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Water Sorption Isotherms of Vegetables as Influenced by Seed Species and Storage Temperature Department of Plant Production, College of Agriculture, King.

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Presentation on theme: "Water Sorption Isotherms of Vegetables as Influenced by Seed Species and Storage Temperature Department of Plant Production, College of Agriculture, King."— Presentation transcript:

1 Water Sorption Isotherms of Vegetables as Influenced by Seed Species and Storage Temperature Department of Plant Production, College of Agriculture, King Saud University, P.O. Box 2460, Riyadh 11451, Saudi Arabia Department of Plant Production, College of Agriculture, King Saud University, P.O. Box 2460, Riyadh 11451, Saudi Arabia Abdullah A. Alsadon

2 IntroductionIntroduction Vegetables are widely grown in Saudi Arabia. Among the common grown crops are beet, cabbage, lettuce and okra.Vegetables are widely grown in Saudi Arabia. Among the common grown crops are beet, cabbage, lettuce and okra. Vegetable seeds are mostly imported by agricultural companies and sold to farmers.Vegetable seeds are mostly imported by agricultural companies and sold to farmers.

3 Some farmers may use part of the seeds to grow their crops and keep the remaining seeds for the following season. Seeds may be stored in farm storage houses in an environment that might not be suitable to preserve seed viability. The dry environments in the central region and humid environment in the coastal areas along with higher temperature all play major influence on seed longevity.

4 Controlling water content of seeds and reducing storage temperature can insure greater seed germinability for several years (Walters et al., 1998). The behavior of moisture sorption isotherms can be illustrated by the relationship between equilibrium moisture content (EMC) and relative humidity (RH).

5 A small change in seed moisture content greatly affect the storage life of seeds (Hanson, 1985). The temperature and moisture content of stored seeds are key factors in seed longevity. The isotherms can be used for determining the approximate RH required for seed storage or drying (Walters and Hill, 1998). Various models have been proposed to describe the relationship between RH and EMC or relationship between RH, EMC and temperature using best fit models to generate three-dimensional response services (Fang et al., 1998).

6 The GAB equation is one of the most widely accepted models for sorption isotherms (Ayaranci et al., 1990; Tsami et al., 1990). The Henderson equation (Henderson, 1952, in Roberts, 1972) predicts greater changes in EMC for a given temperature change at higher RH than at lower RH.

7 The objectives of this study were (a): to determine the water sorption isotherms of four vegetable species at four storage temperatures (5, 15, 25, and 35 ºC), and (b): to correlate experimental sorption data to sorption isotherm equations. The objectives of this study were (a): to determine the water sorption isotherms of four vegetable species at four storage temperatures (5, 15, 25, and 35 ºC), and (b): to correlate experimental sorption data to sorption isotherm equations. ObjectivesObjectives

8 Materials and Methods Seeds of four vegetable species were selected for this study: carrot (Daucus carota L.), cucumber (Cucumes sativus L.), onion (Allium cepa L) and tomato ( Lycopersicon esculentum Mill.).Seeds of four vegetable species were selected for this study: carrot (Daucus carota L.), cucumber (Cucumes sativus L.), onion (Allium cepa L) and tomato ( Lycopersicon esculentum Mill.).

9 Table 1. Seeds sources, initial a w, and initial MC for the four vegetable species. initial MC (db) initial a w Source CultivarSpecies Initial germ. (%) Test date Company 4.360.378806/99Nickerson – Zwaan, Barendrecht, Holland Nantes 2- Tito Carrot 4.280.383952/99California – Ventura, CA, USA. SpecialCucumber 3.100.2748412/98SunSeeds, Porma, ID, USA. Red CreoleOnion 2.880.298852/99Genetics Int. Inc., Modesto, CA, USA. Tanshet Star Tomato a w = water activity = initial MC (db): (g H 2 O/g dry weight).

10 Eight saturated salt solutions (Table 2) were prepared corresponding to a range of water activities from 0.113 to 0.985. Glass desiccators containing the salt solutions were kept in temperature controlled chambers at 5, 15, 25, and 35ºC). The desiccators were tightly sealed from the outside atmosphere using high vacuum silicone grease.

11 Table 2. Water activity of saturated salt solutions at 5, 15, 25, and 35ºC. Water activity (a w ) Z Saturated salt solution 35°C25°C15°C5°C 0.113 Lithium chloride 0.2150.2250.2450.291Potassium acetate 0.3250.3280.3350.336Magnesium chloride 0.4330.432 0.431Potassium carbonate 0.5450.5840.620.644Sodium bromide 0.7520.7530.7550.757Sodium chloride 0.8310.8430.8650.877Potassium chloride 0.9620.9730.985 Potassium sulfate Z = Water activity data were checked for all saturated salt solutions at 25 o C using Aqua Lab. (Model CX-21, readability 1 mg, Decagon Devices Inc., Washington). The data for other temperatures were then adapted from (Winston and Bates, 1960 and Rizvi, 1995).

12 The moisture isotherms of seeds at 5, 15, 25 and 35 º C exhibited a reverse sigmoidal shape.The moisture isotherms of seeds at 5, 15, 25 and 35 º C exhibited a reverse sigmoidal shape. At the first half of the curve (the region with low relative humidity) seeds sorbed relatively lower amounts of moisture (Fig. 1).At the first half of the curve (the region with low relative humidity) seeds sorbed relatively lower amounts of moisture (Fig. 1). The moisture isotherms of seeds at 5, 15, 25 and 35 º C exhibited a reverse sigmoidal shape.The moisture isotherms of seeds at 5, 15, 25 and 35 º C exhibited a reverse sigmoidal shape. At the first half of the curve (the region with low relative humidity) seeds sorbed relatively lower amounts of moisture (Fig. 1).At the first half of the curve (the region with low relative humidity) seeds sorbed relatively lower amounts of moisture (Fig. 1). Results and Discussion

13 Evaluation of vegetable species sorption isotherm response In general, the sorption isotherms curves of the four vegetable species were similar at lower RH (Fig. 2). However, difference between species isotherm curves became obvious when seeds were kept at RH higher than 60%. In general, cucumber seeds had the least sorption response followed by tomato, onion, and then carrot seeds. Thus, it is expected that the longevity of cucumber seeds would be higher than that of the other species in this study. In general, the sorption isotherms curves of the four vegetable species were similar at lower RH (Fig. 2). However, difference between species isotherm curves became obvious when seeds were kept at RH higher than 60%. In general, cucumber seeds had the least sorption response followed by tomato, onion, and then carrot seeds. Thus, it is expected that the longevity of cucumber seeds would be higher than that of the other species in this study.

14 Fig 1 (a) Water sorption isotherms of the four vegetable species at 5ºC.

15 Fig 1 (b) Water sorption isotherms of the four vegetable species at 15ºC.

16 Fig 1 (c) Water sorption isotherms of the four vegetable species at 25ºC.

17 Fig 1 (d) Water sorption isotherms of the four vegetable species at 35ºC.

18 The effect of temperature on sorption isotherms For most seed species, it was found that the increase of temperature increased water activity or RH. At 35ºC, the water sorption was lower at any given RH (Table 3). Seed deterioration manifested by fungus growth was obvious as relative humidity increased. At 97% RH, all seeds deteriorated before reaching equilibrium at 25 and 35ºC (Table 3). Under the conditions of this study, it was found that storing seeds at 5 or 15ºC reduced the possibilities of seed deterioration.

19 Table 3 (a). Experimental EMC (dry weight basis) for the four vegetable species corresponding to various water activities at 5 o C (a w = [RH/100]). __* : Not available since seeds deteriorated due to higher relative humidities at 25 o and 35 o C. Moisture Contents (g H 2 O / g dry weight)Water activity (a w ) tomatoonioncucumbercarrot 5oC5oC 0.0250240.0271660.0299010.0325770.113 0.0399960.0423750.0394050.0424940.291 0.0350250.0392380.0432760.0440510.336 0.0452940.0496260.0482970.0495360.431 0.0757720.0824240.0623240.0723760.644 0.1097280.1247370.0983950.1190780.757 0.1537180.1730260.1328320.1686320.877 0.2577590.3087040.2080520.3174020.985

20 Table 3 (b). Experimental EMC (dry weight basis) for the four vegetable species corresponding to various water activities at 15 o C (a w = [RH/100]). __* : Not available since seeds deteriorated due to higher relative humidities at 25 o and 35 o C. Moisture Contents (g H 2 O / g dry weight)Water activity (a w ) tomatoonioncucumbercarrot 15 o C 0.0574580.0423700.0357100.0429610.113 0.0685540.0504230.0479760.0541530.245 0.0750920.0585220.0542860.0586500.335 0.0788530.0638330.0594420.0641250.432 0.1120160.0950650.0792030.0723560.620 0.1335730.1286370.0953880.1101350.755 0.1682340.1683910.1251310.2837840.865 0.2957850.3342250.219140.3676350.985

21 Table 3 (c). Experimental EMC (dry weight basis) for the four vegetable species corresponding to various water activities at 25 o C (a w = [RH/100]). __* : Not available since seeds deteriorated due to higher relative humidities at 25 o and 35 o C. Moisture Contents (g H 2 O / g dry weight)Water activity (a w ) tomatoonioncucumbercarrot 25 o C 0.0477090.0370060.0294140.0354130.113 0.0610950.0454980.0417510.0467310.225 0.0703480.0547980.0502910.0512320.328 0.0687120.0590840.0540070.0543130.432 0.0937180.0874040.0687400.1171910.584 0.1531300.1486170.1251940.1550890.753 0.2274610.3663810.2509190.2263000.843 __*0.973

22 Table 3 (d). Experimental EMC (dry weight basis) for the four vegetable species corresponding to various water activities at 35 o C (a w = [RH/100]). __* : Not available since seeds deteriorated due to higher relative humidities at 25 o and 35 o C. Moisture Contents (g H 2 O / g dry weight)Water activity (a w ) tomatoonioncucumbercarrot 35 o C 0.0194060.0143050.0112040.0280310.113 0.0202560.0245220.0218880.0241390.215 0.0306100.0354210.0355390.0359390.325 0.0383260.0436630.0397830.0415900.433 0.0447680.0552500.0497830.0534230.545 0.0865060.1047670.0842430.0975610.752 0.1138240.1432860.1065950.1087720.831 __*0.962

23 Fitting sorption data to isotherm models A) GAB equation The GAB (Guggenheim, 1966, Anderson, 1946; de Boer, 1953) equation is one of the most widely accepted models for sorption isotherms (Ayaranci et al., 1990; Tsami et al., 1990) and can be written as follows:

24 Where, Where, EMC= equilibrium moisture content, g water/g dry matter. M m = monolayer moisture content, g water/g dry matter. C= constant related to heat of sorption of monolayer. K= constant related to total heat of sorption. a w = water activity EMC= equilibrium moisture content, g water/g dry matter. M m = monolayer moisture content, g water/g dry matter. C= constant related to heat of sorption of monolayer. K= constant related to total heat of sorption. a w = water activity

25 B- The Henderson Equation B- The Henderson Equation The Henderson equation (Henderson, 1952, in Roberts, 1972 and Toledo, 1991) predicts greater changes in EMC for a given temperature change at higher RH than at lower RH. On the other hand, The Henderson equation can be written as follows: Where, a w is the water activity EMC is equilibrium moisture content (g water/g dry matter), and a and b are constants. The Henderson equation can be written as follows: Where, a w is the water activity EMC is equilibrium moisture content (g water/g dry matter), and a and b are constants.

26 Table 4. Estimated parameters for GAB and Henderson equations for the four vegetable species. Vegetable species Model tomatoonioncucumbercarrot GAB 0.0020.0380.0290.039MmMm 24.91314.622304.10034.517c -0.0370.9190.9320.923k 0.9690.9970.9990.998R2R2 Henderson 36.42419.79849.46522.280a 1.5221.3021.5681.391b 0.9230.9530.9620.949R2R2 R 2 = Correlation Coefficient. Mm = Moisture content (g H2O/g dry weight) c = Constant, a = Constant, b = Constant

27 ________ ------- Fig 2 (a) Diagram of tomato data fitting with GAB ( ________ ) and Henderson (-------) models. RH (%) E,M.C. (g H 2 O/g (dw) oCoC oCoC oCoC oCoC 0 10 20 30 40 50 60 70 80 90 100

28 E,M.C. (g H 2 O /g (dw) oCoC oCoC oCoC oCoC 0 10 20 30 40 50 60 70 80 90 100 ________ ------- Fig 2 (b) Diagram of cucumber data fitting with GAB ( ________ ) and Henderson (-------) models. RH (%) 0 10 20 30 40 50 60 70 80 90 100

29 RH (%) E,M.C. (g H 2 O /g (dw) oCoC oCoC oCoC oCoC ________ ------- Fig 2 (c) Diagram of onion data fitting with GAB ( ________ ) and Henderson (-------) models. 0 10 20 30 40 50 60 70 80 90 100

30 RH (%) E,M.C. (g H 2 O /g (dw) oCoC oCoC oCoC oCoC ________ ------- Fig 2 (d) Diagram of carrot data fitting with GAB ( ________ ) and Henderson (-------) models. 0 10 20 30 40 50 60 70 80 90 100

31 ConclusionConclusion Water sorption isotherms of carrot, cucumber, onion and tomato seeds were highly dependent on temperature. In general, species were not significantly influential on the curves of water sorption isotherms except at higher RH.

32 The experimental data fitted well the two- sorption models of GAB and Henderson equations. Comparing the two models, it is evident that GAB model fits experimental data better than the Henderson model as indicated by the high value of R 2.

33

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