Filter-Paper Aldehyde Detection for Pecan Oil Oxidation Assessment

Pecan oil has high proportion of unsaturated fatty acids. It can be processed using two commercially relevant processing methods: cold pressing and hot pressing which can shape the composition, sensory character, and storage behavior of the resulting oils. Cold-pressed pecan oil retains more functional constituents and is associated with favorable color and acid-value characteristics, while, hot pressing involves roasting or baking before extraction and produces an oil with a more developed roasted flavor profile. However, pecan oil remains vulnerable to oxidative deterioration during storage with light, oxygen, moisture, temperature, and the chemical nature of the lipid phase can all contribute to progressive rancidity, which alter sensory quality and reduce the stability of valuable oil constituents. Assessing oil oxidation outside a laboratory remains difficult. Peroxide value, acid value, p-anisidine value, thiobarbituric acid reactive substances, conjugated dienes, and related indices can provide useful information about oxidative change, but many require reagents, controlled procedures, trained personnel, or analytical instrumentation. Such methods are well suited to formal quality assessment, although they are less convenient when the practical question is simply whether an oil has remained within an acceptable oxidation range during storage. Sensory assessment can address that question to some extent, particularly because rancidity changes odor character. Its interpretation, however, depends on panel training and prior familiarity with the sensory features of fresh and oxidized oils. A further complication arises from the chemistry of lipid deterioration itself. Peroxide value reflects primary oxidation products, whereas the volatile compounds that shape rancid odor are often secondary products produced as hydroperoxides decompose. The relation between chemical oxidation indices, volatile aldehydes, and sensory change therefore needs to be clarified before a simple visual detection strategy can be justified. Acid value may also remain relatively stable even when oxidative deterioration is advancing, making it less suitable as a sole marker for shelf-life assessment in this particular system.

In a recently published paper in Molecules, Dr. Li Cui, and Dr. Haijun Zhu of the Jiangsu Academy of Agricultural Sciences, developed a visual filter-paper sensor to assess oxidation in cold-pressed and hot-pressed pecan oils. The research team began with storage experiments conducted at room temperature, 40 °C, 50 °C, and 60 °C. They selected peroxide value as the principal quality index because acid value changed little during accelerated oxidation and because the authors had previously observed expired pecan oil samples whose acid values did not exceed the relevant national standard. The peroxide-value data for both cold-pressed pecan oil and hot-pressed pecan oil were fitted to first-order kinetic models. They found that, across the tested temperatures, the coefficients of determination ranged from 0.9183 to 0.9841, indicating that the time-dependent increase in peroxide value was described effectively by the first-order kinetic model.

The authors found that storage temperature influenced the rate constant in both oils, with higher temperatures accelerating peroxide formation. Incorporation of the Arrhenius relationship then enabled shelf-life prediction using the national peroxide-value limit of 0.25 g/100 g as the endpoint. The predicted and measured shelf-life values agreed closely and for instance the correlation between predicted and measured shelf life for  cold-pressed oil reached 0.9993, while hot-pressed oil gave a correlation of 0.9866. Also at room temperature, cold-pressed oil reached the peroxide-value limit after 330 days, compared with 100 days for hot-pressed oil. Sensory evaluation linked these chemical changes to the odor characteristics of the oils. The team reported fresh samples to have stronger nut aroma and lower oxidation-associated notes, whereas expired samples were characterized by diminished nut aroma together with stronger oxidized hala flavor and stimulating taste. The pattern differed somewhat between cold-pressed and hot-pressed oils, reflecting their different original sensory profiles. Hot-pressed samples contained roasted notes associated with their processing history, while cold-pressed samples retained a more direct nut-related aroma. The authors treated sensory analysis as a qualitative screening tool rather than a quantitative measurement, since its outcome remained dependent on evaluator training and subjective interpretation.

The team conducted volatile-compound analysis which provided the chemical bridge between peroxide-value increase and the proposed visual sensor. Headspace solid-phase microextraction coupled with gas chromatography-mass spectrometry identified 55 volatile compounds across fresh and expired cold-pressed and hot-pressed oils. Aldehydes emerged as the key oxidation-related group. Hexanal increased in both expired oil types, while compounds such as (Z)-2-heptenal, nonanal, (E)-2-octenal, and (E,Z)-2,4-decadienal also differed substantially between fresh and expired samples. Hexanal and (Z)-2-heptenal showed positive correlations with peroxide value in both oils. Thus, the analytical strategy linked peroxide formation to the accumulation of aldehydes that could be targeted chemically.

The investigators impregnated filter paper with Congo red and hydroxylamine sulfate, then dried to yield a color-sensitive material. Hydroxylamine sulfate reacts with aldehydes to form aldoximes and sulfuric acid. The resulting decrease in pH changes Congo red from its red azo form toward a purple-blue quinone-associated form. This design choice gave aldehyde accumulation a visible consequence: as oxidation-generated aldehydes increased, the paper sensor shifted from red toward purple-blue. Moreover, they optimized the sensor preparation conditions through variation of the filter-paper immersion time in the aldehyde-sensitive solution. A 150-minute immersion produced a sufficiently bright red starting color and was selected for fabrication. For testing, the sensor was fixed inside the cap of a sealed vial containing pecan oil, without direct contact between the paper and the oil, and exposed to the headspace at 35 °C for 24 hours. Fresh cold-pressed and hot-pressed oils retained a red sensor appearance, whereas expired oils produced the purple-blue transition.

The filter-paper sensor developed for pecan oil in the new study provides a simple approach for translating chemical deterioration into an immediately visible quality signal.     A user does not need to handle the oil directly, perform titration, or interpret chromatographic data. This format could be incorporated into small-scale pecan oil packaging and quality-control workflows. For producers handling cold-pressed and hot-pressed oils, the different a* thresholds identified for the two products provide a basis for processing-specific interpretation. Cold-pressed oil was classified as expired when a* reached 11 or lower, while hot-pressed oil reached the corresponding condition at 15 or lower. The distinction matters because the oils did not display identical oxidation behavior or initial volatile profiles. A single visual method could therefore be adapted without treating all pecan oils as chemically equivalent.

The sensor also has value during accelerated storage trials and product-development work. Conventional peroxide-value measurement remains necessary when a quantitative oxidation profile or kinetic model is required. Yet the paper strip offers a convenient complementary screening tool for identifying samples that have moved toward the regulatory peroxide-value limit. In the reported experiments, the obvious red-to-purple-blue color shift of the test paper visible to the naked eye occurs at a peroxide value above 0.25 g/100 g. Slight color deviation can be detected by a colorimeter when the peroxide value reaches approximately 0.20 g/100 g, yet the test paper still appears red to human vision at this concentration, which can serve as an early warning range before reaching the national standard limit. The sensing layer uses ordinary filter paper rather than a more elaborate film substrate, with Congo red and hydroxylamine sulfate providing the functional chemistry. Preparation requires immersion, drying, and storage under dry conditions, while actual measurement involves headspace incubation at 35 °C for 24 hours. That separation between one-time fabrication and later use supports batch preparation of sensors for repeated testing. This connection between oxidation chemistry and visible response gives the sensor a clear basis for practical pecan-oil quality screening.