*2.5. Color*

The *L\** (lightness), *a\** (redness), and *b\** (yellowness) values of sausages were determined by a colorimeter (HunterLab ColorFlex, Reston, VA, USA) with a standard illuminant D65 light source (Xinlian Creation Electronic Co., Ltd., Shanghai, China) and a standard plate (*L\** = 94.52, *a\** = −0.86, *b\** = 0.68). The color values were determined at room temperature by five different areas on the cross-section of sausages. In particular, the area of fat in the sausage slice was not selected. The color differences (Δ*E\**) can be calculated by the following equations [28,30]:

$$\begin{cases} \Delta E \ast = \left(\Delta a^2 + \Delta b^2 + \Delta L^2\right)^{0.5} \\ \Delta a = a \ast\_{\text{control}} - a \ast\_{\text{PO}} \\ \Delta b = b \ast\_{\text{control}} - b \ast\_{\text{PO}} \\ \Delta L = L \ast\_{\text{control}} - L \ast\_{\text{PO}} \end{cases} \tag{1}$$

where Δ*L*, Δ*a*, and Δ*b* are the differences between the *L\**, *a\**, and *b\** values of the control groups and the groups with *PO*, respectively.

### *2.6. Textural Profile Analysis (TPA)*

Texture profile analysis was employed by a texture analyzer (CT3-50kg, Brookfield Engineering Labs, New York City, NY, USA) to calculate the hardness, springiness, gumminess, chewiness, and cohesiveness of the sausages. The sausage was cut into cylinders with a diameter of 2 cm and a height of 1 cm. Then, the cylinder was axially compressed twice until reaching 80% of its initial height with a 20 s pause time between the descent, 30 mm probe retraction, 6 cm/min detection speed, and 100 N force induction. The analysis was repeated at 25 ◦C in triplicate.

### *2.7. Cooking Loss and Water Holding Capacity (WHC)*

Cooking loss was characterized by the method reported by Fu et al. (2016) [30]. Sausage (50 g) was cooked at 80 ◦C for 50 min then cooled at room temperature to determine the weight difference before and after cooking. The cooking loss can be calculated by:

$$\text{Cooling loss} = \left[ \left( m\_1 - m\_2 \right) / m\_1 \right] \times 100\% \tag{2}$$

where *m*<sup>1</sup> and *m*<sup>2</sup> are the weight before and after cooking, respectively.

The WHC of the sample was measured by the method reported by Jridi et al. (2015) with slight modifications [31]. Sausage (10 g) was centrifuged at 12,000 rpm for 30 min at 4 ◦C. Finally, the WHC can be calculated by:

$$\text{WHC} = (\text{W}\_2/\text{W}\_1) \times 100\% \tag{3}$$

where *W*<sup>1</sup> and *W*<sup>2</sup> are the weight of the sample before and after centrifugation, respectively.

### *2.8. Amino Acids Content*

The amino acid content was analyzed according to the method reported by Serrano et al. (2005), which separated the amino acids by cation exchange chromatography via an automatic amino acid analyzer (L8900, Hitachi High-Technologies Corporation, Tokyo, Japan) for measurement [32]. Ninhydrin derivative reagents were used to measure the amino acid content at 570 nm.
