation treatment. Results from this study will help to lay firm groundwork for future studies on D₁₀‐values determination for different pathogens on fruits and vegetables.     

Gamma irradiation as a means to eliminate Listeria monocytogenes from frozen chicken meat

Cells of Listeria monocytogenes ATCC 35152 were sensitive to gamma irradiation in phosphate buffer, pH 7.00 (D₁₀, dose required for 10% survival—0.15 kGy) at 0–5°C. The cells showed higher radiation survival when irradiated under frozen condition, with a D₁₀ of 0.3 kGy. The protection offered by shrimp/chicken/kheema homogenates (100 g litre^?1) was evidenced by even higher D₁₀ values (0.5 kGy) at both 0–5°C and cryogenic temperature. Boneless chicken meat samples were artificially inoculated with L monocytogenes ATCC 35152 cells at low (5 × 10³) colony-forming unit (cfu) g^?1 and high (5 × 10⁶ cfu g^?1) concentrations and irradiated at 1, 3, 4, 6 kGy doses under cryogenic conditions. The efficacy of the radiation process was evaluated by detecting L monocytogenes during storage at 2–4°C in the irradiated samples. These studies, when repeated with three other serotypes of L monocytogenes, clearly suggested the need for a dose of 3 kGy for elimination of 10³ cfu cells of L monocytogenes g^?1 from air-packed frozen chicken meat.     

Combined effect of irradiation and modified atmosphere packaging on shelf-life extension of chicken breast meat: microbiological, chemical and sensory changes

In the present study the combined effect of gamma irradiation (2 and 4 kGy) and modified atmosphere packaging (MAP) (30% CO₂/70% N₂ and 70% CO₂/30% N₂) on shelf life extension of fresh chicken meat stored under refrigeration was investigated. The study was based on microbiological (TVC, Pseudomonas spp., Lactic Acid Bacteria, Yeasts, Brochothrix thermosphacta, Enterobacteriaceae), physicochemical (pH, TBA, color) and sensory (odor, taste) changes occurring in chicken samples. Microbial populations were reduced by 1–5 log cfu/g for a given sampling day depending on the specific treatment. The effect was more pronounced in the case of the combination of MAP (70% CO₂/30% N₂) and the higher irradiation dose of 4 kGy. Of the chemical indicators of spoilage, TBA values for all treatments remained lower than 1 mg malondialdehyde (MDA)/kg meat throughout the 25 day storage period. pH values varied between 6.4 (day 0) and 5.9 (day 25). The values of the color parameters L*, a* and b* were not considerably affected by MAP. Irradiation resulted in a small increase of the parameter a*. Irradiation had a greater effect in extending the shelf life of chicken as compared to MAP. Sensory evaluation showed that the combination of irradiation at 4 kGy and MAP (70% CO₂/30% N₂) resulted in the highest shelf-life extension by 12 days compared to the air packaged samples.     

The Water Activity of Canned Foods

This work reports the a_w values of a diversity of commercially canned foods (54 samples), including fruits and vegetables and cured and uncured meat products. The a_w of canned fruit products (pieces in syrup, purees and juices) ranges between 0.950 and 0.992. Canned cured meats (i.e. deviled ham, meat and liver pastes, corned beef, cooked ham) have a_w values in the range 0.970–0.984, while uncured canned meats display a_w values above 0.982.     

E‐Beam Irradiation for Improving the Microbiological Quality of Smoked Duck Meat with Minimum Effects on Physicochemical Properties During Storage

This study was performed to evaluate the effect of different doses (0, 1.5, 3, and 4.5 kGy) of e‐beam irradiation on the quality parameters (pH, Hunter's parameter, and heme pigment) and stability qualifiers (peroxide value [POV], thiobarbituric acid reactive substances [ TBARSs], and total volatile basic nitrogen [TVBN]) of smoked duck meat during 40 d of storage under vacuum packaging at 4 °C. The initial populations of total bacteria (7.81 log CFU/g) and coliforms (5.68 log CFU/g) were reduced by approximately 2 to 5 log cycles with respect to irradiation doses. The results showed that pH, myoglobin, met‐myoglobin, L^*, a^*, and b^* showed significant differences with respect to different doses and storage intervals; a^* and b^* did not vary significantly because of storage. Higher pH was found in samples treated with 4.5 kGy at 40 d, while the minimum was observed in nonirradiated samples at day 0 of storage. Higher POV (2.31 ± 0.03 meq peroxide/kg) and TBARS (5.24 ± 0.03 mg MDA/kg) values were found in 4.5 kGy‐treated smoked meat at 40 d and the lowest was reported in 0 kGy‐treated meat at initiation of storage (0 d). However, irradiation suppressed TVBN during storage and higher TVBN (7.09 ± 0.32 mg/100 mL) was found in duck meat treated with 0 kGy at 40 d. The electronic nose (e‐nose) effectively distinguished flavor profiles during the different storage intervals. The results showed that different sensory attributes did not vary significantly with respect to the dose of irradiation. We conclude that low dose of e‐beam irradiation and vacuum packaging is beneficial for safety and shelf life extension without affecting the sensory characteristics of smoked duck meat.     

Maintenance of safety and quality of refrigerated ready-to-cook seasoned ground beef product (meatball) by combining gamma irradiation with modified atmosphere packaging

Abstract: Meatballs were prepared by mixing ground beef and spices and inoculated with E. coli O157:H7, L. monocytogenes, and S. enteritidis before packaged in modified atmosphere (3% O₂+ 50% CO₂+ 47% N₂) or aerobic conditions. The packaged samples were irradiated at 0.75, 1.5, and 3 kGy doses and stored at 4 °C for 21 d. Survival of the pathogens, total plate count, lipid oxidation, color change, and sensory quality were analyzed during storage. Irradiation at 3 kGy inactivated all the inoculated (approximately 10⁶ CFU/g) S. enteritidis and L. monocytogenes cells in the samples. The inoculated (approximately 10⁶ CFU/g) E. coli O157:H7 cells were totally inactivated by 1.5 kGy irradiation. D¹⁰‐values for E. coli O157:H7, S. enteritidis, and L. monocytogenes were 0.24, 0.43, and 0.41 kGy in MAP and 0.22, 0.39, and 0.39 kGy in aerobic packages, respectively. Irradiation at 1.5 and 3 kGy resulted in 0.13 and 0.36 mg MDA/kg increase in 2‐thiobarbituric acid‐reactive substances (TBARS) reaching 1.02 and 1.49 MDA/kg, respectively, on day 1. Irradiation also caused significant loss of color and sensory quality in aerobic packages. However, MAP effectively inhibited the irradiation‐induced quality degradations during 21‐d storage. Thus, combining irradiation (3 kGy) and MAP (3% O₂+ 50% CO₂+ 47% N₂) controlled the safety risk due to the potential pathogens and maintained qualities of meatballs during 21‐d refrigerated storage. Practical Application: Combined use of gamma irradiation and modified atmosphere packaging (MAP) can maintain quality and safety of seasoned ground beef (meatball). Seasoned ground beef can be irradiated at 3 kGy and packaged in MAP with 3% O₂+ 50% CO₂+ 47% N₂ gas mixture in a high barrier packaging materials. These treatments can significantly decrease risk due to potential pathogens including E. coli O157:H7, L. monocytogenes, and S. enteritidis in the product. The MAP would reduce the undesirable effects of irradiation on quality, and extend the shelf life of the product for up to 21 d at 3 °C.     

Depression of aw by Soluble and Insoluble Solids in Alginate Restructured Beef Heart Meat

Water activity (a_w), pH, bind and moisture of alginate restructured beef heart meat (BHM) were evaluated using a 2⁵ factorial design based on combinations of beef heart meal and glycerol (10%, 20%), and dextrose, bone meal and glycine (0%, 5%). Effects of these components were significant (p<0.05) for reducing a_w and moisture, and, excluding glycine, for altering pH. The a_w of the BHM control was 0.94, while a_w for 32 treatments ranged from 0.66 to 0.90. Glycine lessened product bind. An intermediate moisture BHM product could be formulated using the hurdle concept and the alginate system for restructuring meat with incorporation of selected soluble (glycerol, dextrose) and insoluble (beef heart meal, bone meal) components.     

Role of deoxyhemoglobin in lipid oxidation of washed cod muscle mediated by trout, poultry and beef hemoglobins

Deoxyhemoglobin content was measured in hemoglobins from trout, chicken and bovine sources between pH 5.5 and 7.5. With decreasing pH, deoxyhemoglobin content of trout was highest, low to intermediate in chicken, and lowest in beef hemoglobin. Each type of hemoglobin was added to washed cod muscle and lipid oxidation assessed during 2?°C storage. The lipid oxidation rate was trout > chicken > beef based on thiobarbituric reactive substances (TBARS) and lipid hydroperoxide formation. There was no significant difference in pro-oxidative activity of chicken compared to turkey hemoglobin. Hemoglobins from trout appeared to oxidize more rapidly compared to chicken hemoglobin in the washed cod muscle model system, as measured by a decrease in redness (a-value) during storage. Loss of red color was slowest in beef samples. These studies suggest that deoxyhemoglobin may be a major catalyst of lipid oxidation at post mortem pH values found in muscle foods, especially in fish and poultry compared to beef.     

Effects of type of packaging material on physicochemical and sensory characteristics of deep‐fat‐fried banana chips

A storage study of deep‐fat‐fried banana chips was carried out for 8 weeks at ambient temperature (27 °C), using four types of packaging material: laminated aluminium foil (LAF), oriented polypropylene (OPP), polypropylene (PP) and low‐density polyethylene (LDPE). The physicochemical and sensory characteristics of the stored banana chips were analysed at weeks 0, 2, 4, 6 and 8. The quality parameters determined were moisture content, water activity (a_w), thiobarbituric acid‐reactive substances (TBARS), texture (breaking force), colour and sensory attributes. The moisture content, a_w, TBARS and breaking force values of all samples increased during storage. The colour also changed during storage, showing higher L and lower a and b values. Samples packed in LAF had the lowest moisture content, a_w, TBARS and breaking force values. The most notable sensory change that occurred during storage was a decrease in crispness. Samples packed in LAF had higher scores than the other three samples, whilst LDPE gave the lowest scores for crispness as well as product colour. There were no significant differences (P > 0.05) in rancid odour among samples packed in OPP, PP and LDPE. However, there were significant differences (P < 0.05) between samples packed in LAF and the other three samples, with LAF giving the lowest rancid odour. © 2002 Society of Chemical Industry     

Development of a method based on ESR spectroscopy for the identification of irradiated beef,pork, and chicken meats

The electron spin resonance (ESR) spintrapping method for the detection of irradiated beef, pork, and chicken was studied using a α-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN) spin trapper in the dose of 0.5–7 kGy. Irradiation caused a significant increase in the ESR signal intensity of samples with hyperfine coupling constants of a ^N=1.57 mT and a ^H=0.25 mT, which correspond to lipid-derived radicals. In contrast, un-irradiated samples exhibited a weak ESR signal with no hyperfine coupling constants. The irradiation-induced lipid radical stability vs. temperature was also studied at room temperature, −4 and −18°C using 3 kGy irradiated beef. Temperature did not affect ESR signal intensity or the hyperfine coupling constants. To investigate the applicability of the proposed procedure for pork and chicken, a comparison of the spectra at the hyperfine coupling constants confirmed the presence of lipid-derived radicals in the samples.     

Development of Shelf-stable Intermediate-moisture Meat Products Using Active Edible ChitosanCoating and Irradiation

Shelf‐stable intermediate‐moisture (IM) meat products were developed using a combination of hurdles such as reduced a_w, active edible coating of chitosan, and irradiation. Chitosan prepared from chitin had a viscosity of 16 c P, molecular weight of 17.54 kDa, and a degree of deacetylation (DD) of 74%. The nitrogen content of the chitosan was estimated to be 7.56%. The antioxidant activity of chitosan increased upon irradiation without significantly affecting its antimicrobial property. The effect of irradiated chitosan coating in terms of its antimicrobial and antioxidant properties in IM meat products immediately after irradiation and during storage was assessed. The a_w of meat products such as mutton sheek kababs and streaky bacon was first reduced to 0.85 ± 0.02. The products were then coated with chitosan and irradiated (4 kGy). No viable bacteria or fungi were detected in chitosan‐coated, irradiated products. In contrast, IM meat products that were not subjected to gamma radiation showed visible fungal growth within 2 wk. The chitosan‐coated products showed lower thiobarbituric acid‐reactive substances (TBARS) than the noncoated samples for up to 4 wk of storage at ambient temperature. The studies thus clearly indicated the potential use of chitosan coating for the preparation of safe and stable meat products.     

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

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

Microbial Inactivation and Electron Penetration in Surimi Seafood During Electron Beam Processing

Electron penetration and microbial inactivation by electron beam (e‐beam) in surimi seafood were investigated. Dose map revealed that 1‐ and 2‐sided e‐beam could efficiently penetrate 33‐ and 82‐mm thick surimi seafood, respectively. Modeling of microbial inactivation by e‐beam demonstrated that 2‐sided e‐beam may control Staphylococcus aureus if the surimi seafood package is thinner than 82 mm. The D_e‐beam value for S. aureus was 0.34 kGy. An e‐beam dose of 4 kGy resulted in a minimum of a 7‐log and most likely a 12‐log reduction of S. aureus. Microbial inactivation was slower when frozen samples were subjected to e‐beam.     

Low dose gamma irradiation of raw meat. I. Bacteriological and sensory quality effects in artificially contaminated samples

Filet américain, consisting of raw, ground meat (beef) mixed with mayonnaise sauce, and the corresponding raw beef (without sauce) were experimentally contaminated with different strains of Salmonella. Yersinia enterocolitica and Campylobacter jejuni and irradiated with doses up to 1.5 kGy.Radiation resistance (D value) was determined immediately after irradiation. For the evaluation of the irradiaion effect on Y. enterocolitica in relation to storage, a number of samples were stored for a few days at 3°C and the microbiologically examined.D values of S. typhimurium, S. anatum, S. panama and S. stanley (strains isolated from retail filet américain) irradiated in filet américain were found to be 0.37, 0.45, 0.41 and 0.61 kGy and in raw beef 0.55, 0.67, 0.66 and 0.78 kGy, respectively. D values of Y. enterocolitica serotypes 0:3, 0:5, 27 and 0:9 irradiated in filet américain were 0.043, 0.065, and 0.080 kGy and in raw beef, 0.10, 0.16 and 0.21 kGy, respectively. D values of C. jejuni (three strains) irradiated in filet américain were 0.11, 0.08 and 0.09 and in beef 0.15, 0.14 and 0.16 kGy, respectively.It is concluded that doses as low as 1 kGy are effective in reducing Salmonella by approximately 1.6–2.7 log cycles in filet américan and 1.3–1.8 log cycles in ground meat. Numbers of Y. enterocolitica and C. jejuni are always reduced by more than 4 log cycles with this dose.On sensory evaluation 38% of samples of filet américain irradiated directly with 1 kGy were not acceptable for a taste panel; preference was for the unirradiated sample in 82% of the cases. However, when beef was irradiated with the same dose prior to the addition of mayonnaise sauce no significant taste differences could be observed between nonirradiated and irradiated samples.     

Application of electron‐beam irradiation pasteurization of ground beef,from steers fed vitamin E fortified diets: microbial and chemical effects

The effects of vitamin E supplementation of diets and electron‐beam irradiation (EBI) processing of ground beef patties on microbial and chemical qualities were investigated during 21 days of storage at 4 °C. Oxidative damage to lipids induced by EBI in ground beef patties containing different fat contents was first determined at 3 day intervals throughout a 7 day storage period at 4 °C. Significantly (P ≤ 0.05) higher values for thiobarbituric acid reactive substances (TBARS) were detected in beef patties of higher fat content (ie at 17 and 30%), which was further enhanced by irradiation at 5 kGy. Since lipid oxidation proceeded to a greater extent in beef patties with higher fat levels, ground beef patties of 30% fat were prepared from steers fed basal (diet I) or basal + 500 IU (diet II) of the antioxidant (vitamin E) supplemented diets. Plasma vitamin E concentrations in cattle fed diets I and II were 1.58 ± 0.42 µg ml^?1 and 2.49 ± 0.40 µg ml^?1 respectively. Patties were processed with three doses (2, 5, or 10 kGy) of EBI and compared with non‐irradiated patties. Microbial indices monitored at 3 day intervals included total aerobic plate count, psychrotrophic counts, and total coliform and Escherichia coli counts. Bacterial growth in ground beef patties stored at 4 °C was significantly (P ≤ 0.05) reduced by EBI at 2 kGy dose. Complete inhibition of bacteria occurred at 5 kGy or higher (P ≤ 0.05) dosage of EBI over 21 days of storage at 4 °C. Quality indices monitored at 3 day intervals throughout a 21 day storage (4 °C) study involving 30% fat ground beef patties made from steers fed vitamin E supplemented diets I and II included TBARS and colour. Results indicated that irradiation at the highest dosages was associated with higher (P ≤ 0.05) TBARS values, which in turn corresponded to lower linoleic acid content. With all three levels of irradiation, Hunter a values of beef patties decreased (P ≤ 0.05) significantly. Lipid oxidation was significantly (P ≤ 0.05) retarded in stored beef patties derived from cattle fed vitamin E (diet II). Copyright © 2003 Society of Chemical Industry     

Influence of γ‐irradiation and maize lipids on the production of aflatoxin B1 by Aspergillus flavus

The effect of γ‐irradiation and maize lipids on aflatoxin B₁ production by Aspergillus flavus artificially inoculated into sterilized maize at reduced water activity (a_w 0.84) was investigated. By increasing the irradiation doses the total viable population of A. flavus decreased and the fungus was completely inhibited at 3.0 kGy. The amounts of aflatoxin B₁ were enhanced at irradiation dose levels 1.0 and 1.5 kGy in both full‐fat maize (FM) and defatted maize (DM) media and no aflatoxin B₁ production at 3.0 kGy γ‐irradiation over 45 days of storage was observed. The level in free lipids of FM decreased gradually, whereas free fatty acid values and fungal lipase activity increased markedly by increasing the storage periods. The free fatty acid values decreased by increasing the irradiation dose levels and there was a significant enhancement of fungal lipase activity at doses of 1.0 and 1.50 kGy. The ability of A. flavus to grow at a_w 0.84 and produce aflatoxin B₁ is related to the lipid composition of maize. The enhancement of aflatoxin B₁ at low doses was correlated to the enhancement of fungal lipase activity.