Moisture sorption isotherm, isosteric heat and adsorption surface area of whole chia seeds

Journal Pre-proof Moisture sorption isotherm, isosteric heat and adsorption surface area of whole chia seeds Sultan Arslan-Tontul PII:

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https://doi.org/10.1016/j.lwt.2019.108859

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LWT - Food Science and Technology

Received Date: 2 October 2019 Revised Date:

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Accepted Date: 18 November 2019

Please cite this article as: Arslan-Tontul, S., Moisture sorption isotherm, isosteric heat and adsorption surface area of whole chia seeds, LWT - Food Science and Technology (2019), doi: https:// doi.org/10.1016/j.lwt.2019.108859. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

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Moisture Sorption Isotherm, Isosteric Heat And Adsorption Surface Area Of Whole Chia Seeds

Sultan ARSLAN-TONTUL1

Selçuk University, Agricultural Faculty, Department of Food Engineering, 42130, Konya, Turkey

1

Selçuk University, Agricultural Faculty, Department of Food Engineering, 42130, Konya, Turkey, Tel: +90 332 2232937; e-mail:[email protected] (Sultan ARSLAN-TONTUL).

1

Abstract

2 3

This study aimed to evaluate moisture sorption isotherm of whole chia seed. The equilibrium

4

moisture content (EMC) of seeds were detected by saturated salt solutions which have the

5

water activity (aw) range of 0.2-0.9. The isosteric sorption heat was calculated by the

6

Clausius-Clapeyron equation using three different sorption temperatures (15 , 25

7

35 ). The adsorption surface area of seeds was also calculated by monolayer moisture

8

content obtained from BET and GAB equation. The EMC content of seeds had an increasing

9

trend and determined as 18-20 g H2O/100g solid at the highest aw level. The whole chia seeds

10

became less hygroscopic with the rising sorption temperatures at constant aw. The moisture

11

sorption isotherm was determined as Type II. The monolayer moisture content was

12

determined as 2.39-2.91 g H2O/100g solid. BET and Peleg were the best-fitted models. The

13

isosteric and net isosteric heat were 77.74 and 34.74 kJ/mol at lowest moisture content,

14

respectively. Additionally, the adsorption surface area changed between 95.31-102.72 m2/g.

15 16

Keywords: water activity; moisture content; sorption isotherm; isosteric heat; oilseed

17

1

and

18 19 20

1. INTRODUCTION

21

Lamiaceae, subfamily Nepetoideae, and genus Salvia. The native land of chia is Guatemala

22

and Mexico, but today, it is cultivated in Australia, Bolivia, Colombia, Guatemala, Peru and

23

Argentina (Grancieri, Martino, & de Mejia, 2019; Moreira, Chenlo, Prieto, & Torres, 2012).

24

Nowadays, the demand for chia seeds is increasing in the food industry due to its high

25

nutritional quality. It is reported that whole chia seed contains high amounts of fatty acids,

26

dietary fibres, proteins, antioxidants, vitamins and minerals (Ayerza & Coates, 2011;

27

Grancieri, Martino, & de Mejia, 2019; Muñoz, Cobos, Diaz, & Aguilera, 2012; Valdivia-

28

López & Tecante, 2015). It is known as one of the most important sources of omega-3,

29

classified as polyunsaturated fatty acids (PUFA) (Ayerza & Coates, 2009, 2011; Moreira,

30

Chenlo, Prieto, & Torres, 2012). It is recommended for the preventing of various chronic

31

diseases such as obesity, cardiovascular diseases, diabetes, and cancer (Grancieri, Martino, &

32

de Mejia, 2019).

Chia (Salvia hispanica L.) is an annual summer herbaceous plant that classified in the family

33 34

In the last decade, chia seeds have been added in the formulation of various foods such as

35

bakery products (Brites et al., 2019; Zhu & Chan, 2018), yoghurt (Kwon, Bae, Seo, & Han,

36

2019), cheese (Munoz-Tebar et al., 2019), cereal bar (Iuliano, Gonzalez, Casas, Moncayo, &

37

Cote, 2019), ice-cream (Campos, Ruivo, Scapim, Madrona, & Bergamasco, 2016) and

38

frankfurter (Fernandez-Lopez et al., 2019) to improve nutritional quality and gain the

39

functionality to the end product. This popularity makes important transportation of chia to all

40

over the world since it is harvested mostly in tropical and subtropical regions. When the

41

storage conditions are not optimised, seeds can quickly deteriorate as oxidatively because of

42

high PUFA content (Bodoira, Penci, Ribotta, & Martínez, 2017; Bordón, Meriles, Ribotta, &

43

Martinez, 2019). Additionally, high relative humidity of environment and water activity of

2

44

food lead to the microbial spoilage (Abdullah, Nawawi, & Othman, 2000). Sorption isotherms

45

can control unstable storage conditions such as relative humidity, temperature and water

46

activity and moisture content of the product.

47 48

A sorption isotherm is a relationship between equilibrium moisture content (EMC) and water

49

activity (aw) at constant temperature and pressure. Sorption isotherms are crucial to know the

50

water sorption mechanism and interactions between food components and water. Therefore, it

51

gives useful information in modelling of the drying process and equipment, optimisation,

52

predicting the shelf life of product, determining critical moisture level, mixing products with

53

various aw and the selection of packaging material (Koua, Koffi, Gbaha, & Toure, 2014;

54

Panjagari, Singh, Ganguly, & Indumati, 2015; Shanker, Kumar, Juvvi, & Debnath, 2019;

55

Soleimanifard & Hamdami, 2018). Besides these practical applications, the isotherm is also

56

important for evaluating the thermodynamic functions of the water, which is adsorbed in

57

foods (Chirife & Iglesias, 1978).

58 59

The isosteric heat of sorption is a thermodynamic parameter calculated from sorption

60

isotherm, carried at least two temperature. It implies the amount of energy required to change

61

unit mass of a product from liquid to vapour at a certain temperature and aw. From the point of

62

view, the energy requirement for drying of a material can be evaluated by isosteric heat. The

63

moisture content at which the net isosteric sorption heat is approximately equal to the latent

64

evaporation temperature of pure water is indicator of bound water. As food is dried to the

65

lower moisture levels, the heat of adsorbed water increase above the vaporization of pure

66

water (Khawas & Deka, 2017; Koua, Koffi, Gbaha, & Toure, 2014).

67 68

In literature, there is only one study investigating the sorption isotherms of whole chia seeds.

69

Moreira, Chenlo, Prieto, & Torres (2012) used the GAB equation (Guggenheim–Andersen–de 3

70

Boer equation) to determine moisture sorption properties of whole chia seeds. On the other

71

hand, to reveal the moisture sorption characteristics of chia and control storage conditions

72

have great importance due to its quick oxidative deterioration with high oil and PUFA

73

content. For this purpose in the designed study, it is aimed to determine the safe storage

74

conditions (humidity and water activity) by evaluating different sorption models and calculate

75

the isosteric heat and water adsorption surface area of chia seeds.

76 77 78 79 80 81

2. MATERIALS AND METHODS 2.1. Material Black chia seeds, which were harvested from Argentina, were obtained by a commercial

82

importer (Yayla Agro Food, Mersin) in Turkey. The seeds removed from broken parts and

83

foreign materials. The dimensions of 20 random seeds were determined as follow; 1.97 mm

84

length, 1.13 width, and 0.89 mm thickness. The proximate analysis of seeds was determined

85

by the procedure of AACC (1999). The total protein, fat, fibre, ash and moisture content of

86

seeds were detected as 20.36, 38.12, 18.28, 4.54 and 7.47 g/100g, respectively. Prior to

87

sorption experiments, seeds were pre-dried in a vacuum oven at 50

88

al., 2012). After the drying, the moisture content and aw were decreased to 1.76 g/100g and

89

0.19, respectively. The seeds were kept in the refrigerator, untill the sorption experiment.

for five days (Moreira et

90 91

Saturated salts solutions of CH3CO2K (CAS No: 127-08-2, Merck, Darmstadt, Germany)

92

MgCl2 (CAS No: 7786-30-3, Sigma, Taufkirchen, Germany), K2CO3 (CAS No: 584-08-7,

93

Merck, Darmstadt, Germany), NaBr (CAS No: 7647-15-6, Carlo Erba, Val de Reuil, France),

94

KI (CAS No: 761-11-0, Merck, Darmstadt, Germany), NaCl (CAS No: 7647-14-5, Sigma,

95

Taufkirchen, Germany), BaCl2 (CAS No: 1026-27-9, Carlo Erba, Val de Reuil, France) and

96

K2SO4 (CAS No: 7778-80-5, Sigma, Taufkirchen, Germany) were used for the obtaining

97

various aw range. 4

98 99

2.2. Sorption procedure

100

The EMC of chia seeds was determined at 15

101

solutions with the aw range of 0.19-0.94 (Table 1). The aw values of each saturated salt

102

solutions were equal to the relative humidity divided by 100 (aw =RH/100). The static

103

gravimetric method was applied for the determination of adsorption isotherms of seeds (Bell

104

& Labuza, 2000). The saturated salt solutions were prepared at boiling water by dissolving the

105

salts until saturation and left them to cool to the room temperature. The saturated salt

106

solutions were placed in desiccators and conditioned for 7 days prior to sorption experiment.

107

The aw of the saturated salt solutions at different holding temperatures were measured by

108

using the aw meter (Aqualab, Washington, USA). Triplicate samples each of 0.45 g (± 0.01 g)

109

were weighed in the beaker and placed in desiccators containing saturated salt solutions. At

110

high aw above 0.6, 2 mL toluene was added in a beaker and it was placed in the desiccators in

111

order to prevent the fungal spoilage of seeds. The sample weighing was performed daily, and

112

EMC was detected when the samples reached constant weight (±0.001) at 15

113 114

, 25

and 35

using eight saturated salt

, 25

and 35

. The moisture content of samples was determined by drying in a drying chamber at 105 to a constant weight.

115 116

2.3. Analysis Of Experimental Data

117

The moisture sorption isotherms of whole chia seeds were determined by plotting of EMC

118

values obtained from each temperature against the corresponding aw. The description of

119

relationship between EMC, equilibrium relative humidity and temperature were verified

120

according to BET (Brunauer–Emmett–Teller) (Aguerre, Suarez, & Viollaz, 1989), GAB (Van

121

den, 1981), Halsey (Halsey, 1948), Henderson (Iglesias & Chirife, 1982), Iglesias & Chirife

122

(Chirife & Iglesias, 1978), Caurie (Chirife & Iglesias, 1978), Oswin (Oswin, 1946), Peleg

5

123

(Peleg, 1993), Smith (Smith & Smith, 1947) and White & Eiring (Sormoli & Langrish, 2015)

124

(Table 2). The curve fitting and regression analysis were performed using a mathematical

125

software program (Origin Lab Corp, Massachusetts, USA). The fittest sorption model was

126

selected by the regarding of minimum root of mean square error (RMSE), minimum means

127

absolute percentage error (P %), and the maximum degrees of freedom adjusted R-square

128

(Radj2) of the fit (Sormoli & Langrish, 2015). RMSE and Radj2 were obtained from

129

mathematical software program, and P was calculated from Equation 1 by experimental (YE)

130

and predicted data (YP) obtained from the fit.

131

∑

=

132

Eq. 1

133 134

2.4. Determination Of Isosteric Heat Of Sorption

135

The net isosteric sorption heat is defined by the difference between total isosteric sorption

136

heat and condensation heat. It was calculated by equation 2 and 3;

137 138

139

(

) ( )

=

=− − ∆!"

Eq. 2

#

Eq. 3

140 141

aw =Water activity

142

T=Selected temperature (15

143

QSt= Isosteric heat of sorption

144

qSt= Net isosteric heat of sorption

145

R=8.314 kJ/molK

146

∆HVap= 43 kJ/mol

, 25

or 35

)

147 148

The heat of sorption was determined from the slopes of ln aw against 1/T plots by linear

149

regression analysis, with the assumption that they are constant over the temperature range 6

150

studied. QSt is a measure of interaction between water vapour and the adsorbent food material

151

(Ayranci & Duman, 2005).

152 153

2.5. Determination Adsorption Surface Area

154

Adsorption surface area of seeds was calculated from equation 4 using monolayer moisture

155

content obtained from BET and GAB as follows (Koua, Koffi, Gbaha, & Toure, 2014);

156

$% = &' ×

) +/-./

× (1.06 × 10

4

56 ) × (6 × 1067 589:;<9:=/589)

Eq. 4

157 158

3. RESULTS AND DISCUSSION

159

3.1. Adsorption Isotherm of Whole Chia Seeds

160

The moisture sorption isotherm of whole chia seeds is given in Figure 1. The EMC content of

161

seeds had an increasing trend by the rising of aw value. It was an expected result caused by

162

increasing of surrounding vapour pressure of food led increasing of the vapour pressure

163

within. This effect was also reported by Moreira, Chenlo, Prieto, & Torres (2012) and

164

Shanker, Kumar, Juvvi & Debnath (2019). The sorption capacity of material is highly related

165

to chemical composition and structure. Materials with hydrophilic structures such as sugar

166

have more water adsorption ability. Lazouk et al. (2015) notified that the composition of

167

oilseed fractions and total moisture content designed the distribution of water in the seed.

168 169

At the highest aw, the seeds were adsorbed 18-20 g H2O/100g solid. This value was lower

170

than that of reported most of the grains, but it showed similarity with oilseeds. The oil content

171

of chia, nearly 30-38 g /100g. It might show hygroscopic effect and limit adsorption of water

172

from the surface. Giner & Gely (2005) found that the EMC content of sunflower was less than

173

wheat, and the researchers explained this result by steric difficulties for water adsorption in

174

the presence of oil. Moreira, Chenlo, Prieto, & Torres (2012) determined EMC of chia as 16.6 7

175

g H2O/100g solid. The EMC of rapeseed was found to be 15 g/100g solid (Lazouk et al.,

176

2015).

177 178

In the study, chia seeds became less hygroscopic with the increasing sorption temperatures at

179

constant aw. It could be a result of that when temperature increases, the water molecules gain

180

more activity which leads to an increase in the intermolecular distance due to the rise in their

181

energy level. Thus, they become less stable and break away easily from the water binding

182

sites of the food. This phenomenon has been reported from previous studies (Bup et al., 2013;

183

Koua, Koffi, Gbaha, & Toure, 2014; Mbarga, Nde, Mohagir, Kapseu, & Nkenge, 2017;

184

Singh, Mishra, & Saha, 2011; Soleimanifard & Hamdami, 2018; Taitano, Singh, Lee, &

185

Kong, 2012).

186 187

As can be seen in Figure 1, the isotherm has sigmoidal shape due to the two bending zone at

188

0.2-0.4 and 0.6-0.8 aw. Therefore, the moisture sorption isotherm of whole chia seeds was

189

determined as Type II according to Brunauer classification. Additionally, C value is an

190

isotherm constant that is calculated from BET equation. C constant higher than one means

191

that the moisture sorption isotherm must be classified in type II (Sormoli & Langrish, 2015).

192

This type has been reported for the various kind of foods such as juice powder (Sormoli &

193

Langrish, 2015), millet grain (Singh, Mishra & Saha, 2011), almond (Taitano, Singh, Lee, &

194

Kong, 2012), whole wheat and rice flours (Abebe & Ronda, 2015), banana flour (Khawas &

195

Deka, 2017) and neem kernel (Mbarga, Nde, Mohagir, Kapseu, & Nkenge, 2017) and chia

196

(Moreira, Chenlo, Prieto, & Torres, 2012). Additionally, type II isotherms are generally

197

described for oilseeds (Al-Muhtaseb, McMinn, & Magee, 2002; Lazouk et al., 2015).

198 199

3.2. Monolayer Moisture Content

8

200

The monolayer moisture content is critical moisture content to control and extend quality

201

shelf life of products. At this moisture level, most of the degradation and food spoilage

202

reactions such as enzymatical browning and oxidation, physical changes in food products

203

such as loss of crispiness, caking and stickiness are slow down. Additionally, it helps

204

determination of the surface potential of moisture sorbed in food (Singh, Mishra & Saha,

205

2011; Sormoli & Langrish, 2015).

206 207

The monolayer moisture content (XM) of whole chia seeds were determined to be 2.39-2.91 g

208

H2O/100g solid according to BET and GAB models. The results were in agreement with

209

Moreira et al. (2012) who calculated the XM of chia seed by the GAB model as 1.5-2.2 g

210

H2O/100g at tested temperatures. Similar results were also reported by containing high oil

211

seeds and nuts. XM content of various nuts (almond, Brazilian nut, cashew, hazelnut,

212

macadamia nut, pecan, pine nut, pistachio, walnut) was determined as 1.1-2.9 H2O/100g for

213

BET and 1.5-3.3 H2O/100g for GAB (Venkatachalam & Sathe, 2006). The reported XM for

214

hazelnut kernel was 2.17- 2.52 (Jung, Wang, McGorrin, & Zhao, 2018). Taitano et al. (2012)

215

determined XM between 2.38-2.48 g H2O/100g in glanced almonds. Lazouk et al. (2015)

216

calculated the XM value of whole rapeseed, sunflower and linseed were 3, 4.9 and 6 g/100g

217

respectively.

218 219

The general opinion is that the XM value decreases with increasing sorption temperatures due

220

to breaking away of water molecules from their sorption sites easily at high energy levels

221

(Samapundo et al., 2007). However, in this study, XM calculated by GAB decreased with

222

increasing sorption temperatures, whereas XM obtained BET was not affected by temperature.

223

This result might be due to the fact that the BET model can only be applied in the aw range of

224

0.1-0.5; therefore, did not represent all experimental data points. Similar results were also

9

225

reported by Sormoli and Langrish (2015) and Mbarga et al. (2017). Additionally, XM obtained

226

by BET was lower than GAB parameters.

227 228

According to Taitano, Singh, Lee, & Kong (2012), XM is critical data for designing storage

229

conditions with minimum changes in the food. Consequently, the obtained data from this

230

study imply that the aw and relative humidity should be lower 0.25 and 25 g/100g,

231

respectively, for a long and quality storage of whole chia seeds.

232 233

3.3. Model equations

234

The results obtained from the regression analysis are presented in Table 3. Some statistical

235

parameters are considered for interpreting the fittest equations. Radj2 is one of these statistical

236

parameters and generally the values are higher than ≥0.98 is acceptable. The studies model

237

except GAB and Iglesias & Chirife (1982) ensured good fitness in expressing sorption

238

isotherm of chia seeds. Interestingly, GAB model has been used for explanation sorption

239

properties most of foods. For example, Moreira, Chenlo, Prieto, & Torres (2012)

240

recommended the GAB equation for fitting the experimental data of chia seed. Koua, Koffi,

241

Gbaha, & Toure, 2014 announced the GAB model as adequately predicted EMC of cassava

242

for the range of temperatures and aw. However, in this study, the fitness of GAB model was

243

low.

244 245

The mean absolute percentage of error (P%) is another statistical fitness parameter. The limit

246

level of P% is controversial. According to Lomauro, Bakshi & Labuza (1985), it should be

247

lower than 5% for good fitness, but in the most of the previous studies it has been reported as

248

10% (Kaymak-Ertekin & Sultanoglu, 2001; Koua, Koffi, Gbaha, & Toure, 2014; Sormoli &

249

Langrish, 2015). From Table 3, when all working temperatures were considered, the fitness of

10

250

model according to P% can be ordered as BET> GAB>Peleg>Oswin>Halsey>Henderson.

251

The goodness to the fit of some models such as Smith and White & Eiring get worse with the

252

increasing sorption temperatures. It can be concluded from the results that these equations can

253

be used only lower sorption temperatures to state moisture properties of chia seeds.

254 255

In most of the literature, the RMSE value of the fittest model was lower than 1. Therefore the

256

nearest RMSE value below 0 to 1 is acceptable for a good model fit. From the results, it can

257

be said that the RMSE value of models was in acceptable range except Iglesias & Chirife

258

(1982). In addition, it can be noted that the lowest values were determined in BET and GAB

259

equations.

260 261

As a conclusion when all statistical parameters were considered, the BET equation gave the

262

best fit to sorption data with the minimum P% and RMSE and maximum Radj2 at 15

263

and 35

264

satisfactorily fitted using the Peleg model in the whole studied a range of aw and temperatures

265

with the values of P<5.32, RMSE<0.392 and Radj2>0.99. Therefore, BET and Peleg model can

266

be applied for adequately predicted EMC of whole chia seeds for the range of temperatures

267

and aw studied. There have been previous studies found the Peleg model as suitable to explain

268

the sorption activities of foods (Khawas & Deka, 2017; Shanker, Kumar, Juvvi, Debnath,

269

2019).

, 25

in aw range between 0.2-0.5. On the other hand, the experimental data were

270 271

3.4. Net Isosteric heat of sorption

272

The isosteric heat of sorption is a useful method for determining the effect of temperature to

273

the foods. It defines as the amount of energy required to change unit mass of a product from

274

liquid to vapour at a particular temperature and aw. The isosteric heat is generally modelled by

11

275

Clasius-Clapeyron equation. The application of this method requires data at least at two or

276

more experimental temperatures. The net isosteric heat of sorption can be used to estimate the

277

energy requirements of drying and provides important information on the state of water in

278

foodstuffs (Koua, Koffi, Gbaha, & Toure, 2014).

279 280

Figure 2 shows the isosteric heat and net isosteric heat of sorption. At the lowest moisture

281

content, QSt and qSt were calculated to be 77.74 and 34. 74 kJ/mol and tended to decrease with

282

increasing the moisture content. Moreira, Chenlo, Prieto, & Torres (2012) found that a 10

283

times increase of moisture content caused to 7.2 times reduces in isosteric heat. Singh, Mishra

284

& Saha (2011) reported that the isosteric heat, calculated using Clausius–Clapeyron equation,

285

varied between 46.76 and 61.71 kJ/mol at moisture levels 7–21 g/100g for barnyard millet

286

grain. The net isosteric heat of sorption decreased from 28 to 5 kJ/kg by the increase of

287

moisture content 2 to 7 g/100g in hazelnut kernels (Jung, Wang, McGorrin & Zhao, 2018).

288

Tarigan, Prateepchaikul, Yamsaengsung, Sirichote & Tekasakul (2006) noted that net

289

isosteric heat decreased until 0 kJ /mol with raising of moisture content.

290 291

Figure 2 clearly illustrated that the decrease of isosteric heat occurred more sharply in the

292

moisture content range of 2-10 g/100g and after that no more change was observed. The

293

similar result was also obtained by Panjagari, Singh, Ganguly & Indumati (2015) who found

294

that the maximum heat of adsorption (93.79 kJ/mol) was obtained between the moisture

295

content of 1–2 g/100g on dry basis. However, between 2 and 5 g/100g moisture, the isosteric

296

heat of sorption decreased sharply, and after that, it was in line. At the high level of moisture

297

content of food, the energy necessary for vaporisation is low due to weak hydrophilic bounds

298

of macromolecules and free water. On the contrary, during drying, moisture content decreases

299

continuously since only the monolayer moisture is left. As a result of this process, the water

12

300

molecules become tightly bound to the surface of food and the sorption sites. At the same

301

time, the heat of sorption increases above the heat of vaporisation of pure water, making it

302

difficult to remove water from the surface (Iglesias & Chirife, 1982; Kaya & Kahyaoglu,

303

2006; Panjagari, Singh, Ganguly & Indumati, 2015; Sormoli & Langrish, 2015). Moreover,

304

Khawas & Deka (2017) attributed the decrease of qSt with the increase in EMC values to

305

strong water-solid interaction and sorption occurred on the less active sites giving lower qSt.

306 307

3.5. Adsorption Surface Area

308

The specific surface area plays an essential role in determining the water-binding capacity of

309

a material (Hidar et al., 2018). Adsorption surface area of chia seeds was calculated by

310

monolayer moisture content obtained from GAB. SA was determined to be 102.72, 97.43 and

311

95.31 m2/g at 15

312

making hydrogen bound capacity of seeds decreased with increasing temperature. This

313

behaviour has been described as a reduction in the number of active sites because of physical

314

and chemical changes induced by temperature (Hidar et al., 2018). The surface interaction,

315

structure and chemical composition affected the water sorption capacity of seeds. Koua,

316

Koffi, Gbaha, & Toure (2014) indicated microporous structure of food lead to increase

317

adsorption surface area. Bup et al. (2013) determined for shea nut as 72.32–175.65 m2/g and

318

reported a significant reduction of the surface area of both raw and cooked kernels with

319

increasing temperature.

, 25

and 35

, respectively. From the results, it can be concluded that

320 321

4. CONCLUSION

322

According to the results, the moisture content of seeds had an increasing trend by the rising of

323

aw value, and the seeds adsorbed 18-20 g H2O/100g solid. Chia seeds became less

324

hygroscopic with the increasing sorption temperatures at constant aw. The adsorption isotherm

13

325

of seeds detected as Type II according to Branuer classification. The XM of whole chia seeds

326

were determined as 2.39-2.91 g H2O/100g solid according to BET and GAB equations.

327

Additionally, the experimental data were satisfactorily fitted using the Peleg model in the

328

whole studied range of water activities. The isosteric heat decreased more sharply at lower

329

moisture contents, and after that no more change was observed. According to SA calculations,

330

it can be concluded that making hydrogen bound capacity of chia seed surface decreased with

331

increasing temperature. Consequently, the obtained data from this study imply that aw and

332

relative humidity should be lower 0.25 and 25 g/100g, respectively for a long and quality

333

storage of whole chia seeds.

334 335

Conflict to interest

336

The author declares that there is no conflict to interest

337 338

Nomenclature

339 340

A, B, C, D, k: Model coefficients

341

GAB: Guggenheim, Anderson, de Boer equation

342

BET: Brunauer, Emmett, Teller equation

343

EMC: Equilibrum moisture content

344

aw: Water activity

345

RH: Relative humidity

346

RMSE: Minimum root of mean square error

347

Radj2: Degrees of freedom adjusted R-square

348

P %: Mean absolute percentage error

349

YE: Experimental equilibrium moisture content

350

YP: Predicted equilibrium moisture content form the fit

351

N: Number of data point

14

352

T: Temperature

353

QSt: Isosteric heat of sorption

354

qSt: Net isosteric heat of sorption

355

R: Universal gas constant

356

∆HVap; heat of vaporization of pure water (kJ/mol water)

357

SA: Adsorption surface area

358

XM: Monolayer moisture content

359 360 361 362 363

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22

Table 1. Water activity (aw) of different saturated solutions of salt at different temperatures. Salt Potassium acetate Magnesium chloride Potassium carbonate Sodium bromide Potassium iodide Sodium chloride Barium chloride Potassium sulphate

15 0.22±0.001 0.34±0.001 0.44±0.003 0.60±0.003 0.70±0.001 0.76±0.001 0.89±0.001 0.94±0.001

aw 25 0.20±0.001 0.33±0.001 0.43±0.001 0.58±0.001 0.69±0.001 0.76±0.001 0.89±0.001 0.94±0.002

35 0.20±0.003 0.31±0.001 0.42±0.001 0.58±0.001 0.69±0.001 0.76±0.001 0.88±0.006 0.94±0.001

Table 2. The model equations for fitting the sorption isotherms of chia seed Model type

Equation

BET

GAB Caurie

=

=

Reference

[ 1−

[ 1−

+

× × × × 1− × + = exp + × = −

Halsey Henderson Iglesias & Chirife Oswin Peleg Smith White & Eiring

× × −1 1−

= 1 − exp − =

+ =

× ×

×

×

Aguerre, Suarez, and Viollaz (1989)

]

Van den (1981) ×

×

] (Chirife & Iglesias, 1978) Halsey (1948) Iglesias and Chirife (1982) (Chirife & Iglesias, 1978)

1−

1− = × + × = − × ln 1 − 1 = + ×

Oswin (1946) Peleg (1993) Smith and Smith (1947) Sormoli and Langrish (2015)

Table 3. Predicted parameters of the fitted models to the experimental data for moisture sorption isotherm of chia seed Model type BET

GAB

Caurie

Halsey

Henderson Iglesias & Chirife Oswin

Peleg

Smith White & Eiring

Temp. ( ) 15 25 35 15 25 35 15 25 35 15 25 35 15 25 35 15 25 35 15 25 35 15 25 35 15 25 35 15 25 15

RAdj2 0.990 0.998 0.996 0.975 0.977 0.978 0.985 0.985 0.978 0.995 0.985 0.986 0.990 0.995 0.984 0.952 0.944 0.981 0.993 0.994 0.991 0.997 0.998 0.995 0.993 0.989 0.986 0.999 0.991 0.990

Model fit parameters P% 0.179 0.559 1.113 2.413 2.351 2.669 11.789 14.160 17.672 1.666 4.875 8.288 3.226 2.947 8.682 13.798 26.332 21.039 6.156 6.183 4.543 3.857 3.523 5.321 7.430 10.227 14.948 2.321 11.203 13.088

RSME 0.001 0.001 0.003 0.004 0.004 0.004 0.603 0.730 1.024 0.068 0.095 0.108 0.044 0.056 0.109 1.227 1.450 1.060 0.339 0.415 0.551 0.284 0.233 0.392 0.466 0.621 0.724 0.232 0.562 0.588

A 0.90 ; k 0.92 ; k 0.91 ; k -0.425 -0.751 -1.410 20.78 5.224 3.094 0.269 0.347 0.474 5.283 3.089 2.378 5.313 4.319 3.539 9.146 8.434 18.090 -0.295 -0.252 -0.768 0.308 0.420 0.515

Model coefficients B C 2.41; XM 8.03 2.39 ; XM 8.47 2.57 ; XM 8.80 2.91 ; XM 5.13 2.76 ; XM 6.42 2.70 ; XM 4.25 3.493 3.864 4.152 1.878 1.419 1.230 2.228 1.876 1.604 1.088 1.178 0.468 0.487 0.551 0.584 1.254 15.990 0.939 19.489 11.077 9.459 6.468 6.528 6.358 -0.277 -0.394 -0.495 -

D 8.172 8.248 1.409 -

Equilibrium Moisture Content (g H2O /100g solids)

25 20 15 10 5 0 0

0.2

0.4

0.6

0.8

1

Water Activity

15 ℃

25 ℃

35 ℃

Figure 1. Moisture sorption isotherms of chia seeds

90 80 Sorption Heat (kJ/mol)

70 60 50 qst Net

40

Qst

30 20 10 0 0

2

4

6 8 10 12 Moisture Content g H2O /100g solids

Figure 2. Net isosteric sorption heat (kJ/mol) of chia seed

14

16

18

Highlights BET and Peleg model gave the best fit to sorption data The monolayer moisture content of seeds was calculated as 2.39-2.91 g H2O/100 g The isosteric heat decreased from 78 to 45 kJ/mol by increasing moisture content The adsorption surface area decreased by increasing sorption temperature

Declarations of interest: none