Flavour precursor development in Cheddar cheese due to lactococcal starters and the presence and lysis of Lactobacillus helveticus.

The rapid release of intracellular enzymes due to autolysis of lactic acid bacteria in the cheese matrix has been shown to accelerate cheese ripening. The objective of this work was to investigate the evolution of the flavour precursors, individual free amino acids (FAAs), free fatty acids (FFAs) and volatile compounds that contribute to the sensory profiles of cheeses at 2, 6 and 8 months of ripening in Cheddar cheese manufactured using starter systems which varied with respect to their autolytic properties. Starter system A contained a blend of two commercial Lactococcus lactis strains (223 and 227) which had a low level of autolysis. System B was identical to A but also included a highly autolytic strain of Lactobacillus helveticus (DPC4571). System C contained only strain DPC4571. Levels of all individual FAAs were elevated in cheeses B and C relative to A after 2 months of ripening. By 8 months of ripening the main FAA were glutamate, leucine, lysine, serine, proline and valine. Levels of C_6:0, C_8:0, C_12:0 and C_18:0 fatty acids did not vary greatly over ripening, while levels of C_4:0, C_10:0, C_14:0, C_16:0 and C_18:1 were elevated in cheeses B and C. Principal component analysis of the headspace volatiles separated cheese A from cheeses B and C. Cheeses B and C had highest levels of dimethyl disulphide, carbon sulphide, heptanal, dimethyl sulphide, ethyl butanoate, 2-butanone, and 2-methyl butanal and were described as having a ‘caramel’ odour and ‘sweet’, ‘acidic’ and ‘musty’ flavour. Cheese A had highest levels of 2-butanol, 2-pentanone, 2-heptanone, 1-hexanol and heptanal and was described as having a ‘sweaty/ sour’ odour and ‘soapy’, ‘bitter’ and ‘mouldy’ flavour. The results highlight the impact of starter lactococci on flavour precursor development and the positive effect of Lb. helveticus and the lysis of this strain on enhancing levels of substrate and flavour precursors early during ripening resulting in early flavour development.     

The bacteriology of fresh and spoiling Lake Victorian Nile perch (Lates niloticus)

A total of 177 bacterial cultures isolated from Lake Victorian Nile Perch (Lates niloticus) were investigated. The flora on newly caught Nile perch consisted of organisms belonging to the genera Moraxella, Alcaligenes, Acinetobacter, Pseudomonas, Aeromonas, Micrococcus and other Gram-positive organisms. 39% were identified as Gram-positive species and 61% were negative in the Gram-reaction. Three cultures out of 53 investigated caused weak rotten off-odours in sterile fish broth and one culture, an Aeromonas spp. produced strong rotten, fishy, hydrogen sulphide off-odours. From Nile perch spoiled at ambient temperature, 15 of the 42 strains isolated caused rotten, fishy, hydrogen sulphide off-odours. These specific spoilage bacteria were all identified as Aeromonas and all reduced trimethylamine oxide to trimethylamine and produced hydrogen sulphide. From spoiled iced Nile perch, 74 out of 82 (90%) of the bacteria isolated were identified as Pseudomonas. A small proportion of these (13 out of 74) produced off-odours in sterile fish broth resembling the spoiling fish. These specific spoilers could not be separated from the non-spoilers based on biochemical activities used in classical taxonomy. While the Pseudomonas spp. isolated did not produce trimethylamine or H₂S, a few of the remaining isolates (two Shewanella putrefaciens and five Aeromonas spp.) did produce these compounds. The role of Shewanella putrefaciens in the iced spoilage of Nile perch was, however, insignificant, since they only very late in the storage reached numbers where their spoilage could be detected.     

Isolation and identification of the volatile sulphides produced during chill-storage of north sea cod (gadus morhua)

The development of unpleasant off-odours and flavours during the chill-storage of trawl caught cod has been well documented¹. Certain of these off-odours have been described as sulphidy. The data reported here suggest that hydrogen sulphide methyl mercaptan and dimethyl sulphide are responsible for the sulphidy off-odours associated with stale chill-stored cod.     

Gas‐producing and spoilage potential of Enterobacteriaceae and lactic acid bacteria isolated from chilled vacuum‐packaged beef

This study aimed to enumerate and identify lactic acid bacteria and Enterobacteriaceae from spoiled and nonspoiled chilled vacuum‐packaged beef and determine their potential to cause blown pack spoilage. These microbial groups were also enumerated in nonspoiled samples and detected in abattoir samples. The potential of isolates to cause ‘blown pack’ spoilage of vacuum‐packaged beef stored at chilled temperature (4 °C) and abuse temperature (15 °C) was investigated. Populations of lactic acid bacteria in exudate of spoiled and nonspoiled samples were not significantly different (P > 0.05), whereas the number of lactic acid bacteria on the surface was significantly higher (P < 0.05) in spoiled samples as compared to nonspoiled samples. The population of Enterobacteriaceae species in exudate and on the surface of samples were significantly higher (P < 0.05) in spoiled packs in comparison with nonspoiled packs. Results of the deterioration potential showed that ‘blown pack’ spoilage was noticeable after 7 days at 15 °C and after 6 weeks at 4 °C for samples inoculated with Hafnia alvei.     

Spoilage potential and sensory profile associated with bacteria isolated from cold-smoked salmon

Off-odours/flavours associated with cold-smoked salmon spoilage are due to the activity of microflora. This study evaluated the spoilage potential of nine bacterial groups (Shewanella putrefaciens, Brochothrix thermosphacta, Aeromonas spp., Lactobacillus alimentarius, Lactobacillus sake,Lactobacillus farciminis, Carnobacterium piscicola, Photobacterium phosphoreum and Serratia liquefaciens) isolated from cold-smoked salmon. Five different isolates from each group were inoculated into sterile cold-smoked salmon blocks, and chemical and sensory changes were studied after five weeks of storage in vacuum packs at 6°C. Bacterial growth was monitored weekly during the storage period. A sensory profile was assigned to each group. Principal component analysis allowed some bacterial species to be characterised by a specific odour, and correspondence factorial analysis discriminated among the species according to their spoilage potential. The bacteria mainly responsible for spoilage were L. sake, L. farciminis and B. thermosphacta, which produced sulphurous, acidic and rancid off-odours respectively. Some strains of S. liquefaciens produced rubbery, cheesy or acidic off-odours. Some P. phosphoreum isolates were characterised by an acidic effect.     

The effect of ageing treatment on the microbiology and storage characteristics of beef in modified atmosphere packs containing 25% CO(2) plus 75% O(2)

The influence of different ageing treatments (1 week hung in air; 1 and 3 weeks in vacuum packs) on the microbiological, colour, odour and rancidity characteristics of beef loin steaks during storage at 1° and 6°C in modified atmosphere packs (MA) containing 25% CO(2) + 75% O(2) was determined. Steaks from the 3-week ageing treatment discoloured more rapidly and developed off-odours sooner than those from the 1-week ageing treatments at both storage temperatures. Evidence of rancidity, using the thiobarbituric acid assay, was only observed in the 3-week aged steaks stored at 6°C. The poorer storage stability of the 3-week aged steaks was explained by higher initial levels of bacteria resulting from growth during ageing. Leuconostocs were the dominant bacteria on most MA samples following storage and appeared the main cause of spoilage, although B. thermosphacta and Pseudomonas spp may have contributed to off-odours on samples aged hung and stored at 6°C.     

The storage life of beef and pork packaged in an atmosphere with low carbon monoxide and high carbon dioxide

Ground beef, beef loin steaks and pork chops were packaged in modified atmospheres of 0.4% CO/60% CO(2)/40% N(2) and 70% O(2)/30% CO(2). In addition ground beef was packaged in clipped chub packs, beef loin steaks were vacuum packaged, and pork chops were packaged in an atmosphere of 60% CO(2)/40% N(2) with each pack containing an O(2) absorber. The packs were stored in the dark at 4 or 8°C for up to 21 days. Meat in 0.4% CO/60% CO(2)/40% N(2) had a stable bright red colour that lasted beyond the time of spoilage. The storage lives in this gas mixture at 4°C, as limited by off-odours, were 11, 14 and 21 days for ground beef, beef loin steaks and pork chops, respectively. The 70% O(2)/30% CO(2) atmosphere resulted in an initially bright red to red colour of the meat, but the colour was unstable and off-odours developed rapidly. The off-odours probably were caused by Brochothrix thermosphacta, which grew in all meat types, or by pseudomonads in ground beef. Meat stored in chub packs, vacuum packs or 60% CO(2)/40% N(2) with an O(2) absorber developed off-odours and microflora similar to those of meat in 0.4% CO/60% CO(2)/40% N(2), but with less acceptable appearances. These results show that a low CO/high CO(2) atmosphere is effective for preserving retail-ready meat. ?     

Influence of particular breed on meat quality parameters, sensory characteristics, and volatile components

The present study demonstrates the effect of breed on meat quality, sensory characteristics, and flavor components. Longissimus dorsi of ‘Angus’ and ‘Hanwoo’ breeds aged for 29 days were used for investigation of the aforementioned characteristics. Our results revealed that intramuscular fat (IMF) was higher for ‘Hanwoo’ beef as compared to ‘Angus’ (p<0.05). A total of 62 flavor compounds were identified, the amount of these compounds varied significantly among ‘Hanwoo’ and ‘Angus’ cattle. The ‘Hanwoo’ beef produced high level of pleasant flavor compounds such as octanal and nonanal derived from oleic acid. Also, the amount of unpleasant flavor compounds such as benzaldehyde derived from linolenic acid was lower than that of ‘Angus’ (p<0.05). Moreover, the tenderness, juiciness, and flavor scores were also higher for ‘Hanwoo’ beef when compared with ‘Angus’. In conclusion, our results clearly demonstrated that the breed had a potential impact on the meat quality, sensory, and volatile components.     

The bacteriology of fresh and spoiling Lake Victorian Nile perch (Lates niloticus)

A total of 177 bacterial cultures isolated from Lake Victorian Nile Perch (Lates niloticus) were investigated. The flora on newly caught Nile perch consisted of organisms belonging to the genera Moraxella, Alcaligenes, Acinetobacter, Pseudomonas, Aeromonas, Micrococcus and other Gram-positive organisms. 39% were identified as Gram-positive species and 61% were negative in the Gram-reaction. Three cultures out of 53 investigated caused weak rotten off-odours in sterile fish broth and one culture, an Aeromonas spp. produced strong rotten, fishy, hydrogen sulphide off-odours. From Nile perch spoiled at ambient temperature, 15 of the 42 strains isolated caused rotten, fishy, hydrogen sulphide off-odours. These specific spoilage bacteria were all identified as Aeromonas and all reduced trimethylamine oxide to trimethylamine and produced hydrogen sulphide. From spoiled iced Nile perch, 74 out of 82 (90%) of the bacteria isolated were identified as Pseudomonas. A small proportion of these (13 out of 74) produced off-odours in sterile fish broth resembling the spoiling fish. These specific spoilers could not be separated from the non-spoilers based on biochemical activities used in classical taxonomy. While the Pseudomonas spp. isolated did not produce trimethylamine or H₂S, a few of the remaining isolates (two Shewanella putrefaciens and five Aeromonas spp.) did produce these compounds. The role of Shewanella putrefaciens in the iced spoilage of Nile perch was, however, insignificant, since they only very late in the storage reached numbers where their spoilage could be detected.     

THE ORIGIN AND OCCURRENCE OF VOLATILE SULPHUR COMPOUNDS IN BRITISH ALES AND LAGERS

A flame-photometric sulphur detector was used to identify, measure and determine the sources of the sulphur volatiles produced during the commercial processing of British ale and lager. Dimethyl sulphide was the main sulphur volatile present in malt but traces of hydrogen sulphide, diethyl sulphide, and dimethyl disulphide were also detected. Hops contained hydrogen sulphide, methyl mercaptan, dimethyl sulphide, diethyl sulphide, methional and dimethyl disulphide. Most of this material extracted into commercial worts was driven off during boiling. Brewing yeasts produced only traces of organo-sulphur volatiles both in laboratory fermentations of wort and during the processing of commercial ales and lagers. In contrast, brewery bacteria, particularly wort spoilage organisms, could generate dimethyl sulphide and sometimes traces of other sulphur volatiles in laboratory cultures. t-Butyl mercaptan was the only organo-sulphur volatile detected in significant concentrations during the primary fermentation and conditioning of commercial beers and this was attributable to the activity of wort-spoilage bacteria. Attempts to identify a volatile compound causing ‘sulphury’ smells in beer were unsuccessful but there was some evidence that it might not contain sulphur.     

The impact of packaging system and temperature abuse on the shelf life characteristics of ground beef

New ground beef packaging systems have warranted investigation of their spoilage and quality characteristics. Furthermore, analysis of ground beef spoilage in modified atmosphere packaging (MAP) and stored at abusive temperature is lacking. This research aimed to determine the effect of packaging systems and temperature abuse on the sensory and shelf-life characteristics of ground beef. Ground beef patties were packaged using polyvinyl chloride overwrap (OW), HI-OX MAP (80% O₂, 20% CO₂), LO-OX MAP (30% CO₂, 70% N₂), CO-MAP (0.4% CO, 30% CO₂, 69.6% N₂), or vacuum (VAC) prior to color, odor, biochemical, and microbial analyses over display. CO-MAP exhibited more desirable color and consumer acceptability throughout display. Lean discoloration and odor scores were lower for anaerobic packaging than aerobic packaging. Microbial results mirrored sensory preferences for anaerobic packaging. These results indicate anaerobic packaging extends shelf-life properties and desirable sensory attributes throughout display and temperature abuse.     

Identification of Spoilage Marker Metabolites in Irish Chicken Breast Muscle Using HPLC,GC–MS Coupled with SPME and Traditional Chemical Techniques

The aim of this investigation was to determine the metabolites of spoilage present on the surface of Irish chicken breast muscle in order to identify biomarkers of microbial spoilage and to verify the results of a previous study which suggested that the increase of free amino acids is the main spectral influence factor leading to the near and middle infrared detection of microbial spoilage. Irish-reared chicken breast muscle samples were individually packed and stored at 4 °C for 8 days under aerobic conditions. Microbiological analysis revealed that Pseudomonas spp. and Brochothrix thermosphacta were the predominant organisms (total viable counts (TVC), 4.24, 6.37 and 8.6 colony forming unity (CFU) g⁻¹ for days 0, 4 and 8, respectively, Pseudomonas 3.2, 5.1 and finally, on day 8 7.4 log CFU g⁻¹). Glucose and L-lactate concentrations decreased but the concentration of water-soluble polypeptides and amino acids increased over storage time. HPLC analysis of free amino acids revealed an increase of the total concentration but the composition of the profiles did not change over time. Headspace analysis detected the following volatile compounds: ethanol, acetone, ethyl acetate, methyl benzoate, heptane, C15, C12, methyl ethyl ketone, carbon disulphide, dimethyl sulphide, hexanal, and toluene. Of interest is the fact that detection of sulphides and an increase of ethanol, acetone, and ethyl acetate concentrations occurred from day 4 to 8. The increase in free amino acids throughout storage and the production of volatile compounds after day 4 require further investigation but are selected as potential biomarkers of microbial spoilage as they could be analytically detected before the accepted levels of sensory spoilage detection.     

Effects of meat pH and the initial numbers of spores of Clostridium estertheticum on the development of blown pack spoilage of vacuum‐packaged beef

To understand how the initial numbers of Clostridium estertheticum spores on, and the concentration of glucose in meat affect the development of blown pack spoilage, beef of pH ≤5.6 and of pH ≥5.8 was inoculated with the spores at various numbers, vacuum‐packaged and stored at 2 °C. For beef of pH ≤5.6, the volumes of packs inoculated with ≤10 spores did not change; and packs inoculated with ≥30 spores started swelling after 35 days, and the rate of volume increase increased with increasing number of inoculated spores. For beef of pH ≥5.8, packs inoculated 0, three or ten spores slackened, and packs inoculated with ≥30 spores became swollen at the end of storage, but to a much lesser degree than the corresponding packs of beef of pH ≤5.6. Glucose was reduced by 21 mm and depleted in the rinse fluids from swollen packs of beef of pH ≤5.6 and of pH ≥5.8, respectively.     

Sensory comparison of commercial low and high oxygen modified atmosphere packed sirloin beef steaks

The objectives of this study were to investigate the effects of using commercially low and high modified atmosphere packaging (MAP) on quality characteristics of sirloin beef steaks. Gas mixtures used included 80% O₂:20% CO₂ (O₂80), 70% O₂/30% CO₂ (O₂70) and 50% O₂/30% CO₂:20% N₂ (O₂50) and for commercial packs O₂75 ± 5%, CO₂25 ± 5%, < 5% N₂-(O₂Com). All samples were packed in similar tray and packaging film formats. The experimental parameters monitored included microbiological, physiochemical and sensory measurements (naïve assessors). Microbial counts did not exceed the recommended legal ranges over the storage period. However the numbers of LAB (lactic acid bacteria) were the highest for commercially-packed samples (O₂Com) in comparison to laboratory packaged samples (O₂80, O₂70 and O₂50). Commercially-packaged samples eventually spoiled due to the occurrence of off-flavours and off-odours and subsequently were the least acceptable to sensory naïve assessors.     

Volatile flavour profile of goat meat extracted by three widely used techniques

Three procedures for the isolation of volatiles from grilled goat meat were compared: dynamic headspace entrainment on Tenax TA, simultaneous steam distillation-extraction, and solid-phase microextraction. Headspace entrainment on Tenax TA extracted the highest number of Maillard-derived volatile compounds. Two hundred and three volatile components were identified; 159 are reported for the first time in goat meat. Most of the volatiles detected (155) were lipid oxidation products, such as hydrocarbons, aldehydes, alcohols, ketones, carboxylic acids and esters. Forty-eight Maillard-derived compounds were identified, comprising pyrazines, pyrroles, thiophenes, furanthiol derivatives, alkyl and alicyclic sulphides, pyridines, and thiazoles. Some reported character impact compounds of cooked meat, e.g., 12-methyltridecanal, (E,E)-2,4-decadienal, methional, and dimethyl trisulphide were identified in the volatile profile of goat meat, together with a series of C₂ to C₅ alkylformylcyclopentenes, which have been reported in cooked chicken, pork, beef and lamb, as being important for the characteristic flavour impression of different animal species.     

Effects of vacuum,modified atmospheres and storage temperature on the microbial flora of packaged beef

The flora growing on beef stored in vacuum and 100% CO₂at 2 or 6°C (experiment 1) and in vacuum and different mixtures of CO₂and N₂at −1 or 2°C (experiment 2) was determined in two separate experiments. Both a high concentration of CO₂and a low storage temperature inhibited bacterial growth, especially of the spoilage bacteria pseudomonads andBrochothrix thermosphacta. No packaging conditions gave growth of coliforms. High numbers (log₈5–6) of lactic acid bacteria after storage in vacuum or gas packs, inhibited growth of pseudomonads andB. thermosphactaduring subsequent storage under retail conditions. Samples of the lactic acid bacteria obtained in experiment 2 were identified by genus-specific rRNA probes. Leuconostocs dominated in vacuum and CO₂packs at both temperatures, whereas carnobacteria dominated in N₂at −1°C.     

Volatile organic compounds in milk and mozzarella: Comparison between two different farming systems

The volatile organic compounds (VOC) and sensory characteristics of milk and mozzarella from farms with different rearing systems were compared. Milk samples were taken from 9 intensive (INT) and 13 semi-extensive farms (EXT) in the grazing period. VOC were analysed by SPME-GC-MS, and sensory evaluation was carried out by paired comparison. The volatile profiles of milks were significantly different: acetone and isopropyl alcohol characterised INT milk, pentanoic and decanoic acids, hexanal, ethylacetate, toluene, dimethyl sulphide characterised milk from EXT farms. Also mozzarella was discriminated by VOC, but only a few compounds derived from milk. Based on sensory evaluation, milk from the two sources can be distinguished by odour and colour, mozzarella only by colour. It was hypothesised that heating during the stretching phase caused volatilisation of many milk volatile compounds and formation new aroma active molecules that overcame the ‘primary odours’ of milk.     

Detection of poultry spoilage markers from headspace analysis with cryoadsorption on a short alumina PLOT column

A rapid and simple diagnostic tool for the early detection of meat spoilage would be invaluable to ensure meat quality during production, distribution, and retail. Recently, we developed an improved purge and trap method for sampling low volatility, as well as volatile, compounds by applying low temperature collection on short alumina-coated porous layer open tubular (PLOT) columns. This method was applied to the analysis of both fresh and spoiled poultry to identify marker compounds that could be used as indicators for poultry spoilage. Samples of chicken breast were crimp-sealed in individual autosampler vials and maintained at 25 °C for either a day or 2 weeks. Two weeks were sufficient to ensure severe spoilage. The headspace was sampled by cryoadsorption for 10, 20, or 30 min; and the analytes were then separated, identified, and quantified with gas chromatography and mass spectrometry. Six potential markers for poultry spoilage were identified in the headspace of spoiled chicken: dimethyl disulphide; dimethyl trisulphide; phenyl sulphide; methyl thiolacetate; allyl methyl sulphide; and 2,4,6-trimethylpyridine. Additionally, isophorone was detected in the headspace of both the fresh and the spoiled chicken; its origin is unknown but is suspected to come from the packaging. The applicability of this method to detect chicken spoilage in a commercial setting was tested by sampling the air above spoiled chicken breast that was maintained in its original retail packaging, as obtained direct from a commercial vendor, for 2 weeks at 25 °C. Sampling was done via a modified, room-temperature approach with an activated PLOT column and a motorised pipette filler/dispenser. Five of the above compounds were also identified with this approach: dimethyl disulphide; dimethyl trisulphide; phenyl sulphide; 2,4,6-trimethylpyridine; and isophorone.     

Influence of different shrinking temperatures and vacuum conditions on the ability of psychrotrophic Clostridium to cause ‘blown pack’ spoilage in chilled vacuum-packaged beef

This study determined the ability of psychrotrophic Clostridium strains isolated from vacuum-packaged beefs and abattoir environments to cause ‘blown-pack’ spoilage of vacuum-packaged beef stored at 2 and 15 °C. The influence of shrinking temperatures (83, 84 and 87 °C) and vacuum pressure (6 and 9 mbar) on the occurrence of such spoilage as well as the effects of simulated transportation (500 km) on the integrity of packages was determined. At 15 °C and 2 °C, twelve and six strains caused ‘blown-pack’ spoilage, respectively. The combination of vacuum pressure (9 mbar) combined with shrinking temperature (87 °C) retarded the occurrence of spoilage. The simulated transportation under the experimental conditions did not affect the integrity of packages. More studies that assess the factors that may contribute for the occurrence of ‘blown-pack’ spoilage should be performed to avoid the occurrence of such spoilage during its shelf-life.