Fruit composition, storage stability, optimal drying conditions and flour characteristics for selected pumpkin varieties
The three economically important species of pumpkin namely Cucurbita maxima, Cucurbita pepo and Cucurbita moschata have many varieties. Pumpkin peel and flesh are key inter and intra specie distinguishing features but there is dearth of knowledge about the peel and flesh characteristics and how these relate to postharvest stability of the different varieties. Pumpkin flesh and seeds greatly contribute to human diet but also promote health through antioxidant effects hence often dried and powdered to increase their usability. However, no study has provided optimum drying temperature time combination for optimal nutritional quality and bioactivity of pumpkin flesh and seeds. This study aimed at understanding fruit characteristics and how they associate with postharvest stability of the major pumpkin varieties grown in Uganda, optimising nutritional quality and bioactivity of flours from flesh and seeds and assessing the suitability of the flours for value addition. To determine the pumpkin fruit varietal characteristics and how they relate to postharvest stability, mature fruits of C. maxima duchesne subsp maxima, C. pepo L var. fastigata and C. moschata decne were stored at ambient conditions (28 oC average temperature and 78% relative humidity) for 8 months. Cell microscopic structure, intercellular space size, sugars, starch, cellulose, hemicellulose, lignin, degree of esterification of pectin, polygalacturonase activity, and cumulative moisture lost were determined monthly. Factor analysis revealed the major changes affecting postharvest stability in the first 3 months to be reduction in sucrose, esterification of pectin, starch, hemicellulose and cellulose, with factor loadings of -0.97, -0.88, -0.87, -0.82 and -0.79, respectively. Moisture loss from the flesh (0.95) and peel (0.94), and change in size of intercellular spaces (0.93) were major changes from 4 to 7 months, while polygalacturonase activity (0.64) was the major factor from 7 to 8 months. Sucrose, starch, cellulose and hemicellulose content and degree of esterification of pectin of different varieties decreased with storage. Glucose and fructose content increased to a maximum at 4 months and then reduced. Polygalacturonase activity and lignin generally increased with storage time. Size of intercellular spaces, and cumulative moisture lost from both flesh and peel increased with storage. The results suggest that postharvest stability of pumpkins is determined by an interplay of factors. The deterministic factor was found to change with storage time. Sucrose breakdown was xvii key during 0 to 3 months, while moisture loss was important at 4 months of storage. Polygalacturonase activity became paramount from 7th month of storage. Of all the varieties studied, C. moschata decne deteriorated fastest. In the first 3 months of storage, the rate of starch breakdown was 0.45, 0.52 and 0.84 g/100 g/month FW in C. maxima duchesne subsp maxima, C. pepo L var. fastigata and C. moschata decne, respectively. The pumpkin varieties also showed a breakdown in sucrose of 0.51, 0.22 and 0.38 g/100 g/month FW respectively, in the first 5 months of storage. C. moschata decne also had the least starch (3.56 g/100 g FW), sucrose (3.15 g/100 g FW), and hemicellulose (1.60 g/100 g FW) contents compared to other varieties. Pumpkin varieties with more lignified peel had lower rates of sucrose breakdown and moisture loss and tended to be more stable postharvest. Varieties with low level of hemicellulose had their cell walls degrade early, while deterioration of pumpkins with highly esterified pectin and closely packed cells was delayed. To select a pumpkin variety to be used in optimisation of drying conditions for maximum nutritional quality and bioactivity of flour from pumpkin flesh and seeds, flesh and seeds of mature C. maxima duchesne subsp maxima, C. pepo L var. fastigata and C. moschata decne, were assayed for proximate composition, α, β, γ, δ-tocopherols and tocotrieonols, total carotenoids, ascorbic acid content, and antioxidant activity. C. moschata decne flesh and seeds had significantly higher (p<0.05) antioxidant activity (% DPPH radicals scavenged) of 44.68% and 53.13%, respectively. Among the three varieties, C. maxima duchesne subsp maxima flesh and seeds had the least antioxidant activity (% DPPH radicals scavenged) of 41.02% and 50.15%, respectively. Fat content of the flesh (0.17-0.23 g/100 g FW) and seed (15.11-19.23 g/100 g FW) and moisture content of the flesh (86.68-90.08 g/100 g FW) and seed (39.85-44.85 g/100 g FW) render all the matrices oil-in-water systems which favours the activity of non-polar antioxidants like tocopherols, tocotrienols and carotenoids. Seed matrices being more non polar favour the activity of polar ascorbic acid unlike flesh matrices. For each variety, β and γ tocopherols existed in same amounts. Hence C. pepo L var. fastigata that was more shelf stable than C. moschata decne and with antioxidant activity second to C. moschata decne was selected for optimisation of drying conditions for nutritional quality and nutraceutical value of pumpkin flesh and seeds.