Doctoral Degrees (Food Science)

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    The efficacy of bacteriophages FO1a and S16 in the reduction of Salmonella on chicken carcasses in a South African poultry processing environment
    (Stellenbosch : Stellenbosch University, 2022-12) Wessels, Kirsten; Gouws, P. A.; Rip, Diane; Stellenbosch University. Faculty of AgriSciences. Dept. of Food Science.
    ENGLISH ABSTRACT: Much of the research surrounding bacteriophages (phages) as a processing aid for the control of Salmonella on chicken meat has been conducted in vitro in the laboratory. Information about the efficacy and application of bacteriophages as part of a hurdle concept in the chicken processing environment is limited. In South Africa, the use of certain antibiotics in live broilers and the use of chlorine-containing antimicrobials in the processing environment, are still permitted as Salmonella control methods. The incidence of Salmonella in chicken meat in South Africa is unclear, but previous research has repeatedly shown that the use of antibiotics and/or chlorine selects for resistance in Salmonella. The aim of this study was to determine the efficacy of a commercial phage cocktail PhageGuard S™ (PGS) (FO1a and S16 phages) in the reduction of Salmonella on chicken carcasses through a validated spraying system in a South African chicken processing plant. This study also investigated the incidence and antibiotic susceptibility profiles of Salmonella isolated from chicken carcasses in the plant. The PGS was applied at a 1% (v/v) concentration onto chicken carcasses via a spraying system (validated specifications: 530 µm nozzle diameter, 200 mesh strainer and 3 Bar pump pressure) after the chlorine spin chilling step. Neck skins samples were collected before the inside- outside wash step (N= 80) and after the PGS application step (N= 160). The neck skin samples were tested for Salmonella presence/absence (EN ISO 6579/A1 (02/2006)) and confirmed using Vitek®. Confirmed Salmonella isolates were screened for antibiotic susceptibility using the Kirby-Bauer disk diffusion method according to M100 from the Clinical and Laboratory Standards Institute (CLSI, 2020). Confirmed Salmonella isolates from neck skin samples collected after PGS application were re-exposed to PGS in the laboratory via a killing assay (Micreos Food Fafety, NL) to determine if the isolates were resistant to the PGS. Before the inside-outside step the Salmonella incidence was 60% with a large portion of these isolates showing resistance to tetracycline (56.3%) and sulfonamide (43.8%). After the combination of the inside-outside wash step, chlorine spin chilling and PGS application, the Salmonella incidence decreased to 23.75%, where more than half of these isolates showed resistance to tetracycline (63.2%) and sulfonamide (55.3%). For the killing assay, all isolates which survived PGS in the processing environment were reduced by 100% in the laboratory, highlighting that the phages were unable to reach the Salmonella via the spray application, and not that the Salmonella was resistant to the phages. The results in this study showed that the multi-drug resistant Salmonella in the chicken neck skins survived a complete immersion in chlorine but were successfully reduced by PGS, making phages a potential solution to many persistent microbial problems. This study also provides valuable insight into implementing phages into the large-scale hurdle concept of a processing environment and highlights the importance of the application method to ensure safe delivery of the phages to the target bacteria for a high efficacy.
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    Multi-stakeholder perspectives on food labels and the future of healthy eating
    (Stellenbosch : Stellenbosch University, 2022-12) Todd, Melvi; Joubert, Elizabeth; Guetterman, Timothy; Sigge, Gunnar; Stellenbosch University. Faculty of AgriSciences. Dept. of Food Science.
    ENGLISH ABSTRACT: South Africa, like many other countries throughout the world, is grappling with the increasing burden of non-communicable diseases, caused in part by an unhealthy diet. This study investigated how changes to the presentation of nutritional information on food labels can potentially help South Africans judge the healthiness of food, thereby potentially guiding them towards healthier choices and beneficial health outcomes. As a point of departure for the study, the value-adding potential of legislating health claims on food labels was considered. Draft legislation to allow health claims have been pending since 2014, and the legislation also does not address any claims pertaining to the health potential of our indigenous food plants, despite notable research spending in this area. Interviews (n = 49) with stakeholders with varied expertise revealed strong evidence for economic value creation potential from health claims, however, this was skewed towards businesses. The findings were less clear about other forms of value and the potential beneficiaries. Developing metrics that people understand and that relate to all forms of value will be required to ensure that health claims on food labels, should they be permitted, do not only benefit a select group of stakeholders. Delving deeper into the question of whether legislating health claims on food labels would be useful for consumers revealed more challenges than opportunities. Persistent impediments to use of the information and unresolved questions about the substantiation and enforcement of health claims were of particular concern. At the same time, no solution appeared apparent for reaching consumers in the informal market with nutritional information. While legislating health claims is not recommended at present, numerous front-of-pack (FOP) label proposals were made by the interview participants. In the final stage of the study, consumers (n = 1 261) were asked to compare FOP labels and a health claim relative to a no-label control, applied to a fictitious cereal product, a commonly consumed processed food in South Africa. Both health warnings and health star rating showed promise in terms of assisting consumers in identifying less healthful products, but effect sizes were largest for the warning label. This outcome requires validation with more product categories, as well as testing in real-world settings. The study culminated with the presentation of a three-phased nutritional labelling strategy for South Africa: ‘Make it clearer’, ‘make it simpler’, ‘make it smarter’ - intended to illustrate the need to take action now whilst simultaneously planning for the future. At a policy level, while South Africa plans for the implementation of FOP labels, we should be actively planning for the labelling of the future – one which will be more technologically enabled. The findings of this study may provide policy-makers and decision-makers, such as the Department of Health, with valuable information about the presentation of nutritional information on food labelling (i.e. selecting FOP labels for further real- world testing), as well as setting the course for the future.
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    The effect of muscle type and ageing on Near Infrared (NIR) Spectroscopy classification of game meat species using a portable instrument
    (Stellenbosch : Stellenbosch University, 2021-04) Dumalisile, Pholisa; Williams, Paul James; Manley, Marena; Hoffman, Louwrens C.; Stellenbosch University. Faculty of AgriSciences. Dept. of Food Science.
    ENGLISH ABSTRACT: Meat and meat products represent a large proportion of the human diet as it is known to provide valuable proteins, and is a good source of minerals, particularly iron, and zinc. Because of its nutritional characteristics it tends to be a commodity of demand to consumers. Game meat offers even higher nutritional attributes than any other red meat category because of its low fat and high protein levels making game meat a highly priced product thereby causing it to be an appealing target for species substitution. Also, fraudsters prefer to use products that are easy to adulterate and difficult to detect. To mitigate the fraudulent substitution of meat products, food authentication and labelling is promoted. The conventional methods of authentication such as DNA based techniques are expensive and slow for the rapidly expanding meat trade. Near infrared (NIR) spectroscopy, a rapid non-destructive, environmentally friendly instrument is thought to be an alternative and cheap solution for on-site meat authentication purposes, although this technology has not yet been evaluated for its suitability to distinguish different South African game species and/or muscles. To evaluate the ability of NIR spectroscopy to distinguish between selected game species’ (impala (Aepyceros melampus), blesbok (Damaliscus pygargus phillipsi), springbok (Antidorcas marsupialis), eland (Taurotragus oryx), black wildebeest (Connochaetes gnou) and zebra (Equus quagga)) Longissimus thoracis et lumborum (LTL) muscle steaks, a handheld MicroNIR™ OnSite spectrophotometer was used in a spectral range of 908–1700 nm. After the spectral data was pre-treated with smoothing, SNV-Detrend, the PCA scores plot revealed two clear clusters separating the medium-sized antelopes and large-sized species. The waveband responsible for the separation as indicated by the loadings line plot situated at 1372 nm, was associated with fat. The developed classification models revealed that the steaks could be distinguished with linear discriminant analysis (LDA), soft independent modelling by class analogy (SIMCA) and partial least squares discriminant analysis (PLS-DA) at classification accuracies ranging from 68 - 100%, 67 - 100% and 70 - 96%, respectively. Also, NIR spectroscopy in combination with multivariate data analysis techniques was used to discriminate between different muscle steaks from longissimus thoracis et lumborum (LTL), infraspinatus (IS) and supraspinatus (SS), biceps femoris (BF), semitendinosus (ST) and semimembranosus (SM) of impala and eland species; and samples from fan fillet (FF), big drum (BD), triangle steak (TS), moon steak (MS) and rump steak (RS) of ostriches. Classification accuracies developed with PLS-DA models ranged from 85 to 100% throughout. It is interesting that good classifications accuracies were achieved when the muscles were grouped according to their anatomical locations, irrespective of the muscle used, PLS-DA models yielded accuracies of 97%, 81% and 92% for eland, impala and ostrich, respectively. Even though NIR spectroscopy in combination with multivariate data analysis techniques could successfully distinguish the different muscle types within animals, and muscles across different species, the instrument did fall short in discriminating the ageing periods of blesbok, eland, and ostrich muscles. However, it is postulated that there is still room for improvement when the device is coupled with machine learning. In summary, the handheld MicroNIR™ OnSite spectrophotometer demonstrated its capability in discriminating between different species of game meat indicating that the instrument could potentially be used in the authentication of game meat.
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    Value addition of pomegranate seed oil: Effect of seed pretreatment methods on yield, quality attributes and functional properties
    (Stellenbosch : Stellenbosch University, 2021-04) Kaseke, Tafadzwa; Opara, Umezuruike Linus; Fawole, Olaniyi Amos; Sigge, G. O.; Stellenbosch University. Faculty of AgriSciences. Dept. of Food Science.
    ENGLISH ABSTRACT: Efficient and cost-effective processing protocols which enhance oil recovery and quality are required by processors in the seed oil industries. Currently, there is no established pomegranate seed oil processing procedure in South Africa, which could be one of the reasons hindering the development of pomegranate seed oil industry, despite the readily available fruit raw material. Furthermore, cold pressing, which is the current and most preferred pomegranate seed oil extraction technique by oil processors and consumers is associated with low recovery of oil. Physical or chemical pretreatment of seeds has been demonstrated to improve oil recovery and quality in other fruit seeds and field crops; however, the application of these pretreatments to enhance pomegranate seed oil extraction efficiency and quality is limited. Therefore, the overall aim of this study was to establish a suitable pomegranate seed pretreatment method for high oil yield, quality, and functional properties. In Theme B, the effect of seed pretreatment on three common pomegranate cultivars in South Africa was examined. To evaluate if blanching as a pretreatment technique for oil extraction adds value to pomegranate seed oil (PSO), seeds of ‘Wonderful’ pomegranate fruit were blanched at 80, 90, and 100 °C for 3 and 5 min. Blanching pomegranate seeds at 90± 2 °C for 3 to 5 min significantly improved oil yield, stigmasterol, punicic acid, total phenolic content (TPC) and 2.2-diphenyl-1-picryl hydrazyl (DPPH) radical scavenging capacity. Given the significance of cultivar on seed pretreatment efficacy and oil quality, blanching (95± 2 °C for 3), microwave heating (261 W for 102 s) and enzyme pretreatment (1.7 %, 40 °C, pH=4.5 and 5 h) were investigated on the seeds of three different pomegranate cultivars (‘Wonderful’, ‘Herskawitz’, and ‘Acco’). Blanching and microwave heating of ‘Wonderful’ and ‘Acco’ seeds improved the oil yield and colour, whilst they enhanced the antioxidant capacity of oil extracted from ‘Herskawitz’ seeds. However, oil extracted from ‘Acco’ after seed enzyme pretreatment exhibited higher oil yield, total carotenoids content (TCC) and, DPPH radical scavenging capacity relative to ‘Wonderful’ and ‘Herskawitz’. The results showed that the quality of PSO from pretreated seeds is cultivar dependent. The study reported in Theme C investigated the influence of blanching (95± 2 °C/3 min) and microwave heating (261 W for 102 s) pomegranate seeds on the quality of oil extracted by cold pressing, the most preferred seed oil extraction technique by oil processors and consumers but with low oil yield. Both blanching and microwave pretreatment of seeds prior to pressing enhanced oil yield, TCC, TPC, DPPH and 2.2-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical scavenging capacity. Although the levels of oil oxidation indices were significantly higher in microwaved than blanched seeds, they remained within the limits of Codex Alimentarius Commission (CODEX STAN 19-1981) standard on cold pressed vegetable oils. The oil palmitic acid, oleic acid, and linoleic acid significantly increased after microwave heating of seeds, whilst punicic acid decreased, which could be attributed to increased heat penetration and oxidation of the fatty acid. Conversely, the fatty acid composition of PSO was not significantly altered by seed blanching, indicating that the nutritional quality of the oil was not affected. Blanching of seeds is, therefore, a valuable step that could be incorporated into the PSO production process. The studies in Theme D, focused on blanched seed storability, oxidative stability, shelf-life and functionality of PSO from blanched seed and blended oils. Blanching pomegranate seed did not cause significant deterioration of the oil quality after seed storage. More so, storing pomegranate seed at 25 and 35 °C for 6 months did not result in a considerable reduction in oil quality, with respect to peroxide value, anisidine value and total oxidation value. In paper 9, the lipid oxidation kinetics, thermodynamics parameters, and shelf-life were estimated, given the improved extractability of bioactive compounds and enhanced antioxidant capacity of PSO from pretreated seeds. The Arrhenius model and activated complex theory were applied to calculate the activation energy (Ea), enthalpy (ΔH‡), entropy (ΔS‡) and Gibbs free energy (ΔG‡), which ranged from 6.2 to 8.59 kJ mol⁻¹, 3.69 to 6.38 kJ mol⁻¹, -146.62 to -155.06 J K⁻¹ mol⁻¹ and 48.13 to 64.74 kJ mol⁻¹, respectively. These thermodynamic parameters showed that the lipid oxidation reactions in all PSO extracts were non-spontaneous, endothermic, and endergonic. Moreover, the developed Arrhenius models established that blanching seeds may increase the PSO shelf-life at 25 °C from 21 to 24 days. In contrast, microwave heating may not change the shelf-life. The higher initial level of peroxide value and increased polyunsaturated fatty acids could explain the insignificant effect of seed microwaving heating on the oil shelf-life. The study provided valuable insights useful in the design of PSO packaging, the establishment of storage conditions and application of novel technologies to preserve the storage life. In paper 10 (Theme D) the functionality and oxidative stability of PSO from unblanched and blanched seeds (95± 2 °C/ 3 min) were investigated by blending with semi-refined sunflower oil (SO) considering the detrimental health effects of synthetic antioxidants. The study showed that blended oils (85: 15 w/w) had better oxidative stability, a lower rate of antiradical activity depletion and concentration of volatile oxidation compounds than SO, although this did not significantly (p > 0.05) vary between the blended oils. Despite this, the formulation of PSO blends is a novel and desirable development for the food industry, which is currently interested in specialty oils and functional foods formulations to improve human nutrition and health. Overall, this study has established that the improvement of PSO quality through seed pretreatment is a function of cultivar. This could be ascribed to the genetic variation of the pomegranate cultivars investigated. The study has also demonstrated that seed pretreatment is essential to improve the performance of safe and green but low oil yielding technologies such as cold pressing. Blanching of pomegranate seed may enhance the oxidative stability and shelf- life of PSO. Furthermore, the study provides scientifically based information that can be used to develop strategies to improve the storability of PSO from pretreated seeds and retain the health promoting properties.
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    X-ray micro-computed tomography (μCT) evaluation of bubble structure to determine quality of dough and bread made from roasted wheat flour
    (Stellenbosch : Stellenbosch University, 2021-04) Germishuys, Zandré; Manley, Marena; Stellenbosch University. Faculty of AgriSciences. Dept. of Food Science.
    ENGLISH ABSTRACT: Roasting of cereals has been shown to improve its sensory properties, increase its shelf life and inactivate proteolytic enzymes promoting increased loaf volume. The breadmaking process is a series of aeration stages which ultimately affects the final crumb structure of the bread. X-ray micro-computed tomography (μCT) is a non-invasive technique capable of producing high quality three-dimensional images enabling microstructural evaluation of food products. The aims of this study were to characterise white flour produced from roasted wheat differing in hardness and protein content using rheological and physicochemical analyses; to determine the optimal roasting conditions that would minimise the effect on the protein properties of the produced white flour; to evaluate the freeze drying of dough as a suitable sample preparation procedure to maintain the structure of the samples during X-ray μCT scanning for efficient analysis of the bubble structure of dough and foam properties of 10 g bread loaves produced from roasted wheat flour; and to evaluate the quality and shelf life of bread loaves prepared from roasted wheat flour by means of C-Cell and texture analysis. Hard, medium and soft textured wheat kernels were roasted for 140 s at 180°C using a forced convection continuous tumble (FCCT) roaster. This resulted in the largest reductions in hectolitre mass (7.36 hg/hl), flour yield (2.33%) and moisture content (2.87%) for the hard wheat. The largest increase in damaged starch (4.54%) and flour ash (0.06%) was observed for the hard and soft wheats, respectively. Roasting resulted in gluten protein changes as a gluten network could not be formed during dough mixing with the roasted flours. The use of composite flours (80% untreated flour and 20% flour from roasted wheat) displayed the largest increase in water absorption capacity (5.2%) for the medium textured wheat. Improved alveograph P/L ratios and higher levels of free starch was observed for the hard and medium textured wheat. The central composite design (CCD) showed no significant differences (p>0.05) for protein content, mixograph peak time and peak height for either the high or low protein roasted wheat. The roasting conditions chosen (based on trends observed) for X-ray μCT evaluation was 90°C and 86 Hz (ca. 130 s) as this combination maximised protein content and peak height and minimised peak time. To evaluate the bubble structure of dough and the foam structure of bread, 20- and 40 min proofed dough as well as 10 g bread samples, produced from roasted wheat flour based on the CCD, were subjected to X-ray μCT. The use of 10 g dough and bread samples enabled scanning at a much higher resolution. A finer crumb structure and softer texture was observed for the bread produced from roasted wheat flour due to decreased strut thicknesses. Lower mixograph peak heights and increased porosity suggested a weaker gluten strength for the roasted wheat samples. The roasting conditions used did not negatively impact the foam properties of the breads. C-Cell analysis showed a coarser crumb structure and a darker crumb colour for breads produced from roasted wheat flour although this did not negatively impact the breads texture. More importantly, texture analysis showed the use of flour produced from roasted wheat resulted in a softer bread (i.e., lower firmness) with an increased shelf life.