**2.4. Chemical structure and properties of carvacrol, cinnamaldehyde and capsaicin**

Carvacrol (C10H14O) is a chemical (see **Figure 1**) found in several plants including: wild bergamot, thyme and pepperwort, but it is most abundant in oregano (*Origanum vulgare*) oil [27]. Carvacrol gives oregano a slightly spicy flavour, is colourless, and has a distinct warm odour. Overall, carvacrol has a promising antibacterial, antiviral, antioxidant, and antifungal impact. Carvacrol also demonstrated significant anti-cancer effects when tested against breast cancer, prostate, lung and mouth cancer cells [28].

**3.1. Dietary formulation, husbandry and sample collection**

The four experiments employed the same dietary formulations (**Table 1**). Birds were fed one of four diets. There were two control diets based on either wheat (WC) or maize (MC) which were formulated to be iso-energetic (12.13 MJ/kg AME) and iso-nitrogenic (215 g/kg CP).

kg/100 kg kg/100 kg

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100 100

The Vitamin and mineral premix contained vitamins and trace elements to meet the requirements specified by the National research Council (1994). The premix provided (units/kg diet): retinol, 12,000 IU; cholecalciferol, 5000 IU; α-tocopherol, 34 mg; menadione, 3 mg; thiamine, 2 mg; riboflavin, 7 mg; pyridoxine, 5 mg; cobalamin, 15 μg; nicotinic acid, 50 mg; pantothenic acid, 15 mg; folic acid, 1 mg; biotin, 200 μg; 80 mg iron as iron sulphate (30%); 10 mg copper as a copper sulphate (25%); 100 mg manganese as manganous oxide (62%); 80 mg zinc as zinc oxide (72%); 1 mg iodine as calcium iodate (52%); 0.2 mg selenium as sodium selenite (4.5%); 0.5 mg molybdenum as sodium molybdate (40%).

**Dietary ingredients Wheat-based diet Maize-based diet**

Maize — 52.86 Wheat 54.68 — Soybean meal (48) 27.49 31.30 Vegetable oil 3.50 1.00 Barley 5.84 6.33 Rye 5.00 5.00 Monodicalcium phosphate 1.43 1.43 Limestone 1.15 1.15 NaCl 0.27 0.33 Lysine 0.15 0.15 Methionine 0.39 0.35 Vitamin mineral premix<sup>1</sup> 0.10 0.10

Crude Protein, g/kg 215 215 ME, MJ/kg 12.12 12.13 Crude Fat, g/kg 47 34 Ca, g/kg 8.4 8.3 Available P, g/kg 4.5 4.4 Lysine, g/kg 12.3 12.3 Methionine + Cysteine, g/kg 9.5 9.5

Calculated analysis (as fed)

ME = metabolisable energy.

**Table 1.** Composition of the control diets.

1

Cinnamaldehyde (C<sup>9</sup> H8 O) is a chemical (see **Figure 1**) that naturally occurs in the inner bark of several tree species from the genus Cinnamomum. Cinnamaldehyde, the principal component of the essential oil of cinnamon bark, gives the cinnamish odour responsible for the characteristic taste and odour of cinnamon spice. Cinnamaldehyde has strong antimicrobial, antifungal and anticorrosion properties [29].

Capsaicinoids (C18H27NO<sup>3</sup> ) are produced by peppers as a protection against certain mammals and fungi (see **Figure 1**). They have no flavour or odour, but act directly on the pain receptors in the mouth and throat. Capsaicin, the most common capsaicinoid, is an irritant for most mammals, including humans, and produces a sensation of burning in any tissue with which it comes into contact [30]. However, birds are not sensitive to the capsaicin [31] and can benefit from the nutritional value of the chilli peppers. Capsicum oleoresin (active ingredient capsaicin) is found in pepper fruits and has antifungal and antibacterial activity [32].

**Figure 1.** Chemical structure (2-D) of (left to right) carvacrol, cinnamaldehyde and capsaicin.
