Equilibrium moisture content of bell pepper

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Equilibrium moisture content of bell pepper a

b

U. S. Shivhare , J. Ahmed & Manpreet Singh

b

a

Department of Food Science and Technology, Guru Nanak Dev University, Amritsar, 143 005, INDIA Phone: 91‐0183‐220667 Fax: 91‐0183‐220667 E-mail: b

Department of Food Science and Technology, Guru Nanak Dev University, Amritsar, 143 005, INDIA Version of record first published: 02 Sep 2009.

To cite this article: U. S. Shivhare, J. Ahmed & Manpreet Singh (2000): Equilibrium moisture content of bell pepper, International Journal of Food Properties, 3:3, 459-464 To link to this article: http://dx.doi.org/10.1080/10942910009524649

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INTERNATIONAL JOURNAL OF FOOD PROPERTIES, 3(3), 459-464 (2000)

EQUILIBRIUM MOISTURE CONTENT OF BELL PEPPER

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U. S. Shivhare*, J. Ahmed, and Manpreet Singh

Department of Food Science and Technology Guru Nanak Dev University, Amritsar 143 005, INDIA *Corresponding author (Tel: 91-0183-220667, Fax: 91-0183-258820, and E-mail: [email protected])

ABSTRACT Equilibrium moisture content (EMC) of bell pepper at selected levels of temperature (30-70°C) and relative humidity (10-75%) of the surrounding air was determined experimentally by the static method. The EMC increased with relative humidity but decreased with increase in temperature. Results demonstrated that Henderson model predicted accurately the variation of EMC with relative humidity at selected temperatures. The standard error was less than 0.077 while correlation coefficient was greater than 0.942 in all cases.

INTRODUCTION EMC is defined as the moisture content of the hygroscopic products such as dried food materials once it has reached in equilibrium with the surrounding air. Vapor pressure present in the food material equals that of surrounding air. EMC plays a significant role in storage and drying applications. EMC indicates whether the product will lose or absorb moisture for the given temperature and relative humidity of the surrounding atmosphere (Brooker et al., 1974). Bell pepper (Capsicum frutescens var. grossum), known as sweet pepper is one of the most liked green vegetables throughout the world. It is used in both conventional and fast food preparations. Bell pepper is a rich source of vitamins; a 100 g edible portion contains about 92.4% moisture, 1.2g protein, 11 mg calcium, 870 I.U. vitamin, 175 mg ascorbic acid, 0.06 mg thiamine, 0.03 mg riboflavin and 0.55 mg niacin. Large sized bell peppers are valued for its crisp texture, delicate but characteristic aroma, and attractive green color in salad and stuffing (Govindrajan, 1986). Bell pepper is available only during the harvesting season due to its perishable nature and its shelf life is limited

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SHIVHARE, AHMED, AND SINGH

to not exceeding a week. Although bell pepper is a vegetable of considerable economic value, processing of bell peppers has not received due attention. Research work was therefore undertaken to study the drying behavior of bell pepper under selected conditions of drying air temperature and pre-treatments. EMC is one of the input parameters for modeling the drying behavior. However there is no information available on the EMC of bell pepper as a function of temperature and relative humidity of surrounding air. The present study was therefore undertaken to experimentally determine the EMC with temperature and relative humidity of the surrounding air using appropriate salt solutions. The specific objectives were: (i) to determine the equilibrium moisture content of bell pepper at selected relative humidity and temperature of the surroundings, and (ii) to describe the variation of equilibrium moisture content with temperature and relative humidity of surrounding air using appropriate mathematical model.

MATERIALS AND METHODS The static method was used to determine the EMC of bell pepper (Rahman, 1995). Dried peppers were used to determine the EMC at 30, 40, 50, 60, 70°C and in neighborhood of 10, 35, 55 and 75% relative humidity which was maintained in the dessicator using selected saturated salt solutions (Large, 1967; Rahman, 1995). Fresh bell peppers of uniform maturity were purchased from the local market at Amritsar. After washing, bell pepper about of a 0.5 cm thick was cut in strips of approximately 4 cm long and lcm wide. Blanching was done in boiling water for 3 min. The blanched samples were immediately cooled to room temperature under running cold water and then spread on sieve tray to drain or dry surface water. Bell pepper strips dried on perforated trays in a cabinet dryer at 50°C. The cabinet dryer was set to the selected temperature and allowed it to reach the desired temperature level. The temperature inside the dryer was measured to within + 1°C. About 3 kg pretreated strips were spread uniformly in single layer on a perforated stainless steel tray. The strips being dried on trays were periodically turned for uniform drying and it took about 6 h to dry the strips to about 10% moisture level. Moisture content was determined by drying sample in an air oven at 103°C till constant mass was obtained (Turhan et al., 1997). The relative humidity and velocity of the air were not regulated. Approximately 7-10g of dried sample was used to determine EMC and experiments were carried out in triplicate. The dessicator containing the saturated salt solution and samples were kept in an incubator at selected temperatures. The samples were weighed after every week. The equilibrium was considered to have been achieved when the change in subsequent reading was less than 10 mg (0.1%). In general it took about 20 days for bell pepper to reach at equilibrium with the surrounding air. Several mathematical models are available in the literature to describe the dependence of EMC on temperature and relative humidity of the surrounding air (Jayas and Mazza, 1993; Rahman, 1995; Myhara et al., 1998). Rahman (1995) has presented a description of these models. While working on okra, Gupta et al. (1999) proposed the following relationship to relate EMC with temperature and relative humidity: Me = A e x p ( B r h / T )

(1)


461

Table 1. EMC of bell pepper at selected temperatures and relative humidities Temperature (°C) 30


40

50

60

70

Relative humidity (%) 12 33 52 75 11 32 49 75 11 31 46 75 11 30 43 75 11 30 40 75

EMC(%) 16.0 27.7 35.8 44.9 13.9 20.0 33.0 42.1 12.0 16.0 21.0 35.0 8.0 13.0 18.3 31.2 5.0 6.8 10.3 15.4

~

Where, M e is equilibrium moisture content (% dry basis), rh denote selective humidity (fraction), T is absolute temperature (K), and A and B represent coefficients.

RESULTS AND DISCUSSION The observed values of the EMC indicated that the EMC increased with relative humidity but decreased with increased temperature (Table 1). Selected two-parameter mathematical models were tested to describe the variation of EMC with relative humidity at any given temperature and standard error and coefficient of the correlation were computed for each model (Table 2). It is obvious from Table 2 that Henderson model (1952) described adequately the EMC data for bell pepper for entire experiment range of relative humidity and temperature of surrounding air. The Henderson model is a thermodynamically derived equation, which has been extensively used relating sorbed water with water activity and is described as (Rahman, 1995): Me = [-ln(l-rh)/A] 1 / B

(2)

Linear form of equation (2) is: ln(A)

(3)


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Table 2. Performance of selected mathematical models for EMC of bell pepper


Model Smith

Oswin

Henderson

Halsey

Chung & Pfost

RH/T

Temperature 30°C 40°C 50°C 60°C 70°C 30°C 40°C 50°C 60°C 70°C 30°C 40°C 50°C 60°C 70°C 30°C 40°C 50°C 60°C 70°C 30°C 40°C 50°C 60°C 70°C 30°C 40°C 50°C 60°C 70°C

Correlation coefficient 0.99 0.97 0.96 0.99 0.97 0.71 0.75 0.92 0.82 0.79 1.00 0.96 0.94 0.99 0.95 1.00 0.94 0.89 0.96 0.92 0.98 0.96 0.97 0.99 0.97 0.94 0.91 0.93 0.90 0.82

Standard error 2.740 5.700 5.120 2.450 2.050 0.130 0.140 0.071 0.132 0.122 0.021 0.071 0.075 0.042 0.077 0.010 0.109 0.132 0.097 0.120 1.240 2.150 1.414 0.721 0.712 0.922 0.830 1.032 1.120 1.670

Standard error

Table 3. Coefficients of Equation 3 T( U C)

1/B

A

30 40 50 60 70

0.437 0.466 0.426 0.553 0.467

4704 2210 2385 338 240

Correlation coefficient 0.996 0.956 0.942 0.989 0.949

0.021 0.071 0.075 0.042 0.077

463


o

30 "C


A 40 °C •

50'C

x

60 °C

O

70 °C

— equation (3)

0

0.1

0.2

0.3 0.4 0.5 Relative Humidity, fraction

0.6

0.7

0.8

Figure 1. Moisture sorption isotherms for bell pepper at selected temperatures

464



Linear regression of equation (3) was carried out using the least-squares technique and coefficients were determined (Table 3). Variation of EMC with relative humidity at selected temperatures is shown in Figure 1. The solid lines represent equation (3). The standard error values were less than 0.077 while the correlation coefficient values were greater than 0.942 for all cases (Table 3). It is observed from Figure 1 that the two parameters Henderson model described well the variation of EMC with relative humidity at selected temperatures. Coefficient A decreased with temperature but variation of 1/B with temperature was not systematic.

CONCLUSION EMC value of bell pepper increased with relative humidity but decreased with temperature. Henderson model described well the variation of EMC of bell pepper with temperature and relative humidity of the surrounding air. Application of the "drying models" usually requires the EMC data of the material pertaining to the condition prevail during drying. The results of this study form the basis for selection of EMC of bell pepper for modelling the convective drying process.

REFERENCES Brooker, D. B., Bakker-Arkema, F. W., and Hall, C. W. 1974. Drying Cereal Grains. AVI Publication, West Port, CT. Govindrajan, V. S. 1986. Capsicum - production, technology, chemistry and quality, Part III. Chemistry of the color, aroma and pungency stimuli. CRC Critical Reviews in Food Science and Technology. 24: 245-355. Gupta, A., Shivhare, U. S., Bawa, A. S., and Singh, S. 1999. Equilibrium moisture content of okra. Institution of Engineers (India). 80: 9-11. Henderson, S. M. 1952. A basic concept of equilibrium moisture. Agricultural Engineering. 33(1): 29-32. Jayas, D. S., and Mazza, D. 1993. Comparison of five, three-parameter equations for the description of adsorption data of oats. Transactions of the ASAE. 36: 119125. Lange, R. A. 1967. Handbook of Chemistry. McGraw Hill Publications. New York. Myhara, R. M., Sablani, S. S., Al-Alawi, S. M., and Taylor, M. S. 1998. Water sorption of dates: modeling using GAB equation and artificial neural network approaches. Lebensm.-Wiss. U.-Technol. 31: 699-706. Rahman, S. 1995. Food Properties Handbook. CRC Press, Boca raton, FL. Turhan, M., Turhan, K. N., and Sahbaz, F. 1997. Drying kinetics of red pepper. Journal of Food Processing and Preservation. 21: 209-223. (Received October 12, 1999; revised February 2, 2000; accepted February 23, 2000)