Abstract
Pre-treatments for drying of wild pomegranate arils were standardized to check discolouration of dried arils. Steam blanching of arils for 30 s followed by sulphur fumigation at 0.3% for 60 min was found suitable as they took minimum time to dry a given tray load, and had minimum non-enzymatic browning, furfural, hydroxyl methyl furfural and moisture contents. The dried arils of the standardized pre-treatment recorded maximum scores for sensory characteristics like colour, texture, taste, aroma and overall acceptability.
Keywords: Wild pomegranate, Dried arils, Sulphuring, Sulphiting, Cabinet drier
Introduction
Wild pomegranate (Punica granatum L.) is the only important wild fruit with great economic significance because of its high acidic nature. It is widely distributed in drier and sub marginal land of mid hill region of outer Himalaya at an elevation of 900 to 1800 MSL. The fruit contains citric acid as the major acid besides malic acid, oxalic acid, succinic acid and tartaric acid (Saxena et al. 1987). But wild pomegranate is highly acidic and hence cannot be used for table purpose but can be a good souring agent for use in curries, chutneys and other culinary preparations in dried form (Phadnis 1974; Chauhan at al. 1994). Lot of work on the preparation of dried arils has been reported from the commercial cultivars of pomegranate (Pruthi and Saxena 1984; Patil et al. 2003; Singh and Sethi 2003) but information on the drying of wild pomegranate arils particularly with pre-treatments is scanty. In India, nearly 1000 tonnes of dried arils are produced annually traditionally from wild pomegranate fruits, valued at Rs 150 crores (Yadav et al. 2006). Arils are dried traditionally without any pre-treatment, as a result poor quality product is produced, which fetches low price in the market. The poor quality of dried arils is due to the discolouration of arils caused by browning during drying and handling. Keeping these in view the present study was undertaken to standardize the pre-treatments for the preparation of good quality dried arils.
Material and methods
Wild pomegranate (Punica granatum L.) fruits harvested at optimum maturity were procured from Narag area of district Sirmour of Himachal Pradesh. Chemicals and other materials used were procured from local market.
Standardization of pre-treatments
The arils were extracted from the selected fruits manually. One kg freshly extracted arils in each treatment was subjected to pre-treatments as given in Table 1.
Table 1.
Pre-treatments of freshly extracted arils
| T1 | Unblanched/Control |
| T2 | SB |
| T3 | SB + 1000 ppm KMS, 10 min |
| T4 | SB + 1000 ppm KMS, 20 min |
| T5 | SB + 1000 ppm KMS, 30 min |
| T6 | SB + 2000 ppm KMS, 10 min |
| T7 | SB + 2000 ppm KMS, 20 min |
| T8 | SB + 2000 ppm KMS, 30 min |
| T9 | SB + 0.3% S fumigation, 40 min |
| T10 | SB + 0.3% S fumigation, 50 min |
| T11 | SB + 0.3% S fumigation, 60 min |
| T12 | 1000 ppm KMS, 10 min |
| T13 | 1000 ppm KMS, 20 min |
| T14 | 1000 ppm KMS, 30 min |
| T15 | 2000 ppm KMS, 10 min |
| T16 | 2000 ppm KMS, 20 min |
| T17 | 2000 ppm KMS, 30 min |
| T18 | 0.3% S fumigation, 40 min |
| T19 | 0.3% S fumigation, 50 min |
| T20 | 0.3% S fumigation, 60 min |
SB Steam blanching for 30 s, KMS Sulphiting with Potassium metabisulphite, S Sulphuring with sulphur powder
In some of the pre-treatments blanching time of arils was standardized by estimating the activity of peroxidase enzyme. Its activity was determined as per the method given by Putter (1974). The enzyme activity per litre of extract was calculated using the formula:
 |
where, ∆t: Time to increase the absorbance by 0.1, min; 6.39: Extinction coefficient of guaiacol dehydrogenation product at 436 nm.
After blanching, arils were fumigated in a chamber by burning sulphur. In sulphiting, blanched arils were dipped in potassium metabisulphite (KMS) solution for varying time at ambient temperature (25 °C). In control, extracted aril samples were directly dried without any pre-treatment.
Drying of arils
The arils of all pre-treatments were dried at 60 ± 2 °C till constant weight in a mechanical cabinet drier (Windson Scientific Works, New Delhi) having 90 × 60 × 90 cm dimensions.
Quality characteristics of fruits and arils
Random sample of 15 fruits were selected. The size of selected fruits was determined with the help of vernier calliper by measuring the length and diameter, and the average size was expressed in millimeter. The average weight of fruits, arils and rind was measured on a top pan balance of Metler Toledo (PB 153-S, Zurich, Switzerland) make. The arils from 15 randomly selected fruits were counted individually and average number of arils per fruit was calculated. The aril: pomace ratio was calculated by dividing weight of aril by the rind. The colour of fruits was observed visually but the colour of arils was compared visually with colour cards of Royal Horticulture Society, London and the card numbers were mentioned along with the colour. Time taken to dry a given tray load was calculated by recording the time required by the material in the tray to attain a constant weight after drying. Yield of dried arils was also calculated by taking into consideration the weight of fresh arils.
Total soluble solids (TSS) were measured by hand refractometer of Erma, Japan make applying correction factor for temperature variation. Dried samples were diluted 2 times with distilled water and the reading was later multiplied by 2. The pH was determined by using a digital pH meter Elico (G163, Hyderabad, India) make after calibration of the meter with buffers of pH 4 and 9. In case of fresh fruit, its pH was determined by taking its juice, whereas, in dried arils sample was prepared by crushing arils and diluting it with sufficient distilled water. Sugars, moisture, total solids, titratable acidity (citric acid), ascorbic acid, anthocyanins, ash content, non-enzymic browning (NEB) and hydroxy methyl furfural (HMF) and pectin were estimated as per Ranganna (1986). The total phenols were determined by the Folin-Ciocalteu procedure (Singleton and Rossi 1965). Furfural was determined by a colorimetric method based upon its reaction with aniline and acetic acid in the presence of acidified SnCl2 (Dinsmore and Nagy 1974).
Pre-treated and dried arils were evaluated for sensory quality by 10 semi-trained panelists on the basis of colour, texture, taste and overall acceptability, on a 9-point Hedonic scale (Ranganna 1986). The experiments were replicated as per replications (n) mentioned in the respective tables. Statistical analysis of data of various atributes including physico-chemical characteristics was carried out by completely randomised design (Mahony 1985) and sensory analysis by randomised block design (Cochran and Cox 1967).
Results and discussion
All the physico-chemical characteristics of fruit and aril presented in Table 2 were within the ranges described by various workers (Parmar and Kaushal 1982; Sharma and Sharma 1990; Chauhan et al. 1994; Kher 1999). The slight differences recorded might be due to the age of the plant, soil moisture conditions of the area from where the samples were procured.
Table 2.
Physico-chemical characteristics of wild pomegranate fruit and arils
| Lengtha, mm | 58.9 ± 1.58 |
| Diametera mm | 46.4 ± 1.00 |
| Weightag | 64.0 ± 3.20 |
| Fruit colour | Yellowish green |
| Arils colour | Red Purple (61 B)b |
| Number of arilsa/fruit | 225.20 ± 14.18 |
| Weight of arilsa/fruit, g | 33.2 ± 2.50 |
| Arils : pomace ratio | 1.0 ± 0.08 |
| Moisture,% | 72.3 ± 0.20 |
| Total solids,% | 27.7 ± 0.20 |
| TSS, °Brix | 20.1 ± 0.08 |
| Reducing sugars,% | 7.1 ± 0.17 |
| Total sugars,% | 7.3 ± 0.16 |
| Titratable acidity,% citric acid | 4.0 ± 0.04 |
| pH | 2.5 ± 0.01 |
| Ascorbic acid, mg/100 g | 21.1 ± 0.38 |
| Anthocyanins, mg/100 g | 7.4 ± 0.10 |
| Pectin,% | 0.7 ± 0.02 |
| Phenols, mg/100 g | 89.8 ± 3.26 |
| Ash,% | 0.9 ± 0.12 |
TSS Total soluble solids
an = 15
bColour card number of Royal Horticulture Society, London
Standardization of blanching time
With the increase in the blanching time the peroxidase activity decreased in the arils from the initial 375.9 to 187.9/l in 150 s in the blanched sample (Table 3). Minute leaching losses in colour and soluble solids were observed in the arils blanched for 30 s. But blanching beyond 30 s led to quite high losses in these attributes. So blanching time of 30 s was found suitable for arils although the enzyme activity was not inhibited completely. Melnick et al. (1944) have reported that steam blanching is preferable to water blanching because of less leaching losses of soluble nutrients in the fruits.
Table 3.
Peroxidase enzyme activity per litre of extract
| Blanching time, sec | Time required in min to increase absorbance by 0.1, ∆t | Enzyme activity = 500 ∆t | Colour and soluble solids loss |
|---|---|---|---|
| 0 | 1.3 | 375.9 | No loss |
| 30 | 1.5 | 333.3 | Very less |
| 60 | 1.7 | 289.0 | Less |
| 90 | 2.0 | 250.0 | More |
| 120 | 2.3 | 214.0 | High |
| 150 | 2.7 | 187.9 | Very high |
Physico-chemical characteristics
Results in Table 4 indicate that blanched arils were dried faster compared to others and the time taken to dry the treated arils ranged from 10 to 11.5 h. However, minimum time of 10 h was taken to dry the arils of T3 to T11, whereas, T1, T18, to T20 took maximum time of 11.5 h to dry a given tray load, which was found at par with T12 to T17. Further, the yield of dried arils ranged from 31.4 to 32.6% and was maximum in T20, which was found at par with T1, T9 to T14, T18 and T19, while minimum was in T8.
Table 4.
Effect of pre-treatments on the physico-chemical characteristics of wild pomegranate arils
| Treatment | Drying time, h | Yield% | Visual coloura | TSS °B |
|---|---|---|---|---|
| T1 | 11.5 | 32.5 | GR (181A) | 41.5 |
| T2 | 10.5 | 31.5 | R (46A) | 37.0 |
| T3 | 10.0 | 31.5 | R (46A) | 37.0 |
| T4 | 10.0 | 31.5 | R (46A) | 36.8 |
| T5 | 10.0 | 31.5 | R (46A) | 36.6 |
| T6 | 10.0 | 31.5 | R (46A) | 36.8 |
| T7 | 10.0 | 31.4 | R (46B) | 36.2 |
| T8 | 10.0 | 31.4 | R (46C) | 36.0 |
| T9 | 10.0 | 32.0 | R (46A) | 38.6 |
| T10 | 10.0 | 32.0 | R (46A) | 38.8 |
| T11 | 10.0 | 32.0 | R (46A) | 39.2 |
| T12 | 11.0 | 32.0 | R (46A) | 39.6 |
| T13 | 11.0 | 32.0 | R (46A) | 39.6 |
| T14 | 11.0 | 32.0 | R (46A) | 39.4 |
| T15 | 11.0 | 31.5 | R (46A) | 39.4 |
| T16 | 11.0 | 31.5 | R (46B) | 39.2 |
| T17 | 11.0 | 31.5 | R (46B) | 38.9 |
| T18 | 11.5 | 32.5 | R (46A) | 40.0 |
| T19 | 11.5 | 32.5 | R (46A) | 40.2 |
| T20 | 11.5 | 32.6 | R (46A) | 40.4 |
| CD 0.05 (n = 3) | 0.9 | 0.6 | – | 0.5 |
Figures in parentheses are square root transformed values
T1-T20: As in Table 1
Moisture content was in the narrow range of 9.0–10.5% (3.0–3.2) and total solids 89.5–91.0% (9.46–9.54) in all samples
GR Greyish Red, R Red, TSS Total soluble solids
aColour card number of Royal Horticulture Society, London
Less time taken to dry the arils in all the blanched arils might be due to the rupturing of their membrane during blanching, thus facilitating faster removal of moisture because of plasmolysis. Higher yield observed in T20 might be due to no leaching losses as blanching was not done in this pre-treatment.
Better colour of arils was retained in all the treatments as compared to the control (Table 4). The red colour (R-46A) was recorded in T2 to T6, T9 to T15, T18 to T20. However, greyish red (dull) colour (GR-181A) of arils was observed in control (T1). The moisture content in the dried arils ranged from 9.0 to 10.5%. It was minimum in T3 to T11 and maximum in T1, T18 to T20. The total solids in the dried arils ranged from 89.5 to 91.0% whereas, the maximum total solids in the dried arils were in T3 to T11 and minimum in T1, T18, T19 and T20. The red colour, minimum moisture and correspondingly high total solids in the above treatments might be due to the combined effect of blanching and sulphuring, which reduced the moisture content by means of exposing the cells by rupturing their membrane, thus facilitating their plasmolysis due to heat and thereby retaining higher total solids. Best colour preservation in the above treatments might be due to SO2 reaction with polyphenols, which prevented development of brown colour in sugar amino system as has been reported by Burton et al. (1963). The retention of best colour might also be due to reduced non-enzymatic reactions as SO2 inhibits brown colour formation at the beginning of the reaction as reported by McWeeny (1984). Moisture content in the dried arils was in the same range as reported by various workers for different cultivars of pomegranate (Patil et al. 2003; Singh and Sethi 2003). The TSS of dried arils ranged from 36.0 to 41.5°B; maximum being in T1 which was significantly higher than other pre-treatments, while minimum was in T8 (Table 4). Moisture and total solids values were in the narrow range of 9.0–10.5% and 89.5–91.0%, respectively in all the treatments.
Chemical characteristics
Maximum values of reducing and total sugars were recorded in T20 (24.1%) and T1 (25.6%), respectively (Table 5). These sugars (21.6% and 22.5%) were minimum in T8. The maximum TSS, and sugars found in T1 and T20 might be due to the low moisture retention as well as absence of leaching losses in the arils. The titratable acidity of dried arils ranged from 13.01 (T2) to 14.6 (T1)%, while pH values, however, were 3.6 in all treatments (Table 5). The highest titratable acidity in the arils found in T1 might be due to lack of leaching losses as a result of no blanching as compared to other pre-treatments.
Table 5.
Effect of pre-treatments on the chemical characteristics of wild pomegranate arils
| Treatment | Phenols mg/100 g | NEB, OD | Furfural, ppb | HMF, ppm | Reducing sugars% | Total sugars% | Anthocyanins mg/100 g |
|---|---|---|---|---|---|---|---|
| T1 | 105.7 | 0.064 | 18.42 | 1.82 | 23.96 (4.85) | 25.64 (5.06) | 33.56 |
| T2 | 109.7 | 0.056 | 16.97 | 1.26 | 21.70 (4.65) | 22.98 (4.79) | 32.01 |
| T3 | 112.5 | 0.039 | 16.25 | 1.21 | 22.79 (4.66) | 22.80 (4.77) | 30.79 |
| T4 | 117.2 | 0.036 | 16.21 | 1.15 | 21.76 (4.66) | 22.74 (4.77) | 30.54 |
| T5 | 119.1 | 0.032 | 16.18 | 1.09 | 21.69 (4.65) | 22.65 (4.76) | 30.49 |
| T6 | 120.1 | 0.031 | 16.16 | 1.11 | 21.75 (4.66) | 22.82 (4.78) | 30.45 |
| T7 | 121.9 | 0.029 | 16.12 | 1.03 | 21.65 (4.65) | 22.60 (4.75) | 30.34 |
| T8 | 123.7 | 0.026 | 16.06 | 0.96 | 21.58 (4.64) | 22.52 (4.75) | 30.20 |
| T9 | 122.4 | 0.027 | 15.88 | 0.98 | 22.78 (4.77) | 24.13 (4.91) | 32.84 |
| T10 | 125.4 | 0.022 | 15.42 | 0.95 | 22.82 (4.77) | 24.25 (4.92) | 32.25 |
| T11 | 133.9 | 0.020 | 14.85 | 0.83 | 22.96 (4.79) | 24.61 (4.96) | 32.12 |
| T12 | 110.4 | 0.043 | 16.90 | 1.22 | 23.15 (4.81) | 25.12 (5.01) | 33.32 |
| T13 | 112.5 | 0.041 | 16.87 | 1.20 | 23.07 (4.80) | 25.01 (5.00) | 33.15 |
| T14 | 115.5 | 0.040 | 16.80 | 1.17 | 23.00 (4.79) | 24.92 (4.99) | 33.05 |
| T15 | 118.1 | 0.039 | 16.83 | 1.17 | 23.00 (4.79) | 25.07 (5.00) | 33.05 |
| T16 | 118.9 | 0.036 | 16.70 | 1.13 | 22.92 (4.78) | 24.82 (4.98) | 31.56 |
| T17 | 121.3 | 0.035 | 16.59 | 1.05 | 22.81 (4.77) | 24.72 (4.97) | 30.61 |
| T18 | 116.2 | 0.036 | 16.42 | 1.17 | 23.65 (4.86) | 25.40 (5.04) | 33.95 |
| T19 | 119.5 | 0.034 | 16.38 | 1.12 | 23.86 (4.88) | 25.26 (5.03) | 33.69 |
| T20 | 121.2 | 0.032 | 16.33 | 1.07 | 24.06 (4.90) | 25.10 (5.01) | 33.46 |
| CD 0.05 (n = 3) | 1.7 | 0.002 | 0.03 | 0.03 | 0.01 | 0.03 | 0.02 |
Figures in parentheses are square root transformed values; NEB Non enzymatic browning; HMF Hydroxymethyl furfural; T1-T20: As in Table 1
pH was 3.6 and the values were in the narrow range of 13.01–14.60% (3.60–3.82) for titratable acidity, 13.02–14–06 mg/100 g for ascorbic, 2.79–2.83% (1.67–1.68) for pectin and 4.15–4.61% (2.04–2.15) for ash for all treatments
Ascorbic acid content of dried arils ranged between 13.02 (T8) and 14.06 (T20) mg/100 g. Anthocyanins were highest in T18 (33.45 mg/100 g) and lowest in T8 (30.2 mg/100 g) (Table 5). Higher retention of ascorbic acid and anthocyanins in the arils of T20 and T18, respectively might be due to lack of leaching losses as no blanching was done.
T11 had maximum (133.9 mg/100 g) phenols, while T1 minimum (105.7 mg/100 g). There was no significant effect of pre-treatments on the pectin content of dried arils. T11 had highest (4.6%) and T20 lowest (4.1%) ash content. Minimum NEB (0.020), furfural (14.8 ppb) and HMF (0.83 ppm) in the dried arils were observed in T11, while maximum NEB (0.064), furfural (18.4 ppb) and HMF (1.82 ppm) were in T1. Pectin values were in the narrow range of 2.79–2.83% in all the treatments. Maximum retention of phenols in the arils of T11 might be due to slower enzymatic reactions in arils as well as the inhibitory effect of SO2 against the enzymes (Frank 1975). Maximum retention of ash in arils of T11 might be due to the fact that less amount of these constituents would have participated in browning reactions. The minimum NEB, furfural, and HMF observed in T11 might be due to the effect of sulphuring, which to some extent prevented formation of NEB products, polymerization of ascorbic acid with sugars to form furfural and degradation of hexose sugars in presence of acids to form HMF by lowering the drying time and moisture of arils. SO2 might have blocked carbonyl group of sugar to render it unavailable for interaction with amino acids in Maillard reaction (Frank 1975; Andrews and Godshall 2002).
Sensory characteristics of dried arils
Maximum scores for colour, texture, taste, aroma and overall acceptability were observed T11, however, it was at par with T8 and T9 with respect to colour, T1, T9, T10, T12, to T14, T18, to T20 with respect to texture and taste and T20 with respect to aroma, whereas T2 had minimum scores for all the sensory characteristics tested (Table 6). The maximum sensory characteristics scores of pre-treated arils for colour, texture, taste, aroma and overall acceptability were obtained for T11 (blanching + sulphuring at 0.3% for 60 min). The reason for the best colour score might be due to less browning observed in arils, while the good texture of arils might be due to low moisture content in arils and quick drying of arils would have retained the taste and aroma of the arils thereby, improving the overall acceptability. Retention of colour and flavour during drying of arils might also be due to the pre-treatment effect of SO2 as has been explained by Joslyn and Braverman (1954).
Table 6.
Effect of pre-treatments on the sensorya characteristics of dried wild pomegranate arils
| Treatment | Colour | Texture | Taste | Aroma | Overall acceptability |
|---|---|---|---|---|---|
| T1 | 7.0 | 8.0 | 8.0 | 7.6 | 7.5 |
| T2 | 7.0 | 7.0 | 7.0 | 7.0 | 7.0 |
| T3 | 7.5 | 7.5 | 7.5 | 7.5 | 7.5 |
| T4 | 7.5 | 7.5 | 7.5 | 7.5 | 7.5 |
| T5 | 7.5 | 7.5 | 7.5 | 7.5 | 7.5 |
| T6 | 7.5 | 7.0 | 7.2 | 7.2 | 7.2 |
| T7 | 7.5 | 7.0 | 7.2 | 7.2 | 7.2 |
| T8 | 7.5 | 7.0 | 7.2 | 7.2 | 7.2 |
| T9 | 8.5 | 8.0 | 8.0 | 8.2 | 8.2 |
| T10 | 8.5 | 8.0 | 8.0 | 8.2 | 8.2 |
| T11 | 8.9 | 8.5 | 8.5 | 8.7 | 8.7 |
| T12 | 8.0 | 8.0 | 8.0 | 8.0 | 8.0 |
| T13 | 8.0 | 8.0 | 8.0 | 8.0 | 8.0 |
| T14 | 8.0 | 8.0 | 8.0 | 8.0 | 8.0 |
| T15 | 8.0 | 7.5 | 7.5 | 7.7 | 7.7 |
| T16 | 8.0 | 7.5 | 7.5 | 7.7 | 7.7 |
| T17 | 8.0 | 7.2 | 7.5 | 7.6 | 7.6 |
| T18 | 8.0 | 8.0 | 8.2 | 8.1 | 8.1 |
| T19 | 8.0 | 8.0 | 8.2 | 8.1 | 8.1 |
| T20 | 8.0 | 8.0 | 8.2 | 8.2 | 8.1 |
| CD 0.05 | 0.7 | 0.8 | 0.7 | 0.4 | 0.4 |
aBased on 9- point Hedonic scale (n = 10 panelists)
Conclusion
After extraction of arils from the wild pomegranate fruits, they can be pre-treated with steam for 30 s followed by sulphuring @ 0.3% for 60 min and dried in a cabinet drier at 60 ± 2 °C for the preparation of best quality dried arils.
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