Impacts of preharvest and postharvest handling and processing on bioactive compounds and functional properties of pomegranate fruit fractions and by-products

Thesis (PhD)--Stellenbosch University, 2016.

ENGLISH ABSTRACT: Pomegranate fruit (Punica granatum L. Punicaceae) is highly valued owing to its high concentration of bioactive compounds found in the arils and peel. In fact, evidence from literature indicates that pomegranate fruit consumption has been associated with reduced risk of life threatening non-communicable diseases such as cancers and cardiovascular disorders. Although substantial amount of research has been reported on the effects of preharvest factors on phytochemical and functional properties of pomegranate, including cultivar and micro-climatic differences, little is known about the effects of postharvest and processing techniques on individual phenolic concentrations of fruit fractions such as arils and peel. The aim of this study was therefore to examine the impacts of preharvest and postharvest handling factors and processing methods on bioactive components and functional properties of pomegranate fruit and by-products. Drying characteristics and a thin-layer drying model for pomegranate peel over a wide temperature range were included in this study given the importance of drying as a commonly applied processing method in the processing of high-moisture products such as fruit. The results showed that concentrations of total phenolic and total tannin as well as radical scavenging activity (RSA) by DPPH assay declined as fruit maturity advanced, while ferric reducing antioxidant power (FRAP), total anthocyanin, total flavonoid and vitamin C concentration increased significantly (P<0.01). Principal component analysis (PCA) demonstrated that fruit grown in areas with lower altitude were associated with higher bioactive compounds at the full ripe stage. The study also showed significant (P<0.05) interaction effect between fruit maturity and altitude of the growing location on the phenolic compounds concentration. Studies on the effect of different extraction methods on phenolic compounds and antioxidant properties of pomegranate juice did not show significant influence (P>0.05) on fructose and total soluble solid concentration of pomegranate juice. Juice obtained from arils plus seed had the lowest citric acid concentration (18.96 g/L) and high juice colour saturation (2.69). Juice obtained by pressing fruit cut in half along the longitudinal axis (halved fruit) had significantly higher total phenolics, total tannins, radical scavenging activity and ferric reducing antioxidant power, which highlights the impact of extraction method on the quality of pomegranate juice. The influence of packaging and long term cold storage of whole pomegranates on phenolic compounds and antioxidant properties of fruit fractions and by-products thereof was also investigated. The result showed that total phenolics in pomegranate juice and peel decreased significantly (P<0.05) with prolonged storage duration regardless of package type. Catechin increased by 65.43% under modified atmosphere package (MAP) while rutin increased by 139.39% in individual shrink wrap package after 4 months of cold storage. Rutin was the predominant flavonoid in peel (3446.24 mg/kg dry matter), and its concentration decreased by 65% in fruit peel stored in MAP at the end of the storage (4 months). The study showed that punicic acid constituted 68.09% of total fatty acids in the seed oil and the concentration did not change significantly after 4 months under MAP and individual shrink wrap packaging, respectively. Fruit peel of whole pomegranates stored in individual shrink wrap package showed poor inhibitory activity against Gram negative bacteria (Klebsiella pneumonia), with minimum inhibitory concentration (MIC) of 1.56 mg/mL while seed oil showed better activity against diphenolase with inhibitory concentration (IC50) of 0.49 μg/mL after 4 months of storage. The effects of drying on the phenolic concentration, antioxidant, antibacterial and anti-tyrosinase properties were also studied. Freeze dried peel extracts had the highest total phenolic, tannin and flavonoid concentration compared to oven dried peel at the temperatures studied (40°C, 50°C and 60°C). Pomegranate peel extracts dried at 50°C showed the highest inhibitory activity with MIC value of 0.10 mg/mL against Gram positive bacteria (Staphylococcus aureus and Bacillus subtili) and monophenolase (22.95 mg/mL). Drying behaviour of pomegranate peels showed that drying time decreased as the oven drying temperature increased. The effective moisture diffusivity of pomegranate peel ranged from 4.05 x 10-10 to 8.10 x 10-10 m2/s over the temperature range investigated, with mean activation energy (Ea) of 22.25 kJ/mol. Empirical models were successfully applied to describe drying kinetics of pomegranate peel and these models could be used as analytical tools for future drying performance assessment.

AFRIKAANSE OPSOMMING: Granate (Punica granatum L. Punicaceae) word hoog aangeskryf weens die hoë konsentrasie van bioaktiewe verbindings wat in die saadomhulsel en skille voorkom. Volgens literatuur is daar bewyse gevind dat granate kan bydrae tot verminderde risiko van lewensgevaarlike kwale soos kanker asook kardiovaskulêre siektes. Alhoewel `n aansienlike hoeveelheid navorsing gerapporteer het oor die effek van voor-oes faktore op fitochemiese en funksionele eienskappe van granate, insluitend kultivar en mikroklimaat verskille, is daar nog min bekend oor die uitwerking van na-oes en verwerkings tegnieke op afsonderlike fenoliese konsentrasies op beide die saadomhulsels en skille. Die doelwitte van hierdie studie was dus om te toets wat die impak van voor-oes en na-oes hantering en verwerking op bioaktiewe verbindings en funksionele eienskappe van granate en neweprodukte is. Uitdroog eienskappe en `n dunlaag drogings model vir granaat skil oor `n wye temperatuur reeks was ook ingesluit in hierdie studie gegewe die belangrikheid van die droog as `n algemeen toegepas verwerking metode in die verwerking van `n hoë-vog bioproduckte. Resultate het gewys dat konsentrasies van totale fenole en tanniene asook die “radical scavenging activity” (RSA) in die DPPH toets afneem tydens rypwording, terwyl “ferric reducing antioxidant power” (FRAP), totale antosianien, totale flavonoïede en vitamien C beduidend toeneem (P<0.01). “Principal component analysis” (PCA) het getoon dat vrugte geproduseer word in areas op laer hoogtes bo seevlak areas geassosieer word met verhoogde bioaktiewe verbindings tydens die voryp stadium. The studie het `n beduidende interaksie tussen vrug rypwording en verskille in hoogte bo seevlak op fenoliese verbindings getoon. Studies oor die uitwerking van verskillende ekstraksie metodes op fenoliese verbindings en antioksidant eienskappe van granaatsap het nie `n beduidende invloed (P>0.05) op fruktose en totale oplosbare soliede inhoud van granaatsap getoon nie. Die laagste sitroensuur inhoud was waargeneem in saadomhulsels plus saad (18.96 g/L) en hoë sap kleur versadiging (2.69). Sap wat van gehalveerde vrugte verky is, het beduidende hoë totale fenole, totale tanniene, RSA en FRAP getoon wat die belangrikheid van ekstraksie metode op granaatsap kwalitiet uitwys. Invloed van verpakking en langtermyn koelstoring op fenoliese verbindings en antioksidant eienskappe van granate en neweprodukte was getoets. Die resultaat het gewys dat totale fenole in granaatsap en skil beduidend afneem (P<0.05) met langdurige stoor, ongeag die tipe verpakking. Catechin het toegeneem met 65.43% onder veranderde atmosfeer verpakking terwyl rutin toegeneem het met 139.39% in afsonderlike kleefplastiek verpakking na 4 maande van koelstoring. Rutin was die oorheersende flavonoïed (3446.24 mg/kg droëmateriaal) in skil, en die konsentrasie het afgeneem met 65% in vrug skil gestor in modified atmosphere packaging (MAP) aan die einde van stoor periode (4 maande). Die studie het gewys dat “punicic” suur 68.09% van die totale vetsure in saadolie uitmaak en dat die inhoud nie beduidend verander het na 4 maande onder MAP en afsonderlike kleefplastiek verpakking nie. Granaatskil wat in afsonderlike kleefplastiek verpakking gestoor is, het swak inhiberende aktiwiteit teen Gram negatiewe bakterieë (Klebsiella pneumonia) getoon (met minimum inhiberende konsentrasie van 1.56 mg/mL) terwyl saadolie beter aktiwiteit teen difenolase met inhiberende konsentrasie (IC50) getoon het met die konsentrasie van 0.49 μg/mL na 4 maande opberging. Uitwerking van uitdroging op die fenoliese konsentrasie, antioksidant, antibakteriële en anti-tyrosinase eienskappe was ook bestudeer (40°C, 50°C and 60°C). Granaat skil ekstrakte wat by 50°C gedroog is, het die hoogste inhiberende aktiwiteit getoon, met die minimum inhiberende konsentrasie waarde van 0.10 mg/mL teen Gram positiewe (Staphylococcus aureus en Bacillus subtili) en monofenolase (22.95 mg/mL). Uitdroginsgedrag van granaat skille het getoon dat droogtyd afneem soos die oonddroog temperatuur toeneem. Die effektiewe vog deurlaatdaarheid van die granaat skil het gewissel van 4.05 x 10-10 to 8.10 x 10-10 m2/s oor die temperatuur reeks wat ondersoek was; met gemiddelde aktiverings energie (Ea) van 22.25 kJ/mol. Empiriese modelle was suksesvol toegepas om die drogingskinetika van granaat skil te beskryf, en dit kan as `n hulpmiddel vir toekomstige uitdroging werkverrigting gebruik word.

Doctoral