Process for recovery of smooth fibre ingredient from pomace : a thesis presented in partial fulfilment of the requirements for the degree of Doctor in Philosophy in Food Technology at Massey University, Manawatu, New Zealand

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2020
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Massey University
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This research aimed to develop a process to convert apple pomace into a food ingredient which can provide functional properties such as water-binding in baked goods, stability in aqueous suspension and smooth mouthfeel. This was achieved by modifying the apple pomace through three main steps: heating, shearing and enzymatic hydrolysis. Firstly, the effect of sample preparation (addition of water to fresh pomace, temperature and shearing apple pomace) on the solubility of pectin was investigated. Secondly, kinetics of main reactions involved in pomace while heating at temperatures between 90-140 °C (10 °C intervals) and incubation times between 0-360 min, were studied at bench scale. These reactions were: solubilisation and depolymerisation reactions of pectin, degradation of sugars and production of secondary products such as organic acids and 5-hydroxymethylfurfural (5-HMF). After that, the kinetics of these reactions were modelled for determining the rate constants and activation energies. The kinetic models of pectin solubilisation and 5-HMF formation were then used for scaling up the hydrothermal process in a more complex heat transfer situation, using a pilot scale retort. Finally, the effect of particle size distribution and molecular weight of solubilised components (mainly pectin) on physicochemical and sensorial properties of pomace material was investigated. Solubilisation of pectin at room temperature was independent of addition of water and shearing treatment of pomace. However, heating at temperatures > 100 °C, combined with increasing the amounts of water added to pomace (from 0 to 8 mL water/ g pomace) resulted in increasing the pectin solubility up to a pomace-water ratio of 1:2. The maximum amount of solubilised pectin (~ 605 μmol galacturonic acid/ g dry pomace) was determined when heating pomace at 130 and 140 °C for 15 and 7 min, respectively. Hydrothermal depolymerisation of pectin through acid hydrolysis and β-elimination reactions also showed temperature-dependent behaviour. Depolymerisation reactions resulted in degradation of pectin polymers into ethanol-soluble forms (galacturonic acid). Depolymerisation seemed more likely to happen from non-esterified sites of pectin polymers, as suggested by the high degree of esterification of the remaining insoluble pectin. Increasing amounts of glucose and fructose were observed in the serum phase of pomace to about five times their initial values when pomace was heated at temperatures >120 °C. This was accompanied by a reduction in sucrose content, suggesting hydrothermal hydrolysis of sucrose to its subunits. A complete conversion of sucrose was recorded at temperatures > 120 °C and times > ~30 min. Glucose, fructose and galacturonic acid underwent further transformation at temperatures > 100 °C forming secondary products of organic acids, (such as acetic acid, formic acid and lactic acid), furfural and 5-HMF. Modelling the kinetics of pectin solubilisation and 5-HMF production resulted in activation energies of 81 and 105 kJ/mol, respectively. The effects of hydrothermal treatment were modelled using COMSOL for heating a slab of pomace in a pilot scale retort with a maximum steam temperature of 125 °C. In this model, heat transfer through the pomace and chemical reactions of pectin solubilisation and 5-HMF production were predicted. The aim for this model was to identify conditions permitting solubilisation enough to double the amount of pectin from the amount initially present at room temperature while limiting the production of 5-HMF to the range permitted in food standards (bulk averaged). The validity of the model was confirmed in the pilot plant condition. Another objective of this research study was to investigate the effects of particle size distributions and molecular weight of pectin on sensory properties and physical stability of pomace samples. A controlled modification of particle size and molecular weight of solubilised pectin was achieved by fractionation of heat-treated pomace into insoluble solid and serum parts. Shearing insoluble particles for 5 min showed significant particle size reduction from 496 μm (initial shearing for 2 min) to 165 μm. Further shearing did not affect the average size of the particles. Microscopy revealed the effect of shearing in separating cell aggregations, resulting in individual cells. Shearing also fractured some individual cells, the oval shape of most cells was still visible. The pectin in the serum phase of pomace before and after heat treatment was analysed for molecular weight distribution. Results confirmed the effectiveness of heat treatment on solubilising high molecular weight pectins into the serum phase. Two ranges of high (between 4000-73 kDa) and low (< 73 kDa) pectin molecular weights were analysed in heat-treated pomace. These two ranges were separated from each other by ultrafiltration. High molecular weight of fraction (with the average size of 380 kDa) was enzymatically hydrolysed into two pectin components with average molecular weights of 30 and 150 kDa. Finally, six pomace ingredients were produced from the pomace fractions with two insoluble particle size distributions (with the average size ranges of 500 and 160 μm) and three ranges of molecular weights (380, 150 and 30 kDa) being blended. Particle size reduction had a significant effect on the physical stability of pomace suspensions. Samples containing particles >496 μm showed phase separation during storage (5 days at 4 °C), while samples with smaller particle size did not show any phase separation.
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The following Figures were removed for copyright reasons: Figures 2.2 (=Redgwell et al., 2008b Fig 6); 2.3 (=Ben-Arie et al., 1979 Fig 1A & B); 2.5 (=Watt et al., 1999 Fig 1); 2.6 (=Berjenholt et al., 2010 Fig 13.4); 2.7 (=Dick-Perez et al., 2011 Fig 8); 2.10 (=Day et al., 2010a Fig 1a-c); 2.14 (=Redgwell et al., 2008a Fig 5); 2.15 (=Espinoza-Munoz et al., 2013 Fig 1b & c); 2.16 (=Day et al., 2010a Fig 1e-g); 2.21 (=Vetter et al., 2003a Fig 1a-c) & 2.22 (=Redgwell et al., 2008a Fig 8).
Keywords
Apples, By-products, Fiber in human nutrition, Pectin
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