## 4 Discussion
[47] observed no effect (\(p>0.05\)) on total cholesterol concentration when using spirulina (1.5% and 2.5%) in laying hen diets. The dietary microalgae supplementation had no influence on egg quality parameters (egg weight and its components). Similar results on egg weight were observed by [46] using chlorella supplementation in laying hens (26-week-old) diets. Other authors, such as [54], used chlorella supplementation in Hy-Line brown laying hens, aged 70 weeks, without any effects on egg weight, but registered the highest Haugh units when supplementing diets with 2.4% liquid chlorella in their study. Our data indicate that the dietary treatments of chlorella and spirulina influenced the yolk coloration, spirulina having a more pronounced effect on enhancing red color. Additionally, longer boiling times result in darker and lower/more negative values for a* (greenish-gray ring) and higher/more positive values for b*. In other studies [34; 46; 54], both chlorella- and spirulina-supplemented diets were confirmed to increase the color of yolk by lutein dosing. The intensity of yolk color can vary depending on the types and concentrations of carotenoids consumed by the laying hens. [PERSON] et al. [55], using chlorella at 12.5 g/kg, noticed a significant intensification of the yellowness of fresh yolk. In the case of boiled eggs, a statistically significant increase in redness was observed. Conversely, an extension of the boiling duration to 10 min resulted in an increase in lightness and a concomitant reduction in yolk coloration. The L* value for fresh yolk indicates that the color of the fresh yolk has a moderately bright appearance. As the boiling time increased, the L* values also increased. The L* value for the 10 min. boiling indicated that the boiled yolk became significantly brighter (\(p\leq 0.0001\)) compared to the fresh yolk. The L* value increased progressively for the 20 min. and 30 min. boiling times, respectively; yolks became lighter as they were boiled for longer durations. The differences in L* values between the boiling time highlight the effect of heat exposure on the lightness of the yolks. This change in lightness can be attributed to structural and chemical transformations that occur during the cooking process, causing the denaturation of the proteins and altering the protein molecules. As a result, the yolks appear brighter or lighter in color [56]. According to [PERSON] and [PERSON] [57], marine biopigments can be categorized into three main groups: chlorophylls, carotenoids, and phycobiliproteins. The rich carotenoid concentration of the pigments zeaxanthin, xanthophylls, and \(\beta\)-carotene offer different greenish, green, golden, red, and brown colors of algae [58]. Other authors [59] tested, in a short-term study, the effects of 1% and 3% spirulina supplementation on color, nutritional value, and stability of yolk. A decreased luminosity and increased redness (\(p=0.0001\)) and yellowness (\(p=0.0103\)) were observed for 1% supplementation, after only 15 experimental days, meaning that the high carotenoid levels present in spirulina are efficiently absorbed by the laying hens' gastrointestinal tract [60]. Dietary supplementation with chlorella at 1% and 2% levels on Hisex Brown laying hens aged 56 weeks revealed a significant increase in total carotenoid deposition by 46% and 119% for the 1% and 2% chlorella groups, respectively. This increase was accompanied by a significant improvement in the yolk egg color, as evidenced by the Roche Fan Yolk Color grade, which registered 5.0 and 6.1 for the 1% and 2% chlorella groups, respectively, compared to 4% for the control group (\(p<0.001\)). These findings suggest that chlorella dietary supplementation can enhance the carotenoid content and improve the color of yolks in laying hens. [PERSON] et al. [47] obtained increases in egg yolk redness from 1.33 (C) to 12.67 (1.5% spirulina) and 16.19 (2.5% spirulina), and a significant yellowness (b*) reduction parameter from 62.1 (C) to 58.17 (1.5% spirulina) and 55.87 (2.5% spirulina).