heat units

 

Heat Units

1. Specific Heat of Fish

- *Definition*: Specific heat is the quantity of heat required to change the temperature of 1g of substance by 1°C.

- *Factors Affecting Specific Heat*: Chemical composition, temperature

- *Formula*: Cf = Cw x W + CL x L + Cm x M

- *Specific Heat Values*:

    - Water: 4.18 KJ/kg°C or 1 kcal/kg°C

    - Fat: 2.075 KJ/kg°C or 0.5 kcal/kg°C

    - Dry matter: 1.505 KJ/kg°C or 0.36 kcal/kg°C

- *Key Points*:

    1. Specific heat depends on chemical composition and temperature.

    2. Specific heat values decrease as fish gets cooler.

    3. Frozen fish requires less heat to change temperature.

2. Thermal Conductivity

- *Definition*: Thermal conductivity is the rate at which heat flows through a material.

- *Factors Affecting Thermal Conductivity*: Chemical composition, physical structure, specific gravity, specific heat, temperature

- *Importance in Fish*: Thermal conductivity determines the rate of chilling, freezing, and thawing in fish.

- *Effect of Temperature on Thermal Conductivity*: Increases as temperature decreases

- *Calculation of Thermal Conductivity*: lf = lo x XW + ln x N

- *Key Points*:

    1. Thermal conductivity determines the rate of heat transfer in fish.

    2. Thermal conductivity is affected by temperature, chemical composition, and physical structure.

    3. Thermal conductivity of ice is higher than that of water.

3. Thermal Diffusivity

- *Definition*: Thermal diffusivity is the rate at which temperature changes occur within a substance.

- *Importance*: Characterizes the rate of temperature equalization in an unevenly heated material.

- *Formula*: α = λ / (cν)

- *Factors Affecting Thermal Diffusivity*: Directly proportional to thermal conductivity, inversely proportional to specific heat and density

- *Thermal Diffusivity Values*:

    - Unfrozen fish: 0.000375 to 0.000475 m²/hr

    - Frozen fish: 3.75×10⁻⁴ to 15×10⁻⁴ m²/hr

- *Key Points*:

    1. Thermal diffusivity characterizes the rate of temperature change.

    2. It depends on thermal conductivity, specific heat, and density.

    3. Thermal diffusivity values differ between unfrozen and frozen fish.

4. Latent Heat

- *Definition*: Latent heat is the quantity of heat required to change the state or phase of a substance without changing its temperature.

- *Example*: Heat required to change ice at 0°C to water at 0°C.

- *Latent Heat of Fusion*: 79.7 cal/g or 80 cal/g (for water/ice)

- *Units*: cal/g, kcal/kg, BTU/lb

- *Calculation of Latent Heat*: Latent Heat = (80 x % water in food) / 100

- *Key Points*:

    1. Latent heat changes the state of a substance without changing its temperature.

    2. Latent heat of fusion is the heat required to change a substance from solid to liquid.

    3. Latent heat can be calculated if the water content of a food is known.