# Application of Natural Antioxidants from Fruits Waste for Improving Egg Quality Characteristics
## 5 Mechanisms of Antioxidants in Laying Hens
### Influence of Genetics on Antioxidant Metabolism  
Other factors that can affect the mechanism of antioxidants in laying hens may include genetic factors, such as the breed or hybrid, which may also determine how antioxidants will be assimilated and deposited into eggs. The various hybrids of laying hens do differ in feed efficiency, nutrient utilization, and egg production characteristics. Some studies [123; 124; 125] reported this important aspect, showing the impact of hybrids on the performances and nutritional quality of the eggs; however, there is still a lack of understanding of the hybrids' ability to metabolize and deposit dietary antioxidants differently. Moreover, it is believed that hens with efficient lipid metabolism are more likely to deposit higher levels of lipophilic antioxidants including carotenoids and vitamin E in their yolk compared with younger ones. To the contrary, in a recent study conducted by [PERSON] et al. [53], it was noted that eggs from early-age laying hens (24 weeks) had a higher intake of antioxidant carotenoids and tocopherols, while the eggs from intermediate-age (42 weeks) and late-age hens (74 weeks) had higher intakes of good quality fats. This might be explained by some hybrids or hens at different ages having poorer nutrient utilization. Moreover, the structure of antioxidant enzymes, like SOD and GPx, may also be important in how the hens cope with oxidative stress as well as how well they utilize dietary antioxidants, as shown by [PERSON] [126]. Beneficial and Potential Detrimental Effects of Using Fruits' Waste on Various Aspects in Laying Hens' Diets  
The literature data showed that various fruit wastes, when added to laying hens' diets, exhibited different effects, as summarized in Table 2.  
Orange waste in different forms used in poultry diets was reviewed by [PERSON] et al. [127]. The authors showed multiple benefits when used in broiler chickens but little about the effects on laying hens. It was reported that up to 20% decreased cholesterol levels in egg yolk, while 0.2% and 0.4% orange peel with dried clover powder affected blood triglyceride, aspartate amino transferase, and alanine amino transferase levels and increased the amount of red blood cells and white blood cells [127]. They also found that 1000 mg/L orange peel extract improved the immune system due to the flavonoid contents of the orange peel, which stimulates the immune system by increasing the anti-inflammatory activity IgM and IgG. More recently, the effect of 9% orange pulp waste was evaluated by [PERSON] et al. [128] and compared with synthetic 0.767 g/kg hesperidin and 0.002 g/kg naringin. The authors reported that the laying rate was higher in the synthetic group (87.6%) when compared with the orange waste-supplemented group (60.8%) and with a concomitant increase in the feed conversion ratio (FCR). Shell quality was also affected by the 9% orange pulp waste, while yolk color was lighter compared to a control diet. However, during the storage time (30 days), the MDA values were significantly lowered (2.99 ng MDA/g yolk) compared with those from a control group (4.19 ng MDA/g yolk), showing the potential of orange waste to increase oxidative stability and shelf life in eggs. However, the optimal level of supplementation to avoid detrimental effects still needs testing. Among numerous types of fruit waste, grape waste (pomace, seed, or meal) was studied in numerous scientific papers; however, there are still gaps to be filled. In a recent study, [PERSON] et al. [111] showed that 5% grape pomace had different effects when tested on different laying hen hybrids (ISA white and ISA brown). Both hybrids had a better laying rate (97.1%) with no effect on egg weight. Grape pomace resulted in eggs with better Haugh Units (96.3 and 99) compared with control eggs (93.8) but decreased the redness parameter in egg yolk.