ARID ZONE JOURNAL OF ENGINEERING, TECHNOLOGY & ENVIRONMENT
AZOJETE - CIGR Section VI Special Issue:

Innovation & Technologies for Sustainable Agricultural Production & Food Sufficiency
AZOJETE, December, 2018. Vol. 14(SP.i4): 202-207

Published by the Faculty of Engineering, University of Maidiguri, Maidiguri, Nigeria.

Print ISSN: 1596-2490, Electronic ISSN: 2545-5818
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*Corresponding author: opara@sun.ac.za, umunam@yahoo.co.uk

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ORIGINAL RESEARCH ARTICLE

DYNAMICS OF POMEGRANATE FRUIT WEIGHT LOSS DURING PRECOOLING AND
AMBIENT STORAGE: A SPATIAL AND TEMPORAL ANALYSIS

M. Mukama1, A. Ambaw2 and U. L. Opara1, 2,*
1Postharvest Technology Research Laboratory, South African Research Chair in Postharvest Technology,

Department of Food Science, Faculty of AgriSciences, Stellenbosch University, South Africa
2Postharvest Technology Research Laboratory, South African Research Chair in Postharvest Technology,
Department of Horticultural Science, Faculty of AgriSciences, Stellenbosch University, South Africa

ARTICLE INFORMATION

Received: October, 2018
Accepted: December, 2018

Keywords:
Punica grabatum
Humidity management
Cold chain
Moisture loss
Liner
Market conditions
Quality

ABSTRACT
In this paper the spatiotemporal profile of weight loss of
pomegranate fruit (cv. Wonderful) was investigated during
precooling and simulated shelf conditions. The effects of relative
humidity (RH) inside the cold room, polyliner inside the packaging
and stack orientation on fruit weight loss were studied. Weight loss
during the precooling operation ranged from 0.17 to 0.25% of the
initial fruit weight and was highest during precooling of stack
without liner and inside non-humidified room (0.25%). It was
observed that fruit weight loss in liner-based packaging was almost
equivalent to room humidification. Results of the shelf life study
demonstrated the importance of room humidification to preserve
fruit quality. Storing fruit in a room at 95% RH minimised weight
loss and best maintained fruit colour, firmness, size and chemical
quality attributes of pomegranates. On the other hand, fruit stored
at ambient condition (65% RH) up to 30 days had excessive weight
loss (up that 29.13±1.49%), which led to shrivel, deformed
appearance and considerably reduced overall visual quality.
©2018 Faculty of Engineering, University of Maiduguri, Nigeria. All rights reserved

1.0 Introduction
Rapid moisture loss is among the main quality problems affecting postharvest life of
pomegranate fruit (Fawole and Opara, 2013; Arendse et al., 2014). On top of losing marketable
fruit weight, fruit that lose moisture above 5% will shrivel; this reduces their visual appeal and
commercial value. Weight loss in fruit is mainly due to transpiration and to a relatively small
extent due to respiratory activity (Waelti, 2010). Large vapour pressure deficit (VPD) between
fruit surface and the surrounding air leads to increased rate of moisture loss.

Cooling pomegranate fruit preserves quality, but weight loss remains a challenge during cold
storage (Arendse et al., 2014). Humidification of the storage room, packaging fruit in plastic films,
fruit coating, shrink wrapping are some of the practiced mitigation measures to moisture loss.
However, humidification can also cause several storage problems affecting fruit. For instance,

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Arid Zone Journal of Engineering, Technology and Environment, December, 2018; Vol. 14(sp.i4):202-207.
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during cold storage of table grapes, humidification increased stem dehydration, browning of
berries, increased the incidence of SO2 injury and package wetting (Ngcobo et al., 2013).

Temperature and humidity control in a stack of fruit is normally based on measurements taken
from specific locations in the storage room. However, these have been reported to be spatially
non-uniform in fruit pallets (Ambaw et al., 2017; Mukama et al., 2017). The amount of weight
loss and the degree of spatial variation in the precooling process of pomegranate fruit are not
clearly understood. Therefore, the objectives of this study were to: 1) measure the spatial and
temporal weight loss profile during precooling of pomegranate fruit, 2) study the effects of
internal packaging, humidification, and package orientation on the moisture loss, and 3)
investigate the physico-chemical quality attributes of pomegranate fruit stored at different shelf
conditions.

2.0 Study Description
2.1 Fruit
Pomegranate fruit (cv. Wonderful) were harvested at commercial maturity from Merwespont
farm in Bonnievale (33°58’12.02” S, 20°09’21.03” E), Western Cape, South Africa and transported
in an air-conditioned vehicle to Postharvest Technology Research Lab at Stellenbosch University.

2.2 Package materials
In this study, corrugated fibreboard carton box (CFC) was used to contain the pomegranate fruit.
The box had 6 semi-circular vent-holes on the long side located at the top and bottom rim of
the side, 2 semi-circular vent-holes at the top rim of the side and 5 circular vent-holes on the
bottom side. Each box contained 12 fruit with average weight of 4.32 ± 0.39 kg per carton. Fruit
weight loss during precooling of two different package designs was investigated: package with
internal polyliner and another without polyliner. Plastic wrapping was done by placing
pomegranates in a single non-perforated 10 µm thick high density polyethylene (HDPE) plastic
film.

2.3 Air suction equipment (ASE)
The ASE was a box with suction fan attached to one end. The suction was generated by using a
centrifugal fan (KDD 10/10 750W 4P-1 3SY, AMS supplies, Sandton, South Africa). The stacked
fruit, as covered with plastic sheet, was placed in front of the ASE so that air was drawn
horizontally through it.

2.4 Cold storage room
The ASE/stack assembly was placed inside a 20 m3 cold storage room equipped with a cooling
unit and a humidifier. The cooling unit had three evaporator fans each creating an air circulation
rate of 1290 m3 h-1. Cold room humidity was controlled using Aqua Room-2 humidifier (Miatec
Inc. 9480SE, Lawnfield Road, Chackamas OR 97105 USA) with 1.4-2.1 bars pressure capacity, 2 L
h-1 liquid capacity, 10 µm droplet size and digital hygrotransmitter sensor (0-100% RH).

2.5 Experiments
2.5.1 Precooling experiments
70 cartons were stacked (7 layers of 10 cartons) on an ISO standard pallet (1.2 m × 1.0 m × 0.1
m). The stack was first equilibrated to ambient air conditions (≈ 17 °C and 65% RH) before being
placed inside the cool storage room. The sides and top of the stack were covered with plastic
sheet so that chilled air was horizontally sucked through the stack by the ASE. Two different

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Mukama, et al. Dynamics of pomegranate fruit weight loss during precooling and ambient storage: a spatial and
temporal analysis. AZOJETE, 14(sp.i4):202-207. ISSN 1596-2490; e-ISSN 2545-5818, www.azojete.com.ng

204

pallet orientations with respect to the ASE were considered: pallet with its 1.2 m side
perpendicular to the air flow and pallet with the 1.0 m side perpendicular to the air flow. Due to
difference in vent-hole proportion between the long and short side of the box, the vent area
along the flow direction of the two different orientations are dissimilar. For the 1.2 m oriented
stack, the vent-hole ratio along the flow direction were 9.45% and 2.15% at inlet and outlet,
respectively. The 1.0 m orientation had vent-hole ratio of 4.24% at both the inlet and outlet end.

All precooling experiments were from the initial ambient condition (≈ 17 °C) down to 7 ± 1.2 °C.
During the experiment fruit pulp temperature was monitored at intervals of 5 minutes using T-
type thermocouples and a 34970a Data Acquisition/Data Logger Switch Unit (Agilet
Technologies, Santa Clara CA 95051, USA). The measured temperature data was used to
calculate the stack average 7/8th cooling time which determined the time to stop the precooling
experiment. Additionally, fruit weight loss was measured by taking initial and final weight of
sample fruit. The temperature and weight loss sampling positions were from stack levels 2, 4,
and 6. A total of 24 measurements (2 pallet orientations × 2 RH conditions × 2 liner conditions ×
3 repetitions) were taken.

2.5.2 Measuring the effect of humidity on shelf life of pomegranate fruit
Two groups of 216 fruit each were equilibrated to ambient condition before the start of the
experiment. Group 1 was kept under high humidity condition (95 ± 1.23 %RH) and group 2
under low humidity condition (65 ± 6.79 %RH). In both cases, the room temperature was kept at
20 ± 0.36 °C. Fruit weight, colour, firmness, size, titratable acidity, total soluble solids and pH
were assessed on a 3 day interval for 30 days.

2.6 Statistical analysis
Statistical analysis was done using Statistica software (Statistica version 12, StatSoft Inc., Tulsa,
USA). Mixed model repeated measures analysis of variance (ANOVA) was done using the VEPAC
module of Statistica 12 at 95% confidence interval. Variations in weight loss were compared
between the package designs, stack levels, stack orientations and fruit position within a stack
level. Statistical significance of the treatments was tested using Duncan’s Multiple Range Test
and means with p < 0.05 were considered significant.

3.0 Results and Discusions
3.1 Weight loss during precooling of pomegranate fruit
On average, weight loss ranged from 0.17 to 0.25% of the initial fruit weight. Clearly, fruit weight
loss was highest (0.25%) for the stack without liner and in a non-humidified cool store (“None” in
Fig. 1 (a)). Stack with plastic liner and in a humidified room (“Liner + Humidification” in Fig. 1 (a))
experienced the lowest weight loss (0.17%). However, the presence of liner always increased fruit
cooling time (Fig 1(b)) such that stacks with liner had SECT higher than 10 h compared with less
than 5 h for stacks without liner.

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Fig. 1. Weight loss of pomegranate fruit (mean ± STDV) during precooling (a) and the corresponding
7/8th cooling time (SECT (b). All humidification was at 95 ± 1.23 %RH. When no humidification, the cool
storage room was kept at ambient humidity condition of 65 ± 6.79 %RH. The weight loss and SECT values
are stack means. Means with different letters are significantly different (p < 0.05).

3.1.1 Effect of stack orientation
The average weight loss of precooled pomegranates in the 1.0 m orientated pallet was 0.20%
while those in the 1.2 m oriented pallet it was 0.22%. The 1.0 m side of the pallet has relatively
lower ventilation compared to the 1.2 m orientation as described in section 2.5.1. Low ventilation
rate results in relatively lower convective mass transfer coefficient from the fruit surface to the
ambient air. This leads to the observed low weight loss profile. However, this also leads to a
relatively higher cooling time.

3.1.2 Spatial variation in weight loss
Spatial variability of the weight loss is important to identify the high and low weight loss regions.
Fruit in the upstream region received the chilled air first, thereby subjecting them to a faster
cooling rate. Increase in air temperature as it moves across a stack causes local variations in heat
loss in the stacked products (Baird et al., 1988) and holding pomegranate fruit at higher
temperatures for longer period causes higher moisture loss rates. There was no significant
difference in weight loss between layers of the stack in all cases.

3.2 Effect of humidity on shelf life quality of pomegranate fruit
3.2.1 Weight loss and fruit shrivel
Pomegranate fruit continuously lost weight throughout the shelf life period. Till day 3, there was
no significant difference between the two RH environments. However, fruit weight loss under the
low RH environment became significantly higher starting from day 6, with losses reaching up to
29.13±1.49% in the low RH environment compared to 5.78±0.44% in the high RH environment.
This shows that the high RH environment reduced the vapour pressure deficit (VPD) between the
fruit and the environment, resulting in a significantly reduced moisture loss from the fruit
(Ngcobo et al., 2013).

Shrivel was observed on fruit from the low humidity environment on day 6. At this stage the
average weight loss was 5.28 ± 0.32% of its initial weight, and by day 9, the dents on the fruit
surfaces were larger. Shrivel is due to loss of turgor pressure in the fruit cell walls as they
continuously lose moisture (Paull, 1999). Under high humidity environment, some fruit slightly
shriveled on day 24.

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Mukama, et al. Dynamics of pomegranate fruit weight loss during precooling and ambient storage: a spatial and
temporal analysis. AZOJETE, 14(sp.i4):202-207. ISSN 1596-2490; e-ISSN 2545-5818, www.azojete.com.ng

206

Changes in fruit colour, size, firmness, titratable acidity, pH, and total soluble solids were more
significantly pronounced in the low humidity storage environment than high humidity
environment (Fig. 2)

Fig. 2. Visual condition of pomegranate fruit 30 days under ambient humidity condition, RH ≈ 65% (top
row) and under humidified condition, RH ≈ 95% (bottom row).

4.0 Conclusion
This study investigated the level of reduction of weight loss achievable by employing liner-based
packaging or room humidification. This study also quantified the spatial variation in weight loss
of pomegranate fruit during precooling operation. Fruit at the back of the stack had higher
weight loss than those at the front. This goes in parallel with the temperature distribution in the
stack as reported in previous studies (Ambaw et al., 2017 and Mukama et al., 2017). The shelf life
study showed the importance of room humidification as a cold chain strategy to maintain
pomegranate postharvest fruit quality. Storing fruit under 95% RH maintained fruit colour best,
minimised weight loss, maintained fruit firmness, fruit size and the chemical quality attributes of
pomegranates. Storing fruit under low RH ambient conditions led to excessive weight loss, which
in turn resulted in excessive shrivel, deformed appearance, and reduced visual quality of fruit.
These findings can be applied in efforts to establish the best storage conditions of
pomegranates to maintain quality and reduce incidence of postharvest losses along the value
chain from harvest to consumers.

5.0 Acknowledgement
This work is based upon research supported by the South African Research Chairs Initiative of
the Department of Science and Technology and the National Research Foundation. The project
was supported through contract research with Agri-Edge Ltd funded by the Department of
Trade and Industry (dti) through the Technology and Human Resources for Industry Programme
(THRIP). We acknowledge the award of postgraduate scholarship to Mr M Mukama by DAAD (In-
Region Scholarship Programme) and the Regional Universities Forum for Capacity Building in
Agriculture (RUFORUM) for support.

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rate and uniformity of cooling of stacked pomegranates: Numerical and experimental
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Arid Zone Journal of Engineering, Technology and Environment, December, 2018; Vol. 14(sp.i4):202-207.
ISSN 1596-2490; e-ISSN 2545-5818; www.azojete.com.ng

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