Examination was made to determine factors on the deterioration rate of deep-fat frying oil, using refractometer. Under constant conditions, this rate was found to be proportional to the heat stability constant, gas-liquid interface area, and oxygen density of air, while to be inversely related to frying oil weight. This should be called heat deterioration law of oils in deep frying. The density of oxygen is proportional to atmospheric pressure, and thus heat deterioration rate can be expressed as follows dRI/dθ=k[S/W]·P/1013…(I)
ΔRI=k[S/W]·θ·P/1013…(II)
In above equations, signs are RI: Refractive Index (2040°C), θ: Heating time (h), k: Heat stability constant (g·cm^-2·h^-1), S/W: Specific surface (cm²/g), P: Atmospheric pressure (hPa). When ΔRI limit (e.g. 12×10^-4 or ΔRI equivalent to 2527% total polar material) was specified, the heat tolerance period could be calculated from equation (II). The range of k at 180°C was from 0.5×10^-4 to 5×10^-4. The relationship between frying temperatures and k was, for example, k=2.3×10^-4, 3.2×10^-4, 4.8×10^-4, at 160°C, 180°C, 200°C, respectively. If the deterioration limit is the same or constant, the following equation can be available.
k₁[S₁/W₁]θ₁P₁=k₂[S₂/W₂]θ₂P₂…(III)
Where, k₁W₁S₁P₁ and k₂W₂S₂P₂ show that the frying conditions of oil are different each other.