1. Introduction

Vitamin and mineral supplementations in layer diet remained indispensable due to their participation in all biochemical processes and chicken gut flora provide very little vitamin synthesis but compete with the host for dietary vitamins [1]. Chicken eggs contain high-quality proteins, carbohydrates, easily digestible fats and minerals, as well as valuable vitamins [2]. The use of quality premix is an important feature of a successful poultry production leading to improved safety, reliability and performance [3] of laying birds and quality of eggs produced. However, in Nigeria like some other developing countries, refrigeration of eggs is seldom practiced and eggs produced are temporarily stored at room temperature until sold to final consumers.

Apart from direct human consumption, eggs are valuable raw materials for the pharmaceutical and cosmetic industries due to their multifunctional properties like foaming, gelling and emulsifying [4] which are highly dependent on interior and exterior characteristics reflected both in the quality and size [5]. The quality is composed of those characteristics of egg that affects its acceptability to consumers such as cleanliness, freshness, weight, shell quality; yolk index, albumen index, Haugh unit and chemical composition [6]. As reported [7-9], egg quality is influenced by management, climatic factors, nutrition, breed and post-lay handling practice. Eggs are highly susceptible to internal quality deterioration during storage [10] depending on shell and internal content [11, 12]. Factors associated with the level of quality loss are time, temperature, humidity, air movement and handling [13]. There is therefore the need for further documentation of the effects of dietary VMP vis a vis possible interaction of RS on DOS of eggs in the hot humid tropical environment. Therefore, this study was aimed at evaluating the proximate composition and physical characteristics of eggs from hens fed different commercial VMP under two RS as affected by the DOS.

2. Materials and Methods

The study was carried out at the Poultry Unit, Teaching and Research Farm, University of Ibadan, Ibadan, Nigeria. 240 twenty weeks old black Bovan nera at the point of lay were allotted to two RS (i.e. BC and DL systems) with 120 birds per RS. In each RS, birds were randomly allotted to six dietary treatments in triplicate of eight birds per replicate making 24 birds per treatment. Five isonitrogenous and isocaloric layer diets were formulated. The composition of the experimental diets is shown in the Table 1. The feed were each supplemented respectively with 0.25% Nutripoult (T1), Hinutrients (T2), Agrimix (T3), Daramvita (T4) and Micromix (T5). The experimental diets and water were given to the birds ad libitum for fifteen weeks. Table 2 shows the composition of the test VMP as shown on the respective labels.

Table 1. Gross Composition (%) of Experimental Diets Fed to Layers

Table 2. Composition per 2.5 kg of Test Vitamin-mineral premixes as shown on the labels

At week 36, a total of 75 eggs were sampled from each RS, they were stored at room temperature for 28 days and were subsequently withdrawn serially for analyses on days 0, 7, 14, 21 and 28. Recorded minimum and maximum ambient temperature in the DOS of eggs were 23.4 and 27.9℃ respectively while the relative humidity ranged between 73 to 87%. Proximate compositions and physical characteristics (external and internal) of the eggs were measured at days 0, 7, 14, 21 and 28 DOS. Each egg was homogenized and the proximate composition determined [14]. Egg length and diameter were measured with electronic venier caliper. Eggs were weighed and broken on a flat surface, and the height of thick albumen was measured with a tripod micrometer. The albumen and yolk were separated, and only yolk was weighed. Shell thickness was measured with micrometer screw gauge after air drying at room temperature. Egg weight loss was determined as the difference between successive weights of eggs at different weighing days. Albumen weight was determined by the difference between egg weight, yolk weight and shell weight while albumen pH was measured with pH meter. Yolk index was estimated from the ratio of yolk height to yolk width. Haugh unit was determined from albumen height and egg weight using the equation as described [15]; HU= 100log (h + 7.6 - 1.7W^0.37) where HU is Haugh unit, h is albumen height (mm), W= egg weight (g).

2.1. Statistical Analysis

Data were analyzed using three way analysis of variance [16] while significant means were separated by least squares method.

3. Results and Discussion

Effect of RS and VMP in the DOS on proximate composition of chicken eggs is shown in Table 3. The crude protein (CP) varied only numerically (P>0.05) among treatments and was not affected by RS. This observation conformed to the documented opinion [17] that egg displayed very consistent composition with regard to its content of total proteins, essential amino acids, total lipids, phospholipids, phosphorus, and iron. Also, protein and total lipid content of eggs were noted [18] not to be affected by the diet of the hen but the types of lipids (fatty acid composition) which in a way contradicted other proximate values of ether extracts (EE), total ash and nitrogen free extracts in Table 3. It was observed that dietary VMP type altered the moisture, CP and EE composition of eggs in this study highly significantly (P<0.01). The CP and EE (11.63 and 7.67 respectively) were significantly higher (P<0.01) in eggs from hen fed diets containing T1 compared to others. In the DOS, CP (11.45 to 11.59), and ash (1.24 to 1.34) increased very significantly (P<0.01) with decreased moisture content. Aside from EE composition, the DOS altered other proximate values highly significantly (P<0.01). The interactions of DOS X VMP only resulted in significantly different (P<0.05) EE values. Also, the interactions of the RS X VMP altered all eggs proximate values highly significantly (P<0.01). However, the interactions of RS X DOS and the RS X VMP type X DOS on egg proximate parameters were all not significantly different (P>0.05).

Table 3. Effect of Rearing Systems, Vitamin-mineral Premixes and the Duration of Storage on Proximate Composition of Chickens Egg

Effect of RS, VMP and the DOS on external characteristics of chicken eggs is shown in Table 4. The net weight loss in eggs from DL was 1.7% which was significantly higher (P<0.05) when compared with those from BC (1.6%). The weight losses significantly (P<0.05) increased with the DOS. Weight losses in eggs during storage were 0, 0.9, 1.7, 2.5 and 3.2% for days 0, 7, 14, 21, and 28 respectively which varied significantly (P<0.05) from one another. Weight losses from eggs of hens fed T1, T2 and T3 were not significantly different (P>0.05) during storage, however, they were significantly higher (P<0.05) when compared with stored eggs from hens on diets T4 and T5. The consistent increased weight loss in egg with advancing DOS agreed with the earlier report [19]. However, authors [20] opined that egg weight did not change in the first 10 days of storage. According to reports [21-24], the losses could be ascribed to the release of carbon dioxide, ammonia, nitrogen, hydrogen sulphide gas and water from eggs. Another report [25] also suggested that reduction in egg weight could be attributed to the loss of humidity from inside the egg due to evaporation effects.

Table 4. Effect of Rearing Systems, Vitamin-Mineral Premixes and Storage Time on External Characteristics of Chicken Egg

Eggs from hens on DL and those fed diets containing VMP 2 were observed to lose more weight compared to others and this could be attributed to the lowered egg shell thickness. It was also noted [26] that thin-shelled eggs lost more moisture compared to when the shell was thick. Also, shell weight and shell thickness of eggs from hen fed supplemental VMP 2 and 3 were observed to be lower in both RS which thus resulted in higher weight losses during storage compared with those fed VMP 5 which recorded lower weight loss. Dietary vitamin D determines the level of calcium metabolism for egg shell formation and vitamin D deficiency has been observed to lead to poor eggshell quality, mainly due to decreased eggshell weight [27].

The effect of RS, VMP type and the DOS on albumen characteristics of chicken eggs is shown in Table 5. The overall albumen height of stored eggs from hens fed T1, T2, and T4 were similar (P>0.05) but differed significantly (P<0.05) from those fed T3. However, albumen height of stored eggs from hens on T3 was similar (P>0.05) to those of T2 though, the overall Haugh unit of all stored eggs on the different dietary VMP were similar (P>0.05). The overall albumen heights were 6.99, 3.92, 3.22, 2.22 and 1.62mm for days 0, 7, 14, 21 and 28 respectively. Also, the Haugh unit values were 83.08, 56.52, 48.03, 30.63 and 15.34 for the corresponding 0, 7, 14, 21 and 28 days respectively of storage. These values significantly decreased (P<0.05) as the DOS increased irrespective of the RS and the VMP used. Much deterioration was observed in overall values of albumen heights (6.99mm to 1.62mm) and Haugh unit (83.08 to 15.38) with the DOS. These results agreed with those in literature [19, 20] on significant decrease in albumen height and Haugh unit with increased DOS.

Table 5. Effect of Rearing Systems, Vitamin-mineral Premixes and the Duration of Storage on Albumen Characteristics of Chickens egg

During storage, some physical and chemical modifications take place in albumen. These are the thinning of the albumen thickness [28], main increase of albumen pH caused by the loss of carbon dioxide from the egg through the pores in the shell [29] and changes occurring in ovomucin [28, 30]. A rapid loss of CO₂ leads to a decrease in quality until the state of gas balance is reached between the inside and outside of the egg [25]. Haugh unit (56.52) was observed to fall in the grade of B (59-30) at days 7 of storage (Table 4) as against the desired grade AA (100-72) or A (71-60). This showed that decline in egg quality was sharp at room temperature in line with the documented report [31].

Also, Haugh unit was related to the DOS (Fig. 1) and the regresion equation was

(1)

This indicated a strong negative and highly significant relationships (P<0.01).

Figure 1. Effects of Duration of Storage on Haugh Unit of chickens Egg

Effects of RS and VMP on albumen pH were not significant (P>0.05). However, albumen pH values significantly increased (P<0.05) with DOS, although there was no significant difference (P>0.05) in the albumen pH obtained at days 21 and 28. The values for these days were statistically higher (P<0.05) compared with the albumen pH at days 0, 7 and 14. Also albumen pH obtained at days 0, 7 and 14 were significantly different (P<0.05). A rapid upsurge in alkalinity of albumen was observed which increased from 8.77 to 9.39 (Table 5). This observation conformed to those reported [19] on increased albumen pH from 7.47 to 9.11 in 10 days of storage.

Effect of RS, VMP and DOS on yolk characteristics of chicken eggs are shown in Table 6. Yolk height, yolk index, and yolk weight values were similar (P>0.05) among the different VMP and the RS. The changes in yolk weight were observed not to be significantly altered (P>0.05) during storage which conformed to earlier findings [20] that yolk and shell weights were not changed by storage. Yolk height and yolk index was observed to decrease significantly (P<0.05) from 15.52mm and 41.94 to 5.41mm and 11.38 respectively with increased DOS. Tabidi [25] reported that reduction rate of yolk index was sharp at room storage. Flattening of yolk was primarily due to increased water content caused by osmotic migration from albumen through the vitelline membrane [28].

Table 6. Effect of Rearing Systems, Vitamin-mineral Premixes and Duration of Storage on Yolk Characteristics of Chickens Egg

Yolk index was related to the DOS (Fig. 2) and the regresion equation was:

(2)

which also indicated a strong negative and highly significant relationships (P<0.01).

Figure 2. Effect of Duration of Storage on Yolk Index of Chickens Egg

4. Conclusions and Recommendations

The RS and the type of dietary VMP both affected the composition and egg quality characteristics in the DOS. The VMP 1 and 5 would be preferred in both RS as they both tend to ensure good albumen height, Haugh unit, yolk height, yolk index, higher shell thickness and lowered weight loss. Quality of eggs was observed to decrease when stored at room temperature. Eggs qualities deteriorated bellow desirable grade before day 7 of storage at room temperature in the hot humid tropics. Thus, alternative methods should be considered for storing excess eggs produced to enhance retention of freshness.