Postharvest technologies for predicting and reducing susceptibility of ‘Marsh’ grapefruit (Citrus paradisi MacFad.) to rind pitting disorder.

Abstract

Citrus fruit is globally one of the most important fruit due to their nutritional value and sensorial attributes, however, they were susceptible to various postharvest disorders, especially during shipping period. Therefore, the aim of this study was to determine the effect of edible coating, carboxymethyl cellulose (CMC) infused with moringa leaf extracts (MLE) on reducing postharvest physiological rind pitting disorder in ‘Marsh’ grapefruit (Citrus paradisi MacFad.). The study also reviewed the literatures on the ability of edible coatings to improve fruit quality and extend shelf life of citrus fruit. Edible coatings recently received attention due to their ability to enhance fruit quality without compromising human health. The first experimental chapter was conducted to evaluate the ability of CMC and MLE as edible coatings to control the disorder in ‘Marsh’ grapefruit. A total of 300 fruit (150 from outside canopy and 150 from inside canopy) were harvested from a commercial orchard at Dole Bolton Citrus Estate in Nkwalini at Showe, KwaZulu Natal, South Africa. Fruit were subjected to different treatments, control (untreated), CMC (0.5%) + MLE (10%), CMC (1%) + MLE (10%), CMC 0.5% and CMC 1%. Treatments were organised in a factorial design. Fruit were stored at 3 ± 0.5 °C and 90-95 % relative humidity (RH), for nine weeks and thereafter taken to room temperature (22 ± 2 °C) for two weeks to simulate shelf life. The physicochemical attributes (total soluble solids. titratable acidity, maturity index, fruit mass loss, fruit colour, rind dry matter) of the fruit were analysed during this period. Rind pitting as well as sensory quality was evaluated at the end of storage period. This study identified that CMC 0.5% + MLE 10% and CMC 1% + MLE 10% reduced postharvest rind pitting disorder incidence compared to CMC 0.5%, CMC 1% and the control treatment. High mass loss contributes largely to rind pitting development, however, edible coatings managed to provide semi-permeable barrier to the fruit. Uncoated fruit had high mass loss which may be due to high water loss from the rind, most probably rind cell collapsed thereby leading to visible pitting in fruit rind. Coated fruit with low rind pitting incidence had low rind dry matter (RDM) compared to uncoated fruit with high rind pitting incidence. This study reported that total soluble solids (TSS) increased with storage time, however, low rate of increase was noticed in coated fruit compared to uncoated fruit. Fruit with higher TSS at the end of storage had high rind pitting incidence compared to fruit with low TSS. Rind colour was expressed as citrus colour index (CCI). Citrus colour index was noticed to increase with storage, however, the rate of increase in coated fruit was lower than that of uncoated fruit. At the end of storage, CCI was therefore higher in uncoated fruit than coated

fruit, while higher CCI was correlated with high rind pitting incidence. These physicochemical quality parameters can be used to predict rind pitting occurrence in ‘Marsh’ grapefruit. The second experimental chapter investigated rind phytochemical quality attributes that can be used as pre-symptomatic markers of rind pitting disorder in ‘Marsh’ grapefruit. Treatments used for this chapter were similar to the abovementioned. Treatments were organised in a factorial design. Visible to near infrared spectroscopy (Vis/NIRS) as a non-destructive technique was used to develop models that can assist in rind pitting disorder prediction. Partial least square (PLS) regression models were developed to predict rind phytochemical quality attributes such as ascorbic acid, phenolics, flavonoids, antioxidant capacity and activity, pigments (chlorophyll a and b, β carotene and total carotenoids) and sugars (sucrose, glucose and fructose), and these models were developed to predict rind pitting disorder of ‘Marsh’ grapefruit. Noticeably, CMC when combined with MLE were able to reduce the incidence of rind pitting disorder when compared to their counterparts. This could be due to the fact that moringa is believed to have high content of flavonoids, phenolics and antioxidants, which may be released to fruit and act as free radical scavengers from the cell matrix and protect fruit from external stress. These studies further investigated the effect of canopy position on susceptibility of rind pitting development. It was found that outside canopy (OC) fruit were more susceptible to rind pitting disorder compared to fruit from inside canopy (IC). This may be due to that OC fruit are exposed to different climate during fruit growth and development which could lead to rind quality stress and damage rind cells. Since OC fruit were more prone to disorder development than IC fruit, it would make financial sense to export fruit to the low demanding market with less penalties if fruit develop pitting prior to destination. Alternatively, fruit with higher chances of developing disorders (OC fruit) must be sent to local markets or fruit may be processed to other sellable products such as juices and dried fruits.