HEAT TRANSFER AND EXTERNAL QUALITY ATTRIBUTES OF 'REGAL SEEDLESS' TABLE GRAPES INSIDE MULTI LAYERED PACKAGING DURING POSTHARVEST COOLING AND STORAGE

Abstract

Postharvest packaging of table grapes is characterized by multi-layer packaging comprising of the carton box, the inner liner film, the individual bunch carry-bag and the SO2 sheet. This multi-layered packaging is required to allow sufficient airflow to efficiently cool packed grapes in order to preserve quality. Total pressure drop and cooling rates of different table grape packaging systems were measured and the percentage contributions of each package component and the fruit bulk were determined. Also, the effects of different carton liners on the cooling rate and external quality attributes of table grapes were investigated. Fruit quality attributes measured included stem dehydration and browning, SO2 injury and decay. On average the liner films contributed significantly higher to total pressure drop (61.04±15.91 %) than other components of the package combinations. Compared with the cooling of bulk grape bunches, the presence of the bunch carry bag increased the half and seven-eighth cooling time by 61.1 % and 97.3 %, respectively. The addition of plastic liners over the bunch carry bag increased the half and seven-eighth cooling time by 168.9 % and 185.2 %, respectively. Non-perforated liners maintained relative humidity (RH) close to 100 % during cold storage and during a 7-day shelf-life period which resulted in delaying the loss of stem quality but significantly (P  0.05) while increasing the incidence of SO2 injury and berry drop during storage compared with perforated liners. INTRODUCTION In most fresh food refrigeration systems, heat is transferred primarily by forced convection, where cold air is forced through food packages; therefore, the temperature and its homogeneity is largely governed by the patterns of the airflow (Smale et al., 2006; Zou et al., 2006). Table grapes are non-climacteric fruit and therefore should be harvested when reaching optimum maturity (Ginsburg et al., 1978; Hardenburg et al., 1986). However, fruit quality tends to deteriorate rapidly during postharvest handling and storage, thus reducing shelf-life during marketing. Deterioration of table grape quality is mainly characterised by stem (rachis) dehydration and browning, fruit weight loss and colour change, accelerated berry softening, berry shatter and high incidence of berry decay due mainly to Botrytis cineria. Table grapes are packed in multi-layered packaging which includes the carton boxes with multiple inner-packaging materials including carton liner films, SO2 pads, moisture absorption sheets and bunch carry bags (Ngcobo et al., 2011). Resistance to airflow may even be higher during table grape cooling due to the fact that berry bunches are packed inside multi-layered packages. It is therefore important to understand the effects of packaging materials and produce on airflow in order to fully understand cooling patterns and moisture transfer properties for the design of efficient cooling systems. The aim of this experiment was to investigate the effects of multi-layered packaging on airflow and cooling patterns and on external quality attributes of table grapes during cooling and storage. MATERIALS AND METHODS