Hot water treatment notably influenced the physico-chemical parameters of Nagpur mandarin fruits. Study revealed that treatment 500C for 90 seconds at 60 days storage period in refrigerated condition had recorded the least (10.41%) physiological loss in weight (PLW) as compared to other treatments. This might be due to the moisture loss from fresh fruit by vapor- phase diffusion and respiration leads to weight reduction. The treatment 500C for 90 seconds showed lower deformation of fruits (11.30 mm) with maximum total soluble solids (11.30%) and Vitamin C content (36.33mg/100ml) which reflects the retention of fruit quality, similar results were obtained by Hong et al. (2007). However, maximum (47.51%) juice recovery with lower deformation (10.20mm) was recorded in fruits treated at 500C for 120 seconds as shown in (Table-1). The similar pattern in changes of physico-chemical parameters were also observed for the fruits stored in ambient condition (Fig. 2).
In treatment 500C for 90 seconds the microbial load recorded after 28 days of storage on Nutrient agar (1.07 log CFU/ml) and Potato dextrose agar (0.69 log CFU/ml) under ambient condition which revelead that, bacterial as well as yeast and mold count was found lower with high temperature and short duration but it increased in fruits not dipped in hot water similar trend was observed in 60 days refrigerated condition (Fig. 4 & 5). The data revealed that the spoilage percent with treatment of 500C for 90 seconds was found to be minimum as compared with control (Fig. 1). The effect of hot water treatment on citrus fruit may be associated with melting and redistribution of natural epicuticular wax on the fruit surface, plugging numerous microscopic cuticular cracks and stomata to improve physical barriers to pathogen penetration (e.g., Botrytis cinerea whose spores can germinate and penetrate the surface of fruit) and transpiration (Porat et al. 2000, Lydakis and Aked, 1998). In fact, natural openings and barely visible cracks in the epidermis of treated fruit were partially or entirely sealed with rearranged natural wax components present on the cuticle, thus limiting sites of fungal penetration into the fruit (Rodov et al. 1995, Schirra and D’hallewin, 1997, Porat et al. 2000, Fallik, 2004). This mechanism can prevent weight loss from transpiration and the development of decay and can potentially assist in maintaining the good appearance and taste in fruit. Similarly, hot water treatment exerted no adverse effects on the sensory attributes of Nagpur mandarin.
Evaluation in Scanning Electron microscope (SEM)
Scanning Electron microscope (SEM) examination of Nagpur mandarin after wet heat treatment (WHT) at 500C for 90 seconds further indicated that the treatment cleaned the fruits and smoothed the fruit epicuticular waxes (Fig.3, A ; C). So that, it covered and sealed the stomata (Fig. 3 A ; D) and microscope cracks on the fruit surface. Several recent studies have confirmed that hot water dips usually for 120-180 seconds at 530C with or without the addition of fungicides were capable of reducing decay in wide variety of citrus cultivars (Rodov et al. 1995, Schirra and D’hallewin 1997).
Cleaning and disinfection of the fruits surface by hot water treatment was confirmed by SEM analysis (Fig.3 A, B, C & D). Another effect of the hot water treatment was that it melted the fruit epicuticular waxes and thus covered and sealed the stomata and cracks on the fruit surface, these could have served as potential pathogen invasion sites. A similar report with hot water treatment on sealing of cracks on the fruit surface was also reported in Fortune mandarins (Schirra and D’hallewin, 1997).
Hot water treatment cleaned the fruit and improved their general appearance without affecting other quality parameters. During the entire storage period, the fruit, particularly that treated at 500C hot water for 90 seconds gained higher scores in color, flavor, texture and lower score in fruit shriveling on a 9 point Hedonic Scale and overall visual quality than untreated controls. Treatment with 500C hot water for 90 seconds markedly improved fruit appearance, making them cleaner and glossier. Similar results of improved general appearance of the fruit without alteration of their quality parameters were reported previously (Schirra and D’hallewin 1997, Porat et al. 2000, Smilanick et al. 2003, Schirra et al. 2004). It can be inferred that hot water treatment with 500C for 90 seconds can significantly minimize the postharvest loss of Nagpur mandarin fruits without any external application of the fungicidal treatment under the refrigerated storage condition, with non pesticidal residual effects promoting the eco-friendly concept for higher marketability.
We authors gratefully acknowledge the guidance and necessary facilities provided by Dr. V. J. Shivankar, Director, ICAR-Central Citrus Research Institute, Nagpur for constant encouragement and providing the necessary facilities and supporting staff members of processing laboratory in carrying out the experiments successfully. Authors also thankful to Dr. K. Karthikeyan, ICAR- NBSS & LUP Nagpur for providing and imaging of samples in scanning electron microscope.