THE EFFECT OF MONO AND POLYGLYCEROL LAURATE ON SPOILAGE AND PATHOGENIC BACTERIA ASSOCIATED WITH FOODS

The antibacterial activity of monolaurin and triglycerol 1,2 laurate alone and combined with three different chelating agents, at different pH (7, 6, 5.5 and 5), different NaCl concentrations (0.5 to 7%) and various combinations of pH and NaCl concentrations was investigated. The activity of these agents against 16 bacteria (7 Gram-positive and 9 Gram-negative) was studied using the spiral gradient end point test to determine the minimum inhibitory concentration (MIC).
Monolaurin was effective against all the Gram-positive bacteria studied, but was only effective against the Gram-negative bacteria in the presence of EDTA. The inhibition was greatest at low pH and high NaCl concentrations. Concern has been expressed about the use of monolaurin in foods because of its poor solubility. Polyglycerol esters are more soluble and antimicrobial activity of these compounds has been observed. Triglycerol 1,2 laurate exerted some inhibitory effects, but was not as inhibitory as monolaurin even in the presence of the chelating agent and under conditions of low pH and high NaCl levels. Other chelators investigated (monoglyceride citrate and sodium citrate) did not promote the inhibitory effect of monolaurin.     

INHIBITION OF SPOILAGE AND PATHOGENIC BACTERIA ASSOCIATED WITH FOODS BY COMBINATIONS OF ANTIMICROBIAL AGENTS

The antibacterial activity of combinations of lysozyme, monolaurin (ML), triglycerol 1,2 laurate (TGL) and butylated hydroxyanizole (BHA) against 7 Gram-positive and 8 Gram-negative bacteria was studied at different pH, NaCl and EDTA concentrations by the spiral gradient end point test. The inhibitory effect of lysozyme in combination with ML was slightly greater for Gram-positive than for Gram-negative bacteria, but their combined effect was not markedly more inhibitory than ML alone. Lysozyme and TGL together were only inhibitory at low pH and high NaCl concentrations in the presence of EDTA. There was an increase in inhibition when lysozyme and BHA were combined. For Gram-positive bacteria, inhibition by ML and BHA together was more marked than when either was present singly. However, ML decreased the antibacterial activity of BHA against Gram-negative bacteria. Similarly, TGL was antagonistic to BHA action against both Gram-positive and Gram-negative bacteria. In general, the inhibition produced by all combinations was greater as the pH decreased and the NaCl concentration increased, especially in the presence of EDTA .     

THE ANTIFUNGAL ACTIVITY OF BUTYLATED HYDROXYANIZOLE AND LYSOZYME

The antifungal activity of butylated hydroxyanizole (BHA) and lysozyme alone and combined with ethylenediaminetetraacetic acid (EDTA), at different pH (3.5, 5.6 and 7), and different NaCl concentrations (0.5 to 20%) was screened on 12 fungi. The spiral gradient end point test was used to determine the minimum inhibitory concentration (MIC). BHA was effective against all the fungi tested in the range of < 12 to 460 μg/mL. In the presence of equal amounts of EDTA, the inhibitory effect was amplified (inhibition occurred between < 12 to 200 μg/mL). The inhibition was more pronounced at pH 5.6 and at higher NaCl concentrations especially when EDTA was present. Lysozyme alone was only effective against Fusarium graminearum PM162 and Aspergillus ochraceus MM 184, but was inhibitory for all fungi, except Candida lipolitica 1591, in the presence of EDTA. The inhibition was greatest at pH 5.6 and higher NaCl concentrations, especially when EDTA was present. These studies show that the antifungal properties of lysozyme and BHA can be greatly influenced by environmental factors.     

HIGH-PRESSURE DESTRUCTION KINETICS OF SPOILAGE AND PATHOGENIC BACTERIA IN RAW MILK CHEESE

Raw milk cheese samples were individually inoculated with Escherichia coli K‐12, Escherichia coli O157:H7 and Listeria monocytogenes, and subjected to high‐pressure (HP) treatments (200–400 MPa) in both pressure hold and pressure pulse modes. Pressure inactivation of E. coli K‐12 at 350 MPa (5 min) at different temperatures demonstrated a strong temperature effect when temperature was 40C and above. Pressure destruction kinetics and pressure sensitivity were evaluated at 25C (where the temperature effect was minimal). HP treatment generally demonstrated a step‐change pressure pulse effect followed by a holding time destruction that was well described by a first‐order model (R² > 0.90) with higher pressures resulting in a faster rate of microbial reduction (smaller D values). The associated D values at an intermediate pressure of 300 MPa were 4.4, 14.5 and 3.6 min for E. coli K‐12, E. coli O157:H7 and L. monocytogenes, respectively, with E. coli O157:H7 thus demonstrating a higher‐pressure resistance than the other two. The corresponding pressure z_p values were 156, 128 and 82 MPa, and ΔV^≠(Arrhenius volume change of activation) values were ?3.6, ?4.4 and ?7.0 (×10^?5 m³/mole), respectively. Pulse mode treatments showed similar pressure resistance trends to pressure hold mode, but were not effective for E. coli O157:H7.     

THE ACTION OF NISIN ON BEER SPOILAGE LACTIC ACID BACTERIA

Within one minute of adding nisin to suspensions of Lactobacilli clumping of cells was observed. This happened with all the strains assayed, irrespective of their sensitivity or resistance to nisin. There was, however, no evidence of cell lysis. Two different assays for cell viability were used to show that the vast majority of cells of sensitive strains were killed within one minute of contact with nisin. The time needed to kill all the cells depended on the concentrations of both nisin and cells. One strain was more resistant and only 50–60% of the cells died within one minute of nisin addition . Using an ATP-bioluminescence assay it was shown that the addition of nisin to both Lactobacilli and Pediococci caused a rapid fall in intracellular ATP levels, which was reflected in a simultaneous appearance of ATP in the extracellular medium. Actively growing cells lost ATP more rapidly than did those in stationary phase. The amount of intracellular ATP lost was affected both by the concentration of nisin used and the sensitivity of the bacteria. It appears that an initial effect of nisin is to make the cell membrane ‘leaky’ as happens with many antibiotics, yeast killer factors (zymocins), colicins, and other gram-positive bacteriocins .     

FORCE-DEFORMATION CONDITIONS ASSOCIATED WITH THE EVALUATION OF BRITTLENESS AND CRISPNESS IN SELECTED FOODS1

The brittleness and crispness of low moisture foods and the crispness of selected high moisture foods were evaluated both instrumentally by Instron fracture tests and using a sensory panel. Brittleness was evaluated sensorially during the “first bite” whereas crispness was identified at a later stage of mastication. The maximum force at fracture of low moisture foods correlated inversely with panelists’ evaluations of brittleness either in the mouth or using the fingers. Panelists' evaluations of crispness in the mouth correlated reasonably well with the initial linear portion of the Instron force-deformation curves when the sample were supported near their ends and also in their middle regions. The shapes of the force-deformation curves for high moisture foods were very much influenced by the way in which samples were supported in fracture tests, the method of fracture and the Instron cross-head speed.     

EFFECT OF TEMPERATURE ON THE QUALITY OF MUSCLE FOODS

Heating/cooking alters a food's flavor precursors yielding a product with a new pleasing taste. While heating to a high temperature destroys most pathogens thereby enhancing safety, it also alters the food's flavor by changing the precursors and final flavor components of the food. For example, heating meat to above 77C not only destroys many foodborne pathogens but also causes major changes in level of desirable flavor precursors. Many of these changes are due to alteration in activity of endogenous hydrolytic enzymes as well as heatinduced structural and functional changes. Heating induces differential production and distribution of flavor principles and flavor precursors. A secondary effect of temperature on food-flavor is its direct affect on analysis of flavor volatiles, i.e., high purge temperatures bias analysis of flavor components by production of new and destruction of existing components. This paper focuses on the structure/function relationship of meat proteins and their proteolytic products as they relate to meat flavor and end-point cooking temperature. The paper also will address the effect of analytical temperature on meat flavor analysis.     

ROLE OF BACTERIAL PROTEOLYSIS IN THE SPOILAGE OF IRRADIATED FLESH FOODS

Microorganisms (Aeromonas hydrophila, Bacillus megaterium, Pseudomonas marinoglutinosa and Salmonella typhimurium) proliferated equally well when incubated in unirradiated and irradiated fish myofibrillar protein fractions. However, formation of Total Volatile Bases was found to be 50–60% less in the irradiated myofibrillar protein fraction. Proteolysis measured in terms of tyrosine release also registered 50% less in irradiated flesh foods as compared to unirradiated samples under identical conditions. Bacterial protease levels were also evaluated after growth in both control and irradiated fish proteins using hemoglobin as substrate which revealed that there is considerable reduction in protease secretion when the bacterium proliferated in irradiated fish proteins. These results suggest that appreciable delay in the spoilage of irradiated flesh foods could be attributed to low bacterial proteolysis.     

EFFECT OF FREEZING AND PACKAGING METHODS ON SURVIVAL AND BIOCHEMICAL ACTIVITY OF SPOILAGE ORGANISMS ON CHICKEN

SUMMARY— A study was conducted to determine effects of freezing, thawing, and subsequent holding at about 5°C on survival or growth of aerobic bacteria on chicken packaged with various materials. Production of fluorescent pigment, and extracellular proteinase and lipase activities were used as indices of the ability of the organisms to produce spoilage. Growth of bacteria was determined by colony counts. Assays for proteolysis were made by means of a dye binding method; lipolysis of chicken fat was determined by titration of free fatty acids. Fluorescent pigment formation was evaluated by means of a photofluorometer. The influence of availability of oxygen on proteolytic and lipoytic spoilage of poultry was studied after chicken was packaged with materials having high or low oxygen permeability and by vacuum packaging. Fluorescent pigment production, proteofytic and lipolytic spoilage of chicken stored at 5°C was directly related to the availability of oxygen provided by the packaging procedure. Bacterial numbers paralleled increases in biochemical indices of deterioration. Freezing of chicken at −29°C for 35 days followed by defrosting and refrigerated storage increased the proportion of biochemically active psychrophiles on the surface of the meat. Vacuum packaging generally limited amount of spoilage as measured by the criteria specified. When samples were analyzed after alternate freezing and thawing, total aerobic bacterial counts were only slightly different from those on chicken frozen continuously for 22-25 days.     

鼠李糖乳杆菌的培养上清液对几个致病菌和腐败菌的抑菌作用研究

研究了Lactobacillus rhamnosus6013菌在MRS中的最佳培养条件，再用琼脂杯扩散法研究该条件下的培养上清液对金黄色葡萄球菌、枯草杆菌、沙门氏菌、大肠杆菌和EHEC0157的抑菌作用，发现该菌的最佳培养条件为37℃，厌氧培养48h，该培养上清液对金黄色葡萄球菌、枯草杆菌、沙门氏菌有良好的抑菌作用。通过HPLC分析得知，乳酸含量高达4．38g/100mL，该浓度的乳酸溶液对金黄色葡萄球菌有很好的抑菌效果。抑菌效果受1mol／L的NaOH溶液，胰蛋白酶，胃蛋白酶处理的影响，表明发酵上清液的抑菌效果可能是乳酸和蛋白质类物质的综合作用。     

EFFECT OF STORAGE ON THE RHEOLOGICAL BEHAVIOUR OF BABY FOODS

The effect of storage temperature on rheological behaviour of four varieties of baby foods was studied. After 24-month storage at 15C and 25C, the constant A in Weltman model increased significantly for the vegetables, meat and fish samples, and decreased significantly for the fruit samples. After 24-month storage at 5C and 15C there were no significant changes in flow behavior index, consistency index and yield stress of the Herschel-Bulkley model for the vegetables, meat, fish and fruit samples. Increase in storage temperature decreased the consistency index and yield stress of the fruit samples.