LIPID OXIDATION OF FISH LIVER OIL AS AFFECTED BY LIGHT, ANTIOXIDANTS AND TEMPERATURE

Crude and refined Mackerel liver oil with or without antioxidants were stored in artificial light or in darkness to assess the effects of photo-oxidative stress on the stability of oils and the antioxidant activity of butylated hydroxy-anisole (BHA) and tert-butylhydroquinone (TBHQ) on those oils. Changes in peroxide value (PV) and thiobarbituric acid value (TBA) were monitored over 18 days at 5C and 40C. The results showed that visible light exposure played the most critical role in the acceleration of fish liver oil oxidation. The addition of antioxidants showed a significant effect in retarding oxidation with TBHQ more effective than BHA. The oxidation rate of the oils decreased in the following order: control with light > control + BHA with light > control without light > control + TBHQ with light > control + BHA without light > control + TBHQ without light. Higher PV and TBA values were observed in refined oils versus crude oils. Increasing storage temperature accelerated the oxidation of only refined oils in light.     

INHIBITION OF LISTERIA MONOCYTOGENES AND OTHER BACTERIA BY A PLANT ESSENTIAL OIL (DMC) IN SPANISH SOFT CHEESE

The inhibitory effects of a mixture of plant essential oils (DMC) were tested in culture media and Spanish soft cheese. The minimum inhibitory concentration (MIC) was determined in plate count agar and tryptose broth, with pH adjusted to 6.5, against Listeria monocytogenes, Listeria innocua, Staphylococcus aureus, Lactobacillus brevis, Micrococcus luteus, Salmonella cholerasuis, Escherichia coli O157:H7 , Enterobacter cloacae, Pseudomonas aeruginosa and Pseudomonas fluorescens. The mixture of essential oils inhibited all Gram-positive bacteria tested at 40 ppm, but higher concentrations were needed to inhibit Gram-negative bacteria, and no inhibitory effect was found against Ps. fluorescens. The effects of DMC against L. monocytogenes and E. coli O157:H7 were evaluated in Spanish soft cheese (Queso Fresco, pH=6.5) stored at 7C. DMC had a bacteriostatic effect against L. monocytogenes at concentrations of 2500 ppm but was ineffective to control the growth of E. coli O157:H7 .     

SURVIVAL AND GROWTH OF LISTERIA MONOCTYOGENES ON BEEF TISSUE SURFACES AS AFFECTED BY SIMULATED PROCESSING CONDITIONS

The numbers of Listeria monocytogenes Scott A were followed on experimentally inoculated lean and fat beef tissue handled under a variety of simulated storage conditions at 5°C, including high and low moisture storage, simulated spray chilling, and vacuum packaging. Under high moisture storage conditions, L. monocytogenes increased approximately 3 log cycles on both lean and fat tissue after 21 days. Similar growth was observed with vacuum packaged lean tissue. Under dry storage conditions, the population decreased approximately 2 logs during the first 14 to 21 days, but the counts remained constant for 42 days. Simulated spray chilling did not affect the growth or survival of L. monocytogenes on either lean or fat tissue .     

INCIDENCE, SURVIVAL AND GROWTH OF LISTERIA MONOCYTOGENES IN READY-TO-USE MIXED VEGETABLE SALADS IN SPAIN

A total of 116 commercial samples of mixed vegetable salads, packaged in plastic bags, were examined for the presence of Listeria monocytogenes during storage at 4C. Commercial products, belonging to 4 different batches, were sampled over a period of one year and stored for 300 h at 4C in the laboratory. Different sets of enrichment and plating media were used to recover organisms, with subsequent identification by Pasco System and serotyping techniques. Out of a total of 70 control (noninoculated) samples, 21 (30%) were observed to contain Listeria monocytogenes, belonging to serotypes 3a and 3b. A study of salad inoculated with 10³ cfu/g Listeria monocytogenes showed that the initial inoculum increased less than tenfold over the experimental period (300 h). During storage of the product, CO₂ reached nearly 30% and O₂ was no longer detected at 60 h, due to the respiration of the vegetables. The pH inside the packages remained around 6. The specific growth rate in the salad was calculated using the Dmodel Program, which gave a rate of 0.003h^-1, a lower figure than that reported by other authors. This study found growth patterns different to those previously reported for salads with separate ingredients. Our results agree with previous reports that modified atmosphere does not greatly inhibit Listeria monocytogenes. This reflects a lower specific growth rate in this food compared to media, and illustrates the difficulty of validating, in complex food systems, mathematic models based on culture media .     

INHIBITION OF LISTERIA MONOCYTOGENES BY LIQUID SMOKE AND ISOEUGENOL, A PHENOLIC COMPONENT FOUND IN SMOKE

The behavior of Listeria monocytogenes was monitored in wiener exudate or Tryptose Broth (TB) in the presence of liquid smoke (CharSol Supreme) or smoke ingredients (i.e., phenols and acetic acid). L. monocytogenes grew in untreated exudate held at 25C, but was inactivated in the presence of 0.2 and 0.6% liq'iid smoke (D-values of 36 and 4.5 h, respectively). Of 11 individual phenols tested, only isoeugenol exhibited antilisterial activity in TB at 37C. Growth of the pathogen in the presence of isoeugenol (100 ppm) was inhibited to a greater extent in TB acidified with acetic acid to pH 5.8 compared to pH 7.0. These studies confirm the potent antilisterial activity of liquid smoke and establish the potential of isoeugenol for controlling the growth of L. monocytogenes in certain processed meats .     

PROBABILITY OF GROWTH OF CLOSTRIDIUM BOTULINUM AS AFFECTED BY STRAIN, CELL AND SEROLOGIC TYPE, INOCULUM SIZE AND TEMPERATURE AND TIME OF INCUBATION IN A MODEL BROTH SYSTEM

Using factorial design experiments and MPN methodology, we evaluated the probability (P) of growth initiation of 17 individual proteolytic (A,B,F) and non-proteolytic (B,E,F) C. botulinum strains (spores and cells) in a model broth as it is affected by strain, cell and serologic type, inoculum size (10⁰–10⁶), temperature (4–47°C) and time of incubation (up to 28 days). Regression analyses of the P of growth of the most capable strains for all conditions tested, allowed the development of quantitative equations relating P of growth initiation by one spore or cell to the variables studied. The close agreement between observed and predicted Ps demonstrated the potential usefullness of the modeling approach in studying microbial interactions with food environments.     

SEMIQUANTITATIVE ANALYSIS BY THIN-LAYER CHROMATOGRAPHY (TLC) OF BIOGENIC AMINES IN DRIED, SALTED AND CANNED FISH PRODUCTS

A thin‐layer chromatography (TLC) method was tested using dried, salted and canned fish products for the identification and semiquantification of biogenic amines: histamine, cadaverine, putrescine, tryptamine, and tyramine. The solvent system was acetone: NH₄OH (95:5) on a precoated silica gel plate, and the developing reagent was 0.2% ninhydrin with 2% acetic acid in methanol. The TLC results were compared with liquid chromatography (LC). For LC levels between 6–9 mg%, TLC gave close to 5 mg%, and for samples with biogenic amines of 10–60 mg%, LC and TLC results are more similar to each other. In case of upper values (> 60 mg%) other methods have to be used or sample dilution is suggested. The proposed TLC was suitable to estimate: cadaverine (5‐40 mg%), and tyramine (10‐20 mg%) in dried salted, and cadaverine (10‐20 mg%), tryptamine (5‐10 mg%), and histamine (10‐60 mg%) in canned fish. With this TLC methodology is possible to screen samples with these amines levels, for routine quality control in laboratories.     

VARIATIONS IN THE SENSORY PROPERTIES OF BEEF AS AFFECTED BY SEX CONDITION, MUSCLE AND POSTMORTEM AGING

The longissimus (LD), biceps femoris (BF), psoas major (PM), semitendinosus (ST) and semimembranosus (SM) muscles from 8 bulls and 8 steers were removed from sides at 1 day postmortem and 7 days postmortem. The sensory and textural properties were evaluated and related to meat composition, meat characteristics and mineral content. Muscles were ranked in order of increasing sensory tenderness ratings: PM>ST>LD>BF>SM. The amount of intramuscular fat was found to be positively correlated to sensory panel tenderness ratings. The amount of intramuscular collagen was found to be negatively correlated to sensory panel tenderness ratings. Muscle concentrations of copper, iron and manganese were found to be positively correlated to tenderness ratings.