Indian Journal of Traditional Knowledge Vol. 12 (4), October 2013, pp. 670-676
Quality characteristics of Ohmic heated Aonla (Emblica officinalis Gaertn.) pulp *A Singh, S Santosh, M Kulshrestha, Khan Chand, UC Lohani & NC Shahi Department of Post Harvest Process and Food Engineering, College of Technology, G B Pant University of Agriculture & Technoloy, Pantnagar – 263 145, US Nagar, UK, India E-mail:
[email protected] Received 06.02.12, revised 17.05.13 Present investigation deals with quality characteristics of aonla pulp subjected to ohmic heating at 11, 13, 15, 17 v/cm voltage gradients. The time and temperature were the process variables which were kept fixed on the basis of preliminary experiments. The product characteristics were evaluated for vitamin C, tannin content, titrable acidity, colour and microbial counts. Results indicated that all the quality parameters were influenced by treatment and KMS addition significantly. Per cent loss for all quality parameter was found to be least in aonla pulp ohmically treated at 17V/cm voltage gradient followed by 15, 13 and 11V/cm while the losses were maximum in conventionally prepared aonla pulp. Ohmically heated aonla pulp at 90°C for 1 minute was observed to be sufficient for safe storage of aonla pulp with little quality losses. In terms of quality, ohmically treated aonla pulp was found to be superior than conventionally prepared one. The quality losses in terms of vitamin C, tannin, titrable acidity and colour lightness were positively correlated to each other. Within the experimental range investigated in this study, aonla pulp ohmically heated at 17V/cm voltage gradients was found to be best in terms of all the quality parameters. Keywords: Ohmic heating, Aonla pulp, Quality changes IPC Int. Cl.8: A61K 36/00, G01, G01K 17/00, B02C, A23N, A47J, G07C 3/14, A47J 36/24
In the processing industries, aonla pulp is being used for preparation of candy, toffees and other edible products. But due to its poor processing techniques, its nutritional properties are not being preserved and thereafter overall quality of the final product goes down. Quality of processed foods can be enhanced by applying proper processing techniques and treatments. Application of ohmic heating towards processing of fruit pulp can be a good approach towards the preserving the quality of pulp as ohmic heating volumetrically heats the entire mass of the food material, thus the resulting product is of far greater quality. Ohmic heating offers several other advantages over the conventional heating methods as it is a direct method and provides rapid cooking, minimum clumping, high-temperature sterilization and almost 100% energy efficient1. Ohmic heating is believed to improve the product quality with minimal structural, nutritional or organoleptic changes 2 and can be used for heating liquid foods containing large particulates, such as fruit pulps, slurries, juices and liquid foods. If ohmic heating is used for aonla processing as a —————— *Corresponding author
non-thermal heat treatment, aonla product will be significantly nutritive. Present investigation deals with the quality assessment of aonla as effected by conventional and thermal heating during its processing. Effect of ohmic heating treatment on aonla pulp was examined to maintain its nutritional qualities. Quality characteristics of aonla pulp relevant to ohmic heating were determined. Materials and methods Experiments were conducted to evaluate the quality of aonla pulp as effected by ohmic heating and conventional heating. Standardization of process parameters (i.e. temperature and voltage gradients) based on quality attributes of aonla pulp were also investigated. The process for making the aonla pulp was standardized first. Independent variables and their ranges were finalized based on the observations and results obtained in preliminary experiments. Voltage gradient and treatments were taken as independent variables for the study. To assess the product quality, ascorbic acid (vitamin C), tannin content, titrable acidity and colour were determined (Flow chart).
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Fruit pieces were grounded into mixer to prepare aonla pulp. Water was added at the time of pulping in 1:1 ratio of fruit to water. In ohmic heating, aonla pulp was then heated ohmically at 90°C for 1 minute to inactivate the polyphenolic oxidase activity using 11, 13, 15 and 17 V/cm voltage gradients. The enzymatic browning was prevented by adding the potassium meta-bisulphite (KMS) at the rate of 2 gm/kg of pulp4 during pulping. Final product (ohmic treated pulp) was compared with respect to quality parameters with conventionally prepared (blanched) aonla pulp and raw pulp, i.e. control.
Flow chart (Experimental design)
Sample procurement Fresh, fully matured, whitish green aonla without any defect on visual infection was procured from local market at Pantnagar. Sample preparation S1 - Aonla was washed and peeled by hand peeler. The peeled aonla was then cut into slices and ground in a mixer with fruit to water ratio 1:13 to obtain pulp (raw sample, R)., S2 - After washing, aonla was blanched and then peeled and cut into slices to make the pulp using 1:1 fruit water ratio (blanched sample, B). Preparation of aonla pulp Aonla pulp was prepared by using conventional method. In case of conventional method, washed aonla fruits were blanched in boiling water for 4 minutes. After cooling to room temperature peeling and destoning of aonla fruits were carried out. Water used for blanching was added at the time of pulping in 1:1 ratio of fruit to water. Also KMS (2gm/kg of pulp) addition was carried out at the time of pulping. Treatment of aonla pulp In case of ohmic heating treatment, washed selected aonla fruits were peeled and cut into pieces.
Physicochemical analysis Colour: Colour of the sample was determined by combination of digital camera, computer, and graphics software5. The L*a*b* values was calculated for determination of the colour of aonla pulp. L* is the luminance or lightness component, which ranges from 0 to 100, and a* (from green to red) and b* (from blue to yellow) are the two chromatic components, which range from -120 to +120. Photographs of freshly prepared aonla pulp were used as a basis for determining the ‘L, a, b’ values in Adobe photoshop. The pixel value tool present in adobe software was positioned to 100 pixel for determining respective ‘L, a, b’ values. These ‘L, a, b’ values were used to calculate the required L*a*b* values by using formulae given below L* = (Lightness/250)*100 a* = (240a/255) – 120 b* = (240b/255) – 120 Vitamin C: Vitamin C was estimated using 2, 6-dichlophenol indophenols visual titration method6. Sample of 10ml was mixed with 3% HPO3 and was filtered. 2-10ml of the HPO3 extract of sample was taken and titrated with standard dye solution to a pink colour (which should persist for at least 15 seconds). At first, preliminary determination about titration was made and then most of the dye required was added rapidly and then accurate titration was carried out. Standardization of dye: Standardization of dye was done to find out the dye factor. Dye solution was filled in a burette. Taking 5 ml of standard working solution in 100 ml of beaker, 5 ml of 3% HPO3 was added and titration was done against dye till pink colour obtained which should persist at least for 15 sec.
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Dye Factor =
0.5 = titre value
mg of vitamin C per 100 gm of sample = Titre value of sample × dye factor × Volume made up × 100 Aliquiot of extract taken for estimation × Weight of sample taken for estimation Titrable acidity: Titrable acidity was estimated by standard method using visual titration method6. Clear filtrate of aonla pulp was prepared using centrifuge. Titration was carried out with 0.1 N NaOH using a few drops of 1% phenolphthalein solution as indicator and titration was done till pink colour was obtained (which should persist at least for 15 sec). This was the titre value. Titre value was calculated for the result as percent anhydrous citric acid.
% titrable acidity =
Preparation of medium: To prepare the nutrient broth, peptone and beef extract was taken and dissolved into 1000 ml of distilled water by warming the flask. PH was adjusted to 7.0 after cooling. Mouth of flask was closed with cotton plugs and sterilized by autoclaving at 15 psig for 15 minutes. Nutrient agar: 3 gm of meat extract, 5 gm of peptone and 25 gm of agar dissolved in 1000 ml of distilled water. Prepared nutrient agar was autoclaved at 15 psig for 20 minute.
Titre value × Normality of alkali × volume made up * × Equivalent wt of Acid × 100 Volume of sample taken for estimaion × Wt. or volume of sample taken × 1000
*This does not apply if the sample is directly taken for estimation as given in juices.
Tannin content: Tannin was estimated using colorimetric method6. Clear filtrate of aonla pulp (0-10 ml) was taken into 100 ml volumetric flasks contain 75 ml of water. 5ml of Folin-Denis reagent and 10 ml Na 2Co3 solution was added into each of the volumetric flasks and volume was made up to 100 ml with water. Solution was properly mixed and colour was measured after 30 min at 760 nm against experimental blank (adjusted to Zero absorbency). Mg of tannic acid was determined by standard tannic acid curve which was prepared by taking tannic acid instead of sample and proceeded as above. Tannin as tannic acid % = mg of tannic acid × Dilution × 100 ml of sample taken for colour developement × Wt. of sample taken × 1000
Microbial analysis: The microbial analysis was carried out for the aonla samples according to the procedure given in7. Sample of aonla pulp (1gm) was weighed aseptically and suspended uniformly in 9 ml of sterilized distilled water as dilution blank. Serial dilutions were prepared. Bacterial population count was carried out for control, blanched and ohmically heated samples by using nutrient agar medium.
Total Plate Count (TPC): Appropriate dilutions of the sample (1 ml) were transferred aseptically to sterile petri-plates in duplicate and mixed well with 10-15 ml of pre sterilized, melted, plate count nutrient agar at 45°C. After solidification, the petri-plates were incubated at 37°C for 24-28 hrs. The average count multiplied by dilution factor was expressed as colony forming unit (CFU) per gram of sample. Statistical analysis: The analysis of variance (ANOVA) tables were generated for each of the response functions. Tukey’s Honestly Significant Difference (HSD) test was performed: for those responses data for which ANOVA was significant, and to determine differences between the means by using SPSS. Results and discussion The quality of aonla pulp was evaluated in terms of selected quality parameters namely, vitamin C, tannin content, titrable acidity, colour and microbial count. 100 % retention of quality parameters for the control samples (raw pulp) was considered indicating no loss occurred during preparation of control samples. Ohmic treated and blanched samples with and without KMS were compared with control sample without KMS only (Fig. 1). Vitamin C Vitamin C varied from 472.27 to 731.3 mg/100 gm of aonla pulp for various treatments (Table 1). Similar
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Fig. 1—Raw, blanched and ohmic heated aonla pulp Table 1- Quality parameters of aonla pulp Treatment
Vitamin C (mg/100 gm)
Tritable acidity (%)
Tannin content (%)
0 % KMS
0.2 % KMS
0 % KMS
0.2 % KMS
0 % KMS
0.2 % KMS
R
731.30±2.34a
ND
2.76±0.02a
ND
1.70±0.02a
ND
11
b
605.60±4.94
b
b
618.64±3.14
2.07±0.02
2.13±0.02
1.23±0.02b
b
1.20±0.01
624.71±3.42
2.10±0.02
2.15±0.02
1.23±0.02
1.26±0.02bc
15
641.79±8.74c
646.16±5.93c
2.15±0.02c
2.21±0.02c
1.26±0.01c
1.30±0.01d
17
671.02±3.12d
678.74±3.22d
2.23±0.02d
2.27±0.01d
1.32±0.01d
1.34±0.01d
B
e
e
e
e
e
1.20±0.02be
1.93±0.01
b
b
616.65±4.12
488.46±4.17
b
b
13
473.63±5.57
b
b
1.97±0.01
1.16±0.01
Values are Mean ± SD of three replicates. Means values followed by the same letter in same column are not significantly different (p>0.05), R = raw anola pulp, B = blanched anola pulp (All the samples treated with KMS (0.2%) were compared to the raw samples (0% KMS))
findings of vitamin C were reported by Tripathi et al. 19888. ANOVA shows that the treatment and KMS addition both have significant (P < 0.05) effect on the vitamin C retention of aonla pulp. The effect of various treatments (F = 2276.45) was found to be more significant than effect of KMS addition (F = 23.53). The effect of interaction (KMS and treatment) was not found significant (P > 0.05) in aonla pulp. Table 1 shows that vitamin C value of all ohmic treated and blanched aonla pulp with and without KMS was significantly (P < 0.05) different from the values of raw pulp. Increase in vitamin C of ohmic treated pulp with and without KMS was not significant (P > 0.05) with increase in voltage gradient from 11 - 13 V/cm, whereas, it increased significantly (P < 0.05) when voltage gradient was increased from 13 - 17 V/cm. The loss of vitamin C in conventionally prepared aonla pulp was found to be quite high (in the range of 33.18 % to 35.2 %) for KMS treated and untreated samples. For ohmically treated pulp, the loss in vitamin C was found in the range of 7.14-17.13 %
which indicates that, vitamin C losses were highly accelerated in pulp treated with 11v/cm voltage gradients for both KMS treated and untreated samples whereas it was found to be least in 17 V/cm treated aonla pulp (Fig. 2a). Percent loss of vitamin C in blanched aonla pulp was found to be within the range as reported by Tandon et al. 20039. KMS addition retained considerable amount of vitamin C in all treatments. Among all treatments, pulp prepared by ohmic heating specially at 17 V/cm voltage gradients was found to be far superior in terms of retention of vitamin C. Titrable acidity Titrable acidity in terms of percent citric acid varied from 1.92 to 2.76 % for various treatments (Table 1). Similar findings for titrable acidity were reported by Barthakur & Arnold, 199110. The ANOVA shows that treatment and KMS addition have significant (P < 0.05) effect on titrable acidity of aonla pulp. Also the variation of titrable acidity
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was found to be influenced more by treatment (Fcal = 1762.57) as compared to KMS addition (Fcal = 52.68). Interaction effect of KMS and treatment was not significant (P > 0.05) on titrable acidity of aonla pulp. Titrable acidity of ohmic treated and blanched aonla pulp with and without KMS was significantly (P < 0.05) different from the raw pulp. Titrable acidity of ohmic treated pulp with and without KMS increased not significantly (P > 0.05) when voltage gradient was increased from 11 - 13 V/cm and thereafter increased significantly (P < 0.05) with increase in voltage gradient from 13 - 17 V/cm. The titrable acidity loss was found to be maximum in pulp prepared by blanching and it varied ranging from 28.62 - 30.07 % (Fig. 2b). In case of ohmically treated aonla pulp, titrable acidity loss followed a decreasing trend with increase in voltage gradient from 11 - 17 V/cm. Among all treatments, ohmically heated pulp especially at 17 V/cm voltage gradients was found to be far superior in terms of retention of titrable acidity. Tannin content Tannin content of aonla pulp varied from 1.15 % to 1.70 % (as a percent tannic acid) for various treatments (Table 1). ANOVA shows that treatment and KMS addition have significant (P < 0.05) effect on tannin content of aonla pulp. Also treatment effect (F = 471.61) was found to be more significant as compared to effect of KMS addition (F = 8). The interaction effect was not significant (P > 0.05) for tannin content of aonla pulp. Tannin content of ohmic treated and blanched pulp with and without KMS was significantly (P < 0.05) different from the values of raw pulp. Tannin content for ohmic treated pulp without KMS increased not significantly (P > 0.05) with increase in voltage gradient from 11-13 V/cm. But tannin content increased significantly (P < 0.05) when voltage gradient was increased from 13 - 17 V/cm. The maximum loss of tannin content was found in blanched aonla pulp in range of 29.42 - 31.76 %, while minimum loss (21.18 - 22.35 %) was found in aonla pulp treated at 17 V/cm voltage gradient (Fig. 1c). However, percentage loss in tannin of blanched pulp with KMS was not significantly (P > 0.05) different from the loss of ohmic treated with KMS at 11 V/cm. For ohmically heated aonla pulp, tannin content loss decreased as voltage gradient increased from 11-17 V/cm. Among all treatments, quality of aonla pulp prepared by ohmically heating
specially at 17 V/cm voltage gradient was found to be far superior in terms of retention of tannin content. Colour The colour of aonla pulp under the influence of various treatments are determined by L*a*b* value (Table 2). The L*a*b* values of control samples was found to be very close to the values reported by (Goyal et al. 2007)4. ANOVA shows that effect of treatments and KMS addition have significant (P < 0.05) effect on L*a*b values. The effect of treatments was found to be more as compared to effect of KMS addition. Interaction of KMS addition
Fig. 2—Per cent loss of (a) vitamin C, (b) titrable acidity, (c) tannin content of aonla pulp
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Table 2—Color values of aonla pulp L*
Treatment
a*
b*
0 % KMS
0.2 % KMS
0 % KMS
0.2 % KMS
0 % KMS
R
48.79±0.44a
ND
-6.62±0.12a
ND
9.30±0.29a
11
b
69.55±0.56
61.87±0.68
13
65.29±0.42c
60.48±0.87b
-8.47±0.34c
15
d
63.04±0.57
c
57.83±0.53
-7.58±0.14
d
17
59.57±0.52e
56.16±0.72c
-7.09±0.16ad
b
d
b
b
-9.42±0.28
b
ND
b
11.58±0.42
13.70±0.26b
-7.81±0.26c
12.12±0.72b
15.60±0.45c
ad
c
14.21±0.32
16.56±0.39c
-6.54±0.15ae
17.39±0.44d
19.58±0.44d
bc
d
-8.51±0.17 -7.06±0.10
b
0.2 % KMS
B 70.32±0.59 66.08±0.34 -9.38±0.25 -8.07±0.11 18.61±0.27 20.45±0.54d Vlues are Mean ± SD of three replicates. Means values followed by the same letter in same column are not significantly different (p>0.05), R = raw anola pulp, B = blanched anola pulp (All the samples treated with KMS (0.2%) were compared to the raw samples (0% KMS))
and treatment had significant (P < 0.05) effect on L*a*b values of aonla pulp. L*a*b values of ohmic treated and blanched sample with and without KMS, were significantly (P < 0.05) different from the values of raw pulp. L* value of ohmic treated pulp without KMS, decreased significantly (P < 0.05) when voltage gradient was increased from 11 - 17 V/cm, while with KMS addition, the value decreased not significantly (P > 0.05) with increase in voltage gradient from 11-13 and 15-17 V/cm. The a* value of ohmic treated pulp with KMS, increased significantly (P < 0.05) with increase in voltage gradient from 11-17 /cm, but without KMS, it increased significantly (P < 0.05) when voltage gradient was increased from 11-15 V/cm and thereafter a* value increased not significantly (P > 0.05) with increase in voltage gradient from 15-17 V/cm. The b* value of ohmic treated pulp without KMS, increased significantly (P < 0.05) when voltage gradient was increased from 13-17 V/cm, while with KMS, it increased significantly (P < 0.05) with increase in voltage gradient from 11 - 17 V/cm. Lightness of the ohmically treated aonla pulp was influenced by KMS addition (Fig. 1). Lightness of pulp was found to be maximum in heated product especially blanched aonla pulp. The a* values was found to be least in ohmically treated pulp at voltage gradient of 17 V/cm and it was maximum in blanched aonla pulp, however, a* value of blanched pulp with and without KMS was not significantly (P > 0.05) different from the value of ohmic treated pulp with and without KMS at 11 V/cm . Also, the a* value of ohmic treated pulp without KMS at 17 V/cm and with KMS at 15 and 17 V/cm was not significantly (P > 0.05) different from the value of raw pulp. It was observed that green colour of aonla pulp was reduced maximum in blanched aonla. KMS addition
increased the b* value for all treatment. The b* value (i.e. blue to yellow) was found to be maximum in blanched pulp followed by ohmic treated aonla pulp, however, b* value of blanched pulp with and without KMS was not significantly (P > 0.05) different from the value of ohmic heated pulp with and without KMS at 17 V/cm. The yellowish colour was found to be maximum in blanched sample. This could be due to the reuse of blanched water for preparation of pulp. Microbial count Total bacterial populations was determined in controlled, blanched and ohmically treated samples of aonla pulp on nutrient agar medium by dilution series and pour plate method. The number of colonies on each dilution series was counted and the plate having colonies ranging in between 30–300 was used for determining number of bacterial population in 1 gm of aonla pulp. The bacterial populations were represented as cfugm-1 of aonla pulp. In ohmic treated samples, all the populations were almost same for bacterial counts. Hence average count indicating the bacterial populations of ohmic treated aonla pulp sample was mentioned. Quality of ohmic treated aonla pulp was superior as compared to conventionally prepared pulp in terms of bacterial populations. Total bacterial count in control samples of aonla pulp was 1.3 × 103 cfugm-1. In case of blanched and ohmic treated samples total bacterial count were about 4.2 × 102 cfugm-1 and 3.4 × 102 cfugm-1 of aonla pulp, respectively, indicating better quality for ohmic treated samples. Significance of the study Processing of aonla fruits (as a pulp) using ohmic heating applications could be one of the options to
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maintain its nutritional quality. Ohmic heater could be used as a potential tool for processing of aonla fruits at small scale/cottage level processing. Same technique may also be used for processing of tomato based purees, fruits and vegetables slurry etc and could be beneficial for small scale enterprenurs. Conclusion On the basis of the experimental data and analysis it may be concluded that all the quality parameters (i.e. vitamin C, tannin content, titrable acidity and colour) were influenced by treatment and KMS addition significantly. It was observed that quality of conventionally prepared and ohmically treated aonla pulp was far superior as compared to controlled pulp samples in terms of bacterial populations. Percent loss for all quality parameter was found to be least in aonla pulp treated with 17 V/cm voltage gradient followed by 15, 13 and 11 V/cm and it was found maximum in conventionally prepared aonla pulp. Aonla pulp ohmically heated at 17 V/cm voltage gradients was found to be best in terms of all the quality parameters which proved that ohmic heating could be the one option to preserve the nutritional qualities of aonla pulp.
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