Characterization of dragon fruit (Hylocereus spp.) components with valorization potential

Subtitle:Karakterisering van potentieel valoriseerbare componenten in pitaya (Hylocereus spp.)

Dragon fruit (Hylocereus spp.), also known as pitaya or pitahaya, is increasingly gaining interest in many countries, including Thailand which is a country with a climate ideal for breeding different varieties of tropical and subtropical fruits in general, and dragon fruit more specifically. The benefits of dragon fruit for human health can be explained by its essential nutrients such as vitamins, minerals, complex carbohydrates, dietary fibres and antioxidants. Dragon fruit is also an essential source of betacyanin which serves as a red/purple pigment with antioxidative properties. In Thailand, most of the fresh dragon fruits are consumed domestically, while some are traded globally as fresh fruit and processed products as juice or puree. As it is, dragon fruit peel and seeds are often considered as waste or by-products from fruit processing and have been less successful at adding value to the fruit. Currently, there is limited literature on dragon fruit properties, its processed products and constituents as well as potential utilization. Therefore, the main objective of this doctoral research was to gain deeper insight in the characteristics of dragon fruit and its derived products and components, specifically of two species of dragon fruit, i.e. white-flesh dragon fruit (H. undatus) and red-flesh dragon fruit (H. polyrhizus). The characteristics of these two species of dragon fruit and their possible application are extensively discussed in the doctoral thesis/research. Chapter 1 provides a state-of-the-art of current and relevant research on dragon fruit with respect to botanical classification, cultivation, economic aspects and chemical composition (particularly nutrients, pigment and antioxidative components) as well as an overview of techniques to process fruits (processing), i.e. freeze-drying and thermal processing. The impact of these processes on quality attributes (e.g. physicochemical and rheological properties) of different fruits is critically reviewed. The review also aims at giving a summarized overview of the most important aspects of seed oil (e.g. chemical composition and lipid oxidation) and cell wall polysaccharides (e.g. structure of pectic and hemicellulosic substances). In Chapter 2, the properties/characteristics of the whole fruit, peel, pulp (seed-free flesh) and puree (flesh containing seeds) of the two species of dragon fruit were performed. Results demonstrated that the peel of both dragon fruit species as well as the pulp and puree of the red-flesh species contained large amounts of betacyanin. As it is, they have the potential to be utilized as a natural colouring agent. Dragon fruit pulp and puree also exhibited significant antioxidative activities. This was even more pronounced for the red-flesh species due to the presence of betacyanin. The pulp and puree showed shear-thinning behaviour due to the presence of mucilaginous components. Throughout this doctoral research, these characteristics of dragon fruit, for example, pigment, antioxidative activities and rheological properties, provide a deeper insight with regard to further valorization of the fruit components and optimization of the fruit processing. Chapter 3 describes the characterization of the freeze-dried pulp and peel of the two species of dragon fruit to be further utilized of dragon fruitU+2019s pigment (betacyanin) as a food additive. It was found that freeze-drying could satisfactorily preserve colour and pigment concentration. The freeze-dried red-flesh pulp and the freeze-dried dragon fruit peel from both species contained high contents of betacyanin. The freeze-dried dragon fruit pulp was well-soluble in water, whereas this was not the case for the freeze-dried dragon fruit peel. Additionally, due to the pH-sensitivity of betacyanin, the influence of pH (1-11) on the colour shift of the freeze-dried dragon fruit peel and pulp was monitored. The colour of the freeze-dried peel was stable within a pH range of 3-7, whereas the freeze-dried red-flesh pulp had high colour stability over a pH range of 1-11 and is available in a convenient form making it suitable for the use as a food colourant. In order to gain insight into the effects of thermal treatment on the characteristics of the white-flesh and red-flesh dragon fruit purees, the antioxidative, rheological, physicochemical and microbiological properties of the purees were investigated at different process conditions (between 50 and 90 °C for 60 min). The results are presented and discussed in Chapter 4. It is interesting to see that the antioxidative properties of the heated dragon fruit puree increased during heating treatment probably because of the superior antioxidative attributes of the dragon fruit seeds present in the puree and the formation of Maillard reaction products. During thermal processing, the L* value (lightness) and b* value (yellowness) of the processed dragon fruit puree can be used to control the quality online. Total colour change (TCC), which is an indicator of different colour between the unheated and heated puree samples, of the red-flesh dragon fruit puree showed a strong negative correlation with betacyanin content. The kinetics of colour changes and betacyanin degradation in the heated dragon fruit puree followed a second-order models. The apparent viscosity of the heated dragon fruit puree increased with heating time and temperature. The rheological data fitted very well with the power-law model, showing shear-thinning behaviour. Thus, the heated dragon fruit puree, particularly the red-flesh puree, offer possibilities to be applied in foodstuffs due to their interesting attributes after thermal treatment. The dragon fruit seeds are considered to have a high antioxidative potential. As it is, oil was extracted from the white-flesh and red-flesh dragon fruit seeds with petroleum ether as cold extraction. The characterization of the seed oil is described in Chapter 5. Dragon fruit seeds contained significant amounts of oil, which accounted for about 32-34% of the dried seed weight. The predominant fatty acids of both dragon fruit seed oils were linoleic acid (C18:2, 45-55%), oleic acid (C18:1, 19-24%), palmitic acid (C16:0, 15-18%) and stearic acid (C18:0, 7-8%), respectively. Dragon fruit seed oil is interesting from a nutritional point of view as it contains a high amount of essential fatty acids, amounting to 56%. The triacylglycerol (TAG) composition in the seed oil was also analyzed. It was shown to contain mainly triunsaturated and diunsaturated TAGs. A significant amount of tocopherols in the dragon fruit seed oil was clearly observed in which U+03B1-tocopherol was the most abundant tocopherol in the oil (72% of total tocopherol content). In addition, a storage assessment of 3 months was performed, monitoring the oxidative and tocopherol stabilities in the dragon fruit seed oil. A low oxidation rate of both dragon fruit seed oils was obtained, while tocopherol content decreased on storage. However, the concentration of tocopherols in the dragon fruit seed oil after 12 weeks remained high content compared to the initial state. Thus, the dragon fruit seed oil could be considered as a good source of essential fatty acids and tocopherols, with a high oxidative stability. In Chapter 6, the activity of pectic enzymes, i.e. pectin methylesterase (PME) and polygalacturonase (PG), of the pulp and peel of white-flesh and red-flesh dragon fruits as well as inactivation of pectic enzymes by thermal treatments (30-90 °C for 10 min) were examined. The untreated white-flesh dragon fruit had a higher PME activity compared to the red-flesh dragon fruit, whereas PG activity was absent in all dragon fruit samples. Results also demonstrated that no significant effects of PME activity at temperature below 70 °C for 10 min in either the pulp or the peel of dragon fruit were observed, while a moderate heat treatment (80 and 90 °C for 10 min) could efficiently inactivate PME. In the last part of this doctoral study, the polysaccharides (pectic and hemicellulosic substances) from cell wall materials of the pulp and peel of white-flesh and red-flesh dragon fruits were isolated and structurally compared, as described in Chapter 7. Prior to cell wall polysaccharide isolation, blanching at 80 °C for 10 min allowed PME inactivation. Subsequently, cell wall material was extracted and sequentially fractionated using various solutions to obtain three different pectic fractions, a hemicellulosic fraction and a remaining residue fraction which could not be solubilized by the procedure used. These polysaccharide fractions were chemically characterized in terms of galacturonic acid (GalA) content, degree of methoxylation (DM), neutral sugar composition, molar mass distribution and affinity towards some specific anti-pectin antibodies. Results showed that the cell wall polysaccharides in the pulp and, to an even greater extent, in the peel of both white-flesh and red-flesh species contained significant amounts of pectic substances. The pectic substances in the dragon fruit peel as well as in the dragon fruit pulp were shown to be lowly methyl-esterified pectin (DM < 39%) and highly water-soluble pectin (38-47%). A higher contribution of pectic homogalacturonan (HG) region in the peel samples was observed compared to the pulp samples, whereas there were no large differences between the pectic substances of both dragon fruit species. Despite the low average DM value of the pulp and peel pectins, pectic substances in both pulp and peel samples showed an affinity for anti-HG antibodies with different specificities, indicating that a wide range of epitopes (including long blocks of unesterified GalA residues as well as a few consecutive esterified GalA residues) was present. In this doctoral research, the key findings of the experimental work are clearly shown and summarized in Chapter 8. Dragon fruit is an excellent source of bioactive compounds such as betacyanin and tocopherol which both have high antioxidative properties and health-promoting functions. Dragon fruit also provides many valuable products and components that could have high potential of being applied in food industry. For example, the freeze-dried dragon fruit pulp could be used as a red/purple colouring agent (betacyanin) as well as an instant juice powder. The heated dragon fruit puree with significant antioxidative activity could serve as a semi-processed product, the dragon fruit seed oil, on the other hand, was defined as a high-value oil. The extracted cell wall polysaccharide from dragon fruit peel demonstrated to be a low methyl-esterified pectin. Thus, the dissemination of the research findings may significantly increase the value of dragon fruit, resulting in an increased revenue for growers and producers.

ISBN:9789059896277

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