3.3 Vapor pressure manometric (VPM) method

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

Electric hygrometers are widely used for obtaining the MSIs of agricultural and

designed hygrometers that are in use. The instrument (Figure 4) consists basically of a sensor, sample chamber and potentiometer. The sensor could use a hygroscopic chemical such as lithium chloride or an ion-exchange resin such as sulfonated polysterne; the conductivity of which changes according to the water activity above the sample. The sensor could be a humidity sensor which is based on capacitance changes in a thin film capacitor. Electric hygrometers give rapid, relatively precise results and are easy to operate. The main problems involved with the use of

i. Evaluation of the equilibration time between the sample and sensor

Crapiste and Rostein [57], Fasina and Sokhansanj [58], Tsami et al. [17] and Arslan and Togrul [59] employed the hygrometric method in studying the moisture sorption behavior of potatoes, alfalfa pellets, model fruit powders and crushed

Diagram of moisture sorption isotherm apparatus utilizing the hygrometer. Source: Fasina and Sokhansanj [58].

food products. There are quite a lot of commercially available and specially

freeze-dried garlic powder, respectively.

ii. Proper temperature control

iii. Need for recalibration for some instrument

3.2 Hygrometric method

Sorption in 2020s

hygrometers are:

chillies, respectively.

Figure 4.

150

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


Figure 5. Schematic diagram of vapor pressure manometric apparatus. Source: Rizvi [6].

3.4 Inverse gas chromatography

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

and

(82.0567 cm3 atm mole<sup>1</sup> K�<sup>l</sup>

of carrier gas (cm3

Figure 7.

153

and Sereno [67].

parameters and peak data to the sorption isotherm:

The inverse gas chromatography (IGC) is a rapid and effective system for studying the thermodynamic properties of a solid taken as the stationary phase in relation to a mobile gas phase containing selected solutes such as water. It is particularly suitable for the study of the lower region of water activity and for products with very low equilibrium moisture contents [66, 67]. With IGC the sorbed solute is injected into the carrier gas stream, and its linear transport is retarded owing to interaction with the product under study, which constitutes the stationary phase. Moisture sorption isotherms are then determined using the chromatographic data obtained and the following equations, which relate chromatograph operating

Moisture Sorption Isotherms and Isotherm Model Performance Evaluation for Food…

<sup>a</sup> <sup>¼</sup> maIads mIpic

<sup>p</sup> <sup>¼</sup> mahRT

where a is the uptake of sorbed water (g/g stationary phase), ma is the mass of water injected (g), m is the mass of stationary phase (g) and Iads/Ipic is the ratio of the areas (A + B)/B calculated from the chromatogram (Figure 7), p is the partial

It has been used successfully to determine the MSIs of homogeneous solid food ingredients like sucrose, glucose and starch [68] and complex heterogeneous foods

Typical gas chromatogram obtained by IGC: 1 = point of injection; 2 = point of emergence of unadsorbed peak (air); 3 = point of emergence of probe peak (water), Ipic = area B; and Iads = area A + B. Source: Manuel Sa

pressure (atm), h is the peak height (detector units), R is the gas constant

/min) and Ipic is the area B in Figure 7.

like bakery products [69], wheat flour [66] and wheat and soy flour [70].

IpicW (3)

), T is the absolute temperature (K), W is the flow rate

(2)

Figure 6. Schematic diagram of vapor pressure manometric system set-up. Source: Ajibola et al. [65].


$$ERH = \frac{(H\_1 - H\_2) \left(\frac{T\_s}{T\_s}\right)}{P\_s} \tag{1}$$

where ERH is the equilibrium relative humidity (%), H1 is the micromanometer reading with sample flask connected to the system (mm of manometric oil), H2 is the micromanometer reading with desiccant flask connected to the system (mm of manometric oil), Ts is the temperature of the environment surrounding the water bath taken as the temperature of sample (K), To is the temperature of the environment surrounding the micromanometer (K) and Ps is the saturated vapor pressure at sample temperature (mm of manometric oil).

The VPM method is rapid and precise but requires the use of vacuum pump, an accurate manometer and closed glass tube system. Proper temperature control is critical to this method, and volatile constituents other than water may contribute to the pressure exerted by the food.

The VPM method has been used to obtain the MSI of cereal grains and rape [61], dry milk [62], sesame seed [63], cowpea [64] and palm kernels [65].

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

### 3.4 Inverse gas chromatography

The inverse gas chromatography (IGC) is a rapid and effective system for studying the thermodynamic properties of a solid taken as the stationary phase in relation to a mobile gas phase containing selected solutes such as water. It is particularly suitable for the study of the lower region of water activity and for products with very low equilibrium moisture contents [66, 67]. With IGC the sorbed solute is injected into the carrier gas stream, and its linear transport is retarded owing to interaction with the product under study, which constitutes the stationary phase. Moisture sorption isotherms are then determined using the chromatographic data obtained and the following equations, which relate chromatograph operating parameters and peak data to the sorption isotherm:

$$\mathbf{a} = \frac{\mathbf{m}\_{\mathbf{a}} \mathbf{I}\_{\text{ads}}}{\mathbf{m} \mathbf{I}\_{\text{pic}}} \tag{2}$$

and

v. The stopcock across the manometer V5 is closed causing the oil in the micromanometer to respond to the vapor pressure exerted by the sample. When the oil level reaches a steady value, the difference is

Schematic diagram of vapor pressure manometric system set-up. Source: Ajibola et al. [65].

vi. The stopcock over the sample is then closed, and the desiccant stopcock is opened to connect the system with the desiccant, causing a change in the height of the manometric oil. After the oil reaches a constant height, the

viii.With the data obtained, the equilibrium relative humidity is calculated using

ð Þ H<sup>1</sup> � H<sup>2</sup>

where ERH is the equilibrium relative humidity (%), H1 is the micromanometer reading with sample flask connected to the system (mm of manometric oil), H2 is the micromanometer reading with desiccant flask connected to the system (mm of manometric oil), Ts is the temperature of the environment surrounding the water bath taken as the temperature of sample (K), To is the temperature of the environment surrounding the micromanometer (K) and Ps is the saturated vapor pressure

The VPM method is rapid and precise but requires the use of vacuum pump, an accurate manometer and closed glass tube system. Proper temperature control is critical to this method, and volatile constituents other than water may contribute to

The VPM method has been used to obtain the MSI of cereal grains and rape [61],

Ps

Ts To 

(1)

vii. The sample is removed from the system, and the moisture content is

ERH ¼

dry milk [62], sesame seed [63], cowpea [64] and palm kernels [65].

micromanometer reading is recorded as H2.

determined using a standard method.

at sample temperature (mm of manometric oil).

the pressure exerted by the food.

152

recorded as H1.

Figure 6.

Sorption in 2020s

Eq. (1):

$$\mathbf{p} = \frac{\mathbf{m\_{\text{a}}} \mathbf{h} \mathbf{R} \mathbf{T}}{\mathbf{I\_{\text{pic}}} \mathbf{W}} \tag{3}$$

where a is the uptake of sorbed water (g/g stationary phase), ma is the mass of water injected (g), m is the mass of stationary phase (g) and Iads/Ipic is the ratio of the areas (A + B)/B calculated from the chromatogram (Figure 7), p is the partial pressure (atm), h is the peak height (detector units), R is the gas constant (82.0567 cm3 atm mole<sup>1</sup> K�<sup>l</sup> ), T is the absolute temperature (K), W is the flow rate of carrier gas (cm3 /min) and Ipic is the area B in Figure 7.

It has been used successfully to determine the MSIs of homogeneous solid food ingredients like sucrose, glucose and starch [68] and complex heterogeneous foods like bakery products [69], wheat flour [66] and wheat and soy flour [70].

Figure 7.

Typical gas chromatogram obtained by IGC: 1 = point of injection; 2 = point of emergence of unadsorbed peak (air); 3 = point of emergence of probe peak (water), Ipic = area B; and Iads = area A + B. Source: Manuel Sa and Sereno [67].

#### Figure 8.

4TE model AquaLab moisture content—water activity measuring instrument. Source: METER Group, Inc. [71].

#### 3.5 AquaLab instrument

AquaLab is the fastest, most accurate and most reliable instrument available for measuring water activity, giving readings in 5 min or less [71]. It is easy to use and provides accurate and timely results. Its readings are reliable, providing 0.003 aw accuracy. The instrument is easy to clean and checking calibration is simple. The photograph of 4TE model of the equipment is shown in Figure 8.
