Development of a membrane filtration method for enumeration of Listeria monocytogenes from soft cheese

A new and simple analytical method has been developed to quantify low levels (≤50 cfu g⁻¹) of Listeria monocytogenes in soft cheese. The technique allows the analysis of 1 g of cheese instead of 0·1 g or 0·01 g using the ISO 11290-2 standard method. The analysis protocol combines filtration of the decimally diluted cheese suspension through a 0·45-μm pore size cellulose ester membrane, and a culture of the filter on a Palcam agar. A tween 80–trypsin treatment used to increase filterability of cheese did not reduce L. monocytogenes counts. The tested method provided more precise results (nearer to the true value) compared to the ISO 11290-2 standard method in the enumeration of L. monocytogenes from artificially contaminated cheese. However, it improves neither repeatability nor reproducibility since the selected medium, Palcam, does not allow distinction between the different Listeria species.     

Inhibitory effect of clove oil on Listeria monocytogenes in meat and cheese

The antilisteric activity of clove oil was examined in meat and cheese at both 30°C and 7°C. At concentrations of 0·5% and 1% clove oil restricted the growth of Listeria monocytogenes in the food items at both temperatures. The inhibitory activity of clove oil was more pronounced at a concentration of 1%.Listeria counts in treated samples were 1–3 log₁₀cfu g⁻¹less compared to controls at different intervals during storage. The results revealed the potential of clove oil as a natural preservative in meat and cheese.     

Contamination of infant powdered milk in use with enterotoxigenic Staphylococcus aureus

One-hundred and ten of the 114 samples of infant milk formulae collected from nursing mothers contained viable staphylococci, with the highest mean count of 1·0 × 10² cfu g⁻¹, from samples collected the day the tin was opened. The highest mean total aerobic plate count was 2·6 × 10³ cfu g⁻¹. Titratable acidity and pH of the reconstituted milk ranged from 0·06 to 0·8 and 6·40 to 6·52 respectively. 52·0% of the 123 isolates were S. aureus, 49·6% produced β-hemolysin and 17·1% produced α-hemolysin. Enterotoxin A was produced by 6·5%, B by 1·6%, C by 2·4%, D by 1·6% and E by 0·8% of the isolates. The total staphylococcal and aerobic plate counts were not affected by either the period elapsed from the opening of tin to sampling or the brand of milk.     

The use of a high pressure waterjet combined with electroimmobilization for the stunning of slaughter pigs: Some aspects of meat quality

It is considered that waterjet stunning may be a humane stunning method to apply in slaughter facilities. An experiment was conducted in a slaughterhouse to examine the effects of waterjet stunning combined with electroimmobilization during exsanguination on the occurrence of haemorrhages in the muscles and on meat quality.Slaughter pigs (n = 31) were stunned by waterjet (3900 bar) in a V-type restrainer and immobilized electrically (40 V) during exsanguination. Control animals (n = 39) were stunned automatically and electrically (600 V) using the same restraint device.At 45 min post mortem the pH in the semimembranosus (SM) and longissimus dorsi (LD) muscles was significantly lower (p < 0·01), while rigor mortis and temperature in the SM and LD were significantly higher (p < 0·05 and p < 0·01, respectively) in the carcasses of pigs stunned with the waterjet as compared to control pigs. At 18 h post mortem the ultimate pH of the LD was lower (p < 0·05), while scatter (fibre optic probe) and filter paper test values of the LD were higher (p < 0·01) in carcasses of pigs stunned with the waterjet. Fewer haemorrhages were observed in the shoulders of pigs stunned with the waterjet.The results of this experiment suggest that waterjet stunning, when combined with electroimmobilization, may be a suitable method to stun pigs in a slaughterhouse. However, further studies are required to improve the meat quality.     

An evaluation of street-vended sliced papaya (Carica papaya) for bacteria and indicator micro-organisms of public health significance

Thirty samples of ripe papaya (Carica papaya) slices, collected in Calcutta from the itinerant roadside vendors were collected over a 3-month period. The papaya were tested for total aerobic plate count (TPC), coliform and fecal coliform counts, and various foodborne pathogens. TPCs ranged from 3·3 to 6·52 log cfu g⁻¹ (average=5·96 log cfu g⁻¹). Eleven samples had counts >6 log cfu g⁻¹. Coliforms were detected in 70% of the samples with the average concentration being 13·5 g⁻¹. The presence ofEscherichia coli was confirmed in 48% of the samples positive for coliforms. Salmonella and Vibrio cholerae were detected in one sample each, and low levels of coagulase-positive Staphylococcus aureus were detected in 17% of the samples. An apparent relationship between high aerobic plate counts, detection of coliforms and the presence of enteric pathogens was observed.     

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.     

Effect of high pressure combined with mild heat or nisin on inoculated bacteria and mesophiles of goat's milk fresh cheese

The effect of pressures ranging from 400 to 500 MPa combined with mild heat on Staphylococcus carnosus inoculated in fresh cheese and the concurrent use of 500 MPa and nisin to inactivate cheese indigenous populations has been studied. Staphylococcus carnosus counts could not be substantially decreased with treatments at 500 MPa at 10 or 25°C for 30 min, whereas treatments at 50°C for 5 min caused a reduction of 7-log₁₀cfu g⁻¹. Multiple-cycle treatments of 500 MPa and times between 15 and 30 min also improved the inactivation rate. Combination of 500 MPa and nisin was the most effective treatment to inactivate cheese indigenous microbiota. Inactivation of Bacillus subtilis spores inoculated in fresh cheese has also been studied. Germination treatments of 60 MPa at 40°C for 210 min followed by vegetative cells inactivation treatments of 500 MPa at 40°C for 15 min caused a lethality of 4·9-log₁₀ofB. subtilis , whereas the same combination of treatments applied at 25°C only caused a 2·7-log₁₀reduction.     

Effect of dietary α-tocopherol supplementation on color and lipid stability in pork

Myoglobin and lipid oxidation are major causes of quality deterioration in fresh pork. A process to enhance color and lipid stability would prove valuable to the pork industry given the current trend of centralized packaging and distribution to retail markets. Our objective was to determine the effects of dietary α-tocopherol (α-Toc) supplementation on color and lipid stability in ground pork, and loin chops stored in modified atmosphere packaging (MAP). Yorkshire crossbred pigs (n=20) were randomized into two groups and fed diets containing 48 (CON) or 170 mg α-Toc acetate/kg feed (VIT-E) for 6 weeks before slaughter. Plasma α-Toc concentration was measured weekly. Post-slaughter, Boston butt shoulders were ground, formed into patties with or without 1.5% salt, and stored fresh at 4°C for 0, 2, 4, or 6 days, and frozen at −20°C for 45 or 90 days. Pork loin chops were packaged aerobically and stored at 4°C for 0, 2, 4 or 6 days, or in MAP at 4°C for 7, 10 or 13 days prior to Hunter L*,a*,b* and TBARS analyses. α-Toc concentration of longissimus dorsi, psoas major, biceps femoris, semimembranosus and semitendinosus muscles was determined. Plasma α-Toc was greater (P<0.05) in VIT-E animals compared with CON and α-Toc concentrations were greater (P<0.05) in all VIT-E muscles compared with CON. TBARS values of both fresh and salted patties were less in VIT-E than in CON meat following 6 days at 4°C; VIT-E TBARS of salted patties were less (P<0.05) after 45 days at −20°C compared with CON. α-Toc supplementation did not influence (P>0.05) color of aerobically packaged or MAP chops, or of fresh or salted pork patties. α-Toc supplementation reduced TBARS formation in fresh and salted pork but had no significant impact on color.     

Analysis for selected pathogens in water used during rinsing of broiler carcasses in small processing operations in Trinidad

The study was undertaken to quantitatively and/or qualitatively determine the level of selected bacterial pathogens in the water used during the rinsing stage at small retail broiler processing operations utilizing a stagnant rinsing system in Trinidad. Water samples (n=6) were collected weekly from thirteen “pluck shops”, across Trinidad, over a 6-week period. Standard media and procedures were used for isolation, detection and quantification of bacterial pathogens. A significant difference was noted for the prevalence (P=0.004) and mean counts (P=0.03) of Campylobacter spp. across counties. Total aerobic plate count ranged from log₁₀ mean±SD, 4.0±1.3 in Caroni to 5.4±1.0 in St. Andrew/St. David and was significantly different (P=0.01). The differences in the prevalence of Campylobacter for tub-style plucking (83.3%) and drum plucking (58.3%) and mean counts for medium sale log₁₀ mean±SD (2.6±0.8) and low sale shops (2.2±0.9), as well as for tub-style plucking (2.5±0.8) and drum plucking (1.8±0.9) were statistically significant (P<0.05). The prevalence of E. coli, the prevalence and mean count of staphylococci were significantly higher (P<0.05) in operations where tub-style plucking was used compared with drum plucking. Since the quality of the in-process rinse water would affect the quality of the final product, it is recommended that the use of running water or a high frequency of changing the rinse water be implemented in these shops.     

Evaluation of buoyant density centrifugation as a sample preparation method for NASBA-ELGA detection ofCampylobacter jejuni in foods

Buoyant densities of four Campylobacter jejuni strains were in the range of 1·084–1·087 g ml⁻¹. Milk (3% fat) and chicken skin homogenates had buoyant densities beneath 1·033 g ml⁻¹. Discontinuous buoyant density centrifugation (BDC) in 40% Standard Isotonic BactXTractor medium respectively succeeded in recovering C. jejuni (5×10³–5×10⁴cfu ml⁻¹) from spiked milk (3% fat) and chicken skin. NASBA–ELGA detection of C. jejuni (5×10²–5×10³cfu ml⁻¹) in 12 different food samples using four different sample preparation methods was performed: RNA extraction by heating (filter and non-filter stomacher bag), RNA extraction by BDC (non-filter stomacher bag), RNA extraction by GuSCN–Triton-X-100 lysis and silica-purification (non-filter stomacher bag). BDC succeeded in overcoming inhibition of the amplification reaction except for one of the soft cheese samples. It was noticed that for chicken skin, chicken meat, pork, chicken sausage, turkey meat, milk (3% fat) and skimmed milk a simple heat treatment at 96°C without any additional precautions to prevent inhibition accomplished NASBA–ELGA detection of the pathogen. The use of a filter stomacher bag improved the quality of the NASBA–ELGA detection signal for beef but did not prevent inhibition of the amplification reaction in the cases of ground beef, prepared minced meat, soft cheese and hard cheese. The silica purification of RNA (which was used as a control treatment) accomplished NASBA–ELGA detection of C. jejuni for all of the latter food homogenates.     

Impact of high-pressure processing on microbial shelf-life and protein stability of refrigerated soymilk

The effects of pressure (400, 500 and 600 MPa), dwell time (1 and 5 min) and temperature (25 and 75 °C) on microbial quality and protein stability of soymilk during 28 days of storage (4 °C) were evaluated under aerobic and anaerobic conditions. After processing and during storage, there were significant differences in total bacterial count (TBC), numbers of psychrotrophs (PSY) and Enterobacteriaceae (ENT), and protein stability between untreated (control) and pressurized samples (P < 0.05). Pressure applied at an initial temperature of 75 °C resulted in a greater suppression in growth of PSY compared to TBC. No ENT was detected in pressurized samples throughout the storage period tested. Dwell time had no significant effect on log reduction of TBC at 25 or 75 °C (P > 0.05). Pressure at 400 MPa (5 min), 500 and 600 MPa (1 and 5 min) produced 100% sub-lethal injury in surviving bacterial populations irrespective of temperature. After 28 days of refrigerated storage, both aerobic and anaerobic pressurized samples had better or similar stability as the control on day one of storage. Soymilk control samples were spoiled after 7 days whereas pressurization increased soymilk shelf-life by at least 2 weeks. Pressure (600 MPa) at 75 °C for 1 min not only significantly reduced initial microbial populations and increased the microbial shelf-life but also extended the protein stability of soymilk (P < 0.05).     

Effect of lactic acid on Listeria monocytogenes and Edwardsiella tarda attached to catfish skin

This study evaluated the use of lactic acid to decontaminate Listeria monocytogenes andEdwardsiella tarda attached to catfish skin with or without mucus. At the highest inoculum levels (10⁴–10⁵cfu skin⁻¹), lactic acid (0·5–2·0%) exposure for 10 min reduced counts of L. monocytogenes firmly attached to catfish skin by 0·9–>1·9 log₁₀cfu skin⁻¹and cells loosely attached by 2·7–>3·7 logs. Counts of E. tarda firmly attached to catfish skin were reduced by 0·9–>3·0 logs and cells loosely attached by 1·5–>3·5 logs. Overall bacterial numbers of lactic acid-treated cells that were firmly attached to skin with mucus were higher than on skin without mucus. Firmly attached L. monocytogenes was more resistant to lactic acid than was firmly attached E. tarda. Catfish skin mucus decreased the antimicrobial effect of lactic acid against attached L. monocytogenes and E. tarda.     

Bacteriological quality of “doubles” sold by street vendors in Trinidad and the attitudes, knowledge and perceptions of the public about its consumption and health risk

The bacteriological loads of “bara”, “channa”, condiments/spices and ready-to-consume “doubles” sold by street vendors in St. George and Caroni counties of Trinidad were determined. Questionnaires were administered to 100 vendors from both counties to determine which of their practices had effect on the bacteriological quality of the products. In addition, a total of 300 randomly selected members of the public in both counties were interviewed using questionnaires, to assess their attitudes, knowledge and perceptions concerning “doubles” consumption. Of a total of 196 samples each of “bara”, “channa”, condiments/spices and ready-to-eat “doubles” tested, the log₁₀ mean (±SD) total aerobic plate count (TAPC) per g was 4.87±5.51, 5.79±6.61, 6.26±6.77 and 6.88±7.91, respectively. The difference was statistically significant (P<0.05; χ²). Applying a recommended maximum standard of log₁₀ TAPC per g of 5.00, 10 (5.1%), 19 (9.7%), 57 (29.1%) and 78 (39.8%) of 196 samples of “bara”, “channa”, condiments/spices and ready-to-consume “doubles”, respectively, were deemed unfit for human consumption. Vendor practices, which significantly affected the prevalence of unfit “doubles” were re-use of leftover product (P=0.01), wearing of hand gloves (P=0.02), availability of water at sale outlet (P=0.0008) and source of water (P=0.0008). Consumption of “doubles” was more common amongst East Indians (69.2%) than other races (30.8%) (P<0.001) and amongst individuals who were aware that “doubles” can cause food-poisoning (61.4%) compared with individuals who did not have the awareness (36.0%) (P<0.03). Age, gender and occupational status of individuals did not significantly (P>0.05) affect consumption of “doubles”. The high prevalence of ready-to-consume “doubles” assessed unfit for human consumption coupled with a significantly higher proportion of consumers than non-consumers being aware that the product could cause food-poisoning, definitely poses a health risk to consumers of “doubles” in the population. Enforcement of sanitary practices during the preparation and sale of “doubles” should reduce the health risk.     

Development and quality evaluation of cooked buffalo tripe rolls

Cooked buffalo tripe rolls (BTRs) were prepared from a combination of buffalo tripe (75%) and buffalo meat (25%) by using mincing (M-BTR) and blade tenderization (BT-BTR). They were stored at 4 ± 1 °C and studied for various physico-chemical, sensory and microbial qualities. Significantly (P < 0.01) higher pH, shear force value, diameter shrinkage and fat percentage were observed in M-BTR and BT-BTR than control product (100% minced buffalo meat). The product yield and drip loss percentage were significantly (P < 0.01) lower in BT-BTR, whereas drip loss percentage was significantly higher in M-BTR compared to controls and BT-BTR. No significant change was noticed in protein and moisture content between the different products. All physico-chemical parameters and sensory evaluation scores of M-BTR were comparable with control. Significant (P < 0.01) increases were noticed in pH, moisture content, thiobarbituric acid and tyrosine values with increasing storage period, whereas the extract release volume decreased significantly. All microbial counts and sensory evaluation scores were within the acceptable limits until 15 days of storage at 4 ± 1 °C in low-density polyethylene pouches stored aerobically.     

Inhibition of Clostridium sporogenes growth in mascarpone cheese by co-inoculation with Streptococcus thermophilus under conditions of temperature abuse

The ability of a biological control system to inhibit the outgrowth of Clostridium sporogenes spores during storage of mascarpone cheese under temperature-abuse conditions was investigated. Challenge studies were carried out on mascarpone cheese artificially contaminated with spores of C. sporogenes (10 cfu g⁻¹), and with or without the coinoculum of a Streptococcus thermophilus strain (10⁵cfu g⁻¹). During storage at 4, 12, and 25°C, the outgrowth of clostridia spores, the growth of S. thermophilus, and the pH changes were evaluated at 10, 20, 30, and 40 days. In mascarpone cheese stored at 4° and 12°C, S. thermophilus and C. sporogenes did not show any growth. The initial pH (6·14) of the product also remained unchanged. During storage at 25°C S. thermophilus grew up to about 10⁷cfu g⁻¹after 10 days, resulting in a pH decrease of mascarpone cheese to values close to 4·5. The cell number decreased progressively during storage reaching values near to 10¹cfu g⁻¹after 40 days, whereas product acidity remained constant. C. sporogenes, when inoculated alone, also grew at 25°C. The cell number increased to levels of about 10⁷cfu g⁻¹after 20–40 days of storage according to the different mascarpone cheese lots used. No growth was found when C. sporogenes was co-inoculated in mascarpone cheese with S. thermophilus and stored at 25°C. The study on the behaviour of C. sporogenes, known as a non-toxigenic variant of Clostridium botulinum, allowed us to obtain useful information for setting up an effective biological control system to inhibit growth of the toxigenic species as well. The use of an additional barrier, besides refrigerated storage, may help to maintain the safety of mascarpone cheese in the event it was exposed to elevated temperatures.     

Phase transitions of pigskin gelatin

Edible films are flexible thin materials based on biopolymers. It is therefore necessary to know the physicochemical properties of those macromolecules in order to obtain films with desirable characteristics. Dried gelatin is a partially crystalline polymer that exhibits glass and helix–coil transitions. The knowledge of phase properties is important for the choice of the type and concentration of the plasticizer to be utilized to obtain a flexible and easy-to-handle film. The objective of this work is to determine the phase transitions of pigskin gelatin as a function of moisture content in the hygroscopic domain. Pure gelatin was conditioned over different saturated salt solutions at 25°C to allows samples with various moisture content. After equilibrium was established, the samples and an empty pan, as reference, were heated twice between −100 and 250°C at the rate of 5°C/min, in a DSC (TA 2010) after quench-cooling with liquid nitrogen. Gelatin was placed in a pan with a perforated cover, and maintained at 105°C for 24 h in the DSC cell before thermal analyses, to obtain completely dried samples. The glass transition temperature of these samples was found to be 220.2°C. The DSC traces obtained in the first scan, except those conditioned at 11% relative humidity, showed a glass transition followed by two endothermic peaks due to two sol–gel transitions in the gelatin. The plasticizing effect of moisture on Tg was observed in all the samples conditioned by absorption and in the second scan with the samples conditioned by desorption. This behavior was well represented by the Gordon and Taylor model, with κ=5.26 and R²=0.96. Also, a plasticizing effect of moisture over the sol–gel transitions was observed. The Flory-Huggins model was applied to experimental data with: χ₁=1.94 and R²=0.999, for the first peak Tm, and χ₁=1.90 and R²=0.989, for the second peak.     

The incidence and antibiotic resistance profiles of Salmonella spp. on Irish retail meat products

Irish retail meat (n=74) and poultry samples (n=106) were tested for the presence of naturally occurring Salmonella spp. The pathogen was detected in 28 poultry (n=106), two pork (n=22) and one cooked meat samples (n=20) examined. Salmonella was not isolated from minced beef or lamb samples tested. Initial counts on samples ranged from 0 to log₁₀2·5 cfu g⁻¹. The most commonly isolated serotype was S. bredeney accounting for 48·4%, followed by S. kentucky (35·5%) and S. enteritidis (6·5%). Salmonella spp. (n=31) isolated from food products were also examined for antibiotic resistance. A total of 155 strains (five strains from each isolate) were tested for resistance to 26 antibiotics using the Bauer method. The percentage of samples showing antibiotic resistance amongstSalmonella isolates were as follows: Riampicin (100%); Tetracycline (92·92%); Oxytetracycline (86·26%); Sulphamethoxazole (86·25%) and Streptomycin (80·92%).     

The antimicrobial effects of long-wave ultra-violet light and furocoumarins on some micro-organisms that occur in cheese brines

Listeria monocytogenes and Escherichia coli O157:H7 have been reported to survive in the brines used to store cheeses like Feta or Teleme, but such brines cannot be heat sterilized as the yeasts and lactic acid bacteria essential for normal cheese maturation are present as well. Long-wave UV light (UVA365 nm), acting in conjunction with photosensitizing compounds (e.g. furocoumarins like psoralen) might have more limited microbiocidal properties, so that, perhaps, pathogens could be eliminated from the cheese brine but not the desired yeasts and lactic acid bateria. In laboratory trials, UVA (intensity 45 W m⁻², exposure time 60 s) with psoralen (5 mg l⁻¹) was active againstListeria innocua —chosen to mimic the behaviour of L. monocytogenes—Escherichia coli O157:H7 andStaphylococcus aureus in a physiologically neutral solution, but E. coli O157:H7 (99% reduction in viable cell count) and L. innocua (99·8% reduction) were slightly less sensitive than S. aureus (99·99% reduction). Yeasts from Feta cheese brines were less affected by the same UVA/furocoumarin system—Debaryomyces hansenii (97·5% reduction) and Yarrowia lipolytica (82·7% reduction), as were typical lactic acid bacteria, namely Lactobacillus paracasei subsp. paracasei (97·8% reduction) and Lactobacillus plantarum (91·9% reduction). A UVA exposure time of 100 s with psoralen (5 mg l⁻¹) was lethal to the ‘pathogens’ but, against the desirable species, onlyYarrowia lipolytica (97·4% reduction) readily survived the same treatment. It was concluded that the UVA/furocoumarin system was microbicidal but not, at least in the form under test, sufficiently selective in its action for use with cheese brines where certain of the microfloras need to be retained.     

The influence of seasonal temperatures on meat quality characteristics of hot-boned, m. psoas major and minor, from goats and sheep

Samples of psoas major and minor muscles were randomly collected weekly from 203 (99 hot and 104 cool seasons) Omani goats, 215 (106 hot and 109 cool seasons) Omani sheep, 212 (104 hot and 108 cool seasons) Somali goats, 242 (127 hot and 115 cool seasons) Somali sheep and 211 (110 hot and 101 cool seasons) Australian Merino sheep slaughtered at the Central Slaughterhouse in Oman to investigate the effect of season on meat quality. The collection period was during November 2004–October 2005 and divided into two seasons according to ambient temperatures and relative humidity. These were termed: cool season (average temperature of 21 °C and 59% relative humidity and hot season (average temperature of 35 °C and 47% relative humidity). Muscles collected during the hot season had significantly (P < 0.05) higher ultimate pH values (5.78) than those collected during the cool season (5.65). Myofibrillar fragmentation index was significantly (P < 0.05) higher for hot season samples (86.88%) than for cool season samples (85.59%). Expressed juice was significantly (P < 0.05) higher for cool season samples (36.84) than for hot season samples (35.74). Goat meat from the hot seasonal group was significantly (P < 0.05) darker than the cold season group based on L* (37.6 vs. 39.6), a* (20.0 vs. 23.3) and b* (3.6 vs. 4.2) colour measurements. These results indicated that high ambient temperatures had caused an increase in muscle ultimate pH leading to significant effects on meat quality.     

The triacylglycerol preparation of conjugated linoleic acid reduces lipid oxidation in irradiated, cooked ground beef patties

It is proposed that conjugated linoleic acid (CLA) would depress the lipid oxidation caused by irradiation of cooked, aerobically stored ground beef patties. The free fatty acid (FFA–CLA) and triacylglycerol (TAG–CLA) preparations of CLA were added at 0%, 1%, 2%, or 4% during the grinding process. Patties were irradiated at 1.5–2.0 kGy and frozen at −20 °C. Subsequently, the patties were tempered to 4 °C, cooked to 70 °C and held at 4 °C for 7 d. Enrichment of ground beef with CLA increased the cis-9,trans-11 and CLA trans-10,cis-12 CLA isomers in ground beef patties, even after cooking. Weight loss (P = 0.03) and percentage fat (P = 0.05) were higher in irradiated beef patties than in control patties. Irradiation decreased the concentration of α-linolenic acid (18:3n − 3) in the ground beef by over 60% (P = 0.07), whereas thiobarbituric acid reactive substances (TBARS) values were higher (P = 0.004) in irradiated beef patties than in control patties. The 1% concentration of added TAG–CLA reduced TBARS in irradiated ground beef patties, whereas 2% and 4% FFA–CLA depressed TBARS (CLA type × percentage interaction P = 0.04). Irradiation increased the cardboard and painty aromatic attributes (P  0.05), and FFA–CLA preparation increased the painty aromatic attribute and afterburn aftertaste, but these effects were not observed with the TAG–CLA preparation (CLA type × treatment interaction P < 0.04). Adding 1% TAG–CLA to ground beef during grinding can reduce lipid oxidation in irradiated, cooked ground beef patties without the negative aftertastes associated with the FFA–CLA preparation.