Abstract
Poultry feathers, consisting largely of keratin, are a low-value product of the poultry industry. The safety and digestibility of a dietary protein produced from keratin (KER) was compared to a cysteine-supplemented casein-based diet in a growing rat model for four weeks. KER proved to be an effective substitute for casein at 50% of the total dietary protein, with no changes in the rats’ food intake, weight gain, organ weight, bone mineral density, white blood cell counts, liver glutathione, or blood glutathione. Inclusion of KER in the diet reduced total protein digestibility from 94% to 86% but significantly increased total dietary cysteine uptake and subsequent liver taurine levels. The KER diet also significantly increased caecum weight and significantly decreased fat digestibility, resulting in a lower proportion of body fat, and induced a significant increase in blood haemoglobin. KER is therefore a safe and suitable protein substitute for casein, and the cysteic acid in keratin is metabolised to maintain normal liver and blood glutathione levels.
Highlights
Keratin is a major component of poultry feathers and has long been used as a source of dietary protein in animal feeds [1]
Components of protein sources (Table 1) were determined by the following methods: nitrogen, Leco total combustion method (AOAC (Association of Official Agricultural Chemists International) 968.06), with CAS protein calculated as 6.38 X nitrogen, and keratin product (KER) and pea (Pisum sativum) flour (PEA) protein calculated as 6.25X nitrogen; fat, Soxtec extraction (AOAC 991.36); moisture, convection oven drying at 105 ̋ C (AOAC 930.15, 925.10); ash, 550 ̋ C furnace (AOAC 942.05); gross energy, bomb calorimetry; amino acids, hydrochloric acid analysis followed by HPLC separation (AOAC 994.12); tryptophan, alkaline hydryolysis followed by HPLC separation; cysteic acid, hydrochloric acid hydrolysis and HPLC separation; total cys, performic acid oxidation, with total native cysteine calculated by subtracting cysteic acid from total cys, normalised for their respective molar weights
Rats did not differ by group in food intake by volume (Table 3), PEA rats had a lower energy intake and a lower body weight gain
Summary
Keratin is a major component of poultry feathers and has long been used as a source of dietary protein in animal feeds [1]. Inclusion of feather meal at a concentration >10% of the diet has been shown to significantly impair growth, weight gain, and food intake [2,3,4]. This is because the keratin protein in feather meal is poorly digestible [4,5,6]. There is evidence that altering dietary concentrations of the sulphated amino acids (SAA) cys and methionine (met) can affect glutathione and its dependent enzymes in specific organs [12,13], with the liver being most strongly affected by dietary protein SAA [14]
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