Acta Universitaria

Mediante regresión lineal con variables ficticias, se determinaron los cambios en altura de albúmina y color de la yema en huevos de supermercado en anaquel durante 1, 3 y 6 días. Los huevos fueron congregados en dos categorías: un grupo con temperatura ambiental mayor a 25 ºC y otro con temperatura inferior a 25 ºC. Se encontró que ambos grupos iniciaban su vida de anaquel con la clasificación cualitativa más alta, pero que al sexto día disminuía a la segunda categoría. Sin embargo, el grupo almacenado por debajo de 25 ºC presentaba un menor efecto sobre la altura de la albúmina. Los cambios porcentuales en el color de yema medidos con L* a* y b* fueron inferiores para el grupo por abajo de 25 ºC. Los resultados indican que cuando la temperatura alcanza o rebasa los 25 ºC, los huevos no deben permanecer en el anaquel más de una semana.

INTRODUCTION

Egg quality is given by a group of characteristics that include size, shell thickness and color, albumin thickness and yolk color. But when consumers are asked about what is the most important quality trait, answers are: freshness followed by yolk color in order of importance (Arias, Fernández & Nys, 1998).

As soon as the egg is laid, physical and mechanical changes occur that affect its quality, one of them is weakening of vitelline membrane that separate yolk and albumin. Consequences of these changes may be assessed by measuring height of albumin using Haugh units (Berardinelli, Ragni, Giunchi, Gradari & Guarnieri, 2008).

Haugh unit is the most common parameter to measure albumin quality and it relates egg´s weight to albumin height, whereas high quality eggs have a higher greater Haugh units score (Alleoni & Antunes, 2001). Nevertheless, correction for egg weight use in the Haugh unit formula adds a bias to the calculation and it is not considered reliable. Because of this situation, albumin height measurement is viewed as a recommended alternative to measure albumin quality due to the fact that it can be measured anywhere with basic equipment (Silversides & Budgell, 2004).

Research suggest that changes in albumin quality over time at room temperature storage are equally well described by albumin height and the Haugh unit score. Therefore, egg weight adjustment use in Haugh unit is unnecessary (Silversides & Villeneuve, 1994).

Ovomucin layer, responsible for the firmness of thick albumin, becomes weak when stored for a long period. Therefore, albumin spreads over a wide range of area in an abnormal manner when an egg is broken, causing an increase in albumin length and width. As a result, albumin height decreases (Alade, Usman & Muhammed, 2013).

Albumin height of eggs reaches a maximum when an egg is laid and decreases when storage time increases (Jin, Lee, Lee & Han, 2011). However, quality can be maintained if immediately after collection an egg is held at temperatures between 0 ºC and 4 ºC (Barbosa, Sakomura, Mendonça, Freitas & Fernandes, 2008). It has been reported that eggs quality suffers no significant changes even after 20 days in storage if they are refrigerated (Estrada, Galeano, Herrera & Restrepo, 2010). Other factors that are considered to affect albumin quality are: strain of bird, genetic selection and also hen age, as albumin quality declines with bird age. It has been reported that infectious bronchitis affects albumin quality; on the contrary, albumin quality is not greatly influenced by bird nutrition (Roberts, 2004).

An important factor that determines acceptability of an egg for the consumer is intensity and uniformity color of yolk (Samli, Agma & Senkoylu, 2005). Even so, it is important to consider that color preferences by consumers differ around the world. A study done in Turkey found that dark color yolk is preferred but also that color is not an important quality trait (Karadas, Grammenidis, Surai, Acamovic & Sparks, 2006). To accurately determine yolk color in a color sphere, it is recommended to use L* (lightness) a* (redness) and b* (yellowness) measurements (Mizrak et al., 2012).

In Mexico, eggs quality is determined by the NMX-FF-079-SCFI-2004 Federal norm. According to albumin height score, product is classified in four categories: Mexico extra (≥ 5.5 mm), Mexico I (4.2 mm – 5.4 mm), Mexico II (2.2 mm – 4.1 mm) and outside of classification (< 2.2 mm). In the case of yolk color, norm states that in all classification color should be between a 9 to 12 score in Roche colorimetric fan. In case of storage, norm only states that storage conditions should be cool and dry and that eggs package displays an expiration date (Secretaría de Economía [SS], 2004).

It has been reported that supermarket eggs have a higher internal quality (Leandro et al., 2005) if they have a shorter display period and a more stable environment. Although if supermarkets do not provide this conditions, quality is compromised. Eggs sold in Mexican supermarkets are mostly displayed under room conditions and because the Mexican norm only recommends that eggs are stored under cool and dry conditions. It is common that eggs are not refrigerated to reduce effect of long time storage. Because of this consumers will buy low quality eggs without them knowing and the likelihood of this happening increase if room temperature of supermarket is high.

It is important to measure how quality of supermarket eggs decays along time, so retailers are better informed and they are able to offer eggs with the internal quality characteristics that consumers expect.

Therefore, objective of this work is to quantify the internal quality change that exhibit supermarket eggs that are shelved up to 6 days in storerooms with high temperature change so that more information is made available to the supplier and consumer of this good.

MATERIALS AND METHODS

The study was carried out between September and December in the city of Mexicali, located in northwest Mexico (latitude 32º 40’, longitude 115º 28’). Climate is characterized as extremely arid with rainfall rarely exceeding 100 mm per year, and an average temperature of 34.7 ºC (–5 ºC in winter and 50 ºC during the summer), and an altitude of 3 m (García, 1981). Eggs used in the study were obtained from a local farm that uses a 72 week production system and were produced by hens from the hy-line strain. After collection eggs were stored at 4 ºC to 8 ºC for less than 12 h before being distributed to four local supermarkets. Eggs were tracked from farm to supermarket using lot number. To classify eggs based on measurements of albumin height and yolk color Mexican egg quality norm NMX-FF-079-SCFI-2004 was employed.

Two groups of eggs were collected from supermarkets storerooms. The first group included eggs from the same lot stored at temperatures above 25 ºC with a temperature range between 25.7 ºC and 30 ºC, while the second included eggs that were stored below 25 ºC. In this case temperature span was between 18.5 ºC to 20.8 ºC, to assure in both groups an adequate number of eggs for subsequent samplings, 100 eggs were collected on the first day they were displayed and the lot number registered, so that samples two and three were part of the same lot. Second collection was done three days later and a sample of 57 eggs was obtained from the above 25 ºC group and 58 eggs from the below 25 ºC. The last sampling was done 6 days after the first, and in this case 57 eggs were collected from the first group and 56 from the second.

Immediately after collection, eggs were placed in a cooler, transported to a laboratory, and analyzed the same day. Albumin height and color values used in this study were the mean of three measurements obtained directly from each egg after it was placed on a chopping board. Height of the albumin was measured using an digital micrometer (Ames, model 03-0230Q, USA) (Juárez-Caratachea, Gutiérrez-Vázquez, Segura-Correa & Santos-Ricalde, 2010), and in the case of yolk color the measurement instrument was a Minolta CM-2002 spectrophotometer with a specular component, a D65 illuminant and a 10º observer (Minolta Camera Co., Ltd, Japan). Yolk color was expressed in the CIELAB dimensions of lightness (L*), redness (a*), and yellowness (b*).

To obtain the albumin height mean and measure yolk color of eggs on the day they were placed on shelves, as well as changes in albumin height and color that eggs in the two groups attained after spending 3 and 6 days on store shelves, the following linear regression model with dummy variables was used (Asteriou & Hall, 2007):

`\mathrm{y_{i}=\beta _{0}+\beta _{1}D_{1i}+\beta _{2}D_{2i}+u_{i}}`.

Where:

β₀: Intercept, which in this case is the main value of dependent variable (albumin height or color) obtained on the day anegg is placed on store shelves.

β₁: The changes in albumin height or color that eggs show 3 days after being placed on the store shelves.

D₁: Dummy variable that takes a value of 1 if the egg is from the group that has been on the store shelves for three days, or takes the value of 1 in any other case (0 and 6 days).

β₂: The changes in albumin height or color that eggs show 6 days after being placed on the store shelves.

D₂: Dummy variable that takes a value of 1 if an egg is from the group that has been on store shelves for six days, or takes value of 1 in any other case (0 and 3 days).

With the use of dummy variable in regression models, it is viable to quantify the effect that nominal independent variable (period on display) had on dependent variable (albumin height or color) (Gujarati & Porter, 2009). Although the number of days that eggs are displayed on showroom may be seen as a continuous variable, because no data between 0 and 3 days and between 3 and 6 days was recorded. It was possible to consider time spent on shelves as a qualitative variable that was given 1 as a value when the variable possessed the attribute (AH on day 0) and 0 otherwise (AH on days 3 and 6) (Asteriou & Hall, 2007). Statistical analysis was done using Megastat program, 2007 version.

RESULTS

Results of ordinary least square estimation of both groups of eggs indicate that after being placed on supermarket shelves, eggs show a statically significate reduction in albumin height (AH) (p < 0.01). However, this decline is even larger in the > 25 ºC group. After 6 days on shelves, eggs from this group reduced their quality 44.23%, while eggs in the < 25 ºC group lost only 32.2%. Table 1 shows that on day 1, eggs in the > 25 ºC group had a mean 8.32 mm AH, which according to the NMX-FF-079-SCFI-2004 norm places them in the highest quality category (Mexico extra). After 3 days, AH mean value dropped to 5.55 (8.3212 – 2.7712) which placed eggs in the upper limit of Mexico I category. On the other hand, after 6 days in shelves eggs had a 4.6317 mean (8.3212 – 3.6895), maintaining them in Mexico I category.

Tabla 1. Albumin height coefficients and confidence interval for the > 25 ºC group.
R = 0.804	R² = 0.646	SE	Confidence interval
Variables	Coefficients	SE	95 % lower	95 % upper
Intercept	8.3212**	0.1236	8.0776	8.5649
Three days	–2.7712**	0.2056	–3.1766	–2.3659
Six days	–3.6895**	0.2033	–4.0904	–3.2886