Characterising water loss in pomegranate fruit cultivars (‘Acco’, ‘Herskawitz’ & ‘Wonderful’) under cold and shelf storage conditions

12 Fruit water loss results in a huge financial loss to the industry due to loss of aesthetic appeal and 13 direct loss of saleable fruit weight. There is currently a limited knowledge on the mechanism of 14 water loss in pomegranate fruits, given their complex structure. Therefore, this study aimed to 15 characterise water loss in the most common export pomegranate cultivars (‘Acco’, ‘Herskawitz’ 16 and ‘Wonderful’) of South Africa . Fruits were stored for 42 d at 7 ºC and 90 % RH and 17 thereafter transferred to shelf at 23 ºC and 58 % RH. Another batch of fruit was immediately 18 stored under prolonged shelf conditions for 16 d. Water loss, respiration rate, arils-peel 19 proportions and moisture content, peel thickness and colour attributes, puncture resistance 20 property and chemical quality attributes of fruit juice were measured. The study revealed that 21 despite the physiological and structural differences among pomegranate cultivars, water loss was 22 similar during the 42 d of cold storage. However, the medium-sized fruits (‘Herskawitz’ and 23 ‘Wonderful’ ) had significantly higher water loss (0.32 ± 0.01 g cm -2 ) than the (‘Acco)’ (0.25 ± 0.01 g cm -2 ) during the prolonged 16 d of shelf storage. The observed maximum 25 water loss of 24.2 % is mainly from the peel proportion. Therefore, research should primarily 26 focus on the peel fraction in addressing the water loss problems of pomegranate fruits. 27


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'Bhagwa', 'Ruby' and 'Wonderful' can lose 20-25 % of the initial fruit weight within 4 weeks at 48 temperature and relative humidity of 22 ⁰ C and 65 % 13,17 . During prolonged cold storage, the 49 fruits 'Bhagwa', 'Ruby' and 'Wonderful' lose between 10 -16 % of their weight within 12 50 weeks at 5 -7 ⁰ C and 90 -95 % RH 13,17-19 . A weight loss above 5 % causes shrivelling 14,20 . 51 Even in the absence of any visible shrivelling, water loss can undesirably affect the visual 52 appearance, flavour and textural properties of the fruits 21 . Excessive water loss results into 53 browning of the peel and arils and hardening of the rind 12,22 . It is important to note that 54 pomegranates are luxurious fruits that sell well in the higher market segment 4 . Therefore water 55 loss can easily cause a huge financial loss to the industry through direct loss of marketable fresh 56 weight and the associated diminished commercial value of affected fruits 22 . 57 Various water loss control techniques have been presented and investigated by many researchers. 58 Storage temperature and relative humidity are important water loss control parameters 13, 23 . while plastic liners cause moisture condensation within the bags promoting fruit decay 18 . 64 Therefore, there is a need to improve the control techniques which to some extent is hampered 65 by the limited knowledge on the characteristics of water loss of pomegranate fruits, given their 66 complex structure. Hence, this study aimed to characterise the water loss of pomegranate fruits 67 based on the fundamental physical and physio-chemical attributes. Secondly, the susceptibility of 68 pomegranate fruit cultivars to water loss was assessed and the contribution of the different parts 69 of the fruit to the water loss was examined. The study was done on the most important export  were packed in ventilated plastic trays cushioned with paper pads and transported using an air-78 conditioned refrigerated truck to the postharvest research laboratory, Stellenbosch University. 79 Sorting was carried out to ensure size uniformity and that the fruits were free from surface 80 defects such as cracks. Fruits were packed in dozens inside single layer display type corrugated 81 fibreboard carton, cushioned with paper trays at the bottom.

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The headspace gas composition (O 2 and CO 2 ) was determined using a closed system 33 . Two 122 fruits were enclosed in an equilibrated hermetically sealed glass jar, in triplicates, for each of the 123 storage conditions. Measurements were taken before and after two hours using a calibrated gas 124 analyser (CheckPoint, PBI-Dansensor A/S, Denmark), for separate setups at low temperature (7 125 ⁰ C) and high temperature (23 ⁰ C).

Fruit firmness 127
Fruit firmness was determined as puncture resistance 13,34 with a 5 mm diameter probe (GÜSS-128 FTA, South Africa). The probe was set to penetrate 8.9 mm into the fruit at 10 mm s -1 . The test 129 was carried out on opposite sides of the fruit cheeks, and the peak force (N) required to puncture 130 the fruit was reported as puncture resistance of 24 readings (2 × 12 fruits (replicates)).
Where A (cm 2 ) is the surface area and (cm) is the geometric mean diameter of the fruit, 148 calculated from the length (L (cm)), width (W (cm)) and thickness (T (cm)) of the fruit.
Where; is water loss per unit fruit mass (%), is water loss per unit surface area of the 154 fruit (g cm 2 ), (g) is the initial fruit mass, (g) is the mass of the fruit after storage days. The respiration rate (RR) was calculated in terms of carbon dioxide production rate ( 2 ) in mL 157 kg -1 h -1 by fitting experimentally obtained data into equation (5) 33 .   The three cultivars differed in mass, specific size dimensions along their L, W, T and overall 194 geometric mean diameter (Table 1). Generally, 'Acco' was the smaller-fruit cultivar 195 compared to 'Herskawitz' and 'Wonderful'. It is import to note that fruit size influences the 196 overall surface area to volume ratio and therefore the rate of water loss to the surrounding 197 environment.

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The water loss profiles of the three pomegranate fruit cultivars are presented in Figure 1.        The variability of physical and physio-chemical attributes of fruits from the three cultivars was 419 summarised in a PCA. Figure 8a-  are characterised by a relatively higher water loss than small-sized fruits ('Acco)' during 500 prolonged storage.

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The study revealed that water loss in pomegranate fruits is primarily and majorly from the peel 502 proportion and that peel related properties such as thickness, moisture content and puncture 503 resistance were significantly influenced by the storage duration. Despite the fact that water loss 504 resulted into deterioration of external aesthetic appeal of the fruits, the edible portion of the fruits 505 (the arils) remained unaffected even at total water loss of 24.2 %. Hence, juice yield was 506 minimally affected by the tested storage duration. Therefore, research should primarily focus on 507 the peel fraction in addressing the water loss problems of pomegranate fruits. This information is 508 helpful to plant breeders in selecting against water loss susceptible cultivars.