Exploiting microdistillation and smartphone-based digital-image colorimetry for determination of protein in foods

Because of their nutritional importance and industrial interest, determination of proteins is usual in food quality control. The Kjeldahl method, the most usual for protein determination and a reference to other indirect methods, is time-consuming and generates significant amounts of waste. Aiming to circumvent these drawbacks, this work proposes a simple, cost-effective, and more environmental friendly procedure based on microdistillation of the ammonium from Kjeldahl digests, absorption on an acid-base indicator solution, and digital-image colorimetry for protein determination in food. Microdistillation was carried out in a simple and inexpensive lab-made apparatus. The digital images were acquired by a smartphone camera under controlled illumination and the intensity of the reflected radiation was converted to the RGB color system using a free app (PhotoMetrix®). Measurements were based on the color change of phenol red from yellow to pink and the G channel (corresponding to the complementary color of the dissociated indicator) was taken as analytical response. A linear response was achieved within 5.0–50.0 mg/L ammonium, equivalent to 0.003–0.03 % protein (r = 0.996), as confirmed by the lack-of-fit test at the 95 % confidence level. Coefficient of variation (n = 10) and limit of detection (99.7 % confidence level) were estimated at 2.3 % and 2 mg kg−1 protein, respectively. A sample throughput of 12 h−1 was achieved with simultaneous sample processing. Protein amounts determined in foods of animal (milk) and vegetal (beans and lentils) origin yielded results in agreement with the micro-Kjeldahl reference procedure at the 95 % confidence level.