Quality change in harvested winter squash by enhancement of calcium status and by use of surface coatings: a dissertation presented in partial fulfilment of the requirements for a Masterate of Horticultural Science, Massey University, Palmerston North, New Zealand
Rots and physiological weight loss cause substantial postharvest losses in harvested winter squash (Cucurbita maxima D. hybrid 'Delica'). This research had both preharvest and postharvest components. Preharvest research was aimed at treatments in the field which would decrease storage rots of squash. Postharvest research was focused at reduction of weight loss which is normally caused by storage conditions during shipment. Calcium concentration affects maintenance of firmness of many fruits and also has a significant effect on fruit resistance to rot development. Enhancement of transpiration of a fruit by an alkaline spray may result in increase of its calcium concentration, since calcium is transported with mass flow of water. Three spray treatments, Na₂CO₃ solution, CaCl₂ solution and H₂O (control), and four painting treatments (Na₂CO₃ solution, CaCl₂ solution, vapour gard solution and H₂O) were applied to squash growing in the field, and the fruit were stored at ambient temperature after harvest. Squash given Na₂CO₃ spray rotted less in the field and tended to rot later during storage than those sprayed CaCl₂ and H₂O, but there was no relationship between the order in which squash rotted and the mineral concentration (calcium, nitrogen and phosphorus) of their tissue. With painting treatments, calcium concentration of both skin and cortex in the squash treated with Na₂CO₃ was higher than those of other three treatments. Enhancement of transpiration changed fruit calcium status, but it was not confirmed if this changed resistance of the fruit to rot development. In the postharvest study, three experiments were carried out. In the first, it was shown that 'Primafresh' coating was the most suitable of a number of materials for reducing weight loss without undue modification of the fruit's internal atmosphere. In the second, squash were given 0, 1, 2, 3, 4, 5, 6 'Primafresh' coatings and stored in a plastic bag at 20°C and 90% RH. Rate of weight loss at 24h intervals, and internal atmosphere composition after 3 days of storage. (O₂, CO₂ and C₂H₄, and CO₂ production of each squash were monitored. Coating significantly reduced weight loss of squash, but the order of weight loss did not relate to the number of coatings applied. Coating markedly decreased internal oxygen concentration ([O₂]ᵢ, mol molˉ¹) and increased internal carbon dioxide concentration ([CO₂] ᵢ, mol molˉ¹) and internal ethylene ([C₂H₄] ᵢ, mol molˉ¹) of squash. The decrease of [O₂]ᵢ and increase of [C₂H₄]ᵢ were dependent on the number of coatings applied, but [CO₂]ᵢ was not. On average over all coating treatments, CO₂ production of coated squash was significantly greater than those of non-coated squash. [O₂]ᵢ, [CO₂]ᵢ and CO₂ production were highly variable in individual squash within each coating treatment. Off-flavour ratings were highly, positively correlated with [CO₂]ᵢ of individual squash. Potential benefits of surface coatings to reduce weight loss of squash appear to be quite limited. Increasing the number of coatings resulted in fermentation without achieving worthwhile reduction of weight loss. Use of a fruit system mathematical model with the information gathered from this thesis and a potential alternative method for reduction of weight loss are discussed. In the third experiment, squash were coated with Primafresh at 2 days intervals to allow physiological equilibration before the following coating was applied. Internal atmosphere composition was measured at 24h intervals. [O₂]ᵢ decreased and [CO₂]ᵢ had increased 24h after each coating. This effect was followed by an increase of [O₂]ᵢ and decrease of [CO₂]ᵢ 48h after each coating, but not to the level present before each coating. This indicated that respiratory response of fruit to the change of internal atmosphere caused by increased skin resistance was not instantaneous. There were a delay between the physical and physiological effects of elevated skin resistance in squash. Results of the second and third coating experiments showed that the sum of [O₂]ᵢ + [CO₂]ᵢ in coated squash was always more than 0.21 mol mol-¹ at any level of (O₂]ᵢ, which is different from the response of other fruit to surface coatings. Two possibilities for this response were discussed. This thesis has shown that both preharvest and postharvest surface coatings can have profound effects on the fruit physiology and thereby affect storage behaviour. Further work will be required if these effects are to be optimised to provide treatments suitable for commercial application.