Sarhad J. Agric, Vol.27, No.2, 2011 189

EFFECTS OF WET HEAT TREATMENT (WHT) DURATIONS ON THE QUALITY OF SWEET ORANGE STORED AT ROOM TEMPERATURE

ABDUR RAB, MUHAMMAD SAJID, SAEEDA and NAJIA

Department of Horticulture, Khyber PakhtunKhwa Agricultural University, Peshawar – Pakistan.

ABSTRACT

The experiment on “effect of wet heat treatment (WHT) durations on the quality of sweet orange stored at room temperature” was conducted at the Agricultural University, Peshawar during the year 2006-07. The fruits were subjected to wet heat treatments for 0-20 minutes and stored at room temperature for 0-75 days. Storage significantly increased weight loss, peel puncture pressure, TSS, acidity, reducing sugars and disease incidence but declined the rind moisture non-reducing sugars and ascorbic acid contents. The modest wet heat treatment (5-10 minutes at 50°C) retarded the storage associated changes in sweet oranges but extended heat treatments (15-20 minutes), reversed the beneficial effects and enhanced the decline in physical and chemical quality attributes. Ascorbic acid, however, continued to decrease even with modest heat treatment.

Key Words: Wet Heat Treatment (WHT), Sweet Orange, Storage, Room Temperature

Citation : Rab, A., M. Sajid, Saeeda and Najia. 2011. Effects of wet heat treatment (WHT) durations on the quality of sweet orange stored at room temperature. Sarhad J. Agric 27 (2): 189-194

INTRODUCTION

Sweet orange is an important member of “citrus” group of fruits. In Pakistan, citrus fruits are grown over an area of 193.2 thousand hectares with a total production of 1472.4 thousand tons (MINFAL, 2007-2008). Citrus fruits are not only important for domestic consumption but also fetch reasonable foreign exchange. Currently, Pakistan exported 214764.6 tons of citrus (Kinnow and others) fruit to different countries (MINFAL, 2007). The demand for sweet orange is very high both in local and international markets; it increases from January onward but the quality of the fruits can not be retained. The improper storage results in rapid loss of sugars, ascorbic acid (Maleki and Sarkissian, 1967) and enhances weight loss (McGornack, 1975). Low temperature storage is generally used to slow down the deterioration in citrus fruits during storage but these are chilling sensitive and hence may be injured by chilling temperatures (Purvis, 1985; Couey, 1989). Chilling injury to citrus fruits may occur at temperatures above 2.5°C and is expressed in increased weight loss, surface pitting, enhanced disease susceptibility (Purvis, 1985). Chilling injury is highly correlated with increased electrolyte leakage of skin tissue (Rab and Saltveit, 1996; Woolf, 1997). Chilling injury can be reduced by a number of postharvest techniques such as heat shocks (Rab and Saltveit, 1996), anaerobic shock treatments (Pesis et al. 1994), or hot air and water treatments (Sanxter et al. 1994; Florissen et al. 1996; Woolf, 1997). Chemical treatments e.g. thiabendazol (Farooqi et al. 1975), proper storage conditions (McGornack, 1975) and packaging materials (BenYahoshia et al. 1979; Kwada and Albergo, 1979) and waxing fruits (Petracek et al. 1998), have been intensively investigated to enhance the shelf life of oranges. Heat treatments have also been used to decrease insect’s infestation (Gould and Sharp, 1992; Sharp and Picho-Martinez 1990), disease incidence (Ben-Yehoshua, et al. 1979; Couey, 1989) and to induce chilling tolerance (Rab and Saltveit, 1996; Florissen, et al. 1996; Ritenour et al. 2004). For most of fruits the required heat treatment effects can be achieved by heating the core of the fruit to 43 to 46.7°C with exposure times varying from 35 to 90 minutes (Gould, 1988; Gould and Sharp, 1992; Sharp and Picho-Martinez, 1990).The present study, therefore, was initiated to evaluate the effects of wet heat treatment durations on the physico-chemical quality attributes of sweet oranges stored at room temperature.

MATERIALS AND METHODS

Oranges were exposed to WHT by immersion for 0-20 minutes in water heated to 50 oC, followed by a gentle breeze from a fan to remove the surface water. These were then packed in cardboard packages having two holes on each side for ventilation. The fruit were then stored at room temperature and data were recorded on physico-chemical quality attributes of fruits at 15 days interval for 75 days. Biochemical quality attributes such as Total Soluble Solid (TSS), Acidity, TSS/Acid ratio, Reducing and Non- Reducing sugars and Ascorbic Acid content were determined according to AOAC (1990). The peel puncturing pressure (PPP) was calibrated with the help of electrometer equipped with a 2mm tip. The penetration fore (puncturing pressure) was determined at the equatorial region of the fruit. For disease incidence 50 fruits in each treatment and replication with no disease symptoms were collected at the end of each storage interval and the data are presented as percent disease incidence. The data calculated on different parameters were subjected to Analysis of Variance (ANOVA) technique to observe the differences between the different treatment as well as their interactions. In cases where the differences were significant, the means were further assessed for differences through Least Significant

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Difference (LSD) test. Statistical computer software, MSTATC (Michigan State University, USA), was applied for computing both the ANOVA and LSD.

RESULTS AND DISCUSSION

To study the effects of wet heat treatment (WHT) durations on quality of sweet orange stored at room temperature was conducted at Horticulture Laboratory, Agricultural University, Peshawar during 2006-07. The data regarding quality attributes of sweet oranges were discussed below.

Weight Loss

The data for weight loss show that it was significantly affected by heat treatment and storage duration but the interaction was non significant (Table I). There has been a continuous increase in weight loss with incremental increase in storage duration. The weight loss increased significantly from the minimum of 0% on day 0 to 11.26% with 15 days storage at room temperature which increased to 13.61% with 30 days storage and finally reached the maximum weight loss (43.54%) after 75 days storage at room temperature (Table I). Wet heat treatment (WHT) duration also significantly affected the percent weight loss in citrus fruits. The weight loss in control treatment (21.66%) decreased initially with WHT duration of 10 minutes (19.43%) and then increased to the maximum of 22.85% with WHT for 20 minutes (Table II). Due to loss of water, the weight loss after harvest is common in different fruits (Akram, et al. 2001; Al-Obeed and Harhash, 2006). Heat treatments have been reported to increase weight loss in Valencia oranges (Citrus sinensis) (Erkan et al. 2005) and other fruits (Aziz et al. 1975) but Vicente et al. (2002) reported that heat treatment retarded weight loss after an initial weight loss (about 2%). It was found that modest WHT (5-15 minutes) decreased weight loss but this effect is reversed by increasing WHT duration.

Table-I Effect of room temperature storage duration on physical quality attributes of sweet orange fruit Storage Duration (Days at 20°C)

Weight Loss (%)

Rind Moisture (%)

Peel Puncture Pressure (mm)

Disease Incidence (%)

0 0.00 f 6.56 a 6.82d 0.00f 15 11.26 e 6.56 a 7.02d 1.33e 30 13.61 d 6.36 a 7.23bc 4.00d 45 24.52 c 5.65 b 7.38b 9.60c 60 33.59 b 5.42 c 7.75ab 17.47b 75 43.54 a 5.32 c 7.97a 28.40a LSD α 0.05 0.976 0.171 0.283 1.27

Rind Moisture Content

The moisture content of the rind remained non-significant for 30 days but increase storage duration to 45 or 60 days resulted in significant decrease (5.65 and 5.42% respectively) in moisture content (Table I). The moisture content of the rind was also non significant between control (6.25%) and WHT for five minutes (6.10%), but increasing treatment duration caused significant decrease in moisture content Table II. Since moisture content of the rind is negatively correlated with water loss, it confirm the observation regarding the effect of heat treatment on weight loss Table I and WHT seems to have a duration dependent inhibitory and promote the effect on moisture loss from the rind.

The Peel Puncturing Pressure (PPP)

The data on the PPP of fruit rind reveal that both storage duration (Table I) and heat treatment (Table II) significantly affected the PPP of the rind. The PPP increased from the minimum of 6.82mm in fresh fruits to the maximum of 7.97mm within 75 days storage at room temperature (Table I). The heat treatment duration also significantly affected it. While heat treatment for 5 and 10 minutes with PPP of 7.18mm and 7.22mm were at par with control (7.10mm), WHT for 15 or 20 minutes resulted in significantly higher PPP (7.49mm and 7.82mm) which may be due to increase water loss from the rind (Table II).

Table-II Effect of wet heat treatment durations on physical quality attributes of sweet orange fruit WHT Duration (Minutes at 50 oC)

Weight Loss (%)

Rind Moisture (%)

Peel Puncture Pressure (mm)

Disease Incidence (%)

0 21.66 b 6.25 a 7.10c 10.67b 5 20.32c 6.10 a 7.18c 6.78c 10 19.43c 5.89 b 7.22c 7.00c 15 21.18bc 5.84 b 7.49b 7.78c 20 22.85a 5.81 b 7.82a 18.44a LSD α 0.05 0.891 0.156 0.256 2.261

Sarhad J. Agric, Vol.27, No.2, 2011 191

Disease Incidence

Percent disease incidence significantly increased with increasing storage. The minimum disease incidence (0%) at 0 day storage increased significantly with increase in storage duration and finally reached the maximum cumulative disease (28.40 %) after 75 days storage, followed by 60 and 45 days storage duration with 17.47% and 9.60% respectively (Table I). (Table II) reveals that heat treatment caused significant but dose dependent influence on disease incidence. The percent disease incidence decreased from 10.67% in control to 6.78 with WHT for 5 minutes. But increasing WHT to 20 minutes enhanced disease incidence to the maximum of 18.44%, which was significantly higher than control. The incidence of disease in fruits generally increases with increasing storage duration (D-Hallewin and Schirra, 2000). A small percentage of 3 to 6% of citrus fruit is prone to rotting after harvest (Tuset, 1987), which may increase to as high as 50%, when condition are favorable for the pathogens (Eckert and Eaks, 1988; Abdel-El-Aziz and Mansour, 2006). The heat treatments have been found to enhance wound healing by promoting the synthesis of lignin like compounds (Brown and Eckert, 1988), that act as physical barriers to the penetration of pathogens (Schirra et al. 2000) or by promoting the synthesis of scoparone that could have anti-fungal properties (Kim et al. 1991). Heat treatments also trigger the synthesis of heat shock proteins (Sabehat et al. 1998; Saltveit et al. 2004) which helps in acquisition of thermo tolerance (Vierling, 1991). Increased exposure to high temperature (beyond 15 minutes) however, increased disease incidence, probably by damaging the waxy layer and the rind tissue (Yousaf and Hashim, 1992, Joyce et al. 2003) or increased cuticle cracks (Eckert and Eaks, 1988), which may also serve as pathogen entry routes.

Total Soluble Solids (TSS)

The minimum TSS (12.46%) were recorded with 15 days storage which was non significant with 0 days storage (12.49%). There was a gradual increase in TSS with increase in storage duration so that the maximum total soluble solids (14.63%) were recorded with 75 days storage followed by 14.11% with 60 days storage (Table III). The increase in TSS during storage seems a normal ripening associated change in citrus fruits (Lee and Kader, 2007; Lurie, 1998; Mohla et al. 2000). The means for heat treatment indicate that the maximum TSS (13.76%) recorded in control treatment decreased non-significantly with WHT duration for 5 minutes (13.33%) but significantly with WHT duration for 10 minutes (13.12%) or more so that it declined to 13.02 and 13.08% with WHT duration of 15 and 20 minutes respectively (Table IV). It has been reported earlier that heat treatments has no consistent effects on TSS in citrus fruits (Ekran et al. 2005; Joseph et al. 2004; Shellie et al. 1993; Shellie and Mangan, 1998), it was found that heat treatments inhibited the increase in TSS. Heat treatment is shown to inhibit the ripening in different fruits (Blackburn et al. 1989; Eaks, 1978; Jiangg et al. 2002; Lauri, 1998). When mature-green Mango fruits were heat-treated by dipping in 50ºC or 55ºC water for 5 min and stored at 25ºC with 90-95% relative humidity, softening slowed down the pectin methylesterase (PME) and polygalacturonase (PG) activities in both peel and pulp tissues (Lurie and Klein, 2006) and other metabolic activities. The influence of WHT may be attributed to the inhibition of ripening and senescence related metabolic activities by heat treatments of citrus fruits (Lurie, 1998, Lurie and Klein 2006).

Table-III Effect of room temperature storage duration on biochemical quality attributes of sweet orange fruit Storage Duration (Days at °C)

TSS (%) Acidity (%) TSS/Acid Ratio

Reducing Sugars (%)

Non Reducing

Sugars (%)

Ascorbic Acid

(mg/100g) 0 12.49b 1.64 a 7.59 d 4.41b 8.70a 39.69a 15 12.46b 1.61 a 7.72 d 4.31b 8.66a 38.04b 30 12.70b 1.53 ab 8.31 cd 4.12b 8.59a 36.03c 45 13.19b 1.42 ab 9.35 c 4.97ab 8.52b 32.57d 60 14.11a 1.18 b 11.97 b 4.59ab 8.34b 27.39e 75 14.63a 0.52 c 28.35 a 5.09a 7.99c 26.25f LSD α 0.05 0.609 0.075 0.947 0.495 0.315 = 0.672

Acidity (%)

During the storage, fruit acidity, though non-significantly, decreased from a maximum of 1.64% on day 0 to 1.42% after 45 days storage and significantly to the minimum of 0.52% with 75 storage at room temperature (Table III). Heat treatment however, had no significant effect on acidity of fruits (Table IV). The results confirm the previous reports that total acidity of citrus fruits were non-significantly affected during storage (Hussain et al. 2004).

TSS/Acid Ratio

TSS acid ratio is critical for taste of the sweet oranges. This ratio increased significantly from a minimum value of 7.59 on day 0 to the maximum of 28.35 after 75 days storage at room temperature (Table III). The TSS/Acid ratio is a function of TSS and acidity, thus the increased TSS while decreased acidity during storage resulted in marked increase in TSS Acid ratio. The WHT significantly affected the TSS/Acid ratio which

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declined from the maximum of 13.43 in control fruits to 11.98 with WHT 5 for minutes. The difference in mean TSS/Acid ratio for WHT durations was, however not significant.

Reducing Sugar

The means for storage duration reveal that minimum reducing sugar (4.41%) at 0 days storage increased with increasing storage duration. The reducing sugars increased to the maximum (5.09%) with 75 days duration (Table III). The heat treatment also resulted in increased reducing sugars with increasing treatment duration. Since the reducing sugar is commonly consumed in respiration it decreased with increasing storage durations. It can be expected to decline during storage but increased reducing sugar with storage could be due to moisture loss which increased its concentration in fruits juice. The WHTs either retarded the decline in reducing sugars or enhanced moisture loss or their conversion from non-reducing sugars, thus, resulting in higher reducing sugars with WHTs.

Non Reducing Sugar

Storage durations significantly affected the non-reducing sugar whereas WHT did not. The mean for storage duration reveal that maximum non reducing sugar (8.70%) was recorded on 0 day storage which decreased significantly to 8.52% with storage for 45 days and their after declined significantly to 8.34 and 7.99% with 60 and 75 days storage, respectively (Table III). The non-reducing sugars are mostly the storage sugars, involved in structural components of the cells. These sugars are then converted to monosaccharide sugars (reducing sugars) with the period of time and used for respiration. Thus, the non reducing sugars seem more protected from both storage and WHT related changes in metabolism.

Ascorbic Acid

The mean for storage duration revealed that maximum ascorbic acid (39.69 mg/100g) was recorded on 0 days which continued to decline with incremental increase in storage duration and finally reached the lowest value of 26.25 mg/100g with 75 days storage at room temperature (Table III). The WHT duration also significantly decreased the ascorbic acid contents of the sweet oranges. The maximum quantity of 34.10 mg/100g was recorded in control treatment followed by heat treatment for five minutes (33.46 mg/100g) with the difference being non significant. Ascorbic acid content of the fruit decreased further 33.33 and 32.59 mg/100g with WHT for 15 to 20 minutes (Table IV). Ascorbic acid is a relatively less stable compound (Kader, 2002) and normally decreases with increase in storage duration (Ansari and Feridoon, 2007; Kaul and Saini, 2000). It is interesting to observe that while other ripening associated changes were inhibited by modest heat treatment in diverse types of fruits such as banana (Blackbourn, et al. 1989; Jiang et al. 2002) avocado (Eaks, 1978) and tomato (Lauri, 1998), the decline in ascorbic acid was further enhanced by WHT. The WHT seems to enhance the decline in ascorbic acid content of the citrus probably degrading the ascorbic acid content.

Table-IV Effect of wet heat treatment duration on bio-chemical quality attributes during 75 day storage at room temperature Heat treatment duration (Minutes at 50°C) Parameters 0 5 10 15 20

LSD at α 0.05

TSS 13.76 a 13.33 ab 13.12 b 13.02 b 13.08 b 0.667 Acidity 1.32 1.31 1.31 1.34 1.31 NS TSS/Acid Ratio 13.43 a 11.98 b 12.03 b 11.54 b 12.8 b 1.038 Reducing Sugars 3.36b 4.60a 4.62a 4.69a 4.65a 0.214 Non Reducing sugars 8.57 8.40 8.43 8.48 8.46 NS Ascorbic acid 34.10a 33.46a 33.17b 33.33b 32.59c 0.936

CONCLUSION AND RECOMMENDATIONS

Storage of citrus fruits at room temperature resulted in increased weight loss, decreased rind moisture, greater peel puncturing pressure (PPP), and enhanced disease incidence. The total soluble solids (TSS) and reducing sugars increased but non-reducing sugars and percent acidity decreased, leading to greater TSS/Acid ratio. The ascorbic acid content of the fruit decreased significantly with increasing storage duration. The heat treatment had a close dependent effect on almost all quality attributes. Modest WHT (for 5-10 minutes) slowed down weight loss, retained the total soluble solids, acidity and TSS/Acid ratio and decreased disease incidence but increased ascorbic acid loss. The increase in wet heat treatment beyond 10 minutes reversed all the beneficial effects but gradually enhanced the ascorbic acid losses. Thus, the modest wet heat treatment can be beneficial for retaining fruits quality; the ascorbic acid is lost even with modest heat treatment.

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