The effect of different vegetable oils on energy content of table eggs yolk

The aim of this study was to analyse the gross energy (GE) value of egg yolk from hens feed with different vegetable oils addition and based on gained results calculate the linear regression between GE and dry matter% (DM) in yolk. Total 66 hens Lohmann Brown Lite were divided in to 11 groups according to concentration and type of used vegetable oil (pumpkin oil 3%, flax seed oil 3%, indian hemp seed oil 2.5% and 5%, grape seed oil 2.5% and 5%, olive oil 2.5 and 5%, apricot seed oil 2.5% and 5%). Hens were in 40 th week of life and were housed per 6 in cage. From each group eight eggs were randomly selected. In total 88 egg yolks were separated and analysed for DM and GE content. The highest yolk GE was detected in group with 3% pumpkin oil addition (1,639 kJ 100 g -1 ) followed by groups with 2.5 and 5% indian hemp seed oil addition 1,632 and 1,632 kJ 100 g -1 , respectively. Difference between these three groups compared to yolk GE content (1,584 kJ 100 g -1 ) in control group was significant (P <0.05). All other added oils increased the yolk GE (P >0.05). GE concentration of yolk can be calculated as follows: GE = -17.34 + (32.73 * DM), R 2 = 0.819.


Introduction
Incubation within poultry hatching is not easy process, during which is necessary to create desirable microclimatic condition, microbiological status of laying eggs (Benková, 2008;Hrnčár & Bujko, 2012), but the appropriate composition of nutrients and energy content in the egg is also very important. The yolk and albumen in the egg supply the developing embryo with nutrients, water and minerals for normal growth. Yolk is an important nutritional component of the avian egg because it contributes 75% of the joules and provides all the lipids, and thus energy for the developing embryo (Noble et al., 1996). Results of Javad et al. (2011) show that egg weight, egg production, fertility and hatchability are significantly affected by diet formulation based on energy, which effects the reproductive performance of hens. Furthermore, according to Herkeľ et al. (2016) the addition of oil in to feed of laying hens affects the egg nutrients as well as the quality of yolk. Besides the importance of yolk for embryonic health and hatchability, plays yolk essential role in human nutrition through increasing the energy value and therewith also nutritional value of whole egg. Therefore, the aim of this study was to determine the effect of six different vegetable oil sources in feedstuff of laying hens on energy content of table eggs yolk.

Material and methods
Egg yolks used for gross energy determination were gained from eggs of Lohmann brown lite hens (during 40 th week of age), which were housed in three-floor cages (six hens in one cage; with a space allowance of 943.2 cm 2 per hen). There were six hens per cage. One cage represents one experimental group. Standard microclimatic condition and 16 hours light regime was used during experiment. Together 11 groups included this experiment. Group names were according to used vegetable oil supplement and its concentration in diet (Table 1).
In this study a commercial oils, as well as commercial feed mixture were used. Control (Con) group feed with commercial mixture for laying hens also included in experiment. All hens had diet and water ad libitum. The nutrient analysis of feed mixtures used in this experiment were realised according to standard methodology (AOAC, 2000) in Laboratory of Quality and Nutritional Value of Feeds at Department of Animal Nutrition (SUA in Nitra) and is shown in Table 2. Feeding of hens according to group was realised from 38 th to end of 40 th week of age. The 40 th week of age was the experimental week. For yolk GE determination eight randomly selected eggs per group were used (together 88 samples).
Yolks were separated, weighed and freeze-dried. After homogenization the DM gravimetrically at t 103 ±2 °C was determined. For GE determination a Calorimeter Leco AC 500 (Leco Corporation, USA) was used (direct calorimetry -measuring of the heat released after combustion of a sample according to manufacturer instruction). Gained results were statistically processed in IBM SPSS v. 20. Descriptive statistic was calculated and differences of means between groups were tested with one way ANOVA (Tukey HSD test). In addition, linear regression between DM (%) and GE content (kJ 100 g -1 ) of yolk was calculated.

Figure 1
Linear regression for GE calculation from dry matter content GE (kj 100 g -1 ) emphasized the importance of energy content of yolk on embryonic health and hatchability. Significant effect of diet composition on hatchability confirmed Javad et al. (2011) and also Pearson and Herron (1981). On the other hand, Antova et al. (2019) attributed different energy value of yolk to the chicken genotype, when they were reared under the same condition and fed with same feed. Whereas, Pištěková et al. (2006) found effect of rearing system of laying hens on egg quality. Also Roztočilová et al. (2018) highlighted the importance of feed quality on qualitative parameters of the eggs. Antova et al. (2019) determined GE of yolk by calorimetric method, which was similar as in this study. They gained yolk GE concentration in area from 1,438 to 1,608 kJ 100 100 g -1 of yolk. Radu-Rusu et al. (2014) published lower yolk energy value 1,208 to 1,330 kJ 100 g -1 of yolk, but they calculate the GE, whereas our results are established by direct calorimetry. Roe et al. (2013) published energy value for raw egg yolk 1,437 kJ 100 g -1 , which was lower compared to our results. Based on gained results of DM and GE content in raw yolk we calculate the linear regression for GE content. As published Vojtaššáková et al. (2000), the gross energy content is calculated according to average concentration of proteins, lipids, saccharides, food fibre, alcohols and variable "c", which for eggs has value 15.70. Calculation of GE according to DM percentage is more easily, than calculation according to determined nutrients as published Vojtaššáková et al. (2000).

Conclusions
The highest yolk gross energy content was determined in hens group fed with addition of 3% pumpkin oil, as well as indian hemp oil 2.5% or 5% addition. Yolk with higher energy content could provide more energy to embryo, which could have positive effect on hatchability of hatching eggs. This is a theme for future experiments in this area. The gross energy content of yolk can by estimated from dry matter content by using formula GE = 17.34 + (32.73 * DM), R 2 = 0.819.