Question 2 of 9: Frying-Oil Degradation from a Transmittance–Wavelength Spectrum, and Optical-Property Sensing

Question 2: Frying-Oil Degradation from a Transmittance–Wavelength Spectrum, and Optical-Property Sensing (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

[Figure not reproduced: Percentage transmittance versus wavelength for the frying fat, redrawn schematically from the source figure. Each curve peaks near 670 nm; the peak height falls from ≈71% (day 4) to ≈25% (day 7) as frying continues. See the official exam paper.]

Why %T falls with days of frying use. Repeated frying at high temperature in the presence of air, moisture (from the food) and catalytic metal surfaces drives thermal oxidation and polymerization of the oil: triglycerides break down into free fatty acids, hydroperoxides, and secondary oxidation products, and the free radicals produced propagate into larger, conjugated and cyclic polymeric species. Both the growing population of suspended/colloidal degradation products (increasing turbidity, hence more light scattered out of the beam path) and the growing concentration of coloured, conjugated-double-bond compounds (increasing true absorbance) reduce the fraction of incident light that reaches the detector at every wavelength, so the whole curve sinks and the peak transmittance falls monotonically from day 4 to day 7.

Why %T peaks at one particular wavelength. Oil is a broadband absorber, not a narrow-line one: it absorbs relatively strongly in the blue/near-UV region (roughly below 500 nm) because of carotenoid pigments and early oxidation/conjugation products, and it absorbs again toward the near-infrared (above ≈800 nm) because of C–H bond overtone and combination vibrations in the fatty-acid chains. Between those two absorption bands sits a genuine transmission "window" in the orange/red part of the visible spectrum (≈650–700 nm here) where neither mechanism is strong, so the fewest photons are absorbed or scattered and %T is at its local maximum — the peak wavelength is therefore a property of the fat's own chromophores, not an artefact of the instrument.

Conclusion from the plot. The systematic, monotonic drop in peak %T with frying time is a directly usable, single-number proxy for cumulative thermal/oxidative degradation of the oil: it correlates with (and can be calibrated against) standard chemical quality indices such as total polar materials (TPM), free fatty acid content and peroxide value, without needing a wet-chemistry titration. Because the peak sits at a fixed wavelength across all four days, a single-wavelength reading at ≈670 nm is sufficient — a full spectral scan is not required for routine monitoring.

Online sensor concept. A simple transmittance sensor — an LED or narrow-band light source at ≈670 nm on one side of a small flow-through (or dip-in) optical cell carrying a continuously sampled slipstream of the fryer oil, and a photodiode on the other side — can log %T continuously while the fryer operates. Because %T falls monotonically and repeatably with degradation, the sensor can be calibrated once (against laboratory TPM/FFA measurements on the same oil at known ages) to convert a live %T reading into an estimated quality index, and the fryer controller can trigger an oil-change alarm automatically once %T drops below a calibrated threshold, replacing a manual dip-strip or send-out-for-titration schedule with continuous, in-line monitoring.

Absorbance and photometric colour index from a spectrophotometer. Once %T is read at a chosen wavelength, absorbance follows directly from the Beer–Lambert definition, $$A = -\log_{10}\!\left(\frac{\%T}{100}\right) = 2 - \log_{10}(\%T),$$ e.g. day 4's peak of 71% transmittance corresponds to \(A = 2-\log_{10}(71) = 0.149\), and this absorbance is directly proportional to the concentration of the absorbing degradation products via Beer's law (\(A=\varepsilon c l\)) once the path length \(l\) is fixed by the cell geometry. A photometric colour index is built the same way but combines absorbance at two or more wavelengths chosen to bracket the visible spectrum (analogous to the Lovibond red/yellow system used for edible-oil colour grading) — e.g. an index such as \(\text{PCI} = A_{420} + 10\,A_{620}\) that weights a blue/violet reading (sensitive to carotenoid loss and early browning) against a red reading (sensitive to the broader polymerization-driven darkening) — and is likewise obtained by scanning the same spectrophotometer to the two specified wavelengths and reading %T (hence A) at each.