Evaporation-Based Low-Cost Method for the Detection of Adulterant in Milk

Abstract

Adulteration of milk poses a severe health hazard, and it is crucial to develop adulterant-detection techniques that are scalable and easy to use. Water and urea are two of the most common adulterants in commercial milk. Detection of these adulterants is both challenging and costly in urban and rural areas. Here we report on an evaporation-based low-cost technique for the detection of added water and urea in milk. The evaporative deposition is shown to be affected by the presence of adulterants in milk. We observe a specific pattern formation of nonvolatile milk solids deposited at the end of the evaporation of a droplet of unadulterated milk. These patterns alter with the addition of water and urea. The evaporative deposits are dependent on the concentrations of water and urea added. The sensitivity of detection of urea in milk improves with the dilution of milk with water. We show that our method can be used to detect a urea concentration as low as 0.4% in milk. Based on the detection level of urea, we present a regime map that shows the concentration of urea that can be detected at different extents of dilution of milk.

Figure 1

Top-view images of the evaporating unadulterated milk droplet with an initial volume = 1 μL at different instants of time: (a) t = 0 s, (b) t = 300 s, (c) t = 330 s (onset of amoeba-shaped pattern formation in the central region), (d) t = 420 s, (e) variation of the thickness of the evaporation-induced deposit obtained using an optical profilometer, and (f) variation of thickness along the central line of deposit (dashed line marked in (e)).

Figure 2

Final evaporative deposits for (a) undiluted milk showing the amoeba-shaped deposition pattern near the center; (b) diluted milk (20%) where the disintegration of structure in the final deposition occurs; (c) diluted milk (50%) with no distinct pattern formation in the droplet interior; and (d) diluted milk (90%). (e) Total evaporation time of milk at different extents of dilution, and (f) initial and evaporated mass (Δm) of the diluted milk.

Figure 3

Final evaporative deposition of undiluted milk–urea mixture with (a) 3.8% urea, where a distinct amoeba-shaped pattern forms in the interior of the deposit, (b) 5.3% urea where the central deposition pattern is absent, and (c) 8% urea where crystallization of urea is observed towards the end of evaporation.

Figure 4

Top-view images of the evaporating droplet comprising 8% added urea in undiluted milk at (a) t = 0 s, (b) t = 350 s, (c) t = 392 s; nucleation of urea crystal initiates in the interior of the droplet; (d, e) t = 397–400 s; the crystal grows along the periphery; (f) t = 407 s (end of crystallization); the arrows represent the outward growth of crystal towards the pinned contact line. Scanning electron microscopy images of the crystallization pattern for cases where the nucleation of urea initiates (g) near the circumference of the droplet, and (h) near the central region of the droplet; the nucleation site is marked by a circle.

Figure 5

Variation of droplet evaporation time with varying initial urea concentration for the (a) undiluted milk–urea mixture and (b) diluted milk (20%)–urea mixture. Initial and evaporated mass for (c) undiluted milk–urea and (d) diluted milk (20%)–urea samples.

Figure 6

Regime map showing the detection level of urea in a milk–water–urea mixture. The insets represent images of evaporative crystallization at the corresponding ranges of dilution.