3.0. Fundamentals of drying
Drying is an old and vital preservation technique that entails reducing the moisture content of food to a level that constrains microbial development and also fosters for the minimization of the rate at which chemical reactions take place. Being a physicochemical and thermophysical operation that allows for removal of excess moisture, water activity in the food is fundamentally reduced. Importantly, before drying food, it is crucial to comprehend the structure of the food material, characteristics of the air utilized, the method for moisture migration, the dryer used, and the drying method. These considerations are vital parameters that determine the methodology that should be used. Additionally, inadequate drying is detrimental to the preservation of food in that it only deteriorates the food quality and lowers the profit margin from trading the products. This implies that selecting a proper dryer and a drying technique are the most vital elements for achieving a commercially viable and feasible food product. Also, characteristics of air, equipment, as well as products are also fundamental considerations to attain effective drying.
The principle of mass and heat transfer usually governs the drying process. Moist food is heated to an appropriate temperature by convection or conduction using external drying medium by radiations, steam, or hot air. The moisture is then vaporized and diffuses into the environment. The removal of moisture is done by either vaporizing moisture from the material's surface or the movement of the moisture from the food's interior to its surface owing to diffusion, vapor pressure gradient, and cell contraction. Also, drying processes can be categorized according to the method used for heat transfer or removal of water vapor. These are:
- Convective or contact drying: The material is in direct contact with the drying moisture, and air from it directly evaporates into the surrounding air. Over 85% of the industrial dryers utilize this method.
- Vacuum drying: The method involves drying the material under vacuum at much lower temperatures in comparison to contact drying. The heat is added indirectly by radiation or conduction.
- Freeze drying: It entails removing water through sublimation. Appropriate pressure and temperature settings are maintained in the dryer for sublimation to occur.
From a mass transport point of view, drying is considered as a process that entails diffusion. In the first period of drying, the water contained in the food material evaporates from the material surface to the air. After a moisture gradient is created, in the food material, second drying commences. The process involves transporting water inside the food material to the surface, which is referred to as the internal diffusion.
3.0.1. Basic Terminologies
Moisture in food play a vital role in maintaining food quality, and this implies that information about the moisture content is vital in deciding for food storage safety. Wet materials can be designated as binary mixtures of a dry solid material and water. The water concentration is expressed as the relative mass fraction of the liquid, which is simplified by the equation below:
XA = mA / mC,
mA is considered as the mass of water while mC is the mass of dry material. As such, the equilibrium moisture content is dependent on air condition, as well as the drying properties of the material. Additionally, moisture content is usually defined as the amount of water present in the food material as a percentage. On wet basis (Xwb), moisture content is expressed as a ratio of the weight of water that is present in food to the total food weight, which is defined by the following equations.
Based on a dry basis, the content of the moisture is primarily expressed as a ratio of the weight of water that is present in the food product to that of dry matter, as defined in the equation below:
The relationship between Xwb and Xdb is defined in the equations below.
In addition, water in food materials is present in three distinct forms, which are unbound, bound, and free water. Unbound water is food's moisture which exerts vapor pressure equal to that of the saturated vapor pressure of pure water in the same temperature. Bound water refers to the content of moisture that primarily exerts a vapor pressure that is deemed to be less than that of pure water and at a similar temperature owing to retention in minor pores, as well as solutions present in cell walls. On the other and, free water refers to the moisture content that is more than the equilibrium moisture content. It can be removed easily by drying under given conditions of humidity and temperature. In essence, free water includes both unbound water and some of the bound water, which is part of the larger capillaries.
3.0.2. Psychrometric Consideration
Most air, which is a mixture of water vapor and dry air, is utilized as a medium in various unit operations including mixing, heating, cooling, and the processes of transport. Additionally, the thermodynamic properties that are shown by moist air are usually referred to as psychormetric properties, and the subject that is involved in studying dry air-water vapor mixture properties is referred to as psychrometry.
18.104.22.168. Dry bulb temperature (Tdb)
One of the properties is dry bulb temperature (Tdb), which is referred to as the temperature of air that is determined to utilize a thermometer.
22.214.171.124. Wet bulb temperature (Twb)
Wet bulb temperature is the temperature that is measured by a thermometer with a bulb wrapped in a wet cloth. The difference between a wet bulb and dry bulb temperatures is referred to as the wet depression. However, the wet bulb temperature (Twb) is usually lower compared to the dry bulb temperature. Additionally, at a 100% relative humidity, both dry and wet bulb temperatures are equivalent, which implies that the wet bulb depression is therefore zero. The wet bulb temperature is considered as an indirect measure of air dryness.
126.96.36.199. Dewpoint temperature (Tdp)
Another property worth consideration is the dew point temperature. The dew point temperature (Tdp) is referred to as the maximum temperature at which air becomes completely saturated with moisture in the cooling process at constant pressure and pressure. Also, further cooling below the dew point temperature often results in the condensation of moisture that is present in the air, which consequently lowers the humidity ration.
188.8.131.52. Absolute humidity (H)
The absolute humidity (H) psychrometric property is also referred to like the humidity ratio of specific humidity, which is defined as mass the of water vapor that is associated with a unit mass of dry air. Even though it is a dimensionless quantity, the units are often quoted as the kilogram of water vapor per kilogram of dry air, which can be obtained using the equation below.
184.108.40.206. Relative humidity (RH)
Additionally, the relative humidity (RH) is defined as the ratio of water vapor (Mv) that is present in moist air to the total mass of water vapor (Mvs) required for saturating it at the same temperature. Similarly, it can be defined as the partial pressure ratio to that of water vapor in the air-vapour mixture to that of the pressure of water vapor of saturated air at the same temperature. It is usually denoted as RH and is expressed using a percentage, as shown in the equation below.
220.127.116.11. Saturation humidity (Hs)
It is the water holding capacity of air at a particular temperature that increases with the increase in temperature. It can also be referred a the maximum quantity of water which air can hold before the water vapor starts condensing back to the original liquid water as a temperature that is constant. The saturation humidity is calculated using equation below.
18.104.22.168 Humid heat (Cp)
The humid heat capacity is a psychrometric property that denotes the heat capacity of moist air. It is usually referred to as the amount of heat that is required in raising the temperature of a kilogram of air along with its associated water vapor by one degree Celsius at a constant pressure. It is estimated using the equation provided below.
22.214.171.124 Humid volume (VH)
The humid volume, which is also referred to as the specific volume of air, is defined as the volume that is occupied by a unit mass of dry air and its associated water vapour. Importantly, it should be noted that humid air is inversely proportional to its density. It is obtained using the equation below.
126.96.36.199. Psychrometric chart
The chart is a described as the graphical representation of the thermodynamic properties of air. Essentially, the properties of air and water vapour mixture is useful in a number of processes. As such, the psychrometric chart is very vital in determining psychrometric properties without the need of calculating the same utilizing the previous equations. Essentially, developing and using equation to determine the psychrometric properties is usually inconveniencing and tedious, which justifies the use of the chart. To avoid the calculations, the psychrometric chart is helpful. It is basically a plot between the humidity ratio (it is the ordinate in the graph) and the dry bulb temperature (it is the abscissa of the graph) with a series of curves that represent values of the percentages of relative humidity superimposed together. It is usually constructed for standard atmosphere pressure, which is 1.01325, and corresponds to the pressure at mean sea level. An example of a psychrometric chart is shown in Figure 1 below, which highlights that knowing two values of air properties usually helps in finding the remaining properties directly using the chart.
3.0.3. Drying Processes
The drying processes is usually divided into three parts, the warm-up period, first drying period, and second drying period.
The material that needs drying is first heated from the initial temperature tp to the wet bulb temperature. The moisture then starts to evaporate from the surface. The mass and heat transfer driving forces are usually non zero. The concentration of water decreases. The process then runs on a drying curve from point A to point B [XA = XA( t ) ]. Essentially, drying of water from the surface slows down the heating of the material as the heat from the air is consumed during the process of evaporation. However, the warm-up period should be relatively short and in some instances, it can be non-catchable.
First drying period.
It is a process characterized of the constant-rate period of drying and begins from point B. At this particular temperature, the material is equal to the wet temperature and usually stays constant, and thus, the added heat is consumed via evaporation of the free water. The drying force and drying rate fA are usually constant. The vapor pressure on the material surface during the constant-rate period is usually equal to the pressure of water vapor measured on the water that is clean and in a similar temperature tw. Essentially, at point C, the moisture content is usually equal to the critical moisture content XAc. In the constant-rate period, the rate of drying usually is dependent on the moisture content, temperature, as well as the mass flow of air. However, it is vital to note that the drying rate is not a function of actual moisture content and bed depth.
Second drying period.
In point C, the water starts percolating, and the material's surface then dries up, and consequently, the first material particles get air contact. The rate of drying starts decreasing, and the temperature of the material starts to rise at point D, which causes a decrease in mass and heat transfer driving forces. The path of the evaporated water vapor is more complicat...
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