Food Process Engineering Drying (Theories and Principles) PDF

ABEN 4510 FOOD PROCESS ENGINEERING DRYING (Theories and Principles) MAY ALISBO-CABRAL Department of Agricultural and Biosystems Engineering College of Engineering Central Luzon State University...

ABEN 4510 FOOD PROCESS ENGINEERING DRYING (Theories and Principles) MAY ALISBO-CABRAL Department of Agricultural and Biosystems Engineering College of Engineering Central Luzon State University 1 Learning Outcome Explain the theories and principle of drying Discuss the trends in drying of AB materials TOPIC OUTLINE 1. Theories and principles of drying 2. Trends in drying AB Materials INTRODUCTION SUNDRYING is the oldest method of dehydration conducted by man However, there are problems associated with sundrying such as susceptibility to contamination, subsequent spoilage as the products is exposed to the elements, tedious and long process compare to modern method. In addition, it requires a very large area of operation and consider as unreliable procedure due to unpredictable weather condition in the country. INTRODUCTION FOOD DRYING is method of food preservation in which food is dried (dehydrated or desiccated) and considered as one of the oldest method of food preservation by the mankind. Drying has the capability to inhibits the growth of bacteria, yeasts and mold through the removal of water. Water is can be traditionally removed through evaporation through air drying, sun drying, smoking or wind drying DRYING removal of moisture unfavorable environment for yeast, molds and bacteria is usually desired to achieve the dehydrated state of a given product in the shortest possible to minimize the chances of spoilage by microbial action and man output drying of foods is often accompanied by changes in the chemical, physical and biological characteristics of the product THEORY OF DRYING mechanism of moisture migration from wet solid to the surrounding medium support the vapor pressure theory The theory hypothesizes that, as the temperature of the product increases, while the moisture content is constant, the water vapor pressure of the product also increases The rate of mass transfer is directly proportional to the vapor pressure difference between the product surface and the absorbing medium States of Water Pure water can exist in three states, solid, liquid and vapour. The state in which it is at any time depends on the temperature and pressure conditions 1. Equilibrium moisture content a hygroscopic material held wet under constant air temperature and humidity will eventually reach a constant equilibrium moisture content, dry EMC, the value of which depends on the temperature and water vapor pressure of the surrounding air Figure shows a plot of a typical sorption isotherm showing the relationship between the relative humidity and the equilibrium moisture content of a product under a constant temperature. Sorption isotherm models Henderson equation Halsey equation Aw = 1 - exp (-KT M") −𝐾1 𝑀 𝐾2 𝐴𝑤 = 𝑒𝑥𝑝 𝑟𝑝 𝑇 𝑀1 where: where: Aw = water activity Aw = water activity K,n = empirical constants K1, K2 = empirical constants T = absolute temperature rp = pore radius M = equilibrium moisture T = absolute temperature M=equilibrium moisture M1 = monolayer moisture Sorption isotherm models In order to estimate the monolayer value, M1, in this equation, the B.E.T. equation is employed. 𝐴𝑤 1 𝐴𝑤(𝐶−1) = + 1−𝐴𝑤 𝑀 𝑀1𝐶 𝑀1𝐶 where: Aw = activity , Pv/Ps M = adsorbed moisture , gm/gm solid M1 = monolayer value, gm/gm solid C =kexp(Q1/RT) K = constant (approximately 1) Q1 = heat of adsorption of monolayer R = gas constant (8.3144J/mole-K) T = absolute temperature, K Sorption isotherm may be developed by equilibrating a given food sample at different relative humidities inside sealed jars with known concentrations of salts, sugars or acids Figure shows a plot of sulfuric acid concentration against the relative humidity that can be achieved when the acid is allowed to equilibrate with the headspace inside its container Sorption isotherm Salts and acids tend to lower the water activity of solutions and thus affect the water activity of the gas phase immediately above it. At higher acid concentrations, lower water activity and hence lower relative humidity is Figure shows a simple but reasonably reached accurate set up to determine the equilibrium moisture of small town samples 2. DRYING STAGES Point A - initial condition of the food material at the time it is exposed to the drying medium. The product adjusts to the drying conditions until it reaches Point B. If the commodity contains a lot of free moisture, point B is followed by a constant drying rate period (CRP) represented by B-C. At the end of this stage, the food reaches its critical moisture (CM) at point C and is immediately followed by the falling rate period (FRP) represented as C-D. After an extended period of drying, the product will reach a constant weight represented by point E. A typical drying curve of foods At this condition the product will be at its equilibrium moisture content (EMC). The drying data may be analyzed to produce a plot of drying rate versus time  Point A : initial condition of the food material Point B: Adjustment of product Point B-C :constant drying rate period (CRP) represented by B-C Point C : Critical Moisture Point C-D :falling rate period (FRP). Point E - constant weight ; equilibrium A typical drying curve of foods moisture content (EMC). DRYING RATE AGAINST DRYING TIME Drying data may be analyzed to produce a plot of the drying rate versus The occurrence and duration of the different stages of drying have been linked to the physical structure of the material and the way in which the liquid phase associated with the solid components of the product. A typical plot of drying rates against drying time CLASSIFICATION OF FOOD MATERIALS ACCORDING TO DRYING CHARACTERISTICS 1. liquid solutions and gels, such as milk, fruit, juices and gelatinized products; 2. capillary-porous rigid materials, like grains, 3. capillary-porous colloidal solids which include fish and meat muscles  CONSTANT DRYING RATE PERIOD During the CRP, the moisture at the surface of the material assumes the state of free surface water so that the rate of evaporation is primarily dependent on the drying air conditions During the period of constant rate, the drying rate of a given piece of material is controlled solely by the conditions of the surrounding Source: shorturl.at/buPRT medium and the material is equal to the rate of moisture loss from a water saturated surface of the same size and shape. CONSTANT DRYING RATE PERIOD The following equation is used to calculate the heat flux, q, towards the evaporating surface. This supplies the necessary heat of vaporization q = hs A (Tinf − To) hs = surface heat transfer coefficient, W/m2-°C A = surface area of evaporation, m2 Tinf = medium temperature, °C To = surface temperature, °C CONSTANT DRYING RATE PERIOD For a constant wet-bulb process, the mass flux may be associated with the heat flux is 𝑑𝑀𝑣 ℎ𝑠𝐴(𝑇𝑖𝑛𝑓 −𝑇𝑜 ) = 𝑑𝑡 𝐿 where: L = latent heat of vaporization The termination of the CRP and the onset of the FRP is marked by the "critical" moisture content. At this point the hygroscopic material has reached the minimum moisture content that can sustain a uniform flow rate of evaporation from the surface. The end of the CRP has also been associated with the decrease in the effective surface area of evaporation and the corresponding increase in the "dry patch" area. FALLING RATE PERIOD As the rate of moisture diffusion within the product decreases and is needed to replenish the moisture at the surface, the rate of drying also decreases and the process enters the falling rate period (FRP). Source: shorturl.at/buPRT FALLING RATE PERIOD The FRP is greatly influenced by the movement of moisture within the material. It is generally accepted that liquid movement in a solid follows the Fickian model as shown in equation 𝑑𝑀 = 𝐷∆𝐶 Source: shorturl.at/buPRT 𝑑𝑡 where: D = diffusion coefficient ∆𝐶 = concentration gradient Moisture independent diffusivity The Fickian equation can be solved analytically as described by Jason (1958). The following assumption to be made: a) the food material is isotropic; b) the effective diffusion coefficient, De, is independent of concentration; c) De does not change with time; and d) De is unaffected by shrinkage.  Moisture independent diffusivity The final form of equation obtained when equation is solved in three dimensions is 𝑘 2 ′ 𝑊 − 𝑊𝑒 8 𝜋 𝐷𝑥 𝐷𝑦 𝐷𝑧 = 2 𝑒𝑥𝑝 − 2 + 2+ 2 𝑊𝑜 − 𝑊𝑒 𝜋 4 𝑎 𝑏 𝑐 where: W = product weight at time, t We = product equilibrium weight Wo = product initial weight Dx,y,z = diffusion coeff. in the x, y, z direction a,b,c = block dimensions k = constant which can vary from 1 to 3 based on the dimensions of the slab Moisture independent diffusivity Another approach to the solution of the Fickian model is by using the Sherwood equation (Sherwood, 1931), which expresses the functional relationship between the variables X [(W- We /(Wo-We)] and t. From the linear portion of the X versus t plot the value of D from equation can be calculated. 𝐾𝑙 2 𝐷= 2 𝜋 A logarithmic plot of unaccomplished weight change against drying time Where: l = slab half thickness Moisture dependent diffusivity Some of the theories that have been applied to the study of moisture movement in foods are as follows: 1) liquid diffusion theory; 2) capillary theory; 3) evaporation-condensation theory; and 4) coupled heat and mass transfer theory. FACTORS AFFECTING DRYING RATES 1. RELATIVE HUMIDITY most important parameters that influences the rate of water vapor migration from the drying front to the drying medium the rate of drying increases with decreasing air relative humidity In the food industry, the relative humidity used ranges from 30 to 60 % Extremely low RH, while promoting very rapid initial drying rates, often leads to the formation of a dried crust (case hardening) which subsequently prevents the removal of moisture from the moist center. For example, heavy salt fish dried at RH below 40 % tended to case harden. The relative humidity also determines the limit to which a product may be dried (EMC). FACTORS AFFECTING DRYING RATES 2. DRY BULB TEMPERATURE In a constant pressure process, the increase in dry-bulb temperature of air results in the decrease of its relative humidity, hence, the increase in its capacity to dry products. Raising the dry-bulb temperature will also increase the vapor pressure at the drying front and facilitate faster drying. The effect of temperature on drying rates is less in comparison to the effect of RH. For food products, it is common to find drying temperatures ranging from 30 to 70°C, depending on the sensitivity of the product to elevated temperatures. FACTORS AFFECTING DRYING RATES 3.AIR VELOCITY the recommended air velocity ranges from 1 to 4 m/s across the product surface. increase in the air velocity tends to increase the rate of drying particularly during the constant rate period. it has been established that during the falling rate period when the moisture diffusion within the product is the limiting factor, the contribution of air velocity to the rate of drying is diminished. the manner of product exposure to the drying medium influences the rate of moisture removal. FACTORS AFFECTING DRYING RATES 3.AIR VELOCITY  Through-flow system - the hot air is forced through from bottom to top, or vice versa, of a drying tray  Cross-flow system the drying medium moves horizontally and parallel to the surface of drying trays. require that a layer of food on the tray be as thin as possible. On the other hand, cross-flow configuration may be used for deeper bed of mainly granular food materials.  For a given tray loading, a through-flow system would encourage faster drying rates as compared to a cross-flow system FACTORS AFFECTING DRYING RATES 4. TYPE OF PRODUCT SUPPORT The most popular means of supporting food products within the drying chamber is using perforated trays particularly in through-flow systems. When used in a cross-flow pattern, perforated bottom trays will allow drying on both sides of the product. Using trays with high thermal conductivity allows faster movement of heat from the drying medium to the product, hence, contributes to increased drying rate. Suspending food materials on skewers within the drying chamber also allows greater surface area of contact between the hot air and the product, thus, contributing to faster drying. The thickness of the product layer on the trays influences the rate of drying. It has been observed that while the drying time increases with tray loading, the drying time increases more slowly than the rate of increase in loading. FACTORS AFFECTINS DRYING RATES 5. PRE-DRYING TREATMENT pre-drying practices also influence the drying process mainly through the physio-chemical changes that they bring to the materials being dried. For example, the size and shape of cut can result in increased surface thereby increasing the drying rate. Salting, osmotic dehydration, blanching FACTORS AFFECTINS DRYING RATES 5. PRE-DRYING TREATMENT Salting will encourage osmotic dehydration that brings down the initial moisture of the product. Osmotic dehydration can also lead to the migration of salts or sugars into the product that will subsequently affect the rate of moisture removal from the material. Blanching is practiced mainly to arrest the browning due to enzymatic activities in plant materials. This procedure, however, can cause starchy granules, when present, to swell and some amount of soluble salts to leach out forming a gelatinous starch laver on the surface of the product. The gelatinous layer can later slow down the rate of drying. FACTORS AFFECTING DRYING RATES 5. NATURE OF PRODUCT Product composition is directly related to the nature by which moisture is held or bound in the solid component of foods. Solutes with high affinity for moisture, such as sugars, salts and soluble proteins, will tend to hold water and consequently reduce the rate of drying of the product. During the early stages of drying, soluble solids are carried to the surface by the migrating moisture. These can form into a crust which subsequently affect the rate of drying. REFERENCES Carpio, 2000. E.V. Engineering for Food Technologist. UPLB Publishing Center. Singh R. P and Heldman, D.R.. 2009. Introduction to Food Engineering Fourth Edition. Elsevier Inc. Retrieved at http://www.ucarecdn.com/fb7332e8-c35a-47b0-9805-051fa171f8fa/. Food Process Engineering and Technology. https://doi.org/10.1016/B978-0-12-812018-7.00003-8# 2018 Elsevier Inc. All rights reserved. Earl and Earl. UNIT OPERATIONS IN FOOD PROCESSING. Retrieved at Unit Operations in Food Processing - R. L. Earle  Thank you and God bless! Keep safe and stay healthy