Evaluation of Torula Yeast ( Candida utilis ) Single Cell Proteins as a Partial Replacement for Soybean Meal in Broiler Diets: Effects on Productive Performance, Meat Quality Traits, and Plasma Metabolome

Over recent years the use of yeast in feedstuffs has gained momentum due to novel developments in single-cell proteins (SCPs). SCPs are considered a sustainable protein source, as their production enable to convert low-value substrates (including industrial/agricultural wastes) into feed, with a related reduced environmental impact. This study provides a first-step insight into the dietary inclusion of torula yeast-derived SCPs in broiler chicken diets. A total of 720 1-day-old male broilers were divided into four groups (10 replicate pens/group): the control group (CON) received a commercial soybean-based diet, while the SCP groups (SCP2, SCP4, SCP6) were fed diets supplemented with 2%, 4%, or 6% SCPs during the grower and finisher phases (15-42 d) as a partial replacement for soybean meal. Growth performance, footpad dermatitis (FPD), plasma metabolome, and breast-meat quality and composition were evaluated. For the entire trial period, performance traits and FPD rates did not differ significantly, however a trend was observed for body weight and daily weight gain which tended to decrease with increasing SCP levels (3083, 3058, 2988, 2942 g/bird, and 72.1, 71.6, 69.8, 68.8 g/bird/day respectively for CON, SCP2, SCP4, and SCP6; p = 0.071; linear fit < 0.01). Breast-meat quality and composition were maintained apart from linearly increased yellowness (b*) in CON, SCP2, SCP4, and SCP6 (7.67, 8.57, 9.11, and 10.38; p < 0.001; linear fit < 0.01). As for plasma metabolites, focusing on differences attributable to SCPs inclusion levels, on day 21, SCP6 reduced Threonine (-34%) and increased Isoleucine (+26%), Ethanol (+91%), Uridine (+67%), Leucine (+24%), and 2,3-butanediol (+60%) compared with CON (p < 0.05). At day 42, SCP6 reduced Arginine (-46%) and Threonine (-17%) and increased 2-aminobutyrate (+51%), Uridine (+116%), Trans-4-hydroxy-L-proline (+29%), and Formate (+25%) (p ≤ 0.05). Overall, the findings demonstrate the feasibility of graded SCPs inclusion in broiler diets, with a potential upper limit of 4%, as indicated by changes in performance and plasma metabolome.