Coolchain simulation

Trays of fruit were exposed to different cooling profiles consisting of 4 different cooling methods; rapid (R), direct (D), gradual (G) or combination of rapid and gradual (C) cooling and 2 subsequent storage temperatures of 0 or 2 °C (Figure 4.1). Rapid cooling was accomplished by placing the trays of fruit in a pallet scale pre-cooler that was set up in a cool room at 0 °C (O’Sullivan et al., 2016). The pre-cooler cooled the fruit rapidly to 0 °C within 12 h. Fruit were subsequently stored at either 0 (R12h,0) or 2 °C

(R12h,2). Direct cooling was achieved by placing the trays of fruit into a cool room set at

0 °C and cooled using a room cooling technique (D3d,0). Fruit were cooled in cool room

by stacking 20 to 24 trays in a single vertical column. Gradual cooling was established by placing trays of fruit into a cool room initially at 16 °C with the set point temperature decreasing to 0 °C (at a rate of 1-2 °C per day) over a period of 2 weeks and subsequently stored at 0 °C (G2w,0). A combination of rapid and gradual cooling was attained by placing

the fruit in a pre-cooler to cool the fruit to 10 °C within 12 h, follow by a gradual cooling to 0 (C1w,0) or 2 °C (C1w,2) within 1 week. Once cooled, all the trays of fruit remained in

Chapter 4: Factors influencing development of chilling injury in ‘Hayward’ kiwifruit during coolstorage 61 T e m per at u re, ° C 0 2 4 6 8 10 12 14 16 18 20 22 24 26 0 4 8 12 16 20 24 0 2 4 6 8 10 12 14 16 18 20 22 24 26 Storage time, d 0 4 8 12 16 20 24 0 4 8 12 16 20 24 R12h,0 R12h,2 D3d,0 G2w,0 C1w,0 C1w,2

Figure 4.1: The proposed cooling profiles to cool fruit to storage temperature at 0 or 2 °C. R12h,0 refers to rapidly cooled to 0 °C within 12 h, R12h,2 refers to rapid cooling to 2 °C within 12 h, D3d,0 refers to direct cooling to 0 °C within 3 d, G2w,0 refers to gradual cooling to 0 °C within 2 w, C1w,0 refers to rapid cooling to 10 °C within 12 h followed by slow cooling to 0 °C within 1 w, C1w,2 refers to rapid cooling to 10 °C within 12 h followed by slow cooling to 2 °C within 1 w.

The temperature and humidity of the cool room and air inside the fruit tray were monitored using iButton data loggers (DS1923, Maxim Integrated, USA) throughout the cooling and storage period. Two data loggers were used per grower, maturity and treatment combination. The data loggers were placed at the centre of the fruit tray, beneath the polyliner and thus measured the air temperature inside the tray. The tray that contained the data logger was located at the centre of the stacked column during cooling and storage. Temperature and humidity were recorded at 15 minutes intervals.

Chapter 4: Factors influencing development of chilling injury in ‘Hayward’ kiwifruit during coolstorage

62

4.2.3. Fruit assessment

Measurements to assess fruit quality (firmness, occurrence and severity of chilling injured fruit, and incidence of rotten fruit) were conducted at fortnightly intervals for the first 60 days (0, 18, 32, 46 and 60 days), and later 3 weeks intervals until 25 weeks of storage (81, 109, 130, 151 and 172 days). The fruit measurements were conducted with a sample size of 30 fruit (a single layer tray) for the first 6 measurement days and 90 fruit (3 single layer trays) for the remaining 4 measurement days.

4.2.3.1. Initial dry matter and soluble solids content

Initial soluble solids content (SSC) was measured using a pocket refractometer (PAL-1, Atago, Tokyo, Japan). SSC was expressed as a percentage on the Brix scale. SSC readings were calculated based on an average of 30 fruit for early maturity fruit and 20 fruit for mid and late maturity fruit. Dry matter (DM) was measured by slicing a 2 to 3 mm thick equatorial slice from the fruit and placing it on a weighed petri dish. The slices were placed in food dehydrator (3000 series, Excalibur, California, USA) at 60 to 65 °C overnight. DM was calculated by the percentage of the dry weight over fresh weight. Petri dish and fruit slices were weighed using a weighing balance with 0.001 g accuracy (Mettler PG-503S, Toledo, Switzerland).

4.2.3.2. Occurrence and severity of chilling injury

Incidence of chilling injury was assessed visually by inspecting for symptom development on the outer pericarp after conducting a single equatorial slice. The symptoms include grainy tissue and a water soaked ring along the outer pericarp (Figure 4.2). The incidence of chilling injury was calculated as a percentage of the total fruit population (90 fruit per grower), with no distinction made for the severity of chilling injury symptoms.

Chapter 4: Factors influencing development of chilling injury in ‘Hayward’ kiwifruit during coolstorage

63

Figure 4.2: Example of severity of chilling injury found along the outer pericarp of ‘Hayward’ kiwifruit. Grainy tissue and water soaked appearance were identified as symptoms of chilling injury. Severity increases from left to right across the figure. Red arrow indicates the chilling injury symptoms.

4.2.3.3. Decay incidence

Decay incidence was assessed visually by inspecting for symptoms of rots which developed on the side or stem end (Figure 3.5). The incidence of decay was calculated as a percentage of the total fruit population (30 or 90 fruit per grower). The water soaked appearance observed along the outer pericarp of a chilling injured fruit is different compared to a severely rotten fruit (Figure 4.3) and thus the ability to differentiate a rotten fruit from a chilling injury fruit is important and achievable.

Chapter 4: Factors influencing development of chilling injury in ‘Hayward’ kiwifruit during coolstorage

64

Figure 4.3: The comparison between a fruit with chilling injury symptom (left) and a rotten fruit (right).

4.2.3.4. Fruit firmness

Kiwifruit were equilibrated to 20 °C overnight before measurement. A penetrometer (QALink Willowbank Electronics Ltd., Napier, New Zealand), fitted with a standard 7.9 mm round Effegi probe and interfaced to a computer was used. A 2 mm slice of skin was removed from an equatorial region before measurement. The probe was set to penetrate the flesh to a depth of 8 mm at 20 mm s-1 with the minimum measurement

being 1 N. The penetrometer was sent to Willowbank Electronics Ltd for calibration before the start of the experiment. Two measurement locations perpendicular to each other were used for each fruit. Fruit firmness readings were calculated based on an average of 30 fruit for the first 6 measurements and 90 fruit for the remaining 4 measurements. Rotten fruit were removed from the population when measuring the fruit firmness.

4.2.3.5. Statistical analysis

The experiments were conducted using a complete random design, with each grower line representing a replicate. Fruit firmness statistical analysis was performed using Minitab Version 15 (Minitab Inc, State College, PA, USA). Data were subjected to a General Linear Model, at each time with storage treatment, fruit maturity and grower as fixed factors. Comparison of means was undertaken using Tukey’s test at p ” 0.05. The

Chapter 4: Factors influencing development of chilling injury in ‘Hayward’ kiwifruit during coolstorage

65 Anderson-Darling test was used to perform a normality test on sample populations. Chi- square analysis was used to analyse the significance of the incidence count of chilling injured fruit data.