Module 1 Thermal properties of food.pdf

Thermal Properties of Foods Module 1 Fisheries Engineering By the end of this module, you will have accomplished the following learning outcomes: Explain the different thermal properties of foods Differentiate the three modes of heat transfer Perform calculat...

Thermal Properties of Foods Module 1 Fisheries Engineering By the end of this module, you will have accomplished the following learning outcomes: Explain the different thermal properties of foods Differentiate the three modes of heat transfer Perform calculations on heat transfer Prepared by: Gardel Xyza S. Libunao College of Fisheries and Ocean Sciences, University of the Philippines Visayas Outline 1. Thermal Properties of foods 1.1. Specific Heat 1.2. Thermal Conductivity 1.3. Thermal Diffusivity 2. Modes of heat transfer 2.1. Conductive Heat transfer 2.2. Convective Heat transfer 2.3. Radiation Heat transfer Outline 1. Thermal Properties of foods 1.1. Specific Heat 1.2. Thermal Conductivity 1.3. Thermal Diffusivity 1.1. Specific Heat Essential part of the thermal analysis of food processing or of the equipment used in heating or cooling of foods How much energy must be supplied to the material or removed from a material in order to increase or decrease its temperature by a given amount? Important in the design of processes such as chilling, freezing, warming, sterilization and cooking 1.1. Specific Heat Apparent specific heat incorporates the heat involved in the change of state in addition to the sensible heat. For processes where a change of state takes place, such as freezing or thawing, an apparent specific heat (latent heat) is used Q (Heat energy) Specific latent heat (L) Mass (m) 1.1. Specific Heat Quantity of heat that is gained or lost by a unit mass of product to accomplish a unit change in temperature, without a change in state Function of the various components in food: moisture content, temperature, and pressure o Specific heat moisture content o Specific heat at constant pressure cp, since pressure is generally kept constant (except in high-pressure processing) 1.1. Specific Heat Function of the various components in food: moisture content, temperature, and pressure o Specific heat moisture content 1.1. Specific Heat In designing food processes and processing equipment, we need numerical values for the specific heat of the food and materials to be used 1. Published data and databases 2. To use a predictive equation 1.1. Specific Heat 1.1. Specific Heat In designing food processes and processing equipment, we need numerical values for the specific heat of the food and materials to be used 1. Published data and databases 2. To use a predictive equation 1.1. Specific Heat Predictive models of specific heat that include temperature dependence: where Xi is the fraction of the ith component, n is the total number of components in a food, and cpi is the specific heat of the ith component 1.1. Specific Heat Specific heat of pure food components as a function of temperature: 1.1. Specific Heat Example 1: Predict the specific heat for a model food with the following composition: carbohydrate 40%, protein 20%, fat 10%, ash 5%, moisture 25% temperature 20°C How to solve? 1. 1.1. Specific Heat Example 1: Predict the specific heat for a model food with the following composition: carbohydrate 40%, protein 20%, fat 10%, ash 5%, moisture 25% temperature 20°C How to solve? 1. 1.1. Specific Heat Example 1: Predict the specific heat for a model food with the following composition: carbohydrate 40%, protein 20%, fat 10%, ash 5%, moisture 25% temperature 20°C How to solve? 2. Include temperature a. Get cpi first – Table 2.9 eqns b. Multiply to given fraction Check Excel computation 1.2. Thermal conductivity Involves the rate of heat transfer The amount of heat that will be conducted per unit time through a unit thickness of the material if a unit temperature gradient exists across that thickness 1. Published data and databases 2. To use a predictive equation 1.2. Thermal conductivity 1.2. Thermal conductivity Involves the rate of heat transfer The amount of heat that will be conducted per unit time through a unit thickness of the material if a unit temperature gradient exists across that thickness 1. Published data and databases 2. To use a predictive equation 1.2. Thermal conductivity For fruits and vegetables with a water content greater than 60%: For meats and fish, temperature 0–60°C, water content 60–80%, wet basis: 1.2. Thermal conductivity Predictive models of thermal conductivity that include temperature dependence: 1.2. Thermal conductivity Density of pure food components as a function of temperature: 1.2. Thermal conductivity Thermal conductivity of pure food components as a function of temperature: 1.2. Thermal conductivity Example 2: Estimate the thermal conductivity of hamburger beef that contains 68.3% water at temperature 20°C How to solve? 1. 1.2. Thermal conductivity Example 2: Estimate the thermal conductivity of hamburger beef that contains 68.3% water at temperature 20°C How to solve? 1. 1.2. Thermal conductivity Example 2: Estimate the thermal conductivity of hamburger beef that contains 68.3% water at temperature 20°C How to solve? 2. Include temperature a. Get ρi first – Table 2.9 eqns b. Get Yi based on Xi and ρi values c. Get ki – Table 2.9 eqns d. Multiply ki and Yi Check excel computation 1.3. Thermal diffusivity Ratio of thermal conductivity, density, and specific heat 1.3. Thermal diffusivity Ratio of thermal conductivity, density, and specific heat Calculated by substituting values of thermal conductivity, density, and specific heat Predictive equation for temperature dependence: 1.3. Thermal diffusivity Thermal diffusivity of pure food components as a function of temperature: Outline 1. Thermal Properties of foods 1.1. Specific Heat 1.2. Thermal Conductivity 1.3. Thermal Diffusivity 2. Modes of heat transfer 2.1. Conductive Heat transfer 2.2. Convective Heat transfer 2.3. Radiation Heat transfer 2. Modes of Heat transfer Heat energy (thermal energy) = sensible + latent forms of internal energy. Heat content of an object is determined by: How does heat transfer from one object to another? Or within an object? 2. Modes of Heat transfer In canning – to thermally sterilize the product, the heat content is raised by transferring heat from heating medium (steam) into the contents of the can 2. Modes of Heat transfer To design the sterilization equipment, we need to know Q (J) → how much heat is necessary to raise the temperature of the content from initial to final temperature 2. Modes of Heat transfer To design the sterilization equipment, we also need to know q (J/s) → rate of heat transfer from steam. Two stages of heat transfer resistances: 1) across the can wall and 2) across the liquid-particle boundary within the can 2. Modes of Heat transfer 2.1. Conductive Heat transfer No physical movement of object undergoing heat transfer Common mode of heat transfer in heating/cooling of opaque solid materials Transfer of energy takes place at a molecular level by: 1. Vibrations of molecules of solid material 2. Drift of free electrons 2.1. Conductive Heat transfer Conductive heat flow in a wall 2.1. Conductive Heat transfer Conductive heat flow in a wall – rate of heat transfer (q) is expressed as: What is k here? 2.1. Conductive Heat transfer Conductive heat flow in a wall – rate of heat transfer (q) is expressed as: 2.1. Conductive Heat transfer Fourier’s law for heat conduction: heat will always conduct from higher temperature to lower temperature the gradient dT/dx is negative: temperature decreases with increasing values of x 2.1. Conductive Heat transfer Example 3: One face of a stainless-steel plate 1 cm thick is maintained at 110°C, and the other face is at 90°C. Assuming steady-state conditions, calculate the rate of heat transfer per unit area through the plate. The thermal conductivity of stainless steel is 17 W/(m °C). The area of the plate is 1m2. Given: Thickness of plate = 1 cm = 0.01 m Temperature of one face = 110°C Temperature of other face = 90°C Thermal conductivity of stainless steel = 17 W/(m °C) 2.1. Conductive Heat transfer Example 3: One face of a stainless-steel plate 1 cm thick is maintained at 110°C, and the other face is at 90°C. Assuming steady-state conditions, calculate the rate of heat transfer per unit area through the plate. The thermal conductivity of stainless steel is 17 W/(m °C). The area of the plate is 1m2. 2.1. Conductive Heat transfer Example 3: One face of a stainless-steel plate 1 cm thick is maintained at 110°C, and the other face is at 90°C. Assuming steady-state conditions, calculate the rate of heat transfer per unit area through the plate. The thermal conductivity of stainless steel is 17 W/(m °C). The area of the plate is 1m2. What is the answer if k = 17 W/(m K)? 2.2. Convective Heat transfer Liquid or gas that comes into contact with a solid body (container) → heat exchange will occur by convection Two types: 1. Forced - involves the use of some mechanical means, such as a pump or a fan, to induce movement of the fluid 2. Natural – occurs due to density differences caused by temperature gradients within the system 2.2. Convective Heat transfer Heat transfer from a heated flat plate, PQRS, exposed to flowing fluid Surface temperature of the plate (Ts) Temperature of the fluid far away from the plate surface is T∞ Rate of heat transfer (q) from the solid surface to the flowing fluid is proportional to the surface area of solid, A, in contact with the fluid h is the convective heat-transfer coefficient (surface heat-transfer coefficient) 2.2. Convective Heat transfer Heat transfer from a heated flat plate, PQRS, exposed to flowing fluid 2.2. Convective Heat transfer Example 4: The rate of heat transfer per unit area from a metal plate is 1000 W/m2. The surface temperature of the plate is 120°C, and ambient temperature is 20°C. Estimate the convective heat transfer coefficient. Given: Plate surface temperature 120°C Ambient temperature 20°C Rate of heat transfer per unit area 1000 W/m2 2.2. Convective Heat transfer Example 4: The rate of heat transfer per unit area from a metal plate is 1000 W/m2. The surface temperature of the plate is 120°C, and ambient temperature is 20°C. Estimate the convective heat transfer coefficient. Given: Plate surface temperature 120°C Ambient temperature 20°C Rate of heat transfer per unit area 1000 W/m2 2.3. Radiation Heat transfer occurs between two surfaces by the emission and later absorption of electromagnetic waves (or photons) requires no physical medium for its propagation solids are opaque to thermal radiation – analysis is concerned with the surface of the material All objects at a temperature above 0 Absolute emit thermal radiation expressed by the equation: 2.3. Radiation Heat transfer Example 5: Calculate the rate of heat energy emitted by 100 m 2 of a polished iron surface (emissivity = 0.06). The temperature of the surface is 37°C. Given Emissivity ε = 0.06 Area A = 100 m2 Temperature = 37°C = 310 K 2.3. Radiation Heat transfer Example 5: Calculate the rate of heat energy emitted by 100 m 2 of a polished iron surface (emissivity = 0.06). The temperature of the surface is 37°C. Given Emissivity ε = 0.06 Area A = 100 m2 Temperature = 37°C = 310 K Stefan-Boltzmann constant = 5.669 x 10-8 W/m2K4 Bonus Opportunity Individual: What are zero-, first- and second-order kinetics?