3. Moisture sorption isotherm measurement techniques

Several methods of determining the moisture sorption isotherm of agricultural and food products have been employed by investigators [25]. Gal [26–28] carried out a thorough review of the methods and pointed out that the basic techniques include the gravimetric, hygrometric, vapor pressure manometric and inverse gas chromatography and special method involving the use of AquaLab.

#### 3.1 Gravimetric method

There are two common gravimetric methods of determining the EMC of agricultural and food products at different temperatures and water activities. One of these methods is the static gravimetric method which involves the placement of the product in an atmosphere with which it then comes into equilibrium (weight loss or gain stops) without mechanical agitation of the air or product. For this method, several weeks may be required for the product to come into equilibrium, and because of the long period of time, mold usually develops on high and intermediate moisture foods at water activities above 0.8. For data obtained at water activities above 0.8 to be reliable, mold growth must be prevented during equilibration. At the point of equilibration, the moisture content is then determined as the EMC. The second one is the dynamic method in which the atmosphere surrounding the product or the product itself is mechanically moved. The dynamic method is quicker but presents the problem of design and instrumentation. The static method has been used extensively and reported to be preferable for obtaining complete sorption isotherms [27]. It has also been recommended as the standard method of determining the moisture sorption isotherms of agricultural and food products [29]. It involves the placement of small sample (10–25 g) of agricultural and food material in vacuum desiccators containing different concentrations of sulfuric acid

Moisture Sorption Isotherms and Isotherm Model Performance Evaluation for Food… DOI: http://dx.doi.org/10.5772/intechopen.87996

(Figure 2a) to maintain the relative humidity (water activity) of the surrounding air at different values from 0 to 100% (0.00–1.00) or saturated solution (Figure 2b) of different salts to achieve different values of relative humidity at a specified temperature. Usually a thermostatically controlled water bath or oven

#### Figure 2.

method (freeze- and spray-drying) of lactose-hydrolyzed milk did not affect the adsorption isotherms but had profound effect on the desorption isotherms. Tsami et al. [17] investigated the effect of drying method on the sorption characteristics of model fruit powder and reported that freeze-dried gel adsorbed more vapor at 25°C than microwave-dried gel, which had a higher sorption capacity than vacuum- and conventionally dried product. Mittal and Usborne [18] determined the moisture sorption isotherms of meat emulsions and showed that their EMC was affected by the fat-protein ratio. Mazza [19] reported that at 40°C and in the monolayer region of the isotherm, the EMC of precooked dehydrated pea was higher than that of raw pea but that at water activities above 0.5, the sorption capacity of precooked pea was lower than that of raw pea. Aviara [20] noted that chemical modification (cross-linking and hydroxypropylation) of cassava, maize and sorghum starches had profound influence on their moisture adsorption and desorption characteristics. While cross-linking lowered the sorptive capacity of the starches, hydroxypropylation enhanced the ability of the starches to sorb or desorb moisture. Palou et al. [21] studied the moisture sorption characteristics of three cookies and two corn snacks whose main composition difference was in fat and total carbohydrate and found the EMC difference at 5% level of significance. Igbeka et al. [22], Ajibola and Adams [23] and Gevaudan et al. [24] studied the moisture sorption characteristics of cassava and presented data that were fitted by different moisture sorption isotherm models. The variance in EMC may be due to the source of the material, product's postharvest and sorption history and varietal differences, methodology of measurement, temperature range and limitations imposed by model selection.

3. Moisture sorption isotherm measurement techniques

chromatography and special method involving the use of AquaLab.

3.1 Gravimetric method

Sorption in 2020s

146

Several methods of determining the moisture sorption isotherm of agricultural and food products have been employed by investigators [25]. Gal [26–28] carried out a thorough review of the methods and pointed out that the basic techniques include the gravimetric, hygrometric, vapor pressure manometric and inverse gas

There are two common gravimetric methods of determining the EMC of agricultural and food products at different temperatures and water activities. One of these methods is the static gravimetric method which involves the placement of the product in an atmosphere with which it then comes into equilibrium (weight loss or gain stops) without mechanical agitation of the air or product. For this method, several weeks may be required for the product to come into equilibrium, and because of the long period of time, mold usually develops on high and intermediate moisture foods at water activities above 0.8. For data obtained at water activities above 0.8 to be reliable, mold growth must be prevented during equilibration. At the point of equilibration, the moisture content is then determined as the EMC. The second one is the dynamic method in which the atmosphere surrounding the product or the product itself is mechanically moved. The dynamic method is quicker but presents the problem of design and instrumentation. The static method has been used extensively and reported to be preferable for obtaining complete sorption isotherms [27]. It has also been recommended as the standard method of determining the moisture sorption isotherms of agricultural and food products [29]. It involves the placement of small sample (10–25 g) of agricultural and food material

in vacuum desiccators containing different concentrations of sulfuric acid

(a) Desiccator containing concentrated sulfuric acid: (1) locking clamp, (2) lid, (3) rubber seal ring, (4) desiccator barrel, (5) sample basket or can, (6) sample basket mounting stand and (7) concentrated sulfuric acid. Source: Spiess and Wolf [29]. (b) Desiccator containing saturated salt solution employed by Kameoka et al. [32] in determining the EMC of brown and rough rice and hull.

(Figure 3) is used to obtain the desired temperature. The water activity of sulfuric acid at different concentrations and temperatures is presented in Table 1, and that of saturated solutions of different salts at various temperatures are presented in Table 2.

#### Figure 3.

Thermostatically controlled water bath or oven for moisture sorption isotherm determination. Source: Spiess and Wolf [29].

Acids are not used extensively because of the danger involved in its handling and

The static gravimetric method involving the use of saturated salt solutions was applied successfully to the determination of MSIs of Jerusalem artichoke [30]; uncooked meat emulsions [18]; ground and short-time roasted coffee [31]; rice [32]; pigeon pea type-17 [33]; cassava [23]; plantain, winged bean seed and gari [34–36]; freeze-dried, osmo-freeze-dried and osmo-air-dried cherries and blue berries [15]; vetch seeds [37]; lupine [38]; high oleic sunflower seeds and kernels [39]; quinoa grains [40]; soya bean [41]; red chillies [42]; chickpea flour [43]; black gram nuggets [44]; sorghum malt [45]; IR-8 rice variety [46]; native and chemically modified starches [20]; and castor seeds [47]. Young [48], Oyelade et al. [49, 50], Al-Muhtaseb et al. [51], Bello [52] and Afkawa [53] applied the static gravimetric method involving the use of different concentrations of sulfuric acid in determining the MSIs of Virginia-type peanuts, maize flour, yam flour, potato, high amylopectin and high amylose starch powders, groundnut and neem seeds and shea nut and desert date kernels, respectively. Bosin and Easthouse [54] suggested the dynamic

the changes that can occur in its composition—it is susceptible to dilution or increase in concentration with time due to the release or absorption of water by the product—thereby effecting a change in the air-water activity. Acids also easily corrode and release fume that can be toxic in the food material. Saturated salts are safer to use, and constant humidity can be maintained by leaving excess salt in the solution. That way, the solution is made to remain saturated thoroughout the duration of the experiment in spite of the release or absorption of water by the product. The use of saturated salt solution, however, requires many salts in order to go thorough the relative humidity (water activity) range of 0–100% (0.00–1.00),

whereas only one acid could be used for the same purpose.

Water activity of saturated salt solutions at different temperatures.

Source: Rizvi [6], Bell and Labuza [56].

Table 2.

149

Salt Temperature (°C)

DOI: http://dx.doi.org/10.5772/intechopen.87996

Moisture Sorption Isotherms and Isotherm Model Performance Evaluation for Food…

20 25 30 35 40 45 50 60 70

Sodium hydroxide 0.09 0.08 0.07 0.07 0.07 0.07 0.07 0.07 0.07 Lithium chloride 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 Potassium acetate 0.23 0.22 0.22 — ————— Calcium chloride ——— 0.22 0.22 0.22 0.21 0.21 0.20 Magnesium chloride 0.33 0.33 0.32 0.32 0.32 0.32 0.31 0.31 0.30 Potassium carbonate 0.43 0.43 0.43 — ————— Magnesium nitrate 0.54 0.53 0.51 0.499 0.48 0.47 0.45 0.45 0.44 Manganese chloride ——— — 0.51 0.5 0.5 0.5 0.5 Sodium bromide 0.59 0.58 0.56 0.55 ————— Sodium nitrite ——— — 0.61 0.61 0.6 0.6 0.6 Sodium chloride 0.75 0.75 0.75 0.75 0.75 0.74 0.74 0.74 0.75 Potassium chloride 0.85 0.84 0.84 0.83 0.82 0.82 0.81 0.8 0.8 Barium chloride ——— — — 0.89 0.89 0.88 0.88 Potassium nitrate 0.95 0.93 0.92 0.91 0.89 ———— Potassium sulfate 0.97 0.97 0.97 0.97 0.96 0.96 0.96 0.95 0.94


#### Table 1.

Water activity of sulfuric acid solution at different concentrations and temperatures.


Moisture Sorption Isotherms and Isotherm Model Performance Evaluation for Food… DOI: http://dx.doi.org/10.5772/intechopen.87996

#### Table 2.

(Figure 3) is used to obtain the desired temperature. The water activity of sulfuric acid at different concentrations and temperatures is presented in Table 1, and that of saturated solutions of different salts at various temperatures are presented in

Thermostatically controlled water bath or oven for moisture sorption isotherm determination. Source: Spiess and

5.00 1.0300 0.9803 0.9804 0.9806 0.9807 0.9808 0.9811 0.9814 10.00 1.0640 0.9554 0.9554 0.9558 0.9562 0.9562 0.9565 0.9570 15.00 1.0994 0.9227 0.9230 0.9237 0.9241 0.9245 0.9253 0.9261 20.00 1.1365 0.8771 0.8779 0.8796 0.8802 0.8814 0.8831 0.8848 25.00 1.1750 0.8165 0.8183 0.8218 0.8218 0.8252 0.8285 0.8317 30.00 1.2150 0.7396 0.7429 0.7491 0.7509 0.7549 0.7604 0.7655 35.00 1.2563 0.6464 0.6514 0.6607 0.6651 0.6693 0.6773 0.6846 40.00 1.2991 0.5417 0.5480 0.5599 0.5656 0.5711 0.5816 0.5914 45.00 1.3437 0.4319 0.4389 0.4524 0.4589 0.4653 0.4775 0.4891 50.00 1.3911 0.3238 0.3307 0.3442 0.3509 0.3574 0.3702 0.3827 55.00 1.4412 0.2255 0.2317 0.2440 0.2502 0.2563 0.2685 0.2807 60.00 1.4940 0.1420 0.1471 0.1573 0.1625 0.1677 0.1781 0.1887 65.00 1.5490 0.0785 0.0821 0.0895 0.0933 0.0972 0.1052 0.1135 70.00 1.6059 0.0355 0.0377 0.0422 0.0445 0.0470 0.0521 0.0575 75.00 1.6644 0.0131 0.0142 0.0165 0.0177 0.0190 0.0218 0.0249 80.00 1.7221 0.0035 0.0039 0.0048 0.0053 0.0059 0.0071 0.0085

Temperature (°C) 5 10 20 25 30 40 50

Table 2.

Sorption in 2020s

Figure 3.

Wolf [29].

Percent H2SO4

Source: Rizvi [6], Bell and Labuza [56].

Water activity of sulfuric acid solution at different concentrations and temperatures.

Table 1.

148

Density at 25°C (g/cm3 )

Water activity of saturated salt solutions at different temperatures.

Acids are not used extensively because of the danger involved in its handling and the changes that can occur in its composition—it is susceptible to dilution or increase in concentration with time due to the release or absorption of water by the product—thereby effecting a change in the air-water activity. Acids also easily corrode and release fume that can be toxic in the food material. Saturated salts are safer to use, and constant humidity can be maintained by leaving excess salt in the solution. That way, the solution is made to remain saturated thoroughout the duration of the experiment in spite of the release or absorption of water by the product. The use of saturated salt solution, however, requires many salts in order to go thorough the relative humidity (water activity) range of 0–100% (0.00–1.00), whereas only one acid could be used for the same purpose.

The static gravimetric method involving the use of saturated salt solutions was applied successfully to the determination of MSIs of Jerusalem artichoke [30]; uncooked meat emulsions [18]; ground and short-time roasted coffee [31]; rice [32]; pigeon pea type-17 [33]; cassava [23]; plantain, winged bean seed and gari [34–36]; freeze-dried, osmo-freeze-dried and osmo-air-dried cherries and blue berries [15]; vetch seeds [37]; lupine [38]; high oleic sunflower seeds and kernels [39]; quinoa grains [40]; soya bean [41]; red chillies [42]; chickpea flour [43]; black gram nuggets [44]; sorghum malt [45]; IR-8 rice variety [46]; native and chemically modified starches [20]; and castor seeds [47]. Young [48], Oyelade et al. [49, 50], Al-Muhtaseb et al. [51], Bello [52] and Afkawa [53] applied the static gravimetric method involving the use of different concentrations of sulfuric acid in determining the MSIs of Virginia-type peanuts, maize flour, yam flour, potato, high amylopectin and high amylose starch powders, groundnut and neem seeds and shea nut and desert date kernels, respectively. Bosin and Easthouse [54] suggested the dynamic

gravimetric method, and Igbeka et al. [22], Roman et al. [25] and Rahman and Al-Belushi [55] utilized it in establishing the MSIs of cassava and potato, apple and freeze-dried garlic powder, respectively.

3.3 Vapor pressure manometric (VPM) method

DOI: http://dx.doi.org/10.5772/intechopen.87996

apparatus using high vacuum grease.

Schematic diagram of vapor pressure manometric apparatus. Source: Rizvi [6].

temperature.

evacuated for 1 min.

Figure 5.

151

The vapor pressure manometric method involves bringing air to equilibrium with the agricultural or food product at a fixed temperature and moisture content and the relative humidity of the air measured as the equilibrium relative humidity (ERH). In this method, the vapor pressure exerted by the moisture in the product is directly measured. As a result, it is taken as one of the best methods of determining the MSI of food [60]. The equilibrium relative humidity is then obtained from the ratio of the vapor pressure in the sample to that of pure water at the same temperature. A schematic diagram of the apparatus and simplified diagram of the system set-up is shown in Figures 5 and 6, respectively. The procedure for determining the

Moisture Sorption Isotherms and Isotherm Model Performance Evaluation for Food…

i. The prepared sample and VPM system are allowed to reach the desired

ii. About 10–50 g of sample is put in the sample flask, and an equal amount of desiccant (CaSO4, CaCl2) is placed in the desiccant flask and sealed on to the

iii. Keeping the sample flask isolated, the system is evacuated to less than 200 μmHg (Rizvi, 1986). The cold strap should be filled with nitrogen prior to evacuation of the system to trap any moisture reaching the vacuum pump.

iv. The space in the sample flask is then connected to the evacuated air space by opening the stopcock over the sample V4 (Figure 6), and the system is again

ERH of agricultural and food products using the method is as follows:
