Thermal Food Processing

14 Thermal Food Processing: New Technologies and Quality Issues
Equation 1.13 gives a rough estimate of the specific heat above the freezing point
of the product.
An empirical equation for the calculation of c p of some different foods is given as 43
c p = 4187 [x w + (γ + 0.001T)(1 – x w) – β exp(–43x w 2.3 )], (1.14)
where the temperature T is in degrees Centigrade (°C) and the numerical values
of the coefficients in Equation 1.14 for some foods are
γ beef = 0.385, b beef = 0.08
γ white bread = 0.350, b white bread = 0.09
γ sea fish = 0.410, b sea fish = 0.12
γ low-fat cheese = 0.390, b low-fat cheese = 0.10.
If detailed composition data are not available, the following simpler model
can be used: 44
c p = 4190 – 2300x s – 628x s 3 , (1.15)
where x s is the mass fraction of solids, and c p is in Joules per kilogram degrees
Centigrade (J/kg °C).
Gupta 45 developed the following correlation to predict the specific heat of
foods as a function of moisture content and temperature considering 15 types of
foods: 10
c p = 2476.56 + 2356x w – 3.79T, (1.16)
where T is in Kelvin (K), and c p is in Joules per kilogram Kelvin (J/kg K), and
x w ranges from 0.001 to 0.80 and T from 303 to 336 K. Equation 1.16 gives fairly
good values for substances like sugar, wheat flour, starch, dry milk, rice, etc. For
substances containing higher moisture (more than 80%), Equation 1.16 shows
higher deviations from reported values.
The specific heat is related to the dielectric properties and the temperature
increase (∆T) through the following equation: 28
πtf ε ε′ tanδV2
∆T =
,
ρ
2 0
cp (1.17)
where t is the temperature rise time (sec), e 0 is the dielectric constant of free
space, and V, the electric field strength, is equal to voltage/distance between
plates (V/cm). Equation 1.17 shows that the specific heat affects the resulting ∆T.