Effects of select copper sources at minimum supplementation levels on nutrient content, off-colors, and blemishes in canned pet food

In the previous research, super-fortification with copper decreased vitamin E content and darkened canned pet food, which prevented the analysis of black blemishes reported in commercial products. The pet food industry has linked these blemishes, which may be concerning to pet owners, to copper supplementation. The objective of this study was to determine the effect of different copper sources included at minimum recommended levels on nutrient content, color, and blemishes in canned pet food. Treatments were arranged in a 2 × 3 + 1 factorial, with 2 levels of copper supplementation [6 and 12 mg/kg dry matter (DM)], 3 copper sources (CG = copper glutamate, CA = copper amino acid complex, and CS = copper sulfate), and a control with no added copper (NC). Diets were analyzed for macronutrients (moisture, crude protein, crude fat, crude fiber, and ash) and micronutrients (calcium, phosphorus, potassium, sodium, magnesium, sulfur, iron, copper, manganese, zinc, and vitamin E). Color was quantified with a CIELAB color space colorimeter wherein L* values closer to 100 represented lighter products and more positive a* and b* values indicated redder and yellower products, respectively. Blemishes were enumerated and their surface area quantified with ImageJ software. Data were analyzed as a general linear mixed model with the fixed effect of treatment and the random effect of production day. P-values less than 0.05 were considered significant. The 12 mg/kg DM treatments (average 14.19 mg/kg DM) contained the highest (P < 0.05) level of copper, followed by 6 mg/kg DM treatments (average 7.59 mg/kg DM) and then NC (0.00 mg/kg DM). Addition of copper decreased (P < 0.05) vitamin E content, except for NC and CS12 which were similar (P > 0.05; average 111.89 mg/kg DM). Lightness (average L* 63.66) was not affected (P > 0.05) by the treatments. Adding copper decreased (P < 0.05) redness, with higher (P < 0.05) a* values for CG6 (9.55) vs. CA6 and CS6 (average 8.50). Yellowness also decreased with the addition of copper, except for CG6 which was similar (P > 0.05) to NC (average 18.70). However, CG6 and CG12 (average 4.05 blemishes/slice of food) contained more (P < 0.05) blemishes than CA6, CS6, and CS12 (average 0.97 blemishes/slice of food). Minimal levels of supplemental copper from CG may enhance overall color preservation but could increase blemish occurrence. No disadvantage was observed for CA vs. CS, indicating that CA could be exchanged for CS in formulations.