Prediction of Moisture Adsorption Characteristics of Dehydrated Fruits Using the GAB Isotherm Model

Rohitha Prasantha BD*

Department of Food Science and Technology, University of Peradeniya, Sri Lanka

*Corresponding author: Prasantha BDR, Department of Food Science and Technology, University of Peradeniya, Faculty of Agriculture, Peradeniya, 20400, Sri Lanka

Received: September 24, 2018; Accepted: October 29, 2018; Published: November 05, 2018

Abstract

The moisture isotherm of fruits can be determined at various temperatures range. The equilibrium moisture content of dehydrated fruits increased sharply as the temperature increased. Adsorption isotherm curves of dehydrated fruit showed the characteristics of a type between II-III shapes. More starch contains fruits show type II isotherm curves whereas high sugar contains fruits show type III isotherm. This phenomenon can further be explained by temperature dependent constants of the GAB isotherm model. Some dehydrated fruits have higher monolayer moisture content and specific active surface area, which could be related to their physico-chemical composition, but both decreased with increase in temperature. The net isosteric heat of sorption was higher in more starch containing dehydrated fruits than high sugar content fruits possibly due to different physico-chemical and structural composition with regards to their sorption characteristics.

Keywords: Fruit; Monolayer moisture; GAB; Isosteric heat; Temperature

Abbreviations

a_w: Water Activity; C, K: GAB Model Parameters; co, ko: Entropic Accommodation Factors; d.b.: Dry Basis (%); e: Temperature Dependence Constant of Monolayer (K); EMC: Equilibrium Moisture Content (g/100 g dry matter); ERH: Equilibrium Relative Humidity (%); g: Monolayer And Multiplayer Water Enthalpy Difference (K); GAB: Guggenheim-Anderson-Boer Equations; hi: Molar Sorption Enthalpy of Free Water (J mol-1); hm: Monolayer Sorption Enthalpy (J mol-1); hn: Multilayer Molar Sorption Enthalpy (J mol-1); i: Multilayer And Free Water Enthalpy Difference (K); m: Moisture Content (g/100 g dry matter); M: Moisture Content (% d.b.); ma: Temperature Dependence Of Monolayer (K); mo: Monolayer Moisture Content (g/100 g dry matter); q_s: Isosteric Heat Of Sorption (J/mol-1); R: Universal Gas Constant (8.314 J mol-1 K-1); r.h.: Relative Humidity (%); S: Active Specific Surface Area (m2 g-1); SD: Standard Deviation; T: Absolute Temperature (K); TSS: Total Soluble Solids Content (oBrix).

Introduction

Dehydration is an important preservation technique with the primary objective of reducing microbial activity, product deterioration and extension of shelf-life during storage. Most fruits contain enough moisture to induce the microbial growth and activity of natural enzymes but by drying can reduce a_w and prevent microbial and enzymatic spoilage. Some physical changes may occur in fruit drying operation such as shrinkage, puffing, sugar crystallization, gelatinization of starch and glass transitions etc. In some cases, desirable or undesirable chemical or biochemical reactions may also occur. Structural collapse in fruits due to moisture removal from the fruits product results significant changes in texture [1]. Therefore, reduction of a_w in the final product is a very important way to ensure the stability of the dried or dehydrated foods. Dehydrated fruits with sufficient low a_w ensure the both low enzymatic activity and microbial spoilage. In general, recommended a_w of most dehydrated fruits is relatively <0.6 for safe storage [2]. However, expose to high drying temperature and significant loss of moisture in fruits cause irreversible stresses in the cellular structure of the fruits and may leads to robust textural changes especially during storage.

The properties such as structure, texture and storage time of a dehydrated fruits depend on the moisture adsorption characteristics and are useful in design and modeling of drying, aeration and storage processes. The relationship between a_w or ERH and EMC of a product is described by the moisture sorption isotherms. Each commodity has a unique set of isotherms and these isotherms are useful to determine the lowest moisture content attainable under the specific drying or storage conditions [3-8]. The monolayer moisture content (mo) is also recognized as the optimum moisture content for good storage stability and to maintain the overall quality of dehydrated products [3]. The shape of the sorption isotherm curve has resulted due to the physical adsorption of moisture into fine porous structure of the food [9]. In general, type II and III isotherms (Brunauer’s classification) show a lower water sorption with an increase in temperature at constant a_w [4,10], indicating loss of hygroscopicity of the fruithigher temperature. During dehydration, structure and physico-chemical nature of the food materials such as starch may undergo in partial gelatinization process [11] and sugar may undergo crystallization at high temperature.

Adsorption Isotherm

The moisture adsorption may reduce significantly due to reduction in moisture affinity sites within the tissue with increasing temperature. The a_w of the dehydrated fruits showed a decrease with increasing temperature, which can be explained with the help of a thermodynamic equation. Isotherm behavior characteristics of sugars and sugar containing dehydrated fruits have been reported by several authors [11-18]. The sharp increase in moisture content of dehydrated fruits could be explained by the solubility of sugars and starcheincreasing a_w (equilibrium) under constant temperature (Figure 1). Adsorption isotherms of many dehydrated fruits that contain high TSS intersect at a_w between 0.5 and 0.8 with increasing storage temperature [10,12,13,16,19,20].

At low moisture, fruits contain high TSS are hygroscopic and tends to absorb large amount water at high a_w and temperature as results of solubilization of sugars [20]. Another reason may associate to high cellulosic and capillary micro-structure of the dehydrated fruits can retain more water within the fruit matrix due to high surface tension [10,14,21]. Therefore, creation of a transient capillary structure along with an increase the number of active binding Site (S) inside the micropores structure of the fruits which helps to adsorb more water [22]. However, at very low moisture content or under very low a_w levels, the transient capillary structure may not play an important role in moisture adsorption.

Modeling of the Sorption Isotherms

Although there are several mathematical models to describe the moisture sorption characteristics, six parameter Guggenheim- Anderson-de Boer equation [23] known as GAB model was frequently used best model to analyze the sorption isotherms of fruits (equation 1) for many ra_w or process biological materials [11-18, 23-28].

It has been shown that parameters K of the GAB model must be in the range of 00 [24, 25, 30]. The coefficients mo, C and K are related to temperature and can be expressed using Arrhenius type equations 2, 3 and 4 [25- 28]. These parameters of GAB models are play an important role in predicting the moisture sorption characteristics of the biological materials.

According to the above equations, g and i values are highly depend on the sorption enthalpy values. Therefore, Arrhenius constants of g and i show the liner relationship with heat of sorption enthalpy values where g = hm-hn/R and i = hi-hn/R.

Therefore, comprehensive temperature dependent six-parameter GAB model fitted more accurately over the wider range of a_wand the temperature between 0.11-0.9 and 25-60oC, respectively [11]. The GAB model has been used successfully to explain the moisture sorption behavior of many foods [15,16,19,25,28,30,31]. The difference in the degree of suitability of moisture sorption models for the different fruits could be due to inherent differences in the fruits particularly in relation to their biochemical composition, processing conditions [19] and the method of dehydration (Table 1).

Table 1: Effect of drying method on monolayer moisture content and isosteric heat of sorption different dehydrated fruits derived from GAB model.




  
    Drying    method
    m_o (g/100g)^†
    Temperature    (^oC)
    q_s(kJ/mol^-1)^�
    S    (m²g^-1)*
    Reference
  
  
    Osmotically    dehydrated fruits
    
    
  
  
    Apples
    11
    30-50
    -
    388.3
    33
  
  
    Raisins
    14
    15-60
    10.6
    494.2
    19
  
  
    Currents
    17.3
    15-60
    5.7
    610.7
    19
  
  
    Prunes
    12.6
    15-60
    18.5
    444.8
    19
  
  
    Apricot
    11.7
    15-60
    14.8
    413.01
    19
  
  
    Pineapple
    7.3-6.8
    20-40
    -
    257.7-219.0
    16
  
  
    Freeze    dried fruits
    
    
  
  
    Raspberry
    7.4
    23
    -
    261.2
    34
  
  
    Strawberry
    5.1
    30
    -
    180
    34
  
  
    Kiwi
    4.7
    30
    
    165.9
    34
  
  
    Blueberries
    17.4
    Apr-45
    13.8
    614.2
    35
  
  
    Passion
    6.4-6.3
    20-50
    40
    
    36
  
  
    Cabinet    dried fruits
    
    
  
  
    Raisins
    2.75
    15-60
    20
    97.07
    19
  
  
    Figs
    9.7
    15-60
    22.5
    342.4
    19
  
  
    Figs
    4.27
    15-60
    13.2
    150.7
    15
  
  
    Prunes
    4.54
    15-60
    20
    160.3
    15
  
  
    Apricot
    4.13
    20-40
    20
    145.8
    15
  
  
    Apple
    6.21
    15-45
    12.3
    219.2
    15
  
  
    Banana
    27.6
    40-70
    16
    974.3
    15
  
  
    Papaya
    13.8-6.2
    May-45
    57.35
    487.1-219.0
    37
  
  
    Solar    dried fruits
    
    
  
  
    Mango
    10.0-11.4
    50-30
    19.5
    353-402.4
    11
  
  
    Jackfruit
    3.6-5.0
    50-30
    33
    127.1-176.5
    11
  
  
    Vacuum    dried fruits
    
    
  
  
    Mango
    3.15
    26-50
    20
    111.2
    21
  
  
    Passion
    6.4-6.3
    20-50
    37
    
    36



Table 1: Effect of drying method on monolayer moisture content and isosteric heat of sorption different dehydrated fruits derived from GAB model.

Shape of the Sorption Isotherm

The sorption isotherms of fruits obtained characteristic type either II or III isotherms with agreement of GAB classification (Figure 1). If parameter C = 2, the GAB equation gives a sigmoidal shape curve with a point of type II inflection [25]. Lewicki [29] reported that GAB model describes well sigmoidal type II isotherms when parameters K and C are kept in the rages of 0.24 0.6.

Download PDF

Citation:Prasantha BDR. Prediction of Moisture Adsorption Characteristics of Dehydrated Fruits Using the GAB Isotherm Model. Ann Agric Crop Sci. 2018; 3(1): 1036.

Instruction for Authors

Submit Your Article