2.11. Application on apricot puree and color measurement

The effect of treatment with AA/P combinations on apricot puree was studied using 100 and 500 mg of AA and 10, 50 and 100 mg of protease preparations per 100 g of fresh apricots. The AA/P combinations were chosen based on the results of PPO inhibition in vitro, where the amount of PPO present in fresh apricots was estimated from the results of the apricot PPO purification (Derardja et al., 2017). The AA/P combinations were prepared in 1 ml of distilled water. The pH and the total soluble solid (TSS) content of the puree samples were 4.38 ± 0.33 and 14.1 ± 0.3 °Brix, respectively.

Apricots were cleaned and deseeded. Then, 100 g were introduced into a Hand Blender Beaker with 1 ml of the freshly prepared AA/P solution. The mixture was blended for 2 min. A control puree was prepared without inhibitors. In addition, to allow for a better comparison, apricot puree was also treated only with AA (50, 100, 500 and 1000 mg), without added protease. After homogenization, apricot purees were packaged in 250 ml plastic boxes made of polypropylene. The boxes were stored at 4 °C in darkness until analysis, for up to 10 days. Apricot puree color was measured immediately after homogenization to determine the initial color (0 min). Afterwards, the color of the apricot puree was determined after 5, 10, 30 min; 1, 2, 4, 8, 16 h; 1, 2, 5 and 10 days. The surface color of treated and untreated purees was measured by a computer vision system (CVS) and expressed as CIELAB Tristimulus coordinates L*, a*, b*.

The computer vision system consisted of a standardized lighting system (photo shooting box) with three LED spotlights (6500 K daylight, 6 W), a Canon EOS-1200D digital camera (18 megapixel resolution and 3x 18–55 mm f/3.5–5.6 Zoom Lens), and a computer with Adobe Photoshop CS6 software (Adobe Systems Inc., USA).

For each assay, 3 g of apricot puree were smeared on a labeled transparent plastic square (70 × 70 × 0.2 mm) and then placed inside the photo shooting box on a white background. All photos were taken in manual mode (1/80, F5.6, ISO400). The measuring procedures of the color were as described by Zhou, Ling, Zheng, Zhang, and Wang (2015). The color values: lightness (L), redness-greenness (a) and yellowness-blueness (b) were obtained from the histogram of the menu bar by using LAB color mode in Adobe Photoshop. The rectangular marquee tool in the main menu was used to select the sample area. Then the L, a, b values of the selected area were transformed to CIELAB (L*, a*, b*) values using the equations (Supplementary B):

The total color difference, ΔE was calculated according to the following equation:

L^∗, a^∗, and b^∗ are the color values of the samples, while L₀^∗, a₀^∗, and b₀^∗ are the color values of the puree which exhibited minimal browning (puree treated with 1000 mg AA at 0 min).