EFFECT OF ATTACHMENT TIME FOLLOWED BY CHLORINE WASHING ON THE SURVIVAL OF INOCULATED LISTERIA MONOCYTOGENES ON TOMATOES AND SPINACH

ABSTRACT

The effect of attachment time (30 min, 24, 48 and 72 h) followed by chlorine washing (200 ppm) on the survival of inoculated Listeria monocytogenes on the surface and subsurface of tomatoes and spinach were studied. The work was done to determine the efficacy of chlorine to decontaminate surface and subsurface pathogens that may have come into contact with produce during preharvest. Tomatoes and spinach leaves were inoculated with a 6 log cfu/mL 18 h culture of L. monocytogenes ATCC 7644 (LM) on the surface and subsurface and incubated at 20C for either, 30 min, 24, 48 or 72 h. LM attached and survived on the surface and subsurface structures of both control and chlorine‐washed vegetables after each attachment time, up to 72 h. Higher levels of LM attachment and survival was however noticed on the subsurface structures. Chlorine had a greater effect on the LM on the surface structures compared to those in the subsurface structures, possibly because chlorine was not able to access the subsurface structures where the pathogens were located. Chlorine was not effective in totally inactivating the surface LM on spinach and tomato. This research indicated that LM could attach to both surface and subsurface structures of both tomatoes and spinach within 30 min and that even after 72 h, it still remained viable.

PRACTICAL APPLICATIONS

This study will inform the produce industry on the ability of L. monocytogenes ATCC 7644 to attach and grow on the surface and subsurface structures of tomato and spinach during post harvest. More attention should be given to this phenomenon because the use of fresh or minimally processed fruits and vegetables are recommended as part of a healthy diet. It also indicates that minimal processors should avoid using vegetables with wounds since L. monocytogenes attached more to the subsurfaces structures of the produce. Moreover, the use of sanitizers such as chlorine is less effective under these conditions. It has also brought to light the inability of chlorine to effectively decontaminate pathogens making it imperative for the produce industry to implement Hazard Analysis Critical Control Point, Good Agricultural Practice and Good Hygiene Practice.     

Centralized bulk pre-packaging of pork retail cuts

The performance of a commercial bulk pre-packaging system was compared to a laboratory system in terms of quality attributes such as microbiology, colour, odour, and acceptability of PVC-overwrapped pork retail cuts. A similar retail shelf life of 3 days was achieved after either 0, 7, or 14 days bulk storage with both packaging systems. The colour of the samples from both systems was pale to normal during the trial. After 14 days storage the samples from both systems were still acceptable and had a fresh meat odour. The results show that the commercial system (Cryovac GFII, Darex Africa (Pty) Ltd, Kemptonpark RSA) may be applied successfully for 100% CO₂ bulk packaging of PVC-overwrapped pork retail cuts.     

Parametric downconversion vs optical parametric amplification: a comparison of their quantum statistics

The extent to which the intense light generated by an unseeded, high-gain optical parametric amplifier retains the desired quantum statistical properties of the individual photon pairs generated by spontaneous parametric downconversion is analysed. It is shown that certain but not all of these properties are retained, with important implications for applications of quantum optics.     

Microbiological quality of shredded Cheddar cheese packaged in modified atmospheres

The microbiological quality of shredded Cheddar cheese packaged in different modified atmospheres with and without oxygen scavengers included in the packaging film was studied with the aim of determining how modified air packaging would affect the mould species present on shredded Cheddar cheese. The film with oxygen scavengers was more effective than the control film against mould growth, whereas the 73% CO₂/27% N₂ atmosphere resulted in the cheese with the best microbiological qualities. The three modified atmospheres and the packaging film influenced the mycoflora of shredded Cheddar cheese as the mould species isolated initially differed, from those isolated at 16 weeks in the six treatments. This study demonstrated that O₂ scavengers were effective in controlling the growth of moulds on shredded Cheddar cheese.     

SURVIVAL AND GROWTH OF ACID ADAPTED ESCHERICHIA COLI STRAINS IN BROTH AT DIFFERENT PH LEVELS

Acid resistance of Escherichia coli O157:H7 strain UT 10 and Escherichia coli ATCC 25922 was determined in brain heart infusion broth at pH 7.4, 4.5 and 2.5. Variations due to acid stress in the counts of both strains were also determined. Acid adaptation enhanced the survival of both strains at pH 4.5, but neither strain could survive after 4 h at pH 2.5. At optimum growth conditions (pH 7.4), E. coli ATCC 25922 exhibited increased viability over E. coli UT 10. At pH 4.5, E. coli UT 10 was more tolerant to low pH than E. coli ATCC 25922. An increase in saturated fatty acids of both AA strains was observed, indicating the importance of lipid modification in enhancing survival at low pH. The results of this study indicated that the food industry should therefore adapt their processing/preservation procedures by taking the most acid tolerant pathogenic E. coli strains into consideration in order to ensure the safety of their products.

PRACTICAL APPLICATIONS

The study will enlighten the industry on the survival of acid adapted pathogens such as E. coli O157:H7 at low pH. It also indicates that current measures used in the preservation of low pH foods can trigger acid adaptation. For this reason, the effectiveness of current preservation measures in controlling foodborne pathogens should be reassessed. The importance of using cells adapted to different pH values in food challenge studies is also highlighted. Since fermented foods, which are generally regarded as safe, have been implicated in foodborne disease outbreaks, more attention should be given to the prevention of contamination of foods with pathogens such as E. coli O157:H7 since they may survive in low pH foods and cause disease.