3.4.2. pH

From Figure 3, we can observe that there are no major differences between the pH of the different treatments, with a tendency for a slight increase along the 10-day period. The reference, i.e., lettuce washed with distilled water, is the sample where the increase of pH is significantly (*p* < 0.05 i.e., *<sup>p</sup>* <sup>=</sup> 8.8 <sup>×</sup> <sup>10</sup><sup>−</sup>6) higher, from 6.16 to 7.00, but still less than a unit of variation for a storage period of ten days. Martin-Diana et al. [8] reported a similar increase in pH in a study with lettuce and carrots and stated that this was a good index for the potential quality maintenance of the products.

According to Beuchat [47], vegetable products retain adequate quality in a pH range of 5–6.5. Our results show that samples treated with whey exceeded this value (6.57) only on day 9, and the samples treated with chlorine exceeded it on day 7 (6.75).

**Figure 3.** The pH measurements of the lettuce after sanitizer treatments, over 10 days in cold storage. Results represent the average of at least three replicate experiments (n = 3) ± SD.

#### 3.4.3. Texture

Texture is a major quality parameter, and together with color they are crucial for consumer acceptance. Texture measurements evaluated two parameters: resistance to puncture as a force in N, and deformation at rupture in length (mm), generally described as firmness and brittleness (the lower the deformation, the more brittle the material is), respectively. Results are shown in Figure 4a,b. As seen in Figure 4a, the firmness of the lettuce shreds was not substantially affected by any of the treatment and variation within 10 days of storage was quite small. As observed with the pH variation, the reference treatment with water is where the highest difference is found—74 mN, which is significantly (*p* = 2.2 <sup>×</sup> 10−6) less firm, and 0.177 mm which is less brittle (*p* = 0.0027) after 10 days in cold storage, i.e., a reduction of 12% in firmness and 7% in brittleness, whereas with whey there was a significant increase in firmness of 69 mN (*p* = 8.2 <sup>×</sup> 10−6), corresponding to a 10% increment and no significant variation on brittleness (*p* > 0.05), which is due to the calcium ions present in the whey solution that might reinforce the walls of the lettuce cells.

**Figure 4.** Firmness (**a**) and Brittleness (**b**) of the lettuce after sanitizer treatments, over 10 days in cold storage. Results represent the average of at least three replicate experiments (n = 3) ± SD.
