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Effect of glycerol on the moisture sorption isotherms of figs
Journal of Food Engineering 93 (2009) 468–473
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Effect of glycerol on the moisture sorption isotherms of figs Asgar Farahnaky *, Sara Ansari, Mahsa Majzoobi Department of Food Science and Technology, School of Agriculture, Shiraz University, Shiraz, Iran
a r t i c l e
i n f o
Article history: Received 3 September 2008 Received in revised form 7 January 2009 Accepted 13 February 2009 Available online 20 February 2009 Keywords: Glycerol Figs fruit Moisture sorption isotherm GAB model BET model
a b s t r a c t In this research the effects of glycerol (at 20% and 40%, w/w) on moisture sorption isotherms of dried figs at 5, 25, and 40 °C were studied. Glycerol had notable effects on the sorption isotherms. The comparison of the samples with and without glycerol showed that at a specific water activity the samples with glycerol contained higher moisture contents. Fitting the experimental data obtained to well known GAB and BET models revealed that these two equations were able to give good fits with R2 of >0.99. The addition of glycerol significantly increases the monolayer water levels of fig–glycerol systems. In addition, glycerol changes the shape and status of the sorption isotherms of figs from sugar type isotherm to sigmoidal shape which is typical of most food systems. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Sorption isotherm is an extremely important tool in food science that describes the relationship between the moisture uptake in a food and the relative humidity of the air with which the food is in equilibrium at a constant temperature (Lagoudaki et al., 1993). The knowledge of moisture sorption characteristics of products would allow correctly specifying the conditions of storage and packaging, predicting shelf life, and understanding the physicochemical changes involved in product making processes. Many food deterioration reactions such as lipid oxidation, enzymatic reactions and non-enzymatic browning, and the growth of important microorganisms depend directly on water activity. They occur more rapidly away from the optimum moisture contents required for storage stability of dehydrated and intermediate moisture foods (Tunc and Duman, 2007). It has been shown that most deteriorative reactions in food systems have the lowest rate at the Brunauer–Emmet–Teller (BET) monolayer which usually corresponds to the water activity (aw) range of about 0.2–0.4. An increase in aw beyond this region will induce an increase in the reaction rate generally by a factor 50–100% for each 0.1 aw change (Labuza et al., 1985). The other application of moisture sorption isotherms is in food engineering in which the accurate computation of equilibrium moisture content values permits designing and optimization of drying process, aeration, storage, and also calculations of drying times (Tunc and Duman, 2007). Formulation of an analytical relation for predicting of sorption behavior of foods presents difficulties due to complexity of their structures. The * Corresponding author. Tel.: +98 711 6138229; fax: +98 711 6289017. E-mail address: [email protected] (A. Farahnaky). 0260-8774/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2009.02.014
addition or removal of water can also change the physical behavior of food materials and cause phase transitions (e.g. dissolution and sugar crystallization). Change of moisture content as a function of aw is determined by a combination of factors, each of which is predominant in a specific region of the isotherm (Tsami et al., 1990). Moisture sorption behavior and effect of temperature on equilibrium moisture content of different foods e.g. cottonseed (Tunc and Duman, 2007), dates (Nabill et al., 2005), mungbean (Chowdhury et al., 2006), some dehydrated fruits like raisin, figs, prunes, and apricots (Ayranci et al., 1990; Tsami et al., 1990; Saravacos et al., 1986), and rice (Iguaz and Virseda, 2007) have been studied and modeled extensively. Several equations have been proposed for modeling of sorption isotherms of food materials, each of these models has had relative success in determining equilibrium moisture content, depending on the aw range or the type of foodstuff. Labuza et al. (1985) noted that no sorption isotherm model could fit data over the entire range of relative humidity as water is associated with the food matrix by different mechanisms in different activity regions. Moreover, the problem in characterizing the physical and chemical properties of food constituents, the complexity of their interactions with water and the effect of water on their internal structures have made accurate modeling of physicochemical sorption phenomena difficult (Chowdhury et al., 2006). Among the proposed models by different researchers, GAB (Guggenheim–Anderson–deBoen) equation has been applied successfully to various foods (Nabill et al., 2005; Kiranoudis et al., 1993; Chowdhury et al., 2006). The fig (Ficus carica, Moraceae), very nutritional and a healthy food, is one of the most widely produced fruits in the world. It is probably originated from Western Asia, and spread to the
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Mediterranean. Figs play an important role in nutrition due to the rich carbohydrate content (almost 65–70%). Indeed, the amount of sugar and vitamins is nearly equal to dates. They contain essential amino acids and are rich in vitamins A, B1, B2, and C and minerals (Doymaz, 2005). The fig is a moderately important world crop, with an estimated annual production of 1,077,211 tons. Turkey and Iran rank first and second, respectively, among fig producing countries (FAO, 2003). In the decade between 1988 and 1997, an average of 14,840 metric tons of dried figs per year was produced in California (USA), with an average value of US$ 16.4 million (Burks et al., 2000). Fresh figs are very sensitive to microbial spoilage, even under cold storage conditions, due to high moisture and sugar contents; thus, they have short shelf life and must be preserved in some way. The fruit is usually consumed fresh or in dried, canned, and preserved forms. Several countries import dried figs or fig paste for different uses. The main exporters of dried figs and paste are Turkey and USA (Sadhu, 1990). Drying – the main process for preserving figs (constituting more than 85% of fig production in Iran) – causes textural changes such as fruit hardening and shrinkage. These could have a negative impact on the marketability of this valuable agricultural commodity. The increase in moisture content of the fig fruit can be regarded as a possible solution to solve textural problems, but increase of water activity can lead to microbial growth and shorten shelf life. Other compounds that bind water in food systems and lower the aw (Meste et al., 2002) can act as texture modifiers in fruits like figs. These humectants are particularly important in the production of intermediate moisture foods (IMF). A wide range of humectants including polyols, sugars and salts are incorporated into food materials in order to lower the water activity to levels prevailing in the intermediate moisture range (usually between 0.5 and 0.9) even in the presence of a considerable amount of water (Sloan and Labuza, 1976). Plasticizers greatly influence the functional properties of the material as they change the organization of the polymer chains. One of their most visible effects is to decrease the glass transition temperature (Tg) of the polymers (Sears and Darby, 1982). Thus, the change in polymer Tg as a function of the molar content of the plasticizer is a classical measure of plasticizer efficiency. Pommet et al. (2005) determined the influence of glycerol and other plasticizers on functional properties of wheat gluten network. Coupland et al. (2000) studied and modeled the effect of glycerol on the moisture sorption behavior of whey protein edible films and reported that the equilibrium moisture content of the protein film is highly determined by the amount of plasticizer present. The BET model was superior to either the Peleg or BET models as a model for the measured isotherms. Similar reports are well documented for small molecules such as low molecular weight sugars, water and glycerol which decrease Tg when mixed with large polymers (Chinnaswamy and Hanna, 1998; Moore et al., 1990; Carvalho and Mitchell, 2001). There is no published report on the moisture sorption isotherms of fig–glycerol mixtures and the main hypothesis of this research is that addition of glycerol to fig will change the sorption isotherms of fig in such a way that it would result in having more water at each specific water activity in comparison with the control devoid of glycerol. Therefore in this study the effect of two levels of glycerol (20% and 40%) on the moisture sorption isotherms of figs at different temperatures (5, 25, and 40 °C) is investigated.
2. Materials and methods 2.1. Raw materials Dried figs (Sabz variety) used in this study was kindly donated by the Estahban Fig Research Centre (Fars province, southern Iran).
The dried figs were packed in sealed polyethylene bags and stored at room temperature (22 °C) before being used for further treatments and experiments. Glycerol and all other chemicals were of analytical grade unless otherwise mentioned. 2.2. Sample preparation Using a sharp knife the dried figs were first cut into small pieces and then ground by a blade type grinder (Mammonlex, Taiwan). To make the glycerol–fig mixtures, glycerol was added to the fig powder at 2 levels (20% 40% w/w, dry basis) while being thoroughly mixed using a laboratory dough mixer (Kenwood, UK) for 10 min. To dry the prepared samples they were put in a vacuum oven set at about 65 °C for 5 h. General composition of dried fig powders was measured according to the AOAC methods (AOAC, 1990). 2.3. Determination of sorption isotherms The static gravimetric method, developed and standardized in the COST 90 project, was used to determine the moisture sorption isotherms of dried figs and fig–glycerol mixtures. Six super saturated salt solutions were used to provide a range of aw from 0.11 to about 0.84 at each specific temperature. Their water activities at 5, 25, and 40 °C were taken from Labuza et al. (1985) and Rizvi (2005) and are given in Table 1. Triplicates of each sample (about 4.0 g) were weighed and placed in air/humidity tight plastic containers containing super saturated salt solutions. For equilibration of samples, the closed containers were then maintained in three incubators equipped with temperature control systems with the accuracy of ±0.1 °C to provide the desired constant temperatures of 5, 25, and 40 °C. The weight of each sample was checked using an analytical balance (with the precision of 0.001 g) initially after three days, and then at one day intervals until a constant weight was reached. The moisture content of control samples was determined by vacuum oven drying at 65 °C (Karmas, 1980), while for the samples containing glycerol low-temperature (45 °C) oven drying with sulfuric acid solution at atmospheric pressure was used, this was due to the evaporation of glycerol as suggested by Favetto et al. (1979). The equilibrium moisture contents were reported on dry weight basis (g water/g dry solid). All moisture sorption experiments were done in triplicates and the average values were used for further data analysis. 2.4. Data analysis Several models have been proposed to correlate the equilibrium moisture contents to relative humidity of environment (Boquet et al., 1978; Rizvi, 2005) which can be divided into several groups; kinetic models based on a multilayer and condensed film (GAB model) (Van den Berg and Bruin, 1981), kinetic models based on an absorbed water monolayer (BET model) (Brunauer et al., 1938), semi-empirical (Halsey model) (Halsey, 1948), and purely empirical models (Oswin) (Oswin, 1946).
Table 1 The water activities (aw) of saturated salt solutions at 5, 25, and 40 °C. Salt
5 °C
25 °C
40 °C
LiCl CH3COOK NaI NaNO2 NaNO3 KCl
0.113 0.291 0.424 0.693 0.785 0.876
0.113 0.225 0.382 0.654 0.742 0.843
0.112 0.216 0.328 0.628 0.713 0.823
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In this study the well known BET (Eq. (1)) and GAB (Eq. (2)) equations were applied to fit the experimental data and to determine the monolayer moisture values of the samples which is an important parameter contributing to food deterioration. The BET isotherm equation is the most widely used model and gives a good fit in the aw range from 0.05 to about 0.5 for many foods and has been approved by the Commission on Colloid and Surface Chemistry of the IUPAC (International Union of Pure and Applied Chemistry). GAB has been suggested to be the most versatile sorption model available in the literature and has been adopted by the European Project Cost 90 on physical properties of foods. In principle, it represents a refined version of BET model. The major advantage of the GAB is that it describes the sorption behavior of nearly all foods from zero to 0.9aw (Rizvi, 2005)
M¼
M 0 C aw ð1 aw Þð1 þ C aw aw Þ
ð1Þ
0
M¼
M 0 C K aw ð1 K 0 aw Þð1 K 0 aw þ C K 0 aw Þ
ð2Þ
where M is the equilibrium moisture content, M0 is the monolayer moisture content, aw is the water activity, C and K0 are model constants. Fitting of experimental data into the above equations was done using the ‘‘Solver” in Excel program (Microsoft Office, 2003). This method was based on minimizing the residual sum of squares (RSS) between the predicted and the measured data. The suitability of the equations was evaluated and compared using the statistical parameters sum of square error (SEE) and the correlation coefficient (R2). The equation giving the smallest RSS and SEE and the highest R2 value was considered to be the best fitted equation (Chowdhury et al., 2006; Foster et al., 2005; Jamali et al., 2006)
RSS ¼
n X
M i;exp M i;pre
2
i¼1
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u n uX M i;exp M i;pre 2 SEE ¼ t n i¼1 3. Results and discussion 3.1. Experimental results Chemical composition of the dried figs used in this research is given in Table 2. They contained about 69% of polysaccharides, 4.3% protein, and 2.46% fat. Glycerol levels added (20% and 40%) were chosen on the basis of preliminary work undertaken, where it was shown that glycerol at levels of 20% and more, is able to soften fig texture. The equilibrium moisture sorption of the samples at different temperatures from 5 to 40 °C (Table 3) showed that temperature increase caused significant reductions in equilibrium moisture levels for all samples with and without glycerol, e.g. the equilibrium moistures for the 40% glycerol sample over supersaturated salt solution of NaNO3 at 5, 25, and 40 °C were 1.15, 0.89, and 0.79 (g water/g dry matter), Table 2 General composition of dried figs used in this research. Moisture content (%) Protein (%) Fat (%) Polysaccharide (%) Ash (%) Total fiber (%)
11.0 4.3 2.46 69.1 3.1 12.1
Table 3 Equilibrium moisture content (EMC, g water/g dry matter) of dried figs with and without glycerol (control, 20% and 40%) at 5, 25, and 40 °C. Fig–G0, control without glycerol, fig–G20, fig with 20% glycerol and fig–G40, fig with 40% glycerol.* aw at 5 °C
0.113
0.291
0.424
0.693
0.785
0.876
Fig-G0 Fig-G20 Fig-G40
0.0359c 0.1099b 0.2348a
0.0447c 0.1670b 0.3221a
0.1043c 0.2612b 0.4530a
0.2620c 0.5275b 0.8307a
0.3671c 0.7285b 1.1481a
0.6118c 1.095b 1.6707a
aw at 25 °C
0.113
0.225
0.382
0.654
0.742
0.843
Fig-G0 Fig-G20 Fig-G40
0.0345c 0.1193b 0.2447a
0.0724c 0.1493b 0.2903a
0.0896c 0.2097b 0.3844a
0.2181c 0.4201b 0.6899a
0.3267c 0.5547b 0.8868a
0.5948c 0.8119b 1.3010a
aw at 40 °C
0.112
0.216
0.328
0.628
0.713
0.823
Fig-G0 Fig-G20 Fig-G40
0.0214c 0.1008b 0.2232a
0.0468c 0.1411b 0.2819a
0.0682c 0.1727b 0.3339a
0.1924c 0.3553b 0.6019a
0.2749c 0.4810b 0.7901a
0.5129c 0.8291b 1.3216a
In each column different superscript letters indicate significant difference (a < 0.05). * Each EMC is the average of three replicates.
respectively. This behavior may be due to excitation of water molecules at higher temperatures. Indeed, as the temperature increases the water molecules get activated due to an increase in their energy level, causing them to become less stable and to break away from the water binding site of the food material, thus decreasing the monolayer moisture content (Chowdhury et al., 2006; Jamali et al., 2006). However, this was slightly different from the other investigators who reported opposite effects for high sugar content foods at greater water activities (Ayranci et al., 1990; Tsami et al., 1990; Saravacos et al., 1986) which could be due to the differences in chemical composition of other fruits compared to figs. The equilibrium sorption moistures of fig samples with and without glycerol (control, figs containing 20% and 40% glycerol) at 5, 25, and 40 °C are presented in Table 3. The equilibrium moisture content at each aw represents the mean value of three replicates. The equilibrium moisture content of all samples increased significantly with increasing aw and the samples containing glycerol had more moisture compared to the corresponding controls at the same temperature. In addition, the samples with 40% glycerol contained more equilibrium moisture than the 20% samples. For instance, at 25 °C the moisture level of the control, 20% glycerol and 40% glycerol samples at water activity of 0.742 were 0.33, 0.56 and 0.89 (g water/g dry matter), respectively. Figs. 1–3 show the effect of glycerol (at 20% and 40% levels) on the equilibrium moisture sorption isotherms of dried figs at 5, 25, and 40 °C. The experimental data are shown along with the lines drawn from the equations obtained by fitting the experimental data into BET and GAB equations (see Section 2 for more details). The shape of the isotherms of control samples is similar to the so called type 1 isotherms which are found for high sugar systems (Ayranci et al., 1990). The equilibrium moisture content increased slowly with water activity until the aw of about 0.65, after which a small increase in humidity led to a large increase in equilibrium moisture content. The equilibrium moistures obtained at 5 °C were in agreement with the data reported by Pixton and Warburton (1976). At low water activities, water can be adsorbed only to surface –OH sites of crystalline sugar. Therefore, moisture content is low at low water activity region, while at high water activities dissolution of sugars occurs and crystalline sugars are converted into their amorphous form. So the amount of water to be adsorbed increases greatly after this transition because of the increase in the number of adsorption sites upon breakage of the crystalline structure of sugars. This form of isotherms is similar to those observed for dried apricot, figs and raisins (Ayranci et al., 1990; Tsami et al.,
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1.6
BET-c
MC(db)-20%
BET-20%
MC(db)-40%
BET-40%
Moisture content (g water/g dry matter)
Moisure content (g water/g dry matter)
2.0
A
MC(db)-c
1.2
0.8
0.4
0.0
1.6
MC(db)-c
BET-c
MC(db)-20%
BET-20%
MC(db)-40%
BET-40%
1.2
0.8
0.4
0.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0.0
1.0
0.1
0.2
0.3
0.4
1.6
B
MC(db)-c
GAB-c
MC(db)-20%
GAB-20%
MC(db)-40%
GAB-40%
Moisture content (g water/g dry matter)
Moisture content (g water/g dry matter)
0.6
0.7
0.8
0.9
1.0
0.6
0.7
0.8
0.9
1.0
2.0
2.0
1.2
0.8
0.4
0.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
aw
2.0
1.6
MC(db)-c
BET-c
MC(db)-20%
BET-20%
MC(db)-40%
BET-40%
A
1.2
0.8 0.4
0.0 0.0
0.1
0.2
0.3
0.4
0.5 aw
0.6
0.7
0.8
0.9
1.0
0.7
0.8
0.9
1.0
2.0
1.6
MC(db)-c
GAB-c
MC(db)-20%
GAB-20%
MC(db)-40%
GAB-40%
B
1.2
0.8
0.4
0.0 0.0
0.1
0.2
0.3
0.4
0.5 aw
1.6
MC(db)-c
GAB-c
MC(db)-20%
GAB-20%
MC(db)-40%
GAB-40%
B
1.2
0.8
0.4
0.0 0.0
0.1
0.2
0.3
0.4
0.5
aw
Fig. 1. Effect of glycerol (at 20% and 40% levels) on the equilibrium moisture sorption isotherm of dried fig at 5 °C. The symbols are experimental data and the lines are from the equations obtained by fitting the experimental data to BET (A) and GAB (B) equations. c: Control or fig without glycerol, 20%: fig with 20% glycerol, 40%: fig with 40% glycerol. Each bar is ±SD.
Moisture content (g water/g dry matter)
0.5
aw
aw
Moisture content (g water/g dry matter)
A
0.6
Fig. 2. Effect of glycerol (at 20% and 40% levels) on the equilibrium moisture sorption isotherm of dried fig at 25 °C. The symbols are experimental data and the lines are from the equations obtained by fitting the experimental data to BET (A) and GAB (B) equations. c: Control or fig without glycerol, 20%: fig with 20% glycerol, 40%: fig with 40% glycerol. Each bar is ±SD.
Fig. 3. Effect of glycerol (at 20% and 40% levels) on the equilibrium moisture sorption isotherm of dried fig at 40 °C. The symbols are experimental data and the lines are from the equations obtained by fitting the experimental data into BET (A) and GAB (B) equations. c: Control or fig without glycerol, 20%: fig without 20% glycerol, 40%: fig with 40% glycerol. Each bar is ±SD.
1990). In addition, it seems that addition of glycerol to dried figs changed the shape and status of sorption isotherms, from normal sugar type isotherm for the control towards the typical sigmoid shape isotherms for 40% glycerol sample associated with most foods. The results of fitting the experimental data into BET and GAB sorption equations (RSS, SEE, and R2) are shown in Table 4. The values of monolayer moisture contents at each temperature as calculated by the BET and GAB equations are also presented in Table 4. The results show that the monolayer moisture content decreases with increasing temperature between 5 and 40 °C. This behavior has been reported for raisin, dried apricot, and fig (Ayranci et al., 1990; Saravacos et al., 1986) and has been attributed in the literature to the reduction in the active sites due to chemical and physical changes (e.g. reduction in hydrogen bonding degree) taking place as the temperature increases, the extent of which depends on the nature of the food (Al-Muhtaseb et al., 2004; Perdomo et al., 2009). The monomolecular moisture contents of the control fig in the temperature range of 5–40 °C, varied from 0.081 to 0.099 g water/g dry solids for BET model and from 0.083 to 0.114 g water/g dry solids for GAB model. These estimated values for M0 are within the reported values for fruits, which vary from 10% to 15% (dry basis) and were found to be much lower than those reported for vegetables, which vary between 18% and 22% (dry basis) (Kiranoudis et al., 1993). Table 5 shows the monolayer moisture content of fig found in this study in comparison with M0 of other selected food materials. Glycerol also had noticeable effect on the amount of monolayer water, which increased with glycerol content. The monolayer moisture content M0 is recognized as the moisture content affording the longest time period with minimum quality loss at a given
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A. Farahnaky et al. / Journal of Food Engineering 93 (2009) 468–473
Table 4 Estimated BET and GAB constants and monolayer moisture levels for dried figs with and without glycerol (control, 20% and 40%) at 5, 25, and 40 °C. Fig–G0 represents control without glycerol; fig–G20 represents fig with 20% glycerol; fig–G40 represents fig with 40% glycerol. Sample
Temperature (°C)
BET constants M0 (%)
Fig–G0 Fig–G20 Fig–G40 Fig–G0 Fig–G20 Fig–G40 Fig–G0 Fig–G20 Fig–G40
5 5 5 25 25 25 40 40 40
8.1 14.4 22.2 9.9 13.8 21.9 8.9 13.4 21.4
GAB constants
C
RSS
4.07 34.75 1519145 2.32 22.75 5732.88 2.71 17.08 742.06
0.0019 0.0131 0.0441 0.00094 0.0025 0.0091 8.4 105 0.00095 0.0044
R2
SEE 2
1.6 10 4.3 102 7.9 102 1.2 102 1.9 102 3.6 102 3.5 103 1.2 102 2.5 102
Table 5 GAB equation monolayer moisture contents of selected food materials compared with fig (found in this study). Food materials
T (°C)
M0 (% dry basis)
Reference
Sunflower seeds Pistachio nuts Sultana raisins Long pepper Apricots Prunes Raisins Currant Figs
20–30 30 20–30 35 20–30 20–30 20–30 20–30 20–30
3.09 3.90 7.70 5.79 11.70 12.60 14.00 17.30 9.70
Mazza and Jayas (1991) Maskan and Karatas (1997) Saravacos et al. (1986) Costa et al. (1998) Tsami et al. (1990) Tsami et al. (1990) Tsami et al. (1990) Tsami et al. (1990) Tsami et al. (1990)
Figs Banana Date (Alig) Date (Deglet Noor) Kiwi
5–40 25–45 5–40 5–40 30
8.3–11.4 50.2 11.18–20.30 9.95–17.44 4.20–5.00
Present study Kiranoudis et al. (1993) Nabill et al. (2005) Nabill et al. (2005) Moraga et al. (2006)
Table 6 Monolayer moisture content (M0) and corresponding aw (i.e. the safest water activity for storage) for figs with and without glycerol according to the GAB model. Temperature (°C)/sample code
Monolayer moisture content, M0 (% dry basis)
aw
5/(Fig–G0) 5/(Fig–G20) 5/(Fig–G40) 25/(Fig–G0) 25/(Fig–G20) 25/(Fig–G40) 40/(Fig–G0) 40/(Fig–G20) 40/(Fig–G40)
11.4 20.2 30.4 9.8 16.4 26.2 8.3 13.4 22.1
0.462 0.318 0.222 0.333 0.239 0.153 0.379 0.196 0.063
temperature. Below this value, the rates of deteriorative reactions, with the exception of the oxidation of unsaturated fats, are minimal. Therefore, at a given temperature, the safest water activity level is that corresponding to M0 or lower (Goula et al., 2008). The safest aw values corresponded to M0 of the samples with and without glycerol, were obtained from the data presented in Table 4 and are tabulated in Table 6. The second parameter of BET model is the sorption energy constant, C, just as the GAB constant, but with slightly different physical meaning (Moraga et al., 2006). It allows us to classify sorption isotherms according to Brunauer’s classification. As the C values obtained in this study were >2, all the sorption isotherms obtained can be classified as type II, like other fruits such as strawberry apple, blackberry, kiwifruit (Moraga et al., 2006), and spray dried tomato pulp (Goula et al., 2008). In the GAB model there is an additional constant for energy, K (a third parameter in addition to M0 and C), which is nearly equal to 1.
0.9938 0.9867 0.9826 0.9966 0.9955 0.9943 0.9995 0.9976 0.9958
M0 (%) 11.4 20.2 30.4 9.8 16.4 26.2 8.3 13.4 22.1
K 0.95 0.94 0.938 0.98 0.958 0.953 1.031 1.019 1.009
C 1.63 5.514 14.43 3.75 11.23 34.57 2.43 15.18 218.6
RSS 0.00059 0.00088 0.0020 0.00029 0.00054 0.00029 1.3105 5.3105 0.00037
R2
SEE 3
9.2 10 1.1 102 1.7 102 6.4 103 8.8 103 6.5 103 1.4 103 2.8 103 7.3 103
0.9980 0.9990 0.9990 0.9989 0.9989 0.9998 0.9999 0.9999 0.9997
It should be noted that, in all sorption series, the monolayer moisture content value obtained by the GAB model was slightly greater than those obtained by the BET model, which is in agreement with the results observed by other authors and summarized by Rahman (1995) and Timmermann et al. (2001). These differences have been attributed to the fact that BET model focuses on surface adsorption in the first layer, while the GAB model takes into account sorbed water properties in the multilayer region (Bell and Labuza 2000). The fitting quality parameters of BET and GAB models (RSS, SEE, and R2) presented in Table 4, indicate that the GAB model gives the smaller RSS/SEE along with higher R2 and so better describes the experimental adsorption data of figs throughout the entire range of water activity (0.11–0.84). This observation is similar to that reported by other researchers, who studied sorption isotherms of various fruits and vegetables. Kiranoudis et al. (1993) determined the equilibrium moisture content of potato, carrot, tomato, green pepper, and onion within the range of 0.1–0.9 water activity at three different temperatures (30, 45, and 60 °C) and found the GAB model satisfactory for the prediction of the experimental data obtained. McLaughlin and Magee (1998) determined the sorption isotherms for potatoes at temperatures of 30, 45, and 60 °C and, among the models tested, the GAB model gave the best fits. According to Al-Muhtaseb et al. (2004), the GAB model adequately represented the sorption isotherms of potato and wheat starch and Akanbi et al. (2006) concluded that the equilibrium moisture contents of tomato slices obey closely the GAB equation. 4. Conclusions The general impact of glycerol on sorption isotherms observed in this research is similar to its effect in glycerol–whey protein mixtures, studied in edible films by Coupland et al. (2000) who reported that glycerol drew water into food systems while keeping its water activity low. Taken together, the equilibrium moisture content of dried figs at each aw is significantly influenced by the amount of glycerol present. The BET and GAB equations were able to fit the experimental data in figs as a high sugar system. This work confirmed that glycerol has a good potential for use as a hygroscopic ingredient in figs, in order to hold more water at a specific water activity. This could be thought of as a promising approach to modify the hard texture of dried figs with long shelf life in terms of microbial stability. More work is needed on the effect of glycerol on textural hardness, sensory characteristics and glass/rubber transition temperature of figs. Acknowledgment This research project was financially supported by Centre of Excellence for Fig Research of Shiraz University.
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Contents lists available at ScienceDirect
Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Effect of glycerol on the moisture sorption isotherms of figs Asgar Farahnaky *, Sara Ansari, Mahsa Majzoobi Department of Food Science and Technology, School of Agriculture, Shiraz University, Shiraz, Iran
a r t i c l e
i n f o
Article history: Received 3 September 2008 Received in revised form 7 January 2009 Accepted 13 February 2009 Available online 20 February 2009 Keywords: Glycerol Figs fruit Moisture sorption isotherm GAB model BET model
a b s t r a c t In this research the effects of glycerol (at 20% and 40%, w/w) on moisture sorption isotherms of dried figs at 5, 25, and 40 °C were studied. Glycerol had notable effects on the sorption isotherms. The comparison of the samples with and without glycerol showed that at a specific water activity the samples with glycerol contained higher moisture contents. Fitting the experimental data obtained to well known GAB and BET models revealed that these two equations were able to give good fits with R2 of >0.99. The addition of glycerol significantly increases the monolayer water levels of fig–glycerol systems. In addition, glycerol changes the shape and status of the sorption isotherms of figs from sugar type isotherm to sigmoidal shape which is typical of most food systems. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Sorption isotherm is an extremely important tool in food science that describes the relationship between the moisture uptake in a food and the relative humidity of the air with which the food is in equilibrium at a constant temperature (Lagoudaki et al., 1993). The knowledge of moisture sorption characteristics of products would allow correctly specifying the conditions of storage and packaging, predicting shelf life, and understanding the physicochemical changes involved in product making processes. Many food deterioration reactions such as lipid oxidation, enzymatic reactions and non-enzymatic browning, and the growth of important microorganisms depend directly on water activity. They occur more rapidly away from the optimum moisture contents required for storage stability of dehydrated and intermediate moisture foods (Tunc and Duman, 2007). It has been shown that most deteriorative reactions in food systems have the lowest rate at the Brunauer–Emmet–Teller (BET) monolayer which usually corresponds to the water activity (aw) range of about 0.2–0.4. An increase in aw beyond this region will induce an increase in the reaction rate generally by a factor 50–100% for each 0.1 aw change (Labuza et al., 1985). The other application of moisture sorption isotherms is in food engineering in which the accurate computation of equilibrium moisture content values permits designing and optimization of drying process, aeration, storage, and also calculations of drying times (Tunc and Duman, 2007). Formulation of an analytical relation for predicting of sorption behavior of foods presents difficulties due to complexity of their structures. The * Corresponding author. Tel.: +98 711 6138229; fax: +98 711 6289017. E-mail address: [email protected] (A. Farahnaky). 0260-8774/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2009.02.014
addition or removal of water can also change the physical behavior of food materials and cause phase transitions (e.g. dissolution and sugar crystallization). Change of moisture content as a function of aw is determined by a combination of factors, each of which is predominant in a specific region of the isotherm (Tsami et al., 1990). Moisture sorption behavior and effect of temperature on equilibrium moisture content of different foods e.g. cottonseed (Tunc and Duman, 2007), dates (Nabill et al., 2005), mungbean (Chowdhury et al., 2006), some dehydrated fruits like raisin, figs, prunes, and apricots (Ayranci et al., 1990; Tsami et al., 1990; Saravacos et al., 1986), and rice (Iguaz and Virseda, 2007) have been studied and modeled extensively. Several equations have been proposed for modeling of sorption isotherms of food materials, each of these models has had relative success in determining equilibrium moisture content, depending on the aw range or the type of foodstuff. Labuza et al. (1985) noted that no sorption isotherm model could fit data over the entire range of relative humidity as water is associated with the food matrix by different mechanisms in different activity regions. Moreover, the problem in characterizing the physical and chemical properties of food constituents, the complexity of their interactions with water and the effect of water on their internal structures have made accurate modeling of physicochemical sorption phenomena difficult (Chowdhury et al., 2006). Among the proposed models by different researchers, GAB (Guggenheim–Anderson–deBoen) equation has been applied successfully to various foods (Nabill et al., 2005; Kiranoudis et al., 1993; Chowdhury et al., 2006). The fig (Ficus carica, Moraceae), very nutritional and a healthy food, is one of the most widely produced fruits in the world. It is probably originated from Western Asia, and spread to the
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Mediterranean. Figs play an important role in nutrition due to the rich carbohydrate content (almost 65–70%). Indeed, the amount of sugar and vitamins is nearly equal to dates. They contain essential amino acids and are rich in vitamins A, B1, B2, and C and minerals (Doymaz, 2005). The fig is a moderately important world crop, with an estimated annual production of 1,077,211 tons. Turkey and Iran rank first and second, respectively, among fig producing countries (FAO, 2003). In the decade between 1988 and 1997, an average of 14,840 metric tons of dried figs per year was produced in California (USA), with an average value of US$ 16.4 million (Burks et al., 2000). Fresh figs are very sensitive to microbial spoilage, even under cold storage conditions, due to high moisture and sugar contents; thus, they have short shelf life and must be preserved in some way. The fruit is usually consumed fresh or in dried, canned, and preserved forms. Several countries import dried figs or fig paste for different uses. The main exporters of dried figs and paste are Turkey and USA (Sadhu, 1990). Drying – the main process for preserving figs (constituting more than 85% of fig production in Iran) – causes textural changes such as fruit hardening and shrinkage. These could have a negative impact on the marketability of this valuable agricultural commodity. The increase in moisture content of the fig fruit can be regarded as a possible solution to solve textural problems, but increase of water activity can lead to microbial growth and shorten shelf life. Other compounds that bind water in food systems and lower the aw (Meste et al., 2002) can act as texture modifiers in fruits like figs. These humectants are particularly important in the production of intermediate moisture foods (IMF). A wide range of humectants including polyols, sugars and salts are incorporated into food materials in order to lower the water activity to levels prevailing in the intermediate moisture range (usually between 0.5 and 0.9) even in the presence of a considerable amount of water (Sloan and Labuza, 1976). Plasticizers greatly influence the functional properties of the material as they change the organization of the polymer chains. One of their most visible effects is to decrease the glass transition temperature (Tg) of the polymers (Sears and Darby, 1982). Thus, the change in polymer Tg as a function of the molar content of the plasticizer is a classical measure of plasticizer efficiency. Pommet et al. (2005) determined the influence of glycerol and other plasticizers on functional properties of wheat gluten network. Coupland et al. (2000) studied and modeled the effect of glycerol on the moisture sorption behavior of whey protein edible films and reported that the equilibrium moisture content of the protein film is highly determined by the amount of plasticizer present. The BET model was superior to either the Peleg or BET models as a model for the measured isotherms. Similar reports are well documented for small molecules such as low molecular weight sugars, water and glycerol which decrease Tg when mixed with large polymers (Chinnaswamy and Hanna, 1998; Moore et al., 1990; Carvalho and Mitchell, 2001). There is no published report on the moisture sorption isotherms of fig–glycerol mixtures and the main hypothesis of this research is that addition of glycerol to fig will change the sorption isotherms of fig in such a way that it would result in having more water at each specific water activity in comparison with the control devoid of glycerol. Therefore in this study the effect of two levels of glycerol (20% and 40%) on the moisture sorption isotherms of figs at different temperatures (5, 25, and 40 °C) is investigated.
2. Materials and methods 2.1. Raw materials Dried figs (Sabz variety) used in this study was kindly donated by the Estahban Fig Research Centre (Fars province, southern Iran).
The dried figs were packed in sealed polyethylene bags and stored at room temperature (22 °C) before being used for further treatments and experiments. Glycerol and all other chemicals were of analytical grade unless otherwise mentioned. 2.2. Sample preparation Using a sharp knife the dried figs were first cut into small pieces and then ground by a blade type grinder (Mammonlex, Taiwan). To make the glycerol–fig mixtures, glycerol was added to the fig powder at 2 levels (20% 40% w/w, dry basis) while being thoroughly mixed using a laboratory dough mixer (Kenwood, UK) for 10 min. To dry the prepared samples they were put in a vacuum oven set at about 65 °C for 5 h. General composition of dried fig powders was measured according to the AOAC methods (AOAC, 1990). 2.3. Determination of sorption isotherms The static gravimetric method, developed and standardized in the COST 90 project, was used to determine the moisture sorption isotherms of dried figs and fig–glycerol mixtures. Six super saturated salt solutions were used to provide a range of aw from 0.11 to about 0.84 at each specific temperature. Their water activities at 5, 25, and 40 °C were taken from Labuza et al. (1985) and Rizvi (2005) and are given in Table 1. Triplicates of each sample (about 4.0 g) were weighed and placed in air/humidity tight plastic containers containing super saturated salt solutions. For equilibration of samples, the closed containers were then maintained in three incubators equipped with temperature control systems with the accuracy of ±0.1 °C to provide the desired constant temperatures of 5, 25, and 40 °C. The weight of each sample was checked using an analytical balance (with the precision of 0.001 g) initially after three days, and then at one day intervals until a constant weight was reached. The moisture content of control samples was determined by vacuum oven drying at 65 °C (Karmas, 1980), while for the samples containing glycerol low-temperature (45 °C) oven drying with sulfuric acid solution at atmospheric pressure was used, this was due to the evaporation of glycerol as suggested by Favetto et al. (1979). The equilibrium moisture contents were reported on dry weight basis (g water/g dry solid). All moisture sorption experiments were done in triplicates and the average values were used for further data analysis. 2.4. Data analysis Several models have been proposed to correlate the equilibrium moisture contents to relative humidity of environment (Boquet et al., 1978; Rizvi, 2005) which can be divided into several groups; kinetic models based on a multilayer and condensed film (GAB model) (Van den Berg and Bruin, 1981), kinetic models based on an absorbed water monolayer (BET model) (Brunauer et al., 1938), semi-empirical (Halsey model) (Halsey, 1948), and purely empirical models (Oswin) (Oswin, 1946).
Table 1 The water activities (aw) of saturated salt solutions at 5, 25, and 40 °C. Salt
5 °C
25 °C
40 °C
LiCl CH3COOK NaI NaNO2 NaNO3 KCl
0.113 0.291 0.424 0.693 0.785 0.876
0.113 0.225 0.382 0.654 0.742 0.843
0.112 0.216 0.328 0.628 0.713 0.823
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In this study the well known BET (Eq. (1)) and GAB (Eq. (2)) equations were applied to fit the experimental data and to determine the monolayer moisture values of the samples which is an important parameter contributing to food deterioration. The BET isotherm equation is the most widely used model and gives a good fit in the aw range from 0.05 to about 0.5 for many foods and has been approved by the Commission on Colloid and Surface Chemistry of the IUPAC (International Union of Pure and Applied Chemistry). GAB has been suggested to be the most versatile sorption model available in the literature and has been adopted by the European Project Cost 90 on physical properties of foods. In principle, it represents a refined version of BET model. The major advantage of the GAB is that it describes the sorption behavior of nearly all foods from zero to 0.9aw (Rizvi, 2005)
M¼
M 0 C aw ð1 aw Þð1 þ C aw aw Þ
ð1Þ
0
M¼
M 0 C K aw ð1 K 0 aw Þð1 K 0 aw þ C K 0 aw Þ
ð2Þ
where M is the equilibrium moisture content, M0 is the monolayer moisture content, aw is the water activity, C and K0 are model constants. Fitting of experimental data into the above equations was done using the ‘‘Solver” in Excel program (Microsoft Office, 2003). This method was based on minimizing the residual sum of squares (RSS) between the predicted and the measured data. The suitability of the equations was evaluated and compared using the statistical parameters sum of square error (SEE) and the correlation coefficient (R2). The equation giving the smallest RSS and SEE and the highest R2 value was considered to be the best fitted equation (Chowdhury et al., 2006; Foster et al., 2005; Jamali et al., 2006)
RSS ¼
n X
M i;exp M i;pre
2
i¼1
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u n uX M i;exp M i;pre 2 SEE ¼ t n i¼1 3. Results and discussion 3.1. Experimental results Chemical composition of the dried figs used in this research is given in Table 2. They contained about 69% of polysaccharides, 4.3% protein, and 2.46% fat. Glycerol levels added (20% and 40%) were chosen on the basis of preliminary work undertaken, where it was shown that glycerol at levels of 20% and more, is able to soften fig texture. The equilibrium moisture sorption of the samples at different temperatures from 5 to 40 °C (Table 3) showed that temperature increase caused significant reductions in equilibrium moisture levels for all samples with and without glycerol, e.g. the equilibrium moistures for the 40% glycerol sample over supersaturated salt solution of NaNO3 at 5, 25, and 40 °C were 1.15, 0.89, and 0.79 (g water/g dry matter), Table 2 General composition of dried figs used in this research. Moisture content (%) Protein (%) Fat (%) Polysaccharide (%) Ash (%) Total fiber (%)
11.0 4.3 2.46 69.1 3.1 12.1
Table 3 Equilibrium moisture content (EMC, g water/g dry matter) of dried figs with and without glycerol (control, 20% and 40%) at 5, 25, and 40 °C. Fig–G0, control without glycerol, fig–G20, fig with 20% glycerol and fig–G40, fig with 40% glycerol.* aw at 5 °C
0.113
0.291
0.424
0.693
0.785
0.876
Fig-G0 Fig-G20 Fig-G40
0.0359c 0.1099b 0.2348a
0.0447c 0.1670b 0.3221a
0.1043c 0.2612b 0.4530a
0.2620c 0.5275b 0.8307a
0.3671c 0.7285b 1.1481a
0.6118c 1.095b 1.6707a
aw at 25 °C
0.113
0.225
0.382
0.654
0.742
0.843
Fig-G0 Fig-G20 Fig-G40
0.0345c 0.1193b 0.2447a
0.0724c 0.1493b 0.2903a
0.0896c 0.2097b 0.3844a
0.2181c 0.4201b 0.6899a
0.3267c 0.5547b 0.8868a
0.5948c 0.8119b 1.3010a
aw at 40 °C
0.112
0.216
0.328
0.628
0.713
0.823
Fig-G0 Fig-G20 Fig-G40
0.0214c 0.1008b 0.2232a
0.0468c 0.1411b 0.2819a
0.0682c 0.1727b 0.3339a
0.1924c 0.3553b 0.6019a
0.2749c 0.4810b 0.7901a
0.5129c 0.8291b 1.3216a
In each column different superscript letters indicate significant difference (a < 0.05). * Each EMC is the average of three replicates.
respectively. This behavior may be due to excitation of water molecules at higher temperatures. Indeed, as the temperature increases the water molecules get activated due to an increase in their energy level, causing them to become less stable and to break away from the water binding site of the food material, thus decreasing the monolayer moisture content (Chowdhury et al., 2006; Jamali et al., 2006). However, this was slightly different from the other investigators who reported opposite effects for high sugar content foods at greater water activities (Ayranci et al., 1990; Tsami et al., 1990; Saravacos et al., 1986) which could be due to the differences in chemical composition of other fruits compared to figs. The equilibrium sorption moistures of fig samples with and without glycerol (control, figs containing 20% and 40% glycerol) at 5, 25, and 40 °C are presented in Table 3. The equilibrium moisture content at each aw represents the mean value of three replicates. The equilibrium moisture content of all samples increased significantly with increasing aw and the samples containing glycerol had more moisture compared to the corresponding controls at the same temperature. In addition, the samples with 40% glycerol contained more equilibrium moisture than the 20% samples. For instance, at 25 °C the moisture level of the control, 20% glycerol and 40% glycerol samples at water activity of 0.742 were 0.33, 0.56 and 0.89 (g water/g dry matter), respectively. Figs. 1–3 show the effect of glycerol (at 20% and 40% levels) on the equilibrium moisture sorption isotherms of dried figs at 5, 25, and 40 °C. The experimental data are shown along with the lines drawn from the equations obtained by fitting the experimental data into BET and GAB equations (see Section 2 for more details). The shape of the isotherms of control samples is similar to the so called type 1 isotherms which are found for high sugar systems (Ayranci et al., 1990). The equilibrium moisture content increased slowly with water activity until the aw of about 0.65, after which a small increase in humidity led to a large increase in equilibrium moisture content. The equilibrium moistures obtained at 5 °C were in agreement with the data reported by Pixton and Warburton (1976). At low water activities, water can be adsorbed only to surface –OH sites of crystalline sugar. Therefore, moisture content is low at low water activity region, while at high water activities dissolution of sugars occurs and crystalline sugars are converted into their amorphous form. So the amount of water to be adsorbed increases greatly after this transition because of the increase in the number of adsorption sites upon breakage of the crystalline structure of sugars. This form of isotherms is similar to those observed for dried apricot, figs and raisins (Ayranci et al., 1990; Tsami et al.,
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1.6
BET-c
MC(db)-20%
BET-20%
MC(db)-40%
BET-40%
Moisture content (g water/g dry matter)
Moisure content (g water/g dry matter)
2.0
A
MC(db)-c
1.2
0.8
0.4
0.0
1.6
MC(db)-c
BET-c
MC(db)-20%
BET-20%
MC(db)-40%
BET-40%
1.2
0.8
0.4
0.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0.0
1.0
0.1
0.2
0.3
0.4
1.6
B
MC(db)-c
GAB-c
MC(db)-20%
GAB-20%
MC(db)-40%
GAB-40%
Moisture content (g water/g dry matter)
Moisture content (g water/g dry matter)
0.6
0.7
0.8
0.9
1.0
0.6
0.7
0.8
0.9
1.0
2.0
2.0
1.2
0.8
0.4
0.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
aw
2.0
1.6
MC(db)-c
BET-c
MC(db)-20%
BET-20%
MC(db)-40%
BET-40%
A
1.2
0.8 0.4
0.0 0.0
0.1
0.2
0.3
0.4
0.5 aw
0.6
0.7
0.8
0.9
1.0
0.7
0.8
0.9
1.0
2.0
1.6
MC(db)-c
GAB-c
MC(db)-20%
GAB-20%
MC(db)-40%
GAB-40%
B
1.2
0.8
0.4
0.0 0.0
0.1
0.2
0.3
0.4
0.5 aw
1.6
MC(db)-c
GAB-c
MC(db)-20%
GAB-20%
MC(db)-40%
GAB-40%
B
1.2
0.8
0.4
0.0 0.0
0.1
0.2
0.3
0.4
0.5
aw
Fig. 1. Effect of glycerol (at 20% and 40% levels) on the equilibrium moisture sorption isotherm of dried fig at 5 °C. The symbols are experimental data and the lines are from the equations obtained by fitting the experimental data to BET (A) and GAB (B) equations. c: Control or fig without glycerol, 20%: fig with 20% glycerol, 40%: fig with 40% glycerol. Each bar is ±SD.
Moisture content (g water/g dry matter)
0.5
aw
aw
Moisture content (g water/g dry matter)
A
0.6
Fig. 2. Effect of glycerol (at 20% and 40% levels) on the equilibrium moisture sorption isotherm of dried fig at 25 °C. The symbols are experimental data and the lines are from the equations obtained by fitting the experimental data to BET (A) and GAB (B) equations. c: Control or fig without glycerol, 20%: fig with 20% glycerol, 40%: fig with 40% glycerol. Each bar is ±SD.
Fig. 3. Effect of glycerol (at 20% and 40% levels) on the equilibrium moisture sorption isotherm of dried fig at 40 °C. The symbols are experimental data and the lines are from the equations obtained by fitting the experimental data into BET (A) and GAB (B) equations. c: Control or fig without glycerol, 20%: fig without 20% glycerol, 40%: fig with 40% glycerol. Each bar is ±SD.
1990). In addition, it seems that addition of glycerol to dried figs changed the shape and status of sorption isotherms, from normal sugar type isotherm for the control towards the typical sigmoid shape isotherms for 40% glycerol sample associated with most foods. The results of fitting the experimental data into BET and GAB sorption equations (RSS, SEE, and R2) are shown in Table 4. The values of monolayer moisture contents at each temperature as calculated by the BET and GAB equations are also presented in Table 4. The results show that the monolayer moisture content decreases with increasing temperature between 5 and 40 °C. This behavior has been reported for raisin, dried apricot, and fig (Ayranci et al., 1990; Saravacos et al., 1986) and has been attributed in the literature to the reduction in the active sites due to chemical and physical changes (e.g. reduction in hydrogen bonding degree) taking place as the temperature increases, the extent of which depends on the nature of the food (Al-Muhtaseb et al., 2004; Perdomo et al., 2009). The monomolecular moisture contents of the control fig in the temperature range of 5–40 °C, varied from 0.081 to 0.099 g water/g dry solids for BET model and from 0.083 to 0.114 g water/g dry solids for GAB model. These estimated values for M0 are within the reported values for fruits, which vary from 10% to 15% (dry basis) and were found to be much lower than those reported for vegetables, which vary between 18% and 22% (dry basis) (Kiranoudis et al., 1993). Table 5 shows the monolayer moisture content of fig found in this study in comparison with M0 of other selected food materials. Glycerol also had noticeable effect on the amount of monolayer water, which increased with glycerol content. The monolayer moisture content M0 is recognized as the moisture content affording the longest time period with minimum quality loss at a given
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A. Farahnaky et al. / Journal of Food Engineering 93 (2009) 468–473
Table 4 Estimated BET and GAB constants and monolayer moisture levels for dried figs with and without glycerol (control, 20% and 40%) at 5, 25, and 40 °C. Fig–G0 represents control without glycerol; fig–G20 represents fig with 20% glycerol; fig–G40 represents fig with 40% glycerol. Sample
Temperature (°C)
BET constants M0 (%)
Fig–G0 Fig–G20 Fig–G40 Fig–G0 Fig–G20 Fig–G40 Fig–G0 Fig–G20 Fig–G40
5 5 5 25 25 25 40 40 40
8.1 14.4 22.2 9.9 13.8 21.9 8.9 13.4 21.4
GAB constants
C
RSS
4.07 34.75 1519145 2.32 22.75 5732.88 2.71 17.08 742.06
0.0019 0.0131 0.0441 0.00094 0.0025 0.0091 8.4 105 0.00095 0.0044
R2
SEE 2
1.6 10 4.3 102 7.9 102 1.2 102 1.9 102 3.6 102 3.5 103 1.2 102 2.5 102
Table 5 GAB equation monolayer moisture contents of selected food materials compared with fig (found in this study). Food materials
T (°C)
M0 (% dry basis)
Reference
Sunflower seeds Pistachio nuts Sultana raisins Long pepper Apricots Prunes Raisins Currant Figs
20–30 30 20–30 35 20–30 20–30 20–30 20–30 20–30
3.09 3.90 7.70 5.79 11.70 12.60 14.00 17.30 9.70
Mazza and Jayas (1991) Maskan and Karatas (1997) Saravacos et al. (1986) Costa et al. (1998) Tsami et al. (1990) Tsami et al. (1990) Tsami et al. (1990) Tsami et al. (1990) Tsami et al. (1990)
Figs Banana Date (Alig) Date (Deglet Noor) Kiwi
5–40 25–45 5–40 5–40 30
8.3–11.4 50.2 11.18–20.30 9.95–17.44 4.20–5.00
Present study Kiranoudis et al. (1993) Nabill et al. (2005) Nabill et al. (2005) Moraga et al. (2006)
Table 6 Monolayer moisture content (M0) and corresponding aw (i.e. the safest water activity for storage) for figs with and without glycerol according to the GAB model. Temperature (°C)/sample code
Monolayer moisture content, M0 (% dry basis)
aw
5/(Fig–G0) 5/(Fig–G20) 5/(Fig–G40) 25/(Fig–G0) 25/(Fig–G20) 25/(Fig–G40) 40/(Fig–G0) 40/(Fig–G20) 40/(Fig–G40)
11.4 20.2 30.4 9.8 16.4 26.2 8.3 13.4 22.1
0.462 0.318 0.222 0.333 0.239 0.153 0.379 0.196 0.063
temperature. Below this value, the rates of deteriorative reactions, with the exception of the oxidation of unsaturated fats, are minimal. Therefore, at a given temperature, the safest water activity level is that corresponding to M0 or lower (Goula et al., 2008). The safest aw values corresponded to M0 of the samples with and without glycerol, were obtained from the data presented in Table 4 and are tabulated in Table 6. The second parameter of BET model is the sorption energy constant, C, just as the GAB constant, but with slightly different physical meaning (Moraga et al., 2006). It allows us to classify sorption isotherms according to Brunauer’s classification. As the C values obtained in this study were >2, all the sorption isotherms obtained can be classified as type II, like other fruits such as strawberry apple, blackberry, kiwifruit (Moraga et al., 2006), and spray dried tomato pulp (Goula et al., 2008). In the GAB model there is an additional constant for energy, K (a third parameter in addition to M0 and C), which is nearly equal to 1.
0.9938 0.9867 0.9826 0.9966 0.9955 0.9943 0.9995 0.9976 0.9958
M0 (%) 11.4 20.2 30.4 9.8 16.4 26.2 8.3 13.4 22.1
K 0.95 0.94 0.938 0.98 0.958 0.953 1.031 1.019 1.009
C 1.63 5.514 14.43 3.75 11.23 34.57 2.43 15.18 218.6
RSS 0.00059 0.00088 0.0020 0.00029 0.00054 0.00029 1.3105 5.3105 0.00037
R2
SEE 3
9.2 10 1.1 102 1.7 102 6.4 103 8.8 103 6.5 103 1.4 103 2.8 103 7.3 103
0.9980 0.9990 0.9990 0.9989 0.9989 0.9998 0.9999 0.9999 0.9997
It should be noted that, in all sorption series, the monolayer moisture content value obtained by the GAB model was slightly greater than those obtained by the BET model, which is in agreement with the results observed by other authors and summarized by Rahman (1995) and Timmermann et al. (2001). These differences have been attributed to the fact that BET model focuses on surface adsorption in the first layer, while the GAB model takes into account sorbed water properties in the multilayer region (Bell and Labuza 2000). The fitting quality parameters of BET and GAB models (RSS, SEE, and R2) presented in Table 4, indicate that the GAB model gives the smaller RSS/SEE along with higher R2 and so better describes the experimental adsorption data of figs throughout the entire range of water activity (0.11–0.84). This observation is similar to that reported by other researchers, who studied sorption isotherms of various fruits and vegetables. Kiranoudis et al. (1993) determined the equilibrium moisture content of potato, carrot, tomato, green pepper, and onion within the range of 0.1–0.9 water activity at three different temperatures (30, 45, and 60 °C) and found the GAB model satisfactory for the prediction of the experimental data obtained. McLaughlin and Magee (1998) determined the sorption isotherms for potatoes at temperatures of 30, 45, and 60 °C and, among the models tested, the GAB model gave the best fits. According to Al-Muhtaseb et al. (2004), the GAB model adequately represented the sorption isotherms of potato and wheat starch and Akanbi et al. (2006) concluded that the equilibrium moisture contents of tomato slices obey closely the GAB equation. 4. Conclusions The general impact of glycerol on sorption isotherms observed in this research is similar to its effect in glycerol–whey protein mixtures, studied in edible films by Coupland et al. (2000) who reported that glycerol drew water into food systems while keeping its water activity low. Taken together, the equilibrium moisture content of dried figs at each aw is significantly influenced by the amount of glycerol present. The BET and GAB equations were able to fit the experimental data in figs as a high sugar system. This work confirmed that glycerol has a good potential for use as a hygroscopic ingredient in figs, in order to hold more water at a specific water activity. This could be thought of as a promising approach to modify the hard texture of dried figs with long shelf life in terms of microbial stability. More work is needed on the effect of glycerol on textural hardness, sensory characteristics and glass/rubber transition temperature of figs. Acknowledgment This research project was financially supported by Centre of Excellence for Fig Research of Shiraz University.
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