## 2 Materials and Methods
### Methods of Meat Quality Assessment  
The research material used to determine the quality of the meat consisted of samples of the _M. longissimus thoracis_, with each weighing approximately 700 g and taken from the level of the 13 th-14 th thoracic vertebrae. Subsequently, the external fat, connective tissue, and tendons were removed from the samples during the cleaning process. To determine the chemical composition of the horsemeat, the following components were analyzed: water content, protein content, fat content, and ash content. The water content was determined in accordance with PN-ISO 1442:2000 [47]. The protein content was determined using the Kjeldahl method, according to which the measured nitrogen content was converted into the protein content in accordance with PN-75/A-04018 [48]. The fat content was determined using the Soxhlet method, following the recommendations of PN-ISO 1444:2000 [49]. The total ash content was determined in accordance with the guidelines contained in PN-ISO 936:2000 [50]. The sensory properties of horsemeat samples were evaluated as described by [PERSON] and [PERSON] [51]. Briefly, 100 g samples were steamed at 95 \({}^{\circ}\)C until their internal temperature reached 80 \(\pm\) 2 \({}^{\circ}\)C, as measured using a digital thermometer with a needle probe (Sous Vide Thermapen; MERA, Warsaw, Poland). Before the sensory evaluation, the samples were cooled down to 20 \(\pm\) 2 \({}^{\circ}\)C and cut perpendicular to the fibers into 1.5 cm thick slices. These slices were stored in disposable plastic boxes with lids, individually coded, and offered in a random order to the evaluation panel, which consisted of six members (three males and three females, aged 26-46 years). These members were experienced in evaluating meat and its products. Each sample was assessed in triplicate by the panel. The sensory analysis was conducted by an evaluation team, who were tested for sensitivity and sensory fitness according to ISO 8586-2:2008 [52] and ISO 8587:2006 [53]. A five-point scale with defined value limits according to the sensory assessment chart (Table 1, Table S1, go to the Supplementary Material) was applied. The evaluation was conducted in a specific laboratory that met the relevant standard requirements [54]. Before testing each sample, the evaluators took a 30 s break and washed their mouths with mineral water. The evaluation was conducted across 10 sessions, and 12 samples were assessed in each session. The pH of the cooled meat was determined using an OSH 12-01 electrode and a CPC-411 pH meter (produced by ELMETRON, Zabraze, Poland), with an accuracy of up to 0.01. The device was calibrated based on buffers with pH values of 4.00 and 7.00. The determinations were made directly in the meat at various time points: 15 min (pH1) after slaughter, and then at 24, 48, 72, 96, and 120 h after slaughter. The pH was measured in the middle section of the harvested muscle. Meat marbling was determined using the point method on a scale from 1 to 5 points, according to the standards of intramuscular fatness of beef (1 point--slight fatness, 2 points--small fatness, 3 points--medium fatness, 4 points--big fatness, and 5 points--strong fatness). This assessment was performed on a 1.5-2 cm thick slice of meat cut from the sample, in daylight, and in a location not exposed to direct sunlight [5; 7; 32; 33]. Marbling was determined in the middle section of the harvested muscle. To further determine the physicochemical characteristics, i.e., the thermal drip and water retention capacity, meat samples were minced twice in a laboratory wolf (HENDI, Warsaw, Poland) using sieves with holes of 4.0 mm in diameter. The obtained meat mass was mixed thoroughly to homogenize the sample. Thermal drip was determined according to [PERSON] and Walczak [44].