Native to the Northern Hemisphere, the trees of the genus Quercus spp. are found between the temperate and tropical latitudes of the Americas, Asia, Europe, and North Africa. These oaks are evergreen, semi-deciduous, or deciduous trees and produce small fruits called acorns. According to the National Institute of Statistics, these trees represent 35% of the forest area in mainland Portugal, where 85% of cork oaks and 92% of holm oaks are found in the Alentejo, while 94% of the remaining species are concentrated in the North and Center regions. For this reason, large quantities of acorns are produced, and it is estimated that the annual production is 401,585 tons. Despite being a fruit rich in polyphenols and gluten-free (resistant) starch, about 55% of the acorns produced are underused. Currently, as starch has multiple applications in the food and non-food industry, the exploitation of acorns as a source of starch would be advantageous. According to the literature, alkaline extraction is the methodology that allows obtaining the highest extraction yields in starch with the highest purity and with the most relevant properties. However, these starches cannot be used as food additives in the European Union due to a lack of approval. Thus, the main objective of this thesis was to value acorns within the scope of the principle of sustainable development through the creation of high-added value and zero-waste ingredients. Different species of acorns (Q. pyrenaica, Q. robur, and Q. ilex) were collected to study the effect of different dehulling methods on their nutritional and phytochemical composition. Each specie was dehulled separately by three different methods: manual, thermal (by thermal shock), and drying. It was found that the carbohydrate content was not significantly affected, but there was a loss of resistant starch of 76, 42 and 62% after thermal dehulling and a loss of 35, 27 and 25% after dehulling by drying compared to manual dehulling for Q. pyrenaica, Q. robur, and Q. ilex, respectively. As Q. robur was the specie with the highest resistant starch/total starch ratio (0.89), as well as the highest phenolic contents (19.3±1.3 mg GA Eq./g DF), hydrolyzable and condensable tannins (73.2±4.6 mg TA Eq./g DF and 0.103±0.01 mg TA Eq./g DF, respectively) comparatively to Q. pyrenaica and Q. ilex, the manually dehulled acorn of the Q. robur specie was selected in the following tests. Then, the optimization of the starch extraction was conducted at room temperature using water as extraction solvent by high hydrostatic pressure (HP) (pressure levels between 0.1 and 500 MPa and extraction times between 5 and 20 min) and pulsed electric field (PEF) (field strengths between 0.1 and 20 kV/cm and times between 35 and 85 μs) technologies. Due to the presence of large tannin content, which are anti-nutritional compounds of great importance in leather production, their extraction was also optimized, as well as other phenolic compounds, to obtain antioxidant extracts with high added value. Regarding the HP extraction, the optimal extraction condition was found at 333 MPa for 17.4 min with a desirability of 79.5%. The starch yield was 35.4±1.1% (w/w DF) and the extracts had a phenolic and hydrolyzable tannin content of 23.1±0.3 mg GA Eq./g DF and 54.1±0.1 mg TA Eq./g DF, respectively, and an antioxidant activity measured by ABTS of 53.6±0.6 mg TX Eq./g DF. As for the PEF extraction, the optimal extraction condition was obtained at 0.1 kV/cm for 63.3 μs with a desirability of 86.5%. The starch yield was 34.5±0.4% (m/m DF) and the extracts had a phenolic and hydrolyzable tannin content of 27.0±1.2 mg GA Eq./g DF and 56.7±0.6 mg TA Eq./g DF, respectively with antioxidant activity of 59.1±1.9 mg TX Eq./g DF. Condensed tannins were not detected in any of the extracts. Compared to alkaline extraction, the results observed under optimal conditions represent an increase in phenolic yields, tannins, and antioxidant activity by 2, 900, and 4% by HP and an increase by 24, 1012, and 15% by PEF, respectively. Thus, these technologies make it possible to obtain aqueous extracts with a higher tannin content than alkaline extraction, thus valuing these compounds. However, it was the PEF extraction technology that made it possible to obtain extracts with a higher content of polyphenols (phenolics and tannins) and with greater antioxidant activity than HP. Concerning starch, although the extraction yields obtained by HP and PEF under optimal conditions were lower than those verified in the respective alkaline extractions (49.7±0.5 and 48.9±1.2% w/w DF for HP and PEF, respectively), these starches are clean-label and can be used more safely as an additive and/or ingredient in human food than alkaline starches. As the extraction yields under optimal conditions were like each other, the impact of technologies on starch properties was studied and the properties were compared to commercial corn starch to try to identify the most advantageous extraction technology. Regarding the starch obtained by HP under optimal conditions, pressurization preserved the type of polymorphism of the acorn starch and did not significantly alter the relative crystallinity and the onset gelatinization temperatures (i.e., it preserved the hydrogen bonds). However, the significant increase in the amylose/amylopectin ratio by 97%, motivated by the cleavage of the amylopectin chains, led to changes in the properties of starches. Pressurization led to a decrease in the solubility and swelling power of the starches relative to the control and led to an increase in the resistance of the gels to deformation. However, no differences were observed in the in-vitro digestibility, pseudo-plastic behavior, or resistance to flow after pressurization. Regarding the starch obtained by PEF under optimal conditions, the amylose and amylopectin contents were not altered, as well as the onset gelatinization temperatures (hydrogen bonds). Although the type of polymorphism was not changed, there was a decrease in relative crystallinity of 17%. PEF treatment did not alter the starch properties concerning the control starch (solubility, swelling power, in-vitro digestibility, and rheology). Compared to commercial corn starch, acorn starches showed lower gelatinization temperatures and enthalpies, better pseudo-plastic behavior, lower in-vitro digestibility, greater resistance to complex flow, and lower resistance to deformation. Overall, acorn starches also showed higher solubility and swelling power up to 80 °C than commercial starch, which encourages the use of acorn starch as an additive in fermented yogurt and dairy products. As extraction by HP and PEF led to obtaining acorn starches with different properties, a dairy product was developed to try to identify the most suitable starch for this purpose. The effect of replacing commercial corn starch with acorn starch extracted by HP and PEF under optimal conditions on the nutritional composition, functional and sensory properties, and shelf-life of chocolate puddings was studied. Acorn starch improved the rheological properties of puddings without negatively affecting their nutritional composition, internal structure, or in-vitro digestibility. The sensory analysis revealed that 61% of tasters preferred the puddings made with acorn starch over commercial corn starch (control). As for the shelf life, acorn starch led to color stabilization and improved texture of the puddings after 28 days of storage at 4°C compared to the control without compromising the microbiological safety. Of the acorn starches used, PEF-extracted acorn starch led to the greatest improvement in pudding texture. In addition, the chocolate pudding made with PEF-extracted acorn starch also had a higher acceptability index than the HP-extracted starch. Thus, PEF-extracted acorn starch seems to be the most suitable for pudding production. In its entirety, the work described in this Doctoral thesis marks important steps in the valorization of acorns in Portugal, that is, in the valorization of its polyphenols and starch. This work will also contribute to the sustainability and transformation of the acorn in a circular economy context, as it demonstrates the potential of using PEF technology, as a sustainable alternative to alkaline extractions, in obtaining products with high added value and of great economic interest.
Date of Award | 18 Sept 2023 |
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Original language | English |
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Awarding Institution | - Universidade Católica Portuguesa
- University of Aveiro
- University of Minho
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Sponsors | Fundação para a Ciência e a Tecnologia |
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Supervisor | Maria Manuela Pintado (Supervisor), Jorge Manuel Alexandre Saraiva (Co-Supervisor) & Elisabete Maria da Cruz Alexandre (Co-Supervisor) |
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