Texture and Weight Loss

Texture was evaluated using a texturometer (Stable Micro Systems, TAXT2i, Surrey, UK) on papaya pulp pieces (2 cm × 2 cm × 3 cm) with and without coating. The force required to penetrate 5 mm of pulp was determined at 0, 5, 10, and 15 days of storage. Determinations were conducted in 6 different sections of the fruit, using a 5 mm probe and test speed of 4 mm/s with automatic return. Fruit weight losses, with and without coating, were recorded during storage time (0, 5, 10, 15 days) using a scale (Torrey, PCR-20, Monterrey, Mexico).

#### pH, Soluble Solids, Titratable Acidity, and Vitamin C

Ten g of papaya pulp was weighed and homogenized in 100 mL of distilled water, and from each sample, 3 pH measurements were taken using a calibrated potentiometer (Hanna, Mod. 209, Woonsocket, RI, USA) at 0, 5, 10, and 15 days of fruit storage, with and without coating [22]. Soluble solids of papaya samples with and without coating were determined by an Abbe refractometer (Atago, DR-A1, Tokyo, Japan) at the different storage times. Titratable acidity was evaluated following AOAC method 942.15A, in which 5 mL of diluted papaya juice in 95 mL distilled water was titrated, with 0.1 M NaOH, and expressed as citric acid percentage [23].

#### Color

Color changes of papaya peel surface were evaluated at the different storage times (0, 5, 10, and 15 days) with and without coating. For this test, sections in which color was determined were marked; thus, measurements were always conducted in the same section. Parameters measured were *L*\* (Luminosity), *a*\* (green to red), and *b*\* (blue to yellow) using a Minolta CR400 colorimeter (Konica Minolta Sensing, Osaka, Japan), a light source D65, and 10◦ angle [24].

#### Volatile Compounds

Volatile compounds determination was carried out according to García-Aguilar et al. [25]. Fruits were placed for 2 h before analysis in hermetic containers. The solid phase microextraction method was applied in this analysis using a pre-equilibrium time of 15 min at 50 ◦C. Solid-phase microextraction was performed with a 75 mm divinylbezene/carboxene/polydimethylsiloxane fiber (Supelco, Bellefone, VA, USA). The fiber was exposed to the interior of the hermetic containers for 20 min at room temperature (~25 ◦C), and then placed in the injection port of a gas chromatograph (Mod. 7890A, Agilent Technologies, Santa Clara, CA, USA) coupled to a quadrupole mass spectrometer (Agilent, 5975C) for 45 min at 230 ◦C for analytes desorption. Compounds identification was carried out using NIST/EPA/NIH Mass Spectra Library version 1.7 (Gaithersburg, MD, USA), considering a similarity >80%. Peak area was obtained by Equation (1):

$$A = \mathcal{W}\_{1/2}h \tag{1}$$

where *A* is peak area, *W*1/2 is width of half peak height, and *h* is peak height.

#### 2.2.5. Image Analysis

Papaya ripening changes were evaluated by image analysis (IA). Samples were illuminated using four TL-D deluxe fluorescent lamps, daylight 18 W (Philips, TL-D, Eindhoven, The Netherlands), and color temperature of 6500 K (D65). Lamps (60 cm long) were arranged in a square shape 35 cm above the sample at an angle of 45◦, and pictures were taken from apical and equatorial areas of the papaya. A digital camera Nikon D3200 (Tokyo, Japan) of 24.2 Mpixels resolution was used to take images without zoom or flash, and were processed following Arzate-Vázquez et al. [26]. Using Image J 1.51j8 program [27], images were changed to 8-bit (250 × 250 pixels, TIFF format) and contrast, correlation, entropy, angular momentum, and fractal dimension parameters were obtained. The gray level co-occurrence matrix complement was used to evaluate papaya texture. Fractal dimensions (FD) were estimated by counting changes of differential boxes from gray level images.

#### 2.2.6. Microbiological Analysis

Total coliforms, fungi and yeasts, and mesophilic aerobic bacteria were quantified. Papayas with and without coating at 0, 5, 10, and 15 days of storage were analyzed in triplicate. Samples were prepared according to NOM-110-SSA1-1994 [28] with some modifications. Papaya weight was recorded, then the whole fruit was placed in a sterile plastic bag containing 100 mL of sterile peptone water (1 g casein peptone, 8.5 g NaCl/L, pH 7.2 ± 0.1), and stirred for 3 min to recover the microorganisms, which were appropriately diluted to carry out population counts (CFU/mL) in triplicate experiments.

#### Total Coliforms

Total coliforms determination was performed by pouring 1 mL of each sample on violet red bile agar, using decimal dilutions where necessary, and incubated at 35 ◦C for 48 h. Red or violet colonies (0.5 mm) surrounded by a precipitated bile area were quantified and expressed as CFU/g [29].
