Doctoral Degrees (Food Science)

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Browsing Doctoral Degrees (Food Science) by browse.metadata.advisor "Caleb, Oluwafemi James"

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    Antimicrobial effects of chitosan and essential oils on postharvest diseases of pomegranate fruit
    (Stellenbosch : Stellenbosch University, 2017-12) Munhuweyi, Karen; Opara, Umezuruike Linus; Caleb, Oluwafemi James; Lennox, Cheryl L.; Sigge, G. O.; Stellenbosch University. Faculty of AgriSciences. Dept. of Food Science.
    ENGLISH ABSTRACT: Pomegranate (Punica granatum L.) fruit diseases often caused by a range of fungi and bacteria, pose significant financial, nutritional and postharvest losses along the value chain. This study aimed at identifying pomegranate postharvest diseases in South Africa and improving the shelf life of whole and minimally processed pomegranates using chitosan and essential oils (EOs). To identify pomegranate postharvest pathogens and their origin in the value chain, samples of leaves and fruit at different development stages were collected from commercial orchards of cultivars ‘Herskawitz’ (mid harvest) and ‘Wonderful’ (late harvest) located in the Western Cape region of South Africa. Fungi were isolated from healthy and intact pomegranate flowers (open, closed, diheased), immature fruitlets buds, immature green fruit and ripe pomegranate fruit as well as leaves. Isolated fungal pathogens were identified using phylogenetic analysis of the internal transcribed spacer (ITS: ITS1 and ITS2) of the nuclear ribosomal DNA and the 5.8S ribosomal RNA genes. Aspergillus niger Tiegh., Cytospora spp., Clonostachys spp., Embellisia eureka E.G. Simmons, Pestalotiopsis spp., Nigrospora oryzae and Rhizopus stolonifer Ehrenb., were associated with non-disinfected plant materials. The major pomegranate spoilage pathogens were isolated from surface sterilised samples and comprised of Alternaria spp., Aureobasidium pullulans, Botrytis spp., Penicillium spp., and Pilidiella granati Sacc. (syn. Coniella granati [Sacc.] Petr. & Syd.). The open flower stage had the highest incidence of spoilage pathogens and the same pathogens were isolated from fruit at postharvest. Pathogenicity tests were carried out on the major postharvest spoilage pathogens isolated namely Botrytis sp., Penicillium sp. and P. granati. This is the first report of P. granati on pomegranate fruit in South Africa. A restriction fragment length polymorphism (RFLP) tool was developed for the detection of pomegranate postharvest pathogens. This tool will be important in the monitoring of pomegranate pathogens in orchards and packhouses. The antifungal activity of crab shell chitosan (0-10 g/L) and fludioxonil (0-1.0 g/L) was tested against Botrytis sp., Penicillium sp. and P. granati previously isolated from pomegranate. Pathogen sensitivity to crab shell chitosan based on the EC50 values (concentration causing a 50% reduction in mycelial growth) showed that P. granati (EC50-0.47 g/L) was the most sensitive followed by Botrytis sp. (EC50-1.19 g/L) and Penicillium sp. (EC50 2.21 g/L). For fludioxonil Penicillium sp. (EC50 0.02 mg/L) was the most sensitive followed by P. granati (EC50 0.48 mg/L) and Botrytis sp. (EC50 0.09 mg/L). Pre-treating wounded fruit (preventive treatment) with chitosan prior to pathogen inoculation gave better disease control (30-66% decay reduction) compared to introducing the chitosan after pathogen inoculation as a curative strategy (18-38%). Applying chitosan (0, 2.5, 7.5 and 15 g/L) as an edible coating on minimally processed pomegranate arils prior to cold storage significantly (P<0.05) lowered counts for mesophilic aerobic bacteria, yeast and moulds. In addition, the chitosan treatment also maintained the physico-chemical attributes of the arils (total soluble solids (TSS), titratable acidity (TA), moisture, colour, firmness, total phenolics, anthocyanins and ascorbic acid). The findings demonstrate that crab shell chitosan can be considered as a potential green fungicide for postharvest disease management of both whole and minimally processed pomegranate fruit. A follow up study to enhance the antimicrobial properties of chitosan was conducted using chitosan as a polymeric carrier of volatile EOs (cinnamon, lemongrass and oregano). The EOs were assayed for antifungal activity against Botrytis sp., Penicillium sp., and P. granati. Lemon grass was the least effective EO as it failed to provide complete inhibition of any of the fungal pathogens while oregano EO was the most potent as it gave complete inhibition of P. granati by both vapour and direct contact methods. In vivo application of chitosan-oregano as an edible coating effectively controlled fungal growth by 59-100% but induced negative effects on the fruit skin. When applied as active film, the chitosan-oregano film still significantly (P < 0.05) reduced fruit decay by 34-100% without elucidating cosmetic damage to the fruit rind. The findings revealed the potential application of chitosan-EO based films in developing antimicrobial based active food packaging systems. To further improve use of EOs in antimicrobial packaging, cinnamon and oregano EOs were encapsulated in β-cyclodextrin (β-CD) and a nanofibrous matrix based on chitosan and polyvinyl alcohol (PVA) to reduce the thermal instability of the EOs and achieve prolonged release. A GC-MS analysis revealed that the β-CD was significantly (P < 0.05) more efficient in encapsulation of cinnamon EO (4.86%), compared to oregano EO (1.75%). However, similar EO concentrations were obtained when the essential oils were incorporated into electrospun chitosan based nanofibres. The EO based β-CD microcapsules and nanofibres both had antimicrobial effect on Botrytis sp. and thus could be used in antimicrobial packaging. This study identified the major postharvest spoilage pathogens of pomegranates in the Western Cape Region of South Africa and further determined that the orchard was the major source of these pathogens. Crab shell chitosan independently and in combination with EOs was shown to effectively control pomegranate postharvest pathogens on whole and minimally processed fruit. While exposing the pomegranate fruit to direct contact with EO emulsions reduced postharvest spoilage it also resulted in a negative cosmetic appearance of the fruit rind. However, exposing fruit to in-direct contact with encapsulated EOs controlled postharvest spoilage without affecting the fruit cosmetic appearance.
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    Modelling and optimization of active modified atmosphere packaging for pomegranate arils
    (Stellenbosch : Stellenbosch University, 2017-12) Belay, Zinash Assefa; Opara, U. L.; Caleb, Oluwafemi James; Mahajan, P. V.; Sigge, G. O.; Stellenbosch University. Faculty of AgriSciences. Dept. of Food Science.
    ENGLISH ABSTRACT: Active modified atmosphere packaging (active-MAP) is a well-proven postharvest technology used to preserve the quality and extend the storage and shelf life of fresh fruit under optimally designed conditions. Successful active-MAP design can be achieved by the mathematical integration of produce physiological characteristics, packaging material properties, and equilibrium gas mixture suitable for the product. The mechanisms by which active-MAP influences fruit quality involve physiological and enzymatic reactions that can be accelerated or reduced depending on the environmental conditions during storage. Therefore, understanding the experimental design and fundamental physiological processes occurring during storage condition are important in the development of an optimal produce-specific active-MAP. Low O2 limit for pomegranate arils was identified at 5 and 10 °C and the responses were monitored using real time respiration rate (RR), respiratory quotient (RQ), emission of volatile organic compounds (VOCs) and microbial growth. The results showed that pomegranate arils could tolerate down to 2.18% O2 during storage at 5 °C and 2.28% O2 at 10 °C. These findings highlighted the importance of selecting appropriate MAP materials with desired permeability to alleviate rapid depletion of O2 and excessive accumulation of CO2 at 10 °C inside the package. The impact of active-MA on quality attributes of ‘Wonderful’ pomegranate arils were investigated at cold (5 °C, 95 ± 2% RH) and ambient storage (20 °C, 65 ± 2% RH) conditions. Low O2 (5-10%) atmospheres significantly maintained antioxidant properties of arils, whereas significantly lower aerobic mesophilic bacteria, yeast and mould counts were found at super-atmospheric O2 (70%). Storing pomegranate arils under ambient condition resulted in quality deterioration and short shelf life. The effects of low O2 and super-atmospheric O2 on RR of ‘Wonderful’ pomegranate arils were analysed at 5 °C. Michaelis-Menten (MM) enzyme kinetic models were applied to determine the inhibition effects of CO2 concentration on O2 consumption rate. The results showed that both storage atmosphere and temperature had significant effects on aril RR. The MM competitive inhibition model best described the effect of CO2 on O2 consumption rate at low O2 and super-atmospheric O2 (R2 > 99%). The findings showed that super-atmospheric O2 had no effect to retard the metabolic process. A simplex lattice mixture design (SLMD) approach was applied to optimize gas composition for storing arils, and effects of temperature on the optimum gas was studied. A special cubical model were developed for the responses (RR, RQ, ethylene production rate and microbial quality) and the coefficients of model parameter estimates (β1, β2, β3, β12, β13, β23 and β123) and ternary contour plots were characterised. The predicted optimium gas mixture (O2:CO2) was 2% O2:18% CO2); and at this atmosphere, the minimum values of RR were 0.26 mL O2 kg-1 h-1 and 0.78 mL CO2 kg-1 h-1, while ethylene production rate was below the detection limit. Under these conditions, the growth of aerobic mesophilic bacteria (3.9 log CFU mL-1), yeast (3.8 log CFU mL-1) and mould (2.3 log CFU mL-1) were quantified. Increasing the storage temperature by 10 °C resulted in a threefold increase in aril RR. Using the same SLMD approach, the optimum gas composition required to maintain individual quality attributes was predicted. Variation in optimal gas mixture for individual quality attributes of arils were observed. According to the model parameter estimates, the optimum gas composition (O2:CO2) was established (6-7% O2:7-8% CO2) for individual sugars, organic acids, antioxidants and colour attributes. The optimum gas composition to maintain aril hardness and volatile compounds (monoterpene and ketones) was 2% O2 and 18% CO2. On the other hand, the optimum atmosphere for aldehydes was 2% O2 and 2% CO2. An integrated designing approach was applied to configure a packaging system for arils capable of modifying atmosphere and in-package relative humidity by designing modified atmosphere humidity package (MAHP) system. Cellulose based NatureFlex® (NF) film, bi-axial oriented polypropylene (BOPP) (PF) film and combinations of the two films were used. The 100% NF package created the lowest in-package RH (60-66%) and the highest reduction of O2, which resulted in arils dryness during storage. The 100% BOPP film resulted in saturated RH and in-package water vapour condensation. The optimized package design using 66% PF and 33% NF films best maintained the overall quality of pomegranate arils. This study demonstrated the potential of SLMD as an innovative tool to optimize the gas composition and improve packaging design for effective cold storage of minimally-processed fresh produce such as pomegranate arils. The results obtained also provide new information on the optimum condition required to maintain specific quality parameters for the commercialization of active-MA for packaging and marketing of pomegranate arils.