Influence of postharvest treatments and storage conditions on the quality of Hass avocados.

The purpose of this study was to investigate the influence of postharvest treatments on the shelf life of avocado. The experimental design consists of pre-packaging (hot water and wax), packaging (low-density polyethylene and biodegradable films), and storage temperatures (ambient and cold storage), which was arranged in a randomized complete block design with three replications. The effects of combined postharvest treatments on the changes in the physical, chemical, and sensory quality of 'Hass' avocados during a 28-day cold chain simulation were evaluated. The quality parameters evaluated included puree viscosity, moisture content, dry matter, pH, total soluble solids, total titratable acid and subjective quality attributes. Storage conditions significantly (P ≤ 0.05) affected the measured parameters. Cold chain conditions (5.5 °C for two days, 5 °C for six days and 4.5 °C for 20 days at 95% relative humidity) offered the most significant benefit in maintaining higher fruit quality. The combination of a wax coating, low-density polyethylene packaging, and cold chain conditions was beneficial in delaying ripening by approximately two weeks with minimum changes in moisture content (9.5%) and TSS (19.0%) and viscosity. Cold storage is essential in improving the shelf life and maintaining the quality of avocado fruits during export.

Introduction

Perishable commodities such as the avocado (Persea americana Mill.) pose a challenge in their supply chain concerning their qualitative and quantitative perishability. This is of great ethical, environmental, and financial concern. The deterioration of perishable commodities, therefore, warrants the complexity of the cold chain management, with great emphasis being placed on controlling and regulating the desired storage conditions (Aiello et al., 2011). The South African avocado industry is primarily based on export, making fruit quality an essential factor (Vorster et al.,1990). Approximately 40% of the South African avocado market is export-orientated (Blakey et al., 2015). Due to the distant export markets, proper postharvest handling must be implemented so as to maintain the avocado quality throughout the export process.

Low temperature is fundamental in extending the shelf life of avocados by retarding the metabolism through reduced respiration rates, ethylene evolution, softening, and colour change (Perez et al., 2004). A deviation of 1 °C in the holding temperature can adversely affect the avocado quality (Milne, 1998). Therefore, strict adherence to the specified cold chain management regime is crucial to maintain an acceptable quality of avocados. The application of a step-down temperature regime, which exposes the avocado to a series of temperatures in decreasing order, has also proven to be beneficial (Milne, 1998). This is often practiced in industry. The interaction of time and temperature is a vital aspect in the quality control of avocados (Vorster et al., 1990). The type of packaging that is used also contributes to the final fruit quality, as efficient packaging will allow for cold air to move uniformly around the fruit horizontally and vertically (Dodd et al., 2007). Studies have shown that the use of low-temperature storage, the application of wax, 1-methylcyclopropene and preventing breaks in the cold chain, were all beneficial in postharvest handling of avocados (Lutge et al., 2012). Based on the literature, it is apparent that numerous studies have been conducted in determining the quality of avocados during the supply chain, with a focus on storage temperature. However, a comparison of the integration of pre-packaging, packaging and storage conditions on the physical, chemical and sensory quality of avocados is deficient. The aim of this experiment is, therefore, to investigate the combined effect of postharvest treatments on the quality of avocados and to provide practical recommendations during avocado postharvest handling in South Africa.

Material and methods

Growing site description

Avocados (Persea americana Mill.), belonging to the ‘Hass’ cultivar, were obtained from the Everdon Estate located in the Karkloof Valley in Howick, KwaZulu-Natal (29°27′S, 30°16′E). The orchard is located in the Phillips' Bioclimatic Group 3, characteristic of cold mesic conditions typical of ‘mist belt’ areas (Moore-Gordon et al., 1995; Moore-Gordon and Wolstenholme, 1996). The orchard experiences mean minimum temperatures of approximately 15 °C in January and approximately 6.7 °C in July, with corresponding mean maximum temperatures of approximately 26.1 °C and 19.4 °C (Moore-Gordon and Wolstenholme, 1996; Mazhawu et al., 2018). The area receives an average annual rainfall of 1 052 mm. Micro-jet irrigation systems supply water to the scheme that has been installed with tensiometers (Moore-Gordon and Wolstenholme, 1996). The predominant soil is a well-drained Hutton prepared by deep-ripping only once. Two mulch dressings are placed around trees annually, which have resulted in a marked improvement to the fruit. Due to the colder subtropical climate in Howick, the avocados grown in these orchards mature at a later stage compared to those grown in the Limpopo province. This enables the Everdon Estate to lengthen its export season, particularly in the case of ‘Hass’ which has a harvest season starting from early July and extending into October or early November each year.

Plant material

Green mature ‘Hass’ avocados were manually harvested by expert harvesters early in the morning to reduce field heat and minimise mechanical injury. Avocados within a mass range of 203–243 g were selected and packed into single layer standard count 18 corrugated cardboard boxes (18 avocados per box) with ventilation. A total of 222 avocados, amounting to approximately 50 kg, were acquired for this experiment. From the commercially harvested avocados, samples were selected based on their uniformity of weight, shape, colour, size and whether they were bruised and blemish-free to be used in the experiment (Mohammed et al., 1999; Maftoonazad and Ramaswamy, 2008; Getinet et al., 2011; Hassan and Dann, 2019). The selected samples were immediately transported to the University of KwaZulu-Natal Food Science and Agricultural Engineering laboratory, which is located 37 km from the packhouse, where sample preparation, treatment, and storage trials were carried out.

Experimental design

A factorial design consisting of three pre-packaging treatments (Avoshine® wax, hot water immersion, and no pre-packaging treatment), three packaging treatments (low-density polyethylene (LDPE), corn starch biodegradable films and no packaging), two temperature and relative humidity (RH) storage regimes (cold chain and ambient) and three replications were arranged in a randomised complete block design (Mohammed et al., 1999; Getinet et al., 2011). The samples were randomly treated and evaluated for their quality immediately after harvest.

Sample preparation and treatments

The avocado samples were visually inspected at the laboratory to ensure that they were not subjected to any damage during transportation and, if they were, the damaged avocados were excluded from the samples (Getinet et al., 2011). All work surfaces, tools, and utensils were cleaned and disinfected. Avocados were treated and tested for each of the two storage regimes on Days 0, 7, 14, 21, and 28.

Pre-packaging treatments

The use of wax (Kremer-Kohne and Duvenhage, 1997) and hot water pre-packaging treatments (Wu et al., 2011) were selected, as these are extensively used for avocados.

Wax coating

The liquid polyethylene wax emulsion, Avoshine®, was used to evenly coat the avocados as described by Hall (2011). Approximately 0.4 ml of Avoshine® wax was used per 250 g of avocado fruit (Blakey, 2012).

Hot water dipping

A hot water bath containing water was initially heated to 80 °C for 30 min to destroy most heat-sensitive micro-organisms and, after that, reduced to 38 °C. The avocado samples were immersed in hot water for five minutes, then removed and dried.

Control

Avocado samples were not subjected to any pre-packaging treatments.

Packaging treatments

The selection of the packaging materials, LDPE, and biodegradable packaging films, were made based on previous findings, which showed their beneficial effects for avocados (Xiao and Kiyoto, 2001; Aguilar-Mendez et al., 2008).

LDPE bag

Soft, flexible, and strong LDPE bags with 20 μm thickness and 250 ✕ 150 mm, high water vapour transmission rate (375–500 g μm m-2.day−1) and high ratio of CO2 to O2 permeability were used (Mangaraj et al., 2009). Micro-perforations (n = 4) at 30 mm intervals were made along the bottom of each bag in two rows, using a 1.13 ✕ 10−3 m diameter needle to allow for the movement of gases and moisture between the micro-environment inside the bag and the surroundings.

Biodegradable cornstarch cellulose bag

Transparent biodegradable corn starch cellulose bags (30 μm thickness, 240 ✕ 100 mm and 45 mm gussets) were used. These bags have a high barrier to air and micro-organisms, which is ideal for the packaging of food (Aguilar-Mendez et al., 2008). Two rows containing four micro-perforations (∅ = 1.13 ✕ 10−3 m) at 20 mm intervals were made along the bottom of each bag. The interval spacing of the micro-perforations was smaller compared to the LDPE bags, due to the difference in width of the bags. Nine avocados from each of the pre-packaging treatments were each placed in a single LDPE bag and sealed at 164.3 mm from the base, to ensure that the volume within the both the LDPE and biodegradable bags had a similar volume. The individually sealed fruits were then placed in the respective storage temperature environment. Figure 1 represents the LDPE and corn starch packaging treatments.

Figure 1

LDPE and corn starch biodegradable flexible films with micro-perforations.

Temperature and relative humidity

A total of 108 avocado samples were stored at 5.5 °C ± 0.01 °C for two days, then at 5 °C ± 0.01 °C for six days and then 4.5 °C ± 0.01 °C for 20 days, allat 95% relative humidity based on a typical Everdon Estate packhouse regime to depict a realistic cold chain. These conditions were controlled in a CTS Climate Test Chamber (Model C-40/100) with a temperature range of -40 °C to +180 °C and a humidity range of 10%–98%. Theoretical temperature and relative humidity fluctuations of the chamber for climatic testing are ± 0.3K and 1.5%, respectively. The total capacity of the testing chamber is 100 L (500 ✕ 500 ✕ 400 mm). Control samples were placed in six corrugated cardboard boxes in a single layer and exposed to ambient temperature (±25.14 °C) and relative humidity (±52.67%) conditions. Two HOBO data loggers (BoxCar® Pro 4.3 software) were used to record the control conditions.

Parameters analysed

Data were collected on Days 0, 7, 14, 21, and 28 during storage. The quality parameters that were analyzed included moisture content, dry matter, total titratable acid, total soluble solids, pH, puree viscosity, and, subjective quality attributes.

Puree preparation

A Braun 300 W MR 400 hand blender was used to blend the diced avocados for two to three minutes until a fine paste was formed as described by Jacobo-Velazquez and Hernandez-Brenes (2011). The puree was sampled for moisture content, dry matter, total titratable acid, total soluble solids, pH, and viscosity.

Moisture content and dry matter

Moisture content (wet basis) and dry matter were determined using 3 g of avocado puree. The samples were dried in an oven at 70 °C for 48 h or until constant weight as described by (Chen et al., 2009).

Total titratable acid, total soluble solids and pH

Approximately 25 g of the avocado puree was added to a beaker containing 25 g (25 ml) of distilled water. The samples were homogenized and filtered through muslin to collect the juice (Maftoonazad and Ramaswamy, 2008). An aliquot of 3 ml of juice was pipetted to a 50 ml beaker, into which two drops of phenolphthalein indicator solution was added. The juice aliquot was titrated with 0.1 N sodium hydroxide (NaOH) till a pink colour was formed and persisted for five seconds while the solution was being stirred, using a magnetic stirrer. Titratable acidity was calculated as the number of milliliters of 0.1 M sodium hydroxide multiplied by an appropriate conversion factor (Equation (1)). A conversion factor of 0.28 was selected, based on linoleic acid, a predominant acid in avocados, as used by Maftoonazad and Ramaswamy (2008).where is the total titratable acid, 0.1 is 0.1 mol of NaOH [N], NaOH is the amount of NaOH added [ml], 0.28 is the conversion factor, and S is the juice sample [ml].

The pH was measured using a standard pH meter, which was calibrated using pH 4 and pH 7 buffer solutions. The pH was determined according to methods by Getinet et al. (2008), Maftoonazad and Ramaswamy (2008), and Jacobo-Velazquez and Hernandez-Brenes (2011).

The total soluble solids were determined using an ATAGO digital portable palette style refractometer (±0.1 % Brix) by placing one to two drops of the juice on the prism (Getinet et al., 2008; Maftoonazad and Ramaswamy, 2008).

Puree viscosity

Puree viscosity was measured using the Anton Paar Rheolab QC Rheometer basic unit (Model 13000) with Rheoplus V3.40 software. The viscosity was measured as a function of shear rate on approximately 16 g of puree samples, which was ramped from 0.01 s−1 to 100 s−1 for 250 s (Tabilo-Minizaga et al., 2005). A graphical representation of the shear stress (pa) and shear rate (s−1) was plotted using the software. The experiment was carried out using the concentric cylinder and cup accessory. The puree samples were filled to the level mark inside the cup. All measurements were carried out at a room temperature of approximately 24 °C.

Subjective quality attributes

Once the avocados were removed from storage, they were inspected and examined visually for mould development or decay. The skin colour was perceived by the human eye in terms of green, purple or black, synonymous to degree of ripening (Hassan and Dann, 2019). Changes in the firmness was inspected by hand-feel for any soft spots (Hassan and Dann, 2019) Any other variances concerning the physical appearance were also observed and recorded such as dark spots or bruises. Based on this each fruit was assigned an overall rating of either ‘good’, ‘fair’ or ‘poor’.

Statistical analysis

The differences between treatments were determined by an analysis of variance (ANOVA) employing the MSTAT-C statistical software, Version 2.10 (MSTAT, Michigan State University). The means were separated using Duncan's Multiple Range Test, with a significance level of 0.05.

Results and discussion

Moisture content and dry matter content

Table 1 displays the changes in the moisture content (MC) of avocados subjected to different pre-packaging, packaging, and storage conditions. The influence of storage conditions and storage period were found to be significant (P ≤ 0.05) on the changes in the MC. Under cold chain conditions, the LDPE maintained higher MC, as compared to biodegradable films. Control avocado samples devoid of pre-packaging and packaging treatments resulted in the highest MC loss at both cold chain and ambient storage conditions of 27.8% at Day 28 and 23.6% at Day 14, respectively. Higher temperatures contributed to a more significant loss in MC, as observed in the works of Ozdemir and Topuz (2004). The lowest reduction in the MC throughout the storage period of 9.5% at Day 28 was recorded for avocado samples pre-treated with wax, packaged in LDPE films and stored at cold chain conditions. Analysis of the MC trends emphasises the additional benefit of using pre-packaging and packaging in reducing the MC, under low-temperature storage. Several authors (Hofman and Jobin-Decor, 1999; Ozdemir and Topuz, 2004; Landahl et al., 2009; Obenland et al., 2012) have found the MC, dry matter (DM) and oil content of avocado fruit to be well correlated. These parameters are widely accepted as avocado fruit maturity indexes due to their consistency during fruit development (Shezi et al., 2020). A delay in the loss of the MC, therefore, implies a delay in the accumulation of oil and dry matter within the avocado, and hence, a delay in the ripening process.

Table 1

Changes in the moisture content (%) of avocados subjected to pre-packaging, packaging and different storage conditions for 28 days.

Treatment			Storage Period (Days)
Pre-packaging	Packaging	Storage Conditions	0	7	14	21	28
	LDPE	CC	54.05a	52.88cd	51.00ef	48.91f	48.18gh
		AT, ARH	55.00a	52.24de	48.02gh	-	-
HWT	Bio	CC	54.05a	53.58ab	50.00f	48.09gh	46.04hij
		AT, ARH	55.00a	51.00ef	48.51fg	-	-
	NP	CC	54.05a	53.10abc	51.15ef	48.28gh	45.40j
		AT, ARH	55.00a	47.02h	43.78lm	-	-
	LDPE	CC	54.05a	54.80a	50.12f	49.85f	48.92f
		AT, ARH	55.00a	50.74ef	47.74gh	-	-
Avoshine®	Bio	CC	54.05a	50.75ef	50.81ef	49.24f	47.79gh
		AT, ARH	55.00a	49.55f	49.23f	-	-
	NP	CC	54.05a	51.62ef	49.02f	48.51fg	45.23jk
		AT, ARH	55.00a	50.83ef	47.21gh	-	-
	LDPE	CC	54.05a	51.04ef	48.47fg	48.48fg	45.74ij
		AT, ARH	55.00a	48.59fg	45.00kl	-	-
NPP	Bio	CC	54.05a	50.43f	49.77f	48.57fg	45.79ij
		AT, ARH	55.00a	51.47ef	48.67fg	-	-
	NP	CC	54.05a	46.76h	46.42h	41.79no	39.00op
		AT, ARH	55.00a	52.48cde	42.00mn	-	-

Means within a column followed by the same letter/s are not significantly different from each other, according to Duncan's Multiple Range Test (P ≤ 0.05). HWT, hot water treatment; NPP, no pre-packaging; Bio, biodegradable corn starch packaging; LDPE, low-density polyethylene packaging; NP, no packaging; CC, cold chain; AT, ambient temperature; ARH, ambient relative humidity.

The percentage of DM in avocado samples increased in both ambient and cold chain storage conditions (Table 2). Hofman and Jobin-Decor (1999) found that the storage of avocados at lower relative humidities of 40% and 60%, as opposed to 80% and 98%, resulted in an increase in the DM content by approximately 1% towards the end of the storage period at 22 °C. An increase in the dry matter is synonymous with avocado fruit maturation and the ripening process (Zauberman and Jobin-Decor, 1995; Hofman and Jobin-Decor, 1999; Shezi et al., 2020). Similar results have been observed in the present experiment, as the relative humidity (RH) of the cold chain conditions was higher (95%) than at ambient conditions (±52.67%). The lower RH at ambient conditions increased the moisture loss from the fruit, thereby increasing the DM content. This was further exacerbated by the higher temperature under ambient conditions. Packaging films were found to be significant (P ≤ 0.05) with LDPE and biodegradable packaged avocados having lower DM than unpackaged avocado samples, at both cold chain conditions and ambient conditions. Pre-packaged avocados displayed a lower DM, compared to control avocado samples without any pre-packaging. Control avocado samples devoid of any pre-packaging and packaging treatments resulted in the highest DM accumulation at both cold chain and ambient storage conditions of 32.8% on Day 28 and 28.9% on Day 14, respectively. The least increase in the DM throughout the storage period of 11.2% at Day 28, was observed for samples pre-treated with wax, packaged in LDPE films and stored at cold chain conditions (P ≤ 0.05). It can be observed that on Day 28 from Tables 1 and 2, samples were no longer viable to be tested as they had long passed their shelf life and decayed.

Table 2

Changes in the dry matter (%) of avocados subjected to pre-packaging, packaging and different storage conditions for a 28 days.

Treatment			Storage Period (Days)
Pre-packaging	Packaging	Storage Conditions	0	7	14	21	28
	LDPE	CC	45.95no	47.12kl	49.00ij	51.09i	51.82gh
		AT, ARH	45.00no	47.76jk	51.98gh	-	-
HWT	Bio	CC	45.95no	46.42mn	50.00i	51.91gh	53.96efg
		AT, ARH	45.00no	49.00ij	51.49hi	-	-
	NP	CC	45.95no	46.90lm	48.85ij	51.72gh	54.60e
		AT, ARH	45.00no	52.98g	56.22bc	-	-
	LDPE	CC	45.95no	45.20no	49.88i	50.15i	51.08i
		AT, ARH	45.00no	49.26ij	52.26gh	-	-
Avoshine®	Bio	CC	45.95no	49.25ij	49.19ij	50.76i	52.21gh
		AT, ARH	45.00no	50.45i	50.77i	-	-
	NP	CC	45.95no	48.38ij	50.98i	51.49hi	54.77de
		AT, ARH	45.00no	47.52jkl	52.79gh	-	-
	LDPE	CC	45.95no	48.96ij	51.53hi	51.51hi	54.26ef
		AT, ARH	45.00no	51.41hi	53.58fg	-	-
NPP	Bio	CC	45.95no	49.57i	50.23i	51.43hi	54.21ef
		AT, ARH	45.00no	48.53ij	51.33hi	-	-
	NP	CC	45.95no	53.24g	55.00cd	58.21ab	61.00a
		AT, ARH	45.00no	49.17ij	58.00abc	-	-

Means within a column followed by the same letter/s are not significantly different from each other, according to Duncan's Multiple Range Test (P ≤ 0.05). HWT, hot water treatment; NPP, no pre-packaging; Bio, biodegradable corn starch packaging; LDPE, low-density polyethylene packaging; NP, no packaging; CC, cold chain; AT, ambient temperature; ARH, ambient relative humidity.

The four-way interaction between pre-packaging, packaging, storage conditions, and storage period had a significant (P ≤ 0.05) influence on the puree viscosity. A substantial increase in the viscosity was observed for avocado samples subjected to ambient storage conditions between Days 7 and 14. In contrast, storage at cold chain conditions maintained the viscosity level as determined at harvest during the first 21 days of storage for all treatments (Table 3). Sakurai and Nevins (1997) explained the decrease in the viscosity and elasticity of avocado tissue as a result of fruit ripening, which is mediated by the action of endo-type hydrolytic enzymes resulting in the breakdown of xyloglucan molecules in the cell wall (Sakurai and Nevins, 1997). This decrease in the elasticity could account for the increase in the viscosity of the puree avocado pulp as the ripening proceeds with time. Control avocado samples devoid of pre-packaging, packaging, and exposed to ambient storage conditions contributed to the greatest increase in the viscosity from 0.03 Pa.s on Day 0–7.11 Pa.s on Day 14. The combination treatment of wax and LDPE films with storage at cold chain conditions maintained the viscosity throughout the storage period. Unripened avocados have high moisture content (Table 1). Therefore, when passing the unripened puree through the muslin, the fluid was more easily separated from the solid components of the puree, which could also be attributed to higher levels of the carbohydrate propectin. Propectin can hold cells together in unripe fruits as determined in unripe guavas (Sanchez et al., 2009). In the present experiment, due to the higher moisture content of the unripe avocado, the puree had larger particles, as opposed to the ripe avocado puree, which had a smooth buttery texture and higher viscosity. A higher viscosity is, therefore, synonymous with fruit ripening, which is accelerated by high ambient temperatures.

Table 3

Changes in the pulp viscosity (pa.s) of avocados subjected to pre-packaging, packaging and different storage conditions for 28 days.

Treatment			Storage Period (Days)
Pre-packaging	Packaging	Storage Conditions	0	7	14	21	28
	LDPE	CC	0.02m	0.03m	0.03m	0.153m	3.71efg
		AT, ARH	0.03m	0.05m	2.29jk	-	-
HWT	Bio	CC	0.02m	0.06m	0.10m	0.06m	4.68cd
		AT, ARH	0.03m	1.27l	2.66ij	-	-
	NP	CC	0.02m	0.06m	0.02m	0.04m	3.06h
		AT, ARH	0.03m	4.91cd	6.08b	-	-
	LDPE	CC	0.02m	0.06m	0.04m	0.06m	0.02m
		AT, ARH	0.03m	0.01m	2.50jk	-	-
Avoshine®	Bio	CC	0.02m	0.06m	0.05m	0.03m	3.50gh
		AT, ARH	0.03m	0.02m	2.63ij	-	-
	NP	CC	0.02m	0.08m	0.04m	0.04m	2.70hi
		AT, ARH	0.03m	0.03m	4.09def	-	-
	LDPE	CC	0.02m	0.03m	0.04m	0.03m	4.40cde
		AT, ARH	0.03m	0.09m	2.01kl	-	-
NPP	Bio	CC	0.02m	0.03m	0.04m	0.06m	3.62fgh
		AT, ARH	0.03m	3.01hi	2.52jk	-	-
	NP	CC	0.02m	0.06m	0.08m	0.06m	5.15cd
		AT, ARH	0.03m	2.37jk	7.11a	-	-

Means within a column followed by the same letter/s are not significantly different from each other, according to Duncan's Multiple Range Test (P ≤ 0.05). HWT, hot water treatment; NPP, no pre-packaging; Bio, biodegradable corn starch packaging; LDPE, low-density polyethylene packaging; NP, no packaging; CC, cold chain; AT, ambient temperature; ARH, ambient relative humidity.

pH value

Aguirre-Joya et al. (2017) stated that Hass avocado fruit have a characteristic pH near neutrality, as can be seen in Table 4. Storage conditions and storage periods significantly (P ≤ 0.05) influenced the avocado pH value. A general decline in the pH value was observed for avocado samples subjected to the different postharvest treatments, specifically those stored under ambient conditions. This trend is in agreement with the findings of Maftoonazad and Ramaswamy (2008), Jacobo-Velazquez and Hernandez-Brenes (2011) and Aguirre-Joya et al. (2017). Pre-packaging treatments had a higher significance (P ≤ 0.05) than packaging films. Wax-coated avocado samples had higher pH values when stored under both cold chain and ambient conditions. Jacobo-Velazquez and Hernandez-Brenes (2011) reported the decline in the pH value to be attributed to the movement of organic acids from intercellular locations to the avocado puree. Besides, the increase in acidity could be due to the increase in the concentration of free fatty acids as a result of trigliceride lipolysis (Jacobo-Velazquez and Hernandez-Brenes, 2011). An increase in the acidity can be one of the changes associated with avocado deterioration. Avocado samples devoid of pre-packaging and packaging treatments displayed the lowest pH values. However, it should be highlighted that hot water pre-treated samples, packaged in biodegradable films and stored under ambient conditions, exhibited a decrease in pH values over a 14-day storage period by 10.8%, compared to a decrease by 9.9% in control avocado samples. The lower pH values of hot water treated avocado samples can be attributed to tissue damage. The pH values of hot water treated samples without packaging and stored under cold chain conditions fluctuated during the 28-day storage. While cold storage proved to be highly beneficial for avocados, the use of hot water treatments appeared to have reduced the pH more than in the control samples, which is not desirable.

Table 4

Changes in the pH of the pulp of the avocado subjected to pre-packaging, packaging and different storage conditions for 28 days.

Treatment			Storage Period (Days)
Pre-packaging	Packaging	Storage Conditions	0	7	14	21	28
	LDPE	CC	6.74a	6.60cde	6.58de	6.55e	6.53fg
		AT, ARH	6.76a	6.50gh	6.44hi	-	-
HWT	Bio	CC	6.74a	6.68c	6.62cde	6.59cde	6.46h
		AT, ARH	6.76a	6.52g	6.03k	-	-
	NP	CC	6.74a	6.55e	6.58de	6.54ef	6.58de
		AT, ARH	6.76a	6.60cde	6.19j	-	-
	LDPE	CC	6.74a	6.73ab	6.66c	6.61cde	6.58de
		AT, ARH	6.76a	6.76a	6.59cde	-	-
Avoshine®	Bio	CC	6.74a	6.72abc	6.71abc	6.69bc	6.65c
		AT, ARH	6.76a	6.65c	6.52g	-	-
	NP	CC	6.74a	6.67c	6.69bc	6.66c	6.61cde
		AT, ARH	6.76a	6.69bc	6.61cde	-	-
	LDPE	CC	6.74a	6.63cd	6.58de	6.57de	6.53fg
		AT, ARH	6.76a	6.73ab	6.12jk	-	-
NPP	Bio	CC	6.74a	6.63cd	6.65c	6.63cd	6.60cde
		AT, ARH	6.76a	6.53fg	6.41i	-	-
	NP	CC	6.74a	6.63cd	6.61cde	6.58de	6.58de
		AT, ARH	6.76a	6.61cde	6.09jk	-	-

Means within a column followed by the same letter/s are not significantly different from each other, according to Duncan's Multiple Range Test (P ≤ 0.05). HWT, hot water treatment; NPP, no pre-packaging; Bio, biodegradable corn starch packaging; LDPE, low-density polyethylene packaging; NP, no packaging; CC, cold chain; AT, ambient temperature; ARH, ambient relative humidity.

Total soluble solids

The storage conditions and the storage period significantly (P ≤ 0.05) affected the total soluble solids (TSS) of avocados stored for the 28 days (Table 5). A general increase in the TSS was observed in avocados subjected to the different treatments throughout the storage period. The increase in the TSS occurred at a faster rate at ambient conditions compared to the cold chain storage conditions. The increased temperature and reduced RH under ambient storage conditions may have contributed to the increased hydrolysis of carbohydrates stored within the avocado fruit into soluble sugars. This results in a higher TSS and a reduction in the avocado shelf life, which is undesirable. Packaging treatments displayed a higher significance (P ≤ 0.05), compared to pre-packaging on the avocado TSS. Packaged avocado samples, generally demonstrated lower TSS values, compared to unpackaged control samples at both cold chain conditions and ambient conditions. This is in agreement with Tefera et al. (2007) and Workneh et al. (2011). However, Aguirre-Joya et al. (2017) found no distinct change in the TSS of avocado fruit in both refrigerated and room storage conditions but rather a general increase in the TSS values during storage. Avocados treated with wax, LDPE, and control storage conditions displayed the lowest rate of increase in the TSS of only 19.0% from Day 0 to Day 28 indicative of slower senescence and ripening and a longer shelf life.

Table 5

Changes in the total soluble solids (°brix) of avocados subjected to pre-packaging, packaging and different storage conditions for 28 days.

Treatment			Storage Period (Days)
Pre-packaging	Packaging	Storage Conditions	0	7	14	21	28
	LDPE	CC	2.90h	3.20g	3.27fg	3.20g	4.20bcd
		AT, ARH	2.95h	3.07gh	4.40bc	-	-
HWT	Bio	CC	2.90h	2.95h	3.00h	3.10gh	3.87d
		AT, ARH	2.95h	3.07gh	4.30bc	-	-
	NP	CC	2.90h	3.47ef	3.80de	3.80de	4.47bc
		AT, ARH	2.95h	4.70ab	5.00a	-	-
	LDPE	CC	2.90h	3.00h	3.27fg	3.30efg	3.45ef
		AT, ARH	2.95h	3.10gh	3.95d	-	-
Avoshine®	Bio	CC	2.90h	3.13gh	3.53ef	3.40efg	3.70e
		AT, ARH	2.95h	3.10gh	4.57abc	-	-
	NP	CC	2.90h	3.13gh	3.27fg	3.30efg	3.90d
		AT, ARH	2.95h	2.63hi	4.30bc	-	-
	LDPE	CC	2.90h	3.20g	3.45ef	3.50ef	3.50ef
		AT, ARH	2.95h	2.90h	3.70e	-	-
NPP	Bio	CC	2.90h	3.13gh	3.30efg	3.37efg	3.83de
		AT, ARH	2.95h	4.15cd	4.00d	-	-
	NP	CC	2.90h	3.00h	3.10gh	3.27fg	4.30bc
		AT, ARH	2.95h	3.67e	4.80ab	-	-

Means within a column followed by the same letter/s are not significantly different from each other, according to Duncan's Multiple Range Test (P ≤ 0.05). HWT, hot water treatment; NPP, no pre-packaging; Bio, biodegradable corn starch packaging; LDPE, low-density polyethylene packaging; NP, no packaging; CC, cold chain; AT, ambient temperature; ARH, ambient relative humidity.

Total titratable acid

Table 6 presents the changes in the total titratable acid (TTA) of the avocado pulp. The storage conditions and the storage period were highly significant (P ≤ 0.05) in terms of the avocado TTA. A general increase in the TTA was observed for all treatment conditions. The increase in the TTA was observed to occur at a faster rate under ambient conditions for a 14-day storage period, compared to cold chain conditions for a 28-day storage period. This shows that the avocado samples at cold chain conditions had an addition 14 day shelf life compared to those at ambient conditions. Maftoonazad and Ramaswamy (2008) observed a similar trend of a more rapid rise in the TTA at higher temperatures of avocado samples. Pre-packaging and packaging were found to have a significant (P ≤ 0.05) influence on the changes of the avocado TTA. Pre-package avocados displayed lower TTA values, compared to non-pre-packaged samples. Similarly, packaged samples demonstrated lower TTA values than unpackaged samples. The four-way interaction between pre-packaging, packaging, storage conditions, and the storage period was found to significantly (P ≤ 0.05) effect the avocado TSS. The increase in the TTA corresponds to a decrease in the pH. The lowest rate of increase in the TTA from 1.9 to 3.7 occurred in samples coated with Avoshine®, packaged in LDPE film and stored at cold chain conditions for 28 days. In contrast, the highest rates of increase in the TTA from 1.9 to 11.5 and from 1.7 to 7.5 were observed for control samples stored at the cold chain and ambient storage conditions, respectively. This, therefore, illustrates the benefit of pre-packaging and packaging treatments in maintaining the quality of avocado fruit.

Table 6

Changes in the total titratable acid (mg.ml−1) of avocados subjected to pre-packaging, packaging and different storage conditions for 28 days.

Treatment			Storage Period (Days)
Pre-packaging	Packaging	Storage Conditions	0	7	14	21	28
	LDPE	CC	1.9k	3.6hi	3.9h	3.7h	5.0def
		AT, ARH	1.7k	4.9ef	8.4bc	-	-
HWT	Bio	CC	1.9k	3.1hij	3.1hij	2.3jk	2.3jk
		AT, ARH	1.7k	3.8h	8.1bcd	-	-
	NP	CC	1.9k	3.4hi	4.1gh	3.6hi	4.2fgh
		AT, ARH	1.7k	2.0k	8.4bc	-	-
	LDPE	CC	1.9k	3.1hij	3.2hij	3.4hi	3.7h
		AT, ARH	1.7k	1.4kl	5.1de	-	-
Avoshine®	Bio	CC	1.9k	3.4hi	3.4hi	4.4fg	7.0bcd
		AT, ARH	1.7k	1.9k	6.2d	-	-
	NP	CC	1.9k	3.1hij	3.6hi	4.4fg	5.8d
		AT, ARH	1.7k	5.8d	6.5cd	-	-
	LDPE	CC	1.9k	2.2jk	3.0hij	4.4fg	6.2d
		AT, ARH	1.7k	4.5fg	5.3de	-	-
NPP	Bio	CC	1.9k	2.6ij	2.6ij	8.7b	8.1bcd
		AT, ARH	1.7k	4.5fg	4.3fg	-	-
	NP	CC	1.9k	2.5j	2.7ij	8.4bc	11.5a
		AT, ARH	1.7k	5.0def	7.5bcd	-	-

Means within a column followed by the same letter/s are not significantly different from each other, according to Duncan's Multiple Range Test (P ≤ 0.05). HWT, hot water treatment; NPP, no pre-packaging; Bio, biodegradable corn starch packaging; LDPE, low-density polyethylene packaging; NP, no packaging; CC, cold chain; AT, ambient temperature; ARH, ambient relative humidity.

Avocado samples stored at ambient conditions succumbed to more mould development, especially in the packaged treatment. The micro-environment within the packaging was conducive for the proliferation of mould, due to the higher temperature and RH. Droplets of moisture began to collect in packaged samples at ambient conditions, indicating a loss in moisture from the avocado, which resulted in excessively higher PWL. The visual comparison between avocado samples stored under ambient conditions and cold chain conditions revealed a higher percentage of marketable fruit at the lower temperature (Table 7). The colour change of the skin from a green to purple/black was most pronounced at ambient conditions due to the excessively higher temperatures. Control avocado samples devoid of pre-packaging and packaging treatments exhibited the darkest skin colour and showed extreme softening. These fruits were discarded after 14 days of storage due to excessive softening and decay. In general, avocado samples coated with wax and packaged in LDPE films displayed aesthetically appealing fruit, with a glossy exterior, particularly at cold chain conditions, and were able to remain in storage for the entire 28-day period.

Table 7

Changes in the subjective quality attributes subjected to different postharvest treatments.

Treatment			Final state of avocado	Rating
	LDPE	CC	Dull exterior, remained firm and green	Good
		AT, ARH	Dull exterior, soft, darkening of the skin, mould development, shrivelling	Poor
HWT	Bio	CC	Dull exterior, remained firm and green	Good
		AT, ARH	Dull exterior, soft, darkening of the skin, mould development, shrivelling	Poor
	NP	CC	Dull exterior, remained firm and green	Fair
		AT, ARH	Dull exterior, soft, darkening of the skin, high degree of shrivelling	Poor
	LDPE	CC	Shiny exterior, slight softening and darkening of the skin	Excellent
		AT, ARH	Shiny exterior, softening and darkening of the skin, slight mould development	Fair
Avoshine®	Bio	CC	Shiny exterior, remained firm and green	Excellent
		AT, ARH	Shiny exterior, softening and darkening of the skin, slight mould development	Fair
	NP	CC	Shiny exterior, slight softening and darkening of the skin	Very good
		AT, ARH	Shiny exterior, softening and darkening of the skin, slight mould development	Poor
	LDPE	CC	Dull exterior, slight softening and skin darkening	Fair
		AT, ARH	Dull exterior, excessive softening, darkening of the skin, mould development, shrivelling, condensation within packaging	Poor
NPP	Bio	CC	Dull exterior, slight softening and skin darkening	Fair
		AT, ARH	Dull exterior, excessive softening, darkening of the skin, mould development, shrivelling, condensation within packaging	Poor
	NP	CC	Dull exterior, softening and skin darkening	Poor
		AT, ARH	Dull exterior, most excessive softening, darkening of the skin, mould development, shrivelling	Very bad

HWT, hot water treatment; NPP, no pre-packaging; Bio, biodegradable corn starch packaging; LDPE, low-density polyethylene packaging; NP, no packaging; CC, cold chain; AT, ambient temperature; ARH, ambient relative humidity.

Conclusion

The storage conditions and the storage period were found to have the greatest influence on all of the avocado quality parameters. The simulation of a realistic cold chain, incorporating stepping down the temperature from 5.5 °C to 4.5 °C over a 28 days, has proven to preserve the postharvest quality of avocados, compared to storage at ambient conditions. Based on the results, samples stored at cold chain conditions had a shelf life of 14 days more, compared to those at ambient conditions. The lower temperature was instrumental in reducing the rate of increase in DM and the subsequent moisture loss, lowering the viscosity, reducing the rates of pH reduction and in mediating the increase in TSS and TTA. The combination of the pre-packaging wax coating and LDPE film at cold chain storage conditions resulted in the least change in MC and DM quality parameters by 9.5% and 11.2%, respectively. The lowest reduction in the pH of 1.3% was observed under this treatment, which can be associated with the lowest increase in the TTA. Likewise, the lowest increase of 19.0% in the TSS was also observed. It can, therefore, be deduced that the combined use of Avoshine® coating, LDPE films, and cold chain storage conditions is beneficial in preserving the postharvest quality of avocados, by delaying the ripening process and consequently extending the shelf life of avocados by up to two weeks more, compared to storage at ambient conditions.

Declarations

Author contribution statement

A. Kassim: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

T. S. Workneh: Conceived and designed the experiments; Contributed reagents, materials, analysis tools or data.

Funding statement

This work was supported by the [SFH20110808000023600] and the .

Competing interest statement

The authors declare no conflict of interest.

Additional information

No additional information is available for this paper.