FTec 143 Food Engineering 2 (Module 1 Lesson 1) PDF

FTec 143 Food Engineering 2 Module 1 Lesson 1 Food Evaporation: Concepts and Theories Ayrton John V. Bantay Faculty, Department of Mechanical Engineering Affiliate Faculty, Department of Food Science and Te...

FTec 143 Food Engineering 2 Module 1 Lesson 1 Food Evaporation: Concepts and Theories Ayrton John V. Bantay Faculty, Department of Mechanical Engineering Affiliate Faculty, Department of Food Science and Technology Lesson Outcomes At the end of the lesson, the students will be able to: 1. Describe and explain food evaporation process; 2. Enumerate food properties that are affected during evaporation process; 3. Describe the occurrence of boiling point elevation, 4. Solve problems involving calculation of the changes of the physical properties of food and steam during evaporation; and 5. Interpret the Dühring chart, steam table, and Mollier’s diagram. Introduction We can view food material as entity that made of solid and non-solid components. The non-solid components of food include water. Water is an essential part of any living things to live. The amount of water in food could hinder or enhance microbial growth that would eventually leads to spoilage. The separation of water from food can be done by several means like drying, centrifugation, filtration and evaporation. Filtration and centrifugation basically separates suspended solids from the solvent component using physical force. On the other hand, drying and evaporation remove moisture in vapor form from food material using heat. Introduction Evaporation takes place in many food processes like drying, baking, cooking, and more. However; we will define evaporation as a unit operation employed to separate water from a dilute solution, with or without the presence of suspended solids, to obtain a concentrated product. The removal of moisture from food is done by introducing energy, such as heat, that will boil the water into steam separating it from the food. Concentrated products include evaporated milk, condensed milk, pastes, jams, jellies, and more. Introduction There many ways to evaporate water from foods and the most common method is atmospheric heating. This is because atmospheric heating can provide ample energy to evaporate water without employing any complex system. However, this advantage also poses potential problems to foods since this system is poorly controlled and inefficient in supplying energy. Factors to Consider in Food Evaporation Foods being diverse and sensitive entities respond to heat energy differently. There are heat sensitive foods that will produce off-flavor, discoloration, and loss of its nutritive and functional qualities when subjected to relatively extreme heat. Thus, processors should be aware on the types of food they are handling and how to process them. There are few considerations to check before evaporating feed material, these are: 1. Maximum allowable temperature for processing – foods are generally sensitive to extremely high temperature. Foods that are sensitive to heat should be process using low temperature process. Knowledge in thermal processing helps processor to attain food safety without destroying food quality. Factors to Consider in Food Evaporation 2. Food composition – Food components would include, water, sugar, protein, vitamins, and more. Sugar in food can undergo caramelization when exposed to extreme heat. Milk protein coagulates in similar manner when exposed to heat. In addition, Functional properties like vitamins and antioxidant will be denatured when exposed to high heat. These are just few scenarios food will underwent when their components are poorly judged; therefore, they should be accounted during the design of evaporation process and equipment. Factors to Consider in Food Evaporation 3. Source of heat energy – Direct fire or steam are the two most common means of evaporating water. Household scale processing use direct fire to evaporate water while industrial processing uses steam. The advantage of fire is that it is almost a pure energy, at 600oC or higher, that it heats up the food material almost immediately but posing inherent problems like uneven heating, burning, and even destruction of food. Use of direct fire is necessary in creating aromatic flavors especially when caramelization and roasting is required. On the other hand, steam is a convective heat that its energy content can be controlled systematically. It can be use for both heat sensitive and non-sensitive foods. Other sources of energy for evaporation are refrigerants. Steam Properties Steam is the basic source of energy in food processing. This is because water is abundant, non-toxic, non-corrosive, high latent energy content, stable, recyclable, sustainable, and cheap. Steam can be manipulated so as to provide just the right energy required for food processing. The quality of steam is dependent to the temperature and pressure provided during boiling process. Steam can be classified as wet steam, saturated steam and superheated steam. At any given pressure or temperature, superheated steam contains the most energy seconded by saturated steam. The energy of steam is called steam enthalpy. Steam Properties P-v-T diagram of water (Cengel and Boles, 2008) Steam Properties The behavior of how water properties changes are summarized in Figure 27. Water from food evaporates when the vapor pressure of water exceeds the atmospheric pressure. In doing so, water needs an ample supply of heat to increase its temperature and transform into its gaseous form – the steam. The energy required to heat up the product is called sensible heat while the energy needed to boil it is called latent heat (vaporization). Sensible heating at constant pressure (say atmospheric) starts from anywhere between points B and C of Figure 28 until it reach its boiling temperature at C. Line ACE is the saturation region which is the totality of saturated liquid line, wet-steam region, and saturated vapor line (Figure 29). Steam Properties The behavior of how water properties changes are summarized in Figure 27. Water from food evaporates when the vapor pressure of water exceeds the atmospheric pressure. In doing so, water needs an ample supply of heat to increase its temperature and transform into its gaseous form – the steam. The energy required to heat up the product is called sensible heat while the energy needed to boil it is called latent heat (vaporization). Sensible heating at constant pressure (say atmospheric) starts from anywhere between points B and C of Figure 28 until it reach its boiling temperature at C. Line ACE is the saturation region which is the totality of saturated liquid line, wet-steam region, and saturated vapor line (Figure 29). Steam Properties Steam Properties Wet steam is any steam found under the liquid vapor region (Figure 29). By definition, wet steam is any steam that has quality (or moisture, y) greater than zero but less than 100%. Saturated steam is any steam with a quality of 100% (moisture = 0%). It is any steam found from the critical point down along the saturated vapor line (Figure 29). Steam quality refers to the fraction of water that completely turns into vapor while moisture is the remaining water in liquid form that is found in the steam. When the quality of the steam reached 100% it becomes saturated steam. The sum of quality (x) and moisture (y) is the total mass of steam: 𝑥+𝑦 =1 Steam Properties The properties of steam has long been studied and are tabulated and charted as standard reference materials. Use of chart like the Mollier’s Diagram can give you any steam properties at any given two thermodynamics properties. Wet steam enthalpy (h) can be computed as: ℎ = ℎ𝑓 + 𝑥ℎ𝑓𝑔 ℎ = ℎ𝑓 + 𝑥 ℎ𝑔 − ℎ𝑓 ℎ = ℎ𝑔 − 𝑦 ℎ𝑔 − ℎ𝑓 Where: hf = enthalpy of saturated liquid hg = enthalpy of saturated vapor hfg = latent heat of vaporization Steam Properties Superheated steam contains more energy than both wet steam and saturated steam. Steam is said to be superheated when it actual temperature is higher compared to its saturation temperature at constant pressure. The difference between these temperatures is called degree superheat, oSH. There many tables for the superheated properties of steam but the use Moiller’s diagram is more convenient since it reduce the number of interpolation works. Example Find the enthalpy of the steam with 93% quality at 142oC. Changes in Food Properties Foods are compounds made of heterogeneous aggregates of substances – may it be pure substance or not. Evaporation permits the removal of moisture from food but with minimal unwanted changes to its sensory and nutritional characteristics. As the moisture content of food decreases its thermal properties also changes. There are two important food properties that have to be accounted during evaporation calculations. These are the specific heat capacity, boiling point, viscosity, thermal conductivity, and microbial growth in food. Changes in Food Properties Specific Heat Capacity Specific heat is the energy required to raise the thermal state of a product. Pure water has an average specific heat of 4.187kJ/kg-K at atmospheric condition. The specific heat of food is the cumulative specific heat of all components: 𝐶𝑝 = ෍ 𝑚𝑖 𝑐𝑝𝑖 Where mi and Cpi are the mass fraction and specific heat of the ith component, respectively. Changes in Food Properties Specific Heat Capacity In general, among naturally occurring substances, water has the biggest specific heat. Therefore, as evaporation continues the specific heat of the concentrate drops together with its declining moisture content. The change in specific heat of the product being evaporated is given emphasis, especially in energy requirement calculation, in processes wherein heating and evaporation take place simultaneously like that in batch-type evaporation. The energy given to a process with the assumption that the feed has constant specific heat may lead to over-cooking or waste of energy. On the other hand, under-process product may be produced when the end- product specific heat will be used. To compensate the change in specific heat, the average of both the feed and product’s specific heat can be a good measure of the lumped specific heat for energy requirement calculation. Example Apple juice with 5% solids has a specific heat of 4.01 kJ/kg-K and after an evaporation process its solid content becomes 28%. Determine the specific heat of the concentrated apple juice. Solution: Solution Boiling Point Elevation Boiling point is the temperature of the liquid wherein its vapor pressure becomes greater than the overlying atmospheric pressure. Boiling point of the liquid product varies than that of pure water. The presence of solute tends to increase the boiling point of a liquid due to changes in its chemical potential. The increase in boiling point or boiling point elevation is a colligative property; thus, governed by defined equations (Berry et al., 1980; Heldman & Lund, 2007): Boiling Point Elevation Water activity and moisture content have very complex relationship models with different products. May authors tried to use molality as an estimate of water activity in computing boiling point elevation although have less accuracy at higher concentration. Thus, assuming water activity is equal to mole fraction in cases like milk and sugar solution: Boiling Point Elevation Boiling Point Elevation The complexity of determining the boiling point of concentrated solution becomes easier using the Dühring Chart. Dühring Charts are experimental plot the boiling points of a variety of solutions at different concentrations relative to the boiling point of water. An example of these plots is found in figure for salt (NaCl) solution. Boiling Point Elevation Dühring chart for NaCl solution Example Find the boiling point of 10% Sodium Chloride solution subject to a vacuum of -10 psi. Solution: Solution Solution Solution Solution The new boiling point estimated is 73.0oC. Viscosity Viscosity or fluid consistency is also affected during evaporating process. Water being removed from the solution concentrated the amount of solute packing them together creating a thicker solution. Excessive thickening of the solution leads to increase in paddling/pumping power and decrease in heat transfer efficiency. Arrhenius model of viscosity has been discussed previously in Food Engineering 1. The model suggests that heating a product decreases its viscosity. Continuous type evaporator therefore experiences thinning of product during heating process which can make pumping easier. However, batch type evaporators experience thickening during heating process because heating and evaporation are present simultaneously. Thus, these set-ups require the need of mixing paddles. Viscosity In practice, for Newtonian liquids like fruit juices and highly diluted solutions a viscosity range of 0.5 – 50.0 mPa-s is allowed inside the calandria (this term will be discussed later in evaporator parts). Working viscosity range of viscous Newtonian and Non-Newtonian liquids have complex viscosity range depending on their effective specific volume of solute and are included in this module. Density Unlike viscosity, density of concentrated solution may decrease or increase. If the suspended solids have densities greater that water then the concentrate becomes denser and vice versa. This can be explained by looking at equation: 1 𝑤𝑖 =෍ 𝜌 𝜌𝑖 Say a solution with water and suspended solid, equation can be reduced to: 1 𝑤𝑤 𝑤𝑠 = + 𝜌 𝜌𝑤 𝜌𝑠 𝜌𝑤 𝜌𝑖 𝜌= 𝑤𝑤 𝜌𝑖 + 1 − 𝑤𝑤 𝜌𝑤 Density The change in product density is also apparent when during the heating process. Water for instance has density varying from 960 kg/m3 to 1000 kg/m3 at temperature range of 100oC to 5oC. Several researchers correlated different food densities at different temperatures. Milk density for instance has been studied by Pisecky in 1997 according to components like water, fat, and nonfat solids (Heldman & Lund, 2007): Density The change in product density is also apparent when during the heating process. Water for instance has density varying from 960 kg/m3 to 1000 kg/m3 at temperature range of 100oC to 5oC. Several researchers correlated different food densities at different temperatures. Milk density for instance has been studied by Pisecky in 1997 according to components like water, fat, and nonfat solids (Heldman & Lund, 2007): Thermal Conductivity The change on the thermal conductivity of the product affects directly the heat transfer efficiency of evaporation system. The increase of feed temperature and decrease in moisture content as evaporation progress drastically affects the thermal conductivity of each and every individual components of food. For continuous type evaporators, where boiling is not allowed inside the calandria, increase in temperature is the main cause of the changes of the thermal conductivity of the product. Most food macro molecules such as protein and carbohydrates together with water and minerals (ash) have their thermal conductivity increase with temperature except for fat. Thermal Conductivity Thermal conductivity of selected food macro molecules at different temperatures  God Bless you… Thank you… The first virtue in a soldier is endurance of fatigue; courage is only the second virtue. ~Napoleon Bonaparte