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ABSTRACT Stirred yogurt is a cultured dairy product produced by mixed cultures of lactic acid bacteria. It is a semi-viscous liquid whose rheological properties are major quality determinants. These are influenced by several manufacturing factors such as heat treatment of the yogurt milk. Improvement of the properties of stirred yogurt made under different manufacturing conditions was the key focus in this research. The research included an investigation of the key factors involved in the development of nodulation which is an unsightly defect, as consumers expect stirred yogurt to have a smooth consistency. This research consisted of three main experimental parts: 1. Effect of different heat treatments on the properties of stirred yogurt; 2. Factors involved in nodulation in stirred yogurt; and 3. The physical and chemical nature of nodules. The types and levels of heat treatment have a major influence on the properties of stirred yogurt. In this study, yogurt milk was treated at pasteurization and UHT conditions. The milk heated at pasteurization, 80-95oC, and UHT, 130-145oC, conditions had similar levels of whey protein denaturation, approximately 85-99%, while the milk treated at low temperature for a long time, 65oC for 4 hr, had a significantly lower level of denaturation (~55%). Yogurt made from milk pasteurized at 85oC for 30 min and 92oC for 7 min and UHT treated milk at 130-145oC for 5 s had similar hardness and viscosity while yogurt made from milk treated at 65oC for 4 hr had significantly lower hardness and viscosity than corresponding yogurts from high heat treatments. In addition, the water-holding capacity and syneresis of yogurts made from either pasteurized or UHT milk were not significantly different. Yogurt (made from either pasteurized or UHT-treated milk) enriched with non-dairy ingredients, gelatin, inulin and sugar, showed higher hardness, viscosity, water-holding capacity than yogurt made with only dairy ingredients. These yogurts also showed no syneresis. This can be largely attributed to the gelatin which improves the texture, binds additional water and prevents syneresis. The formation of nodules in yogurt has been reported to be influenced by several factors. In this study, the heat treatment of the yogurt milk, the types and levels of sugar added, and the type of starter cultures were found to significantly influence the level of nodulation in the stirred yogurts. Severe pasteurization heat treatments, at temperatures < 100ºC, caused more nodules than mild heat treatments at temperatures in this range. The type of heat treatment was also important, with yogurt made from UHT-treated milk showing much less nodulation than yogurt made from pasteurized. This was observed when the levels of whey protein denaturation in the pasteurized and UHT milk were similar, indicating that the extent of whey protein denaturation alone is not a major factor in the development of nodules. Sucrose added either before or after heat treatment of yogurt milk also affected the extent of nodulation. Increasing the amount of added sucrose from 0 to 6.5% caused correspondingly higher numbers of nodules. In addition, there was a positive synergistic effect between heat treatment and sugar addition on nodulation; the highest numbers of nodules appeared when the yogurt milk was severely heated and sugar was added at the highest level, 6.5%. Addition of lactose or fructose instead of sucrose did not promote nodule formation while glucose caused nodulation in a similar manner to sucrose. The use of different starter cultures affected the numbers of nodules; yogurts made with the culture ABT 10 had much less nodulation than those made with ABT5 and ABT6, even under heating conditions and sugar addition conducive to nodule development. The amount of exopolysaccharide (EPS) produced by the starter cultures, ABT5 and ABT10, did not correlate with the extent of nodulation in the yogurt. Disturbance during yogurt fermentation by changing the temperature (from 37 to 42 or 45 to 42oC) when the gel was forming increased the numbers of nodules while refrigerated storage of yogurt and altering the pH during heat treatment (from pH 6.46 to 6.90) did not affect the number of nodules. Ultrasonication of the yogurt milk caused a reduction in the numbers of nodules in the yogurt. This was attributed to disruption of clumps of starter culture bacteria, thus preventing excessive localised build-up of acid around the bacterial clumps which could form the nucleus of nodules. The major conclusions from this study are that heat treatment of the yogurt milk and the level and type of added sugar are important factors affecting the extent of nodulation of stirred yogurt. Other factors such as the type of starter culture bacteria and their degree of clumping are also significant. Optimisation of these factors would allow yogurt manufacturers to minimise nodulation in stirred yogurt.

Tomatoes (Lycopersicon esculentum cv. Santa) are climacteric fruit that continue to ripen after harvest. Once the fruit are separated from the plant, the physiological reactions that control the ripening process are altered, so that tomato quality is altered. The earlier the stage of ripening at harvest, the larger the discrepancy between the tomatoes left to ripen on the plant and those harvested while still unripe. Quality is lost as the tomatoes lose moisture, firmness, nutrients and stored carbohydrates. Other aspects such as colour and flavour development and cell wall modifications also affect the quality of unripe harvested tomatoes. Environmental factors that affect the postharvest quality include the temperature, humidity and atmospheric conditions in which the tomatoes are stored. Hormonal manipulation (introducing or blocking ethylene) also has an effect on the final fruit quality. This research project concentrated on the changes that occur in quality during post harvest ripening. Comparisons were made on how different post harvest treatments and storage protocols influenced the quality and longevity of tomato fruit. The purpose of this study was to determine storage conditions that would prolong the shelflife of tomatoes so that they can be acceptable for consumption for up to 40 days after harvest. The best results were obtained from tomatoes that had not completely turned red at time of harvest. These tomatoes could maintain their acceptability best when they were stored at 12°C under controlled atmosphere conditions. 1-MCP treatment was also effective in limiting quality losses for up to a week after harvest, and avoiding exposure to ethylene gas can also be recommended. Heat treatment was not at all successful in improving the longevity of the fruit. / Prof. C.S. Whitehead

The behaviour of lead concentrate particles oxidizing in a stagnant gas reactor has been examined and a mathematical model which predicts the kinetics of galena particles developed. The effects of oxygen concentration, particle size, furnace temperature and concentrate composition were studied. The results showed that the concentrates all exhibited sharply defined ignition points. The ignition points were found to be strongly dependent on oxygen concentration. Reductions in ignition temperature of up to 100 K were observed when the concentrates were reacted in pure oxygen rather than in air. Iron composition was found to have a similar effect on ignition temperature. The modelling results predict short reaction times for ignited particles (less than 100 milliseconds). Particles which ignited in air attained predicted temperatures in excess of 2600 K. In oxygen, particle temperatures were calculated to be greater than 2800 K. Good agreement was found between experimental results and model predictions. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Samples of raw whole milk were heated at L5, 50 and 75 C for 0, 30, 60, llU and 240 min. After heat treatment each sample was subdivided and portions held at O C for 0, 30, 60 and 120 min. In a second experiment samples of milk were heated at 25 and 50 C for the same lengths of time as before but this time at 75 C for only 5, 10, 15 and 20 min. A constant holding time of 30 min was used after the heat treatments. Following the various heat treatments pH of each sample was measured and rennet clotting properties of milk at a constant rennet concentration were determined using the Formagraph instrument. The 75 C heat treatment of both experiments was left out from the statistical analysis. Results obtained in the first experiment indicate that cold storage at O C for up to 120 min has no significant effect on pH or on rennet-clotting properties of milk. Milk that was heated at 75 C for 30 min or longer did not coagulate. Results of the second experiment show that increase from 25 C to 50 C and extended heating time both reduce pH and clotting time. However, curd firming rate and cutting time are only affected by the length of time that the milk is heated. When milk is heated at 75 C coagulation time, curd firming rate and cutting time of curd are severely retarded.

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