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Edible coatings controlling preservative migration from surface to food bulk could inhibit surface microbial growth which is often the main cause of spoilage for many food products. In this project we focused our attention upon methylcellulose, hydroxypropyl methylcellulose, and chitosan as the structural component for such edible films. These films were generally transparent and effective at thicknesses in the order of 20 to 100 μm. We expect them to have little impact on the sensory properties of a food. Permeability cell measurements were used to evaluate the effect of coating composition. Further film characterization included film thickness and electron microscopy studies. To gain an understanding of the permeation process, the permeability tests were done at 5, 24, 32, and 40°C. Among these polysaccharide films, methylcellulose was the most promising diffusion barrier with a permeability constant of 3.4 and 1.4xl0⁻⁸ (mg/sec cm²)(cm)/(mg/ml) at 24 and 5°C, respectively. These barrier properties were enhanced by the incorporation of lipids into the film formulation. The permeability of sorbates in methylcellulose and hydroxypropyl methylcellulose emulsified with lauric, palmitic, stearic and arachidic acid was found to depend upon the polysaccharide, the fatty acid chain length, and the number of fatty acid double bonds. Potassium sorbate permeation increased in the following order lauric>palmitic>stearic>arachidic acid. The effect of the double bond type, i.e. cis vs. trans was also determined. The permeability rate of potassium sorbate increased in the order of oleic>elaidic>stearic acid. The effect of temperature on potassium sorbate permeability was analyzed using an Arrhenius activation energy model for the permeation process. Permeability determinations at four different temperatures showed excellent agreement with this model and suggest that the permeation process is diffusion controlled. Electron microscopy studies showed the absence of pores, channels or other defects which might be introduced during casting, drying, handling or permeability determination. This observation is consistent with our hypothesis that potassium sorbate permeation is diffusion controlled. Furthermore, our experimental data suggest that the diffusion is controlled by the properties of the solvent embedded in the film. Further studies are required to confirm this hypothesis. The effect of casting technique was examined by coating a pure polysaccharide film with a fatty acid mixture or bees wax and by laminating a fatty acid mixture or hydrogenated palm oil between two layers of pure polysaccharide films. Unfortunately, most of these films cracked easily and could not be tested in our permeability cell. On the other hand, hydroxypropyl methylcellulose films coated with bees wax showed exceedingly low potassium sorbate permeability values. These modifications of the polysaccharide film properties reduced the potassium permeability down to 10⁻⁹ to 10⁻¹¹ (mg/sec cm²)(cm)/(mg/ml) depending upon temperature, film composition and film casting technique. A simplified procedure previously published was used to evaluate surface microbial stability enhancement. With this information a food processor can select the appropriate film, application procedure and film thickness to achieve the desired shelf life under ambient or refrigerated storage conditions. / Graduation date: 1988

The use of edible coatings in combination with antimicrobial agents enhances the microbial stability of foods. In this study we evaluated the potassium sorbate and sorbic acid permeability of an edible methylcellulose (MC) - palmitic acid (PA) film with a MC:PA ratio of 3:1. Permeability cell measurements were used to evaluate the effect of pH and water activity (a [subscript w]) on the film permeation rate by sorbic acid and potassium sorbate. For films with a thickness of 55-66 pm, potassium sorbate permeability increased from 2.3x10⁻¹⁰ to 2.0x10⁻⁸ (mg/sec cm²)/(cm)/(mg/mL) as a [subscript w] increased from 0.65 to 0.80. Films were not stable at a [subscript w] levels above 0.80. The permeability of the film to sorbic acid decreased from 3.3x10⁻⁸ to 9.1x10⁻¹⁰ (mg/sec cm²) (cm)/(mg/mL) when pH in the permeability cell was increased from 3 to 7. This permeability decrease with pH could balance the loss of effectiveness of sorbates due to the lowering of the percentage of undissociated molecules at high pH. Another characterization of the MC-PA film was the evaluation of the effect of relative humidity (RH) on its oxygen transmission rate (OTR). In general, the film was highly permeable to oxygen (OTR values at 24°C, 660 to 1400 mL O₂ (STP)/m² 24h atm, average film thickness of 55 μm). Therefore, there is no risk to develop anaerobic conditions on food surfaces coated with the MC-PA film. There was no relative humidity effect on OTR in the 0-60% RH range; whereas, the OTR doubled when the RH was increased from 60 to 80%. These results agree well with the moisture uptake of the MC-PA film. Its moisture sorption isotherm shows a large moisture content increase at RH levels larger than 60%. The application of methylcellulose-based films as moisture permeability barriers in simulated sundae ice cream cones showed that a MC-PA film practically stopped moisture transfer from the ice cream to the sugar cone. Sugar cone crispness was retained for a period longer than three months when stored coated at -10°F and +10°F. The crispness of commercial sundae ice cream cones is retained for periods much less than three months. / Graduation date: 1990

The effect of acids on the growth, survival and detection of Salmonella enteritidis PT4 is particularly important in view of the number of outbreaks in which mayonnaise has been implicated as a vehicle. Capacitance measurement was compared with colony counting procedures for the enumeration and determination of sub-lethal injury of Salmonella enteritidis during storage under varied conditions of pH, acidulant and temperature. Capacitance monitoring was shown to offer an improved technique for the measurement of sub-lethal injury in cell populations. Higher levels of sub-lethal injury were detected by the extension of capacitance detection time than were indicated by differential colony counts on selective and non-selective media. The extension of detection time noted with sub-lethally injured cell populations was shown to be due to an extended lag phase when cells were placed in the capacitance growth medium and not the result of delayed detection of the growth of a small, uninjured sub-population. Plots of percentage survival and extension of detection time in survivors gave similar curves for acetic and lactic acid. These acids showed both greater lethality and greater ability to inflict sub-lethal injury than the stronger citric or hydrochloric acid. Sub-lethal injury and lethality were not simply related, as little sub-lethal injury was observed with the stronger acids even under conditions that were ultimately highly lethal. The results indicate that weak organic acids cause more reversible damage to cellular sites prior to death: an observation that has implications for choice of resuscitation procedures when examining acidified foods. Injured cells were found to contain lower levels of ATP than healthy unstressed cells. Inhibition with chloramphenicol did not appear to increase injury and total protein patterns for injured and uninjured cells were similar, suggesting that protection afforded by the synthesis of stress proteins is not a significant factor in this case. The recovery of sub-lethally injured cells in various pre-enrichment and selective enrichment media using capacitance detection times and colony counts on selective and non selective media was determined. Buffered peptone water appeared more effective in recovering injured Salmonella compared to Lactose broth. Additionally, selenite cystine was shown to recover cells faster than the other two selective broths tested; the Muller-Kauffmann Tetrathionate broth and the Rappaport Vassiliadis enrichment broth. In this study, different injury conditions were applied; these included acid stress, heating and freezing. According to the lag phases of injured cell populations, short pre-enrichment is not recommended in the present study.

Please read the abstract in the section 00front of this document / Thesis (PhD (Food Science))--University of Pretoria, 2001. / Food Science / unrestricted

No description available.

Varying concentrations of the food preservatives nisin and lysozyme were adsorbed onto glass surfaces chemically modified to exhibit different degrees of hydrophobicity. The antimicrobial activity of the adsorbed preservatives was evaluated by documenting the ability of Listeria monocytogenes to adhere and grow on the glass surfaces. A bioluminescence protocol was developed to effectively enumerate bacterial cells adhered to glass. Lysozyme adsorption onto glass surfaces was monitored by labeling with ¹²⁵I. Results indicated that synergy was present for 0.9/0.1 molecular ratio of nisin/lysozyme. Synergistic effect was increasing gradually with the increase of nisin in the ratios tested. This trend was observed on both surface types. However, the magnitude of synergy was more pronounced on hydrophobic surfaces than on hydrophilic ones. Results from protein radiolabeling showed that lysozyme was adsorbed with higher mass to hydrophilic surfaces than to hydrophobic ones. / Graduation date: 1999

The effects of various pressure treatments (OK, 30K, 60K, 75K psig) and packing medium (water or cocktail sauce) on shucked oysters were investigated. The pH, moisture content, microbiological tests (including aerobic plate count (APC) and anaerobic plate count (ANPC)), enzyme assays (i.e. α-amylase, β-amylase, lipase and peroxidase activities) were conducted to determine the quality of pressure treated oysters during a 6 week shelf-life study. The moisture content in water-packed oysters under OK, 30K, 60K and 75K psig pressure treatments was slightly increased during storage, while that in cocktail sauce-packed samples was significantly lower than in water-packed samples. Addition of cocktail sauce lowered the pH in oysters, which effectively inhibited the microbial growth, but altered the appearance. The microbial shelf-life of water-packed oysters with pressure treatment of 60K and 75K psig was extended several weeks compared with the controls while 30K psig had less of an effect. Pressure treatments did not inhibit enzyme activities in oysters, however, the addition of cocktail sauce was significant in inhibiting the enzyme activities in this study. / Graduation date: 2000

Pasteurization radiation offers a new means of extending the refrigerated storage life of foods. This procedure avoids many of the undesirable changes which result when sterilization radiation is used and still eliminates most of the spoilage organisms. Shrimp was treated with 5 ppm chlortetracycline and subjected to pasteurization levels of gamma radiation and stored at 38°F. The levels of irradiation used were 0.5 and 0.75 megarad. These levels were based on the flavor threshold of irradiation intensity. The quality of the stored irradiated shrimp was determined by subjective evaluation, chemical analyses and microbiological examinations. The storage life of the shrimp irradiated at 0.5 megarad was extended to 5 weeks, compared with 1 week for the unirradiated samples held at the same temperature. Throughout the ten week storage period, the samples which received doses of 0.5 megarad and CTC and those which received 0.75 megarad with and without CTC remained in good condition. / Graduation date: 1963