No description available.

Thesis (MEng)--University of South Australia, 1996.

Thesis (MEng)--University of South Australia, 1996.

Thesis (M.S.)--University of Missouri-Columbia, 2004. / Typescript. Includes bibliographical references (leaves 166-176). Also available on the Internet.

Thesis (M.S.)--University of Missouri-Columbia, 2004. / Typescript. Includes bibliographical references (leaves 166-176). Also available on the Internet.

D. Tech. Mechanical Engineering, Mechatronics and Industrial Design / Heat pump drying (HPD) is a proven efficient method in food drying industries and more efficacious than traditional South African industrial and agricultural drying methods, providing high energy efficiency with controllable temperature, air flow, air humidity and large energy saving potentials. In the last decade the market of heat pump (HP) systems for water heating and space cooling/heating has been well developed in South Africa, but the development of heat pumps (HPs) for industrial and agricultural drying has been very slow. In the case of optimization of HPD systems, most literature studies concentrate on optimization of the drying parameters; however it is important to note that the overall performance of the HPD system, which consists of two subsystems, the HP and drying system, depends on working conditions and the two subsystems interact with each other. It is therefore important that the HP and the dryer should not be examined or optimized separately. Therefore, in this study, an air source heat pump system was designed, constructed, installed and analysed as a single integrated unit for drying fruits. The main objective of the study was to carry out experimental investigations on optimization of an air source heat pump for drying South African fruits.

No description available.

Thesis (MSc Food Sc )--Stellenbosch University, 2000. / ENGLISH ABSTRACT: Drying of fruit is an age-old preservation technique that even to this day, remains popular. On the foreign markets there is a trend towards dried fruit products that are softer and more palatable. Based on this, the South African dried fruit industry has launched a range of "soft eating", high moisture dried fruit (HMOF) products. These products have a moisture content of ca 36% (w/w) and a water activity (aw) of ca 0.85, both of which are considerably higher than that of traditional dried fruit. It is this higher moisture content that makes the product susceptible to serious microbial spoilage and colour deterioration in spite of the use of sulphur dioxide. Currently, the shelf-life of these products is approximately three months and to ensure a quality export product, the shelf-life must be at least nine months. During this study the microbial composition of five South African commercially available HMDF products (raisins, prunes, nectarines, cling stone peaches and apricots), was determined. It was found that high moisture dried (HMO) raisins and prunes had the highest bacterial counts, while apricots were more susceptible to yeast and mould spoilage. The most frequently isolated microbes were members of the endospore forming Bacillus group and osmotolerant yeasts like Zygosaccharomyces. Members of the genera Salmonella, Staphylococcus and even anaerobic endospore formers were present in certain samples of the HMO prunes and raisins. From these results it was concluded that the current pasteurisation method of 7 h at 80°C used by the industry is not effective in successfully eliminating spoilage microbes. High moisture dried nectarines and prunes were used in challenging studies, where four indicator organisms (B. cereus, Z. rouxii, Escherichia coli and Penicillium chrysogenum) were selected to evaluate the shelf-stability of these two HMOF products over six months at 5° and 25°C. The results showed higher bacterial counts for the fruit stored at 25°C (1 200 cfu.g ̄ ¹ versus 450 cfu.g ̄ ¹ at 5°C), as well as extensive Maillard browning of the nectarines at the higher storage temperature. Furthermore, it was found that the pasteurisation method (7 h at 80°C) was not effective in eliminating the endospore former B. cereus, P. chrysogenum and Z. rouxii, which may thus lead to serious spoilage implications. The E. coli strain did not survive in either of the HMOF products and no counts were found even before pasteurisation. From these results it was clear that the current pasteurisation method of HMOF products is not effective in terms of the prevention of spoilage, and that storage of the product at lower temperatures would lead to a better quality product. The impact of different preservatives (lysozyme, pimaricin, sorbate, benzoate and an imidazole compound) on the survival and growth of B. cereus and Z. rouxii, were evaluated in nectarine puree prepared from dried nectarines. Lysozyme showed the best inhibitory effect (70% inhibition) on B. cereus over a 12 h incubation period, followed by benzoate, sorbate, the imidazole compound and lastly, pimaricin. The data obtained for the inhibitory effect of the preservatives on Z. rouxii showed that benzoate and sorbate showed. the best growth inhibitory effect (100% inhibition after 24 h). Pimaricin gave a 100% inhibitory effect after 48 h while the imidazole compound gave a 89% inhibition. From the different pasteurisation time/temperature combinations applied to eliminate the endopspore former B. 'cereus, it was found that pasteurisation of 150 min at 90°C showed the best inhibition results. However, the colour of the product was unstable. It was, therefore, decided to also evaluate the use of modified atmosphere packaging (MAP), as part of the product's processing. It was found that the colour of the HMO nectarines was stable and it is thus recommended that a time/temperature combination of 150 min at 90°C together with MAP, is used to ensure a shelf-stable HMOF product. / AFRIKAANSE OPSOMMING: Die droging van vrugte is 'n eeu-oue preserveringstegniek wat vandag steeds baie gewild is. Daar is 'n tendens in die oorsese markte na droëvrugte wat sagter en smaakliker is. Gebaseer op hierdie tendens het die Suid Afrikaanse droëvrugtebedryf 'n nuwe "soft eating", hoë vog droëvrug (HVDV) produkreeks die lig laat sien. Hierdie produkte het 'n voginhoud van ca 36% (m/m) en 'n water aktiwiteit (aw) van ca 0.85. Dit is die hoër voginhoud wat lei tot mikrobiologiese bederf en verbruining. Tans is die rakleeftyd van die produkte ongeveer drie maande, terwyl uitvoer standaarde 'n rakleeftyd van minstens nege maande vereis. Tydens hierdie studie is die mikrobiese samestelling van vyf Suid Afrikaans beskikbare HVDV produkte (rosyne, pruimedante, nektariens, taaipit perskes en appelkose) bepaal. Dit is bevind dat hoë vog droë (HVD) rosyne en pruimedante meestal hoë bakterie tellings het, terwyl appelkose meer vatbaar is vir giste en misiliêre fungi. Die organismes wat meestal geïsoleer is, was lede van die endospoorvormende Bacillus groep en osmotolerante giste soos Zygosaccharomyces. Lede van die genera Salmonella, Staphylococcus en selfs anaerobiese endospoorvormers is gevind in van die HVD pruimedante en rosyne. Vanuit hierdie resultate was dit duidelik dat die huidige pasteurisasie proses van 7 h by 80°C, wat deur die industrie gebruik word, nie effektief is om bederf organismes suksesvol te elimineer nie. Hoë vog droë nektariens an pruimedante is gebruik in uitdaging studies, waar vier indikator organismes (B. cereus, Z. rouxii, Escherichia coli en Penicillium chrysogenum) gekies is, om die rakleeftyd van die twee HVDV produkte oor 'n opbergingstydperk van 6 maande by 5° en 25°C te evalueer. Die resultate het hoër bakteriese tellings vir vrugte wat by 25°C (1 200 kve.g ̄ ¹ versus 450 kve.g ̄ ¹ by 5°C) opgeberg is getoon, asook dat Maillard verbruining by HVD nektariens plaasvind by die hoër opbergingstemperatuur. Verder is bevind dat die pasteurisasie metode (7 h by 80°C) nie effektief was om die endospoorevormer B. cereus, P. chrysogenum en Z. rouxii te elimineer nie, wat ernstige bederf implikasies tot gevolg kan hê. Die E. coli stam het nie in een van die twee HVDV produkte oorleef nie en geen tellings is gevind nie, selfs nie voor pasteurisasie nie. Die resultate het duidelik getoon dat die huidige pasteurisasie proses vir HVDV nie effektief is om mikrobiese bederf te voorkom nie, terwyl 'n lae opbergingstemperatuur aanbeveel word om 'n beter kwaliteit eindproduk te verseker. Die impak van verskillende preserveermiddels (lisosiem, pimarisien, sorbaat, bensoaat en 'n imidasool verbinding) ten opsigte van die groei en oorlewing van B. cereus en Z. rouxii is geëvalueer in nektarien puree. Lisosiem het die beste inhibisie effek (70% inhibisie) t.o.v. B. cereus getoon gedurende die 12 h inkubasie periode, gevolg deur bensoaat, sorbaat, die imidasool verbinding en laastens pimarisien. Die data verkry vir die inhibitoriese effek van die preserveermiddels t.o.v Z. rouxii, het getoon dat bensoaat en sorbaat die beste groei inhibisie getoon het (100% inhibisie na 24 h). Pimarisien het 'n 100% inhibisie getoon na 48 h, terwyl die imidasool verbinding 89% inhibisie getoon het. Van die verskillende pasteurisasie kombinasies wat aangewend is om die endopsoorvormer B. cereus te elimineer, is dit bevind dat 'n pasteurisasie van 150 min by 900G die beste inhibisie resultate getoon het. Die kleur van die nektariens was egter onstabiel. Gevolglik is daar besluit om gemodifiseerde atmosfeer verpakking (MAV) te evalueer as deel van die produk se prosessering. Dit is bevind dat die kleur van die HVD nektariens stabiel was en daar word verder aanbeveel om 'n tyd/temperatuur kombinasie van 150 min by 90°C, saam met MAV te gebruik, om 'n kwaliteit eindproduk te verseker.

The main objective of this study was to determine the effect of fruit ripeness and drying method on the oxidative stability and micro-component content of avocado oil extracted with supercritical carbon dioxide (SC-CO2). A secondary objective was to determine the effect of fruit ripeness, method of fruit drying and extraction method on the extractability of avocado oil with hexane and SC-CO2. For the oil extractability study, unripe and ripe avocado fruit pieces were either freeze-dried or oven-dried (80oC) and extracted with hexane or SC-CO2. For both extraction methods, oil yield was higher from ripe fruit than from unripe fruit. Scanning electron microscopy (SEM) indicated structural degradation during ripening, making the oil more available for extraction in ripe fruit. Oil from freeze-dried samples was in most cases more extractable than from oven-dried samples possibly through formation of rigid structures due to starch gelatinisation and dehydration and protein crosslinking around the oil cells during oven drying. Oil yield was higher with hexane than with SC-CO2 extraction because hexane is less selective, permeates the whole plant material and leads to a more complete extraction, while SC-CO2 may create paths of least resistance in the plant material where it moves preferentially, thus leading to a less complete extraction. For oxidative stability studies and micro-component characterisation, oil extractions were performed on an industrial scale SC-CO2. extractor. For all treatments (unripe freeze-dried, ripe freeze-dried, unripe oven-dried, ripe oven-dried), oil was divided into four fractions and analysed for fatty acid profile, peroxide value (PV), anisidine value (AV), free fatty acids (FFA), oxidative stability index (OSI), colour, tocopherol, sterol, chlorophyll, carotenoid and total unsaponifiable content. Oil from ripe, freeze-dried avocado had relatively lower levels of chlorophyll, carotenoids and tocopherols, than oil samples from the other treatments. This may be due to relatively higher lipoxygenase levels in ripe fruit which may bring about higher oxidative breakdown of these components. Also, the activity of lipoxygenase may be preserved under the lower temperature conditions of freeze-drying, but inactivated at high temperature during oven-drying. Intensity of blue and red on the Lovibond colour scale of all oil samples as well as chlorophyll and carotenoid content increased with progressive extraction. These pigments are presumably extracted in the latter stages of extraction because they are located in chloroplasts, chromoplasts and idioblast cells with thicker membranes than the parenchyma cells where triglycerides are located. Levels of total sterols, total tocopherols and their isomers did not show any specific trends with progressive extraction, which could be related to their location in cell membranes where they would be extracted concurrently with the triglycerides. Levels of total unsaponifiables were mostly higher in the first than the latter fractions. This could be due to the early elution of non-polar waxes which are highly soluble in SC-CO2 and highly available due to their location on the surface of the avocado skin. The fatty acid profile of the avocado oil was not influenced by the degree of ripeness or drying method and therefore did not affect the OSI. Oleic acid increased while linoleic acid decreased with progressive extraction. Compared to the changes observed in levels of some of the micro-components, the changes in fatty acid levels with progressive extraction were relatively small and the fatty acid profile alone could not explain the OSI of the oil. Oil from oven-dried avocado had lower PVs but higher AVs than oil from freeze-dried fruit indicating more advanced oxidative deterioration in oil from oven-dried samples than from freeze-dried samples. FFA levels were higher in oil from ripe, freeze-dried fruit. Levels of hydrolytic enzymes increase during fruit ripening and are preserved during freeze-drying while they are inactivated during oven-drying. FFA levels decreased with progressive extraction. Free fatty acids are very soluble in the SC-CO2 and due to their location on the surface of the plant material, they could be extracted early in the extraction. Oil from oven-dried fruit had relatively higher OSI compared to the other treatments. The OSI of all samples increased with progressive extraction. There was a significant negative correlation between FFA and OSI for both drying methods. AV correlated positively with OSI for oil from oven-dried fruit and negatively for oil from freeze-dried fruit. AV contributed the most to the prediction of OSI in oven-dried fruit, while FFA contributed the most in freeze-dried fruit. It was suggested that the high OSI of oil from oven-dried fruit, despite its high AV, may be due to the presence of compounds with high antioxidant activity in the oil formed through the high temperatures of the oven-drying process. Therefore, using multiple regression techniques, predictive models were developed to determine the effect of the micro-components on the oxidative stability of the oil. The OSI correlated positively with chlorophyll (0.83) and carotenoids (0.80). The models indicated that chlorophyll and carotenoids were the most important variables in predicting the oxidative stability of avocado oil extracted with SC-CO2. This might be due to the antioxidant effect of carotenoids and the possible formation of pheophytin and pyropheophytin, thermal breakdown products of chlorophyll, which exert antioxidant effects in oil. / Thesis (PhD (Food Science))--University of Pretoria, 2008. / Food Science / unrestricted

The study on dehydration of frozen saskatoon berries and the need for dried fruits has been strategically identified in the prairies. Our motivation was to find a suitable method for dehydration in order to extend saskatoon berry shelf life for preservation. Microwave, convection and microwave-convection combination drying processes were identified to finish-dry saskatoon berries after osmotic dehydration using sucrose and high fructose corn syrup (HFCS) sugar solutions. Osmotic dehydration removes moisture in small quantities and also introduces solutes into the fruit that acts as a preservative and also reduces the total drying time. <p>Due to the very short harvesting season of saskatoon berries, an accelerated process like microwave combination drying can bring down the moisture to safe storage level, immediately after harvest. Untreated and osmotically dehydrated berries were subjected to convection (control), microwave and microwave-convection combination drying conditions at different product drying temperatures (60, 70 and 80C) until final moisture content was 25% dry basis. A laboratory-scale microwave combination dryer was developed, built with temperature and moisture loss data acquisition systems using LabView 6i software. Thin-layer cross flow dryer was used for convection-only drying and for comparison. <p>Drying kinetics of the drying processes were studied and curve fitting with five empirical equations including Page equation, was carried to determine drying constant, R2 and standard error values. The microwave-combination drying method proved to be the best for drying saskatoon berries. Dehydrated product quality analysis by means of color changes, rehydration ratio measurements and observed structural changes with scanning electron microscope technique were the factors in drying method selection for saskatoon berries.<p> This research was instrumental in the modification and development of a novel drying system for high-moisture agricultural materials. Microwave-convection combination drying at 70oC, yields good results with higher drying rates and better end-product quality.