Browsing by Author "Kritzinger, Imke"

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    Postharvest moisture loss in Japanese plums
    (Stellenbosch : Stellenbosch University, 2019-04) Kritzinger, Imke; Lotze, Elmi; Theron, K. I.; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.
    ENGLISH ABSTRACT: Plums exported from South Africa reach overseas markets after a long sea freight period. Yet consumers still expect fruit to be in perfect condition upon arrival at the supermarket. While care is taken to limit moisture loss throughout the handling chain, fruit still show the negative effects thereof. Reduced fruit quality due to moisture loss may lead to rejection of export consignments at overseas markets, causing major financial losses for South African producers. The aim of this study was to investigate the role of the fruit cuticle in determining moisture loss and susceptibility to shrivel development in Japanese plum cultivars. Peel permeability differed between farms, seasons, cultivars, orchards and developmental stage. In general, the water vapour permeance of the peel was higher in cultivars that are susceptible to moisture loss and shrivelling. However, this was not true in all cases and measuring pre-harvest water vapour permeance of the peel to predict shrivel susceptibility was only successful in some cultivars. Lenticel numbers differed between seasons and cultivars and clearly contribute to moisture loss, but this contribution differs between cultivars. As the number of open lenticels could not explain all the variation in peel permeability between cultivars, cuticle composition must play an important role in determining peel permeability. Cuticular composition differed significantly between cultivars and seasons. The compound 2,4-bis (dimethyl benzyl) phenol was present in high concentration in both cultivars. We propose that the combination of a rigid cuticle, due to high phenol content, fewer tri-hydroxy acids, and high primary alcohol content, and its smaller intercellular spaces, reduces ‘Songold’ cuticle deformation due to excessive postharvest moisture loss. Since the hypodermal cells of ‘Songold’ are closer together, their dehydration and collapse might not lead to significant shrinkage compared to the other cultivars. The cuticle is rigid, which means that it is less likely to collapse when the supporting cells underneath it shrink and collapse due to moisture loss. Packaging solutions to reduce moisture loss need to be optimized for individual cultivars since they vary so much in terms of susceptibility to moisture loss and shrivel. Using Low-Density Poly-Ethylene packaging with 92 or 72 micro-perforations might be a viable option to reduce moisture loss, while still preventing excessive in-package humidity, decay and chilling injury. In seasons when high rates of moisture loss are experienced, the use of these bags might reduce the number of consignments rejected at overseas markets. This study showed the complex interplay of different cuticle characteristics in response to or as a result of, moisture loss. It would be interesting to investigate how environmental signals lead to a certain cuticular response – which genes are involved, how these genes activated and so forth. Elucidating some of the mechanisms involved in the functioning and response of this complex biopolymer might enable manipulation of the cuticle to improve fruit quality and extend shelf life or to select and breed cultivars that are not prone to cuticular defects.
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    A study of broken stones in Japanese plums (Prunus salicina Lindl.)
    (Stellenbosch : Stellenbosch University, 2015-12) Kritzinger, Imke; Jooste, M.; Lotze, Elmi; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.
    ENGLISH ABSTRACT: In order to export South African plums to overseas markets strict quality standards must be maintained. Among these quality classifications are specifications about the presence of cavities and pieces of broken stone/pit within the flesh of the fruit. If more than 10% of the fruit in a carton are affected by severely broken stones or large flesh cavities, the fruit have to be marketed as Class 2. A substantial amount of plums destined for export from South Africa is affected by broken stones and thus have to be marketed as Class 2. Lower prices are attained for Class 2 fruit, therefore, the presence of broken stones has a detrimental effect on the income generated from these fruit. The main aim of this study was to gain a better understanding of broken stone development and compare the growth characteristics of Japanese plum cultivars differing in their susceptibility to broken stones. Stone breakage in ‘Laetitia’ was observed as soon as stone hardening was initiated. At the start of stone hardening the parts of the stone that are still ‘soft’ are not strong enough to resist the pulling forces of the growing mesocarp and the stone is subsequently pulled apart. Regression analysis indicated that lengthwise growth of the fruit, fresh weight of the endo- and mesocarp, minimum orchard temperature and orchard night temperature, and relative humidity (RH) early in the growing season could possibly be used to predict the incidence of broken stones at harvest. Differences in the incidence of broken stones were observed between ‘Laetitia’, ‘Sapphire’ and ‘Songold’ plums and between seasons. Furthermore, significant differences were observed in the density of the endocarp in different parts of the stone. For ‘Laetitia’ and ‘Songold’, stone breakage was observed when rapid increases in stone density coincided with rapid increases in fruit growth. The stones broke in positions where an interface exists between high and low density parts in the stone and when rapid radial growth takes place in the direction where the stone is least dense. In contrast, in ‘Sapphire’, stone breakage was observed before the stones had started to lignify, indicating that the endocarp was pulled apart by the expanding flesh because it was too soft to withstand the strong pulling forces created by the flesh. The incidence of broken stones was influenced by environmental factors, as higher temperatures during the stone development and hardening period could lead to more complete endocarp formation (more stone cells are formed under such conditions). Such fruit would thus have higher endocarp density, which, if coupled with rapid radial growth, could lead to a higher incidence of broken stones. Foliar and/or root applications of calcium nitrate and potassium silicate were applied to ‘Laetitia’ plums to determine whether the incidence of broken stones could be reduced by increasing the strength of the endocarp cell walls. However, no such effect was observed. Hence, neither calcium nor silicate treatments can be recommended for reducing broken stones in plums.