Moisture sorption isotherms and chemical composition of omani dates

Moisture sorption isotherms and chemical composition of omani dates

PII: Journal of Food Engineering X(1998) 471-479 0 1998 Elsevier Science Limited. All rights reserved Printed in Great Britain SO260-8774(98)00060-O ...

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PII:

Journal of Food Engineering X(1998) 471-479 0 1998 Elsevier Science Limited. All rights reserved Printed in Great Britain SO260-8774(98)00060-O 0260-8774/98 $19.00 + 0.00

ELSEVIER

Moisture Sorption Isotherms and Chemical Composition of Omani Dates Robert M. Myhara,a* Mark S. Taylor,a Bogdan A. Slomin& & Ismail Al-Bulushi” UDepartment of Food Science and Nutrition, Sultan Qaboos University, Box 34, Postal Code 123, Al-Khod, Sultanate of Oman ‘Department of Animal Science, University of Manitoba, Winnipeg, MB R3T-2N2, Canada (Accepted 5 April 1998)

ABSTRACT The chemical composition and water sorption isotherms of two Omani date varieties, Fard and Khalas, were determined. Compositional analysis showed that Khalas contained more glucose (44.69f0.04%) than Fard (43.55 fO.70%), but less non-starch polysaccharide (NSP). Khalas had 3.95 f 0.04% and Fard had 4.44 + 0.03%. Moisture sorption isotherms conducted at three temperatures displayed a crossing effect due to the dissolution of crystalline sugars at higher temperatures and moisture contents. Modeling with the GAB equation predicted monolayer moisture contents (X,) of 13.9% for Fard and 14.4% for Khalas. Isostetic heat of sorption, for both varieties, varied from 9.4 to - 1.6 kJlmo1 as the moisture content changed from 5.0 to 40.0%. Differences in moisture sorption behavior of the two varietieswere attributed to compositional differences. Compositional data alone, could not act as a universal moisture sorption model since other complicating factors, such as crystalline sugar dissolution, were involved. 0 1998 Elsevier Science Limited. All rights reserved. INTRODUCTION

Among the agricultural commodities of commercial importance in Oman, dates are of particular interest. Traditional dry dates, known in Arabic as Tamr, are the shelf stable dried fruit of the date palm Phoenix dactylifera. Dried dates are microbiologically stable when their water activity (a,) is reduced below 0.6 (Beauchat, *Author to whom correspondence should be addressed. Tel. (968) 514179; E-mail: [email protected] 471

R. M. Myhara et al.

472

1981). This critical value is achieved by removing water from the fresh fruit. How much water to remove depends, to a large extent, upon the chemical composition of the date. In the area surrounding Oman, some 12 varieties of dates are commonly produced (Ahmed et al., 1995). These date varieties can vary significantly in their chemical composition, especially the amounts of reducing, non-reducing sugars, as well as the amount and composition of dietary fiber (Ahmed et al., 1995; Mustafa et al., 1986). This variation in chemical composition may have a large effect upon their moisture sorption behaviour. The relationship between a, and moisture content (at a constant temperature) is described practically and theoretically by a moisture sorption isotherm. Determination of the moisture sorption behavior of fruit such as dates, can be used directly to solve food processing design problems, predict energy requirements, determine proper storage conditions and aid in new food product formulations (Myhara et al., 1996). The objectives of this study were to determine the moisture sorption behaviour of the date varieties Fard and Khalas and relate, if possible, moisture sorption behavior to chemical composition. THEORETICAL Isothermal

moisture

CONSIDERATIONS

sorption

The prediction of moisture sorption behavior through modeling has been underway for some time. The Brunauer-Emmett-Teller (BET) plot (Brunauer et al., 1940) and its modified version the GAB equation as described by Labuza et al. (1985) have produced particularly successful models. The GAB equation (eqn (1)) relates a,, equilibrium moisture content (EMC) and temperature effects. X,&Ku,

EMC =

[( 1 - K%)( 1 -

Kuw+CKaw)l

(1)

where EMC is the equilibrium moisture content, X, is the monolayer moisture content, a, is water activity, C is a constant related to the first layer heat of sorption and K is a factor related to the heat of sorption of the multilayer. C = &exp(AH,lRT),

K = Koexp(AHklRT).

(2)

Both C and K are defined in eqn (2), where T is absolute temperature (K), R is the universal gas constant (8.314 J/mol/K), and AH, and AH, are heat of sorption functions. AH, = HM - HN,

AHk = H, - HN.

(3)

In eqn (3), HM is the sorption heat of the monolayer, HN is the sorption heat of the multilayer and H, the heat of condensation of water (taken to be ca. 43 kJ/mol at 25°C). Evaluation of the parameters X,, C,, AH,, I& and AHK can be carried out using direct non-linear regression analysis, comparing experimentally derived results, with results predicted by eqn (1). The loss function minimizes the value of the total sum

Moisture sorption isotherms and chemical composition of Omani dates

473

of squares (SS,) between the experimentally derived results and the predicted results. Direct non-linear regression analysis has been shown to be superior to both linear and indirect non-linear regression analysis (Maroulis et al., 1988). The performance of various minimization techniques may be evaluated by comparing their percent relative mean square (%RMS) and standard error (S.E.), where d.f. is the degrees-of-freedom for the regression. /

%RMS=-

Isosteric

moisture

loo N i= ’ I

N

I$i-Yil

-

Yi

SE. =

N

(tii J;g,



d.f.

YJ2

(4)

sorption

(at a constant equilibrium moisture The relationship between a, and temperature content) is described by a moisture sorption isostere. The isosteric heat of sorption (QJ, a measure of the interaction between absorbate and absorbent, can be determined by the integrated form of the Clausius-Clapeyron equation [eqn (5)] (Ayranci et al., 1990):

(5) at constant equilibrium peratures T, and T2.

moisture

contents.

MATERIALS

a,l

and aw2 are water activities at tem-

AND METHODS

Sample preparation

The dates used were of the Fard and Khalas varieties grown at Nizwa, Oman during the 1994 season. De-stoned date samples, macerated in a mortar and pestle, were spread to a thickness of 1.0-1.5 mm on glass slides. Prepared slides were freezedried for 48 h. Freeze-dried samples contained approximately 2.5% moisture. Sorption

procedure

Adsorption isotherms were determined by placing freeze-dried samples into sealed 1 1 glass jars that contained saturated salt solutions (sorbostat). Each sorbostat maintained a constant equilibrium relative humidity (ERH) as described by Spiess & Wolf (1987). Every second day, samples were removed and weighed until weight loss or gain became 10.001 g for two successive weighings. To study temperature effects, the sorbostats were maintained at 15, 25 and 45 & 0.2”C by a Haake thermostatic water bath. The accuracy with which each saturated salt solution was able to maintain ERH was verified using a water activity meter (Model CX-2, Decagon

R. M. Myhara et al.

474

Devices. Pullman, WA). Thymol was used to prevent mold growth at high ERHs. Each experiment was replicated 10 times (five for Fard and Khalas, respectively). Data processing Experimentally derived equilibrium moisture data were fitted to the GAB model through minimization of the loss function. GAE? parameter estimation, using QuasiNewton and simplex, was carried out using the NONLIN module of Systat (Systat Inc. Evanston, IL). Compositional analysis of dates Moisture content, protein and dry ash determinations were all standard methods of analysis (AOAC, 1990). Non-starch polysaccharides (NSP) were determined by gasliquid chromatography (component neutral sugars) and by calorimetry (uranic acids) using the technique of Englyst & Cummings (1988). Dates destined for individual sugar analysis were held at 100°C for 10 min, then prepared as described by Naczk et al. (1992). A 10 ~1 sample was injected onto an HPLC system (model HP1090), equipped with a model HP1047A refractive index detector (Hewlett Packard, Palo Alto, CA) and a Nova-Pak Radial-Pak cartridge column (4 pm, 100 x 8 mm) (Waters Associates, Milford, MA, U.S.A.). Chromatographic conditions were identical to those described by Myhara et al. (1988) except that the flow rate was 1.0 ml/min, the column and detector temperatures were 40°C external standards of fructose, and glucose and sucrose were used for quantification. For compositional data, an independent t-test was conducted to measure the differences between means. RESULTS

AND DISCUSSION

Compositional results The chemical composition of both Fard and Khalas dates are shown in Table 1. The dry matter found in both varieties consisted primarily of fructose and glucose. Khalas contained less fructose (40.94 f 0.24%) than Fard (42.25 f 0.23%) (JJc 0.05), while Khalas contained more glucose (44.69 +0.04%) than Fard (43.55 +0.70%) (~~0.05). Glucose has a lower solubility in water than fructose, thus at low water contents, more glucose may exist in a crystalline form. Sugars in a crystalline form cannot bind as much water as those sugars found in solution. This higher proportion of crystalline glucose in Khalas, may affect its moisture sorption behavior. The total NSP content (Table 1) of Fard (4.44_+0.03%) was higher than that of Khalas (3.95 &O&l%) (p x0.05). These figures are consistent with other studies showing the relatively low fiber content of dates (Sawaya et al., 1983) when compared to other fruit. Most of the difference in NSP content between the date varieties in this study was due to the higher proportion of pectin (uranic acid) in Fard (1.55%) than in Khalas (1.10%). Non-starch polysaccharides, being polyhydroxyl in nature, can bind substantial amounts of water. The higher proportion of NSP in Fard, may also affect its moisture sorption behavior. Total crude protein and ash levels were similar for both varieties (p > 0.05).

Moisture sorption isothemzs and chemical composition of Omani dates

475

TABLE 1 Compositional Data for the Date Varieties Fard and Khalas Fard

Khalas

42.25 f 0.23ab 43.55 + 0.70b 0.53 * 0.01 4.44 f 0.03b 0.17 0.54 0.16 0.34 1.68 1.55 2.39kO.12 1.94f0.12

40.94 f 0.24b 44.69 + 0.04b 0.57 f 0.07 3.95 * 0.04b 0.21 0.58 0.13 0.21 1.71 1.10 2.44 & 0.33 1.60+0.01

Components (% dry basis) Free sugar Fructose Glucose Sucrose Fiber-total Arabinose Xylose Mannose Galactose Glucose Uranic acids Protein Ash a f Standard deviation. ‘Values within each row are significantly

different

($ ~0.05).

Isothermal sorption data Moisture

sorption

isotherms

Fig. l(a,b), respectively. The containing a high sugar content

for Fard and Khalas at 15, 25 and 45”C, are shown in isotherms have a shape (Tsami et al., 1990).

typical

of food

materials

a ‘6. ‘A

FardIS’C Fard 15X

g

30

E s s 0 f

20

-a 5 Khrlrc 45% -‘b Khrlrs 25’C ‘4 Khrlrs WC

% B s g

10

= s W

-0.0

a

b

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.5

4v

Fig. 1. (a) Moisture sorption isotherms of date variety Fard at 15, 25 and 45°C: -, A 45°C. (b) Moisture sorption isotherms of date variety o 15°C; . . ., q 25°C; --, o 15°C; . . ., q 25°C; -, A 45°C. Khalas at 15, 25 and 45°C: -,

R. A4. Myhara et al.

476

Water is more strongly bound at lower temperatures, with a concomitant increase in a, as temperatures increase. This trend, evident in this study at lower a, levels, is due to water sorption onto primary sites of hydrophilic compounds present in the dates (Saravacos et al., 1986). At higher a, levels a reversal of this occurs where a, levels are lower at a constant EMC, as temperatures increase. The result is a crossing of moisture sorption isotherms conducted at differing temperatures. Dates contain large quantities of fructose and glucose, which at low temperatures and equilibrium moisture contents may at least partly, be present in a crystalline form. The crossing phenomenon described above is due to the increased solubility of sugars as temperatures increase. The increased quantity of sugars in solution, bind larger amounts of water, thus decreasing water activity. Similar results have been obtained (Saravacos et al., 1986; Ayranci et al., 1990; Tsami et al., 1990) for other high sugar fruit. As glucose concentrations increase, this crossing should occur lower down the curve at lower a, values. In this study, the isotherms of Khalas crossed at a lower a, value than for Fard [Fig. l(a,b)]. Khalas contained a larger percentage of glucose than Fard. Isothermal sorption models Data derived by direct nonlinear regression analysis from the present study, together with data from other studies are compared in Table 2. The percent relative mean square and standard error in the present study compare favorably. The monolayer moisture content (X,) can be defined as the moisture content at which all available hydrophilic sites are surrounded by a monolayer of water. In this study it was found that date varieties Fard and Khalas attained this moisture level at 13.9%d.b. and 14.4%d.b., respectively (Table 2). These monolayer moisture levels are similar to those of raisins and prunes (Table 2). Fard, having a larger percentage

Direct Non-linear

Regression

TABLE 2 Analysis of Some Sorption

Data According to the GAB Equa-

tion Dried fruit

co

AHc (kJlmo1)

K.

Al& (wlmol)

S.E.”

%RMP

0.0002 0.0019 0.0006 0.8140 0.0156 1.2440 0.0573

22.50 14.60 18.50 1.11 10.50 -0.08 5.70

1.22 1.44 1.42 1.31 1.30 1.28 1.58

-0.531 - 0.980 - 0.994 - 0.800 - 0.793 - 0.746 - 1.341

0.900 0.710 0.680 1.064 1.280 0.581 1.350

N/A

Prunes’ Dates(Fard) Raisins’ Dates (Khalas) Currantsc

9.7 11.7 12.6 13.9 14.0 14.4 17.3

Blueberries’ Dates (Khadrawy)d

17.4 18.4

0.0048 0.2460

13.80 3.49

1.12 1.92

-0.653 - 2.124

1.909 1.822

Figs”

Apricots’

(c72b.)

“Standard error. bPercent relative mean square. ‘Data taken from Lim et al. (1995). dData taken from Rygg (1948). N/A: not applicable.

N/A N/A 9.191

N/A 9.011

N/A 5.434 3.952

Moisture sorption isotherms and chemical composition of Omani dates

477

of NSP, should have a larger X, value. The fact that it does not indicates that other factors, such as crystalline sugar dissolution, are complicating the relationship between total carbohydrate content and water activity. Isothermal thermodynamic issues The evaporation of water from dates requires energy to overcome the heat of evaporation of pure water, plus the energy needed to overcome hydrogen bonding between water and polyhydroxyl compounds. The enthalpy required to achieve this is low at high water contents and increases at lower water content. AH, represents the difference in enthalpy between the monolayer and multilayer sorption. In the present study, AH, values were estimated at 1.11 and -0.08 kJ/mol (Table 2) for Fard and Khalas, respectively. The positive AH, value for Fard indicates stronger bonds between solids and monolayer water molecules than between monolayer and multilayer water molecules. For Khalas the slight negative value is a result of the endothermic dissolution of sugar crystals. Fruits such as dates, which may contain large amounts of crystalline sugars may yield unusual GAEI parameters at higher equilibrium moisture contents, as is the case with Khalas. Isosteric thermodynamic issues As moisture content increased for both date varieties (Fig. 2(a,b), Qst decreased from 8.71 kJ/mol for Fard (9.38 kJ/mol Khalas) at 3.8% moisture, to - 1.57 kJ/mol for Fard (- 1.64 kJ/mol Khalas) at 40% moisture. These trends, similar to those of apricot, fig and raisin (Ayranci et al., 1990) indicate weaker interactions between date solids and water, as well as the endothermic dissolution of sugars at higher equilibrium moisture contents.

10.00y

1I

6.00 6.00

8

Fard

4.00

E •i d

2.00

a”

-4.00

a

4.00 0

6

10

16

20

26

30

36

40

Equilibrium Moisture Content (X d.b.)

46

b

1



..”

-

6

10

...”

16

...

20

26

30

..-

36

46

46

Equilibrium Moisture Content (X d.b.)

Fig. 2. (a) Isosteric heat of sorption for date variety Fard, (b) isosteric heat of sorption for

date variety Khalas.

478

R. M. Myhara et al.

Conclusion Compositional analysis showed that the date varieties Fard and Khalas differed significantly in terms of glucose and fructose levels, as well as the amount and type of NSP. Moisture sorption isotherms of both date varieties displayed a crossing effect due to the dissolution of crystalline sugars at higher temperatures and moisture contents. The points at which this crossing occurred differed between the two varieties. Both date varieties differed isothermally and isosterically, in regards to their moisture sorption behavior. The difference in moisture sorption behavior of the two varieties must be due to the difference in their chemical composition. These differences cannot, however, be directly used to predict moisture sorption behavior, since other factors such as crystalline sugar dissolution must also be taken into account. Further research into the effects of other such factors must be undertaken before a truly universal moisture sorption model, based upon chemical composition, can be developed. ACKNOWLEDGEMENTS Sultan Qaboos University paper number SQU-141296. REFERENCES AOAC (1990). O#icial Chemists, Arlington, Ahmed, I.A., Ahmed, varieties as influenced

Methods of Analysis, 15th edn. Association of Official Analytical VA, U.S.A. A.W.K. & Robinson, R.K. (1995). Chemical composition of date by the stage of ripening. Food Chem., 54, 305-309.

Ayranci, E., Ayranci, G. & Dogantan, Z. (1990). Moisture sorption isotherms of dried apricot, fig and raisin at 20°C and 36°C. J. Food Sci., 55(6), 1591-1593. Beauchat, L.R. (1981). Microbial stability as affected by water activity. Cereal Foods World, 26(7), 345-349. Brunauer, S., Deming, L.S., Deming, W.E. & Teller, E. (1940). On theory of the Van der Waal’s adsorption of gases. J. Am. Chem. Sot., 62,1723-1732. Englyst, H.N. & Cummings, J.H. (1988). Improved method for the measurement of dietary fiber as non-starch polysaccharides in plant foods. J. Assoc. Ofic. Analyt. Chem., 71, 808-814. Labuza, T.P., Kaanane, A. & Chen, J.Y. (1985). Effect of temperature on the moisture sorption isotherms and water activity shift of two dehydrated foods. .I. Food Sci., 50, 385 Lim, L.T., Tang, J. & He, J. (1995). Moisture sorption characteristics of freeze dried blueberries. J. Food Sci., 60(4), 810-814. Maroulis, Z.B., Tsami, E. & Marinos-Kouris, D. (1988). Application of the GAB model to the moisture sorption isotherms for dried fruits. J. Food Engng, 7,63-78. Mustafa, A.B., Harper, D.B. & Johnston, D.E. (1986). Biochemical changes during ripening of some Sudanese date varieties. J. Sci. Food Agric., 37, 43-53. Myhara, R., Nilsson, B.J., Skura, B.J., Bowmer, E.J. & Cruikshank, P.K. (1988). Gas production from melibiose, raffinose and white bean extracts by bacteria of human fecal origin. J. Can. Inst. Food Sci. Technol., 21, 245-250. Myhara, R., Taylor, M. & Al-Bulushi, I. (1996). The moisture sorption isotherms of Omani dates. In 10th International Drying Symposium, Kracow, Poland, July 29-August 3, 1996. Naczk, M., Myhara, R.M. & Shahidi, F. (1992). Effects of processing on the oligosaccharides of oilseed and legume protein meals. Food Chem., 45(3), 193-197.

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Rygg, G.L. (1948). Storage humidity for dates. Report of 25th Annual Date Growers Institute. Date Growers Institute, Coachella Valley, CA, U.S.A. Saravacos, G.D., Tsiourvas, D.A. & Tsami, E. (1986). Effect of temperature on the water adsorption isotherms of sultana raisins. J. Food Sci., 51(2), 381-383. Sawaya, W.N., Khalil, J.K., Safi, W.N. & Al-Shalhat; A. (1983). Physical and chemical characterization of three Saudi date cultivars at various stages of development. J. Can. Inst. Food Sci. Technol., 16(2), 87-91. Spiess, W.E.L. & Wolf, W. (1987). Critical evaluation of methods to determine moisture sorption isotherms. In Water Activiq: Theory and Applications to Food, eds L.B. Rockland & L.R. Beuchat, Marcel Dekker, New York, pp. 215-234. Tsami, E., Marinos-Kouris, D. & Maroulis, Z.B. (1990). Water sorption isotherms of raisins, currants, figs, prunes and apricots. J. Food Sci., S(6), 1594-1597.