treatments, and control charts derived therefrom illustrate maintenance within limits of the inevitable quality loss which accompanies storage of eggs. Knowledge ...
RESEARCH NOTE QUALITY CONTROL CHARTS FOR STORAGE OF EGGS HULYA KAHRAMAN-DOGAN, LEVENT BAYINDIRLI and MUSTAFA OZILGEN’
Food Engineering Department Middle East Technical University 06531 Ankara, Turkey Accepted for Publication May 1, 1994
ABSTRACT Modified Shewhart charts were constructed with weight loss, Haugh units and yolk index for storage of untreated, soda lime, water glass or oil coated and thermostabilized eggs. The data obtained showed differences between the treatments, and control charts derived therefrom illustrate maintenance within limits of the inevitable quality loss which accompanies storage of eggs. Knowledge of the trends make it possible to readjust the storage conditions to prolong the period before violation of acceptable limits comes into play,
INTRODUCTION The Shewhart control charts consist of means and range charts and are used in quality control with measured properties. The mean of the replicate measurements may be calculated as:
x = -1 ”
C xi i= 1
x
Where is the sample mean, xi’s are the values of the individual items constituting the sample, and n is the number of the items in the sample set. The means charts are constructed to control the means of the replicate samples within ‘Corresponding author: Mustafa Ozilgen, International Fax: 90 (312) 210 1270.
Journal of Food Quality ( I 994) 17 495-501. All Riglifs Reserved. Copyright 1994 by Food & Nutririon Press, Itic.3 Trumhull, Connecticut.
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the required upper (UCL) and lower (LCL) control limits. The range of a sample is defined as:
where and correspond to the maximum and the minimum rneasurements of the quality factor in the ithsample set. The mean value of the ranges may be calculated as:
where k is the number of the sample sets. The range charts are constructed to assure that the range of the individual samples are confined within predetermined limits. In such a quality control plan the central line (CL,) and the upper (UCL,) and lower (LCL,) control limits of the means chart may be calculated as (Miller and Freund 1985):
k
C L , = r1; Z T i
(4) i=1 -
3R
CL, - d&
LCL,
=
UCL,
= CL, +
3R
il&
Where d, is a correction factor depending on the sample size n (Miller and Freund 1985). The central line (CLR) and the upper (UCLR) and lower (LCLR) control limits of the means chart may be calculated (Miller and Freund 1985):
CL, = R LCL, = D, R UCL, = D, R
D, and D, are correction factors applied to sets LCL, and UCL,, In any statistical quality control plan the ranges and the means of the samples are tried
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to be maintained within the predetermined upper and lower control limits. If the measurements tend to violate these limits the process is modified, e.g., storage temperature or humidity may be adjusted to correct the causes of the violation. Coating or thermostabilization slows down deterioration of the eggs but cannot eliminate it. Equations (4) - (9) are used in the processes where the mean and the range of the measured quality factors are maintained within the prespecified constant limits, and may not be suitable for use in quality control of the stored shell eggs, where deterioration is inevitable but at reasonably slow rates. In that case acceptance may persist for quite some time. In the present study we modified Eq. (4) - (9) then checked their validity under different storage applications by using the weight loss, Haugh units and the yolk index as the quality factors of the eggs.
MATERIAL AND METHODS The eggs with identical biological origin and 65 +5 g of initial weight were purchased from the Poultry Research Institute (Ankara, Turkey) within a few hours after laying. Clean eggs with no apparent defects were used in the experiments. The eggs were coated by dipping into soda lime (1.26 g/l Ca(OH),, 0.1 % NaC1) or water glass (sp.gr. 1.34, 25 % (v/v) Na,SiO, solution) for one hour or into corn oil for 3 min (Kahraman-Dogan 1993). Thermostabilized eggs were dipped into hot water at 58C for 15 min. The treated eggs were drained and dried at room temperature (20+2C). The untreated control and the shell treated eggs were stored at room temperature in the cardboard egg boxes. Experiments lasted for 45 days. The sample size, n, was six with weight loss, 3 with Haugh units or yolk index measurements. The weight determinations were done with a laboratory scale (Sartorious, model PT 120, Germany, sensitive to 0.01 8). The interior quality of the eggs were quantified with the Haugh units (HU) (USDA 1972): H U = 100xlog{7.57+albumen height(mm) - 1.7x[egg weight (g)])o.37}(10)
The yolk characteristics were quantified with the yolk index (YI) (Stadelman 1977):
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RESULTS AND DISCUSSION A storage process may be controlled by directly measuring properties of the store room or those of the eggs. With the quality factors of the eggs, it would be possible to determine the rate of the quality loss. If the rate of the quality loss is greater than expected, the storage conditions i.e., the storage temperature or humidity, etc., may be adjusted. In the present study variations in the egg quality were followed with weight, Haugh units and yolk index changes.
Weight Loss The average weight decreased with time linearly. The central line of the means chart was CL,
=
CL,, - a t
where CL, represents the fitted sample means, CL,, is the initial average weight of the sample, a is the coefficient of the decrease in weight. Values of the sample mean weights were calculated with Eq. (1). Constants CL,, and a were calculated with linear regression to minimize the sum of square error. It was also seen that the range of the weight loss was proportional with the average weight loss of the samples during the storage period:
where k represents the time dependent sample means, a is the initial average range of the samples, b is the coefficient of increase in range in proportion with the sample mean. Constants CL,, and a were calculated with linear regression to minimize the sum of square error. Equation (12) was substituted in Eq. (13) for then a time dependent expression was obtained for the range of the samples:
x,
where CL, represents the fitted sample ranges, CL,, is the initial range of the samples, 0 is the coefficient of the increase of the ranges with time. Values of the constants CL,, and 0 were calculated with linear regression to minimize the sum of square error, Eq. (14) is given in Table 1B for the specific experimental conditions. Equations (12) and (14) were substituted in Eq. (5) and (6) for CL, and CLR, respectively to calculate UCL, and LCL, (Table 1A). Equation (14) was substituted in Eq. (8) and (9) to determine UCLR and LCLR, respectively.
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Since the ranges of the samples change with time, different slopes were observed with the means chart lines pertaining to the same experimental conditions and LCL,, CL, and UCL, were not parallel (Table 1). The negative values of parameter CL,, and CL,, implies that weight loss started with some delay in the related experiments. It is not uncommon to observe negative values especially with LCL, and LCL, while constructing the Shewhart control charts. These negative values are replaced with zero when they are physically meaningless. Weight loss starts with some delay, the overlapping or crossing control limits imply that the control charts may not be used for an initial short period of a few days.
TABLE 1. CONTROL CHART PARAMETERS FOR WEIGHT LOSS A) Means chart parameters
Group Control Oil coated Soda lime coated Water glass coated Thermostabilized
LCL, -0.0 14+0.085t 0.009+0.014t 0.039+0.045t 0.047+0.055t -0.039+0.001t
CLX -0.2 9 7+02 3 8t 0.031+0.017t -0.182t0.179t 0.02 1+0Ol5t -0.350+0.E32t
UCL, -0.592+0.59 1t -0.027+0.047t -0.674+0.402t -0.428+0.951t -0.660+0.463t
B) Range chart parameters Group Control Oil coated Soda lime coated Water glass coated Thermostabilized
LCLR 0 0 0 0 0
CLR -0.216+0.112t -0.043+0.022t -0.359+0.163t -0.063+0.007t -0.223+0.169t
UCL, -0.432+0.224t -0.086+0.044t -0.719+0.327t -0.126+0.014t -0.454+0.338t
TABLE 2 . CONTROL CHART PARAMETERS FOR HAUGH UNITS A) Means chart parameters
Group Control Oil coated Soda lime coated Water glass coated Thermostabilized
LCL, 62.7-1.15t 75.5-0.30t 67.3-0.98t 60.8-0.92t 75.7-0.80t
CLX 76.7-1.15t 84.7-0.30t 79.1-0.98t 74.3-0.92t 82.9-0.80t
UCL, 90.7-1.15t 93.9-0.30t 91.0-0.98t 88.7-0.92t 9 1.0-0.80t
CLR 12.7 9.0 11.6 12.1 7.0
UCL* 32.7 23.2 29.9 31.3 17.9
B) Range chart parameters Group Control Oil coated Soda lime coated Water glass coated Thermostabilized
LCLR 0 0 0 0 0
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H. KAHRAMAN-DOGAN, L. BAYINDIRLI and M. OZILGEN
TABLE 3. CONTROL CHART PARAMETERS FOR YOLK INDICES A) Means chart parameters
Group Control Oil coated Soda lime coated Water glass coated Thermostabilized
LCL, 43.9-0.76t 45.1-0.17t 42.6-0.5% 44.2-0.73t 43.6-0.72t
CLX 47.6-0.76t 48.7-0.17t 47.3-0.59t 48.8-0.73t 48.2-0.73
UCL, 51.2-0.76t 52.2-0.17t 51.9-0.59t 53.3-0.73t 5 2 7-0.72
CLR 3.6 3.5 4.5 4.4 4.5
UCLR 9.2 8.9 11.7 11.5 11.5
B) Range chart parameters
Group Control Oil coated Soda lime coated Water glass coated Thermostabilized
LCLR 0 0 0 0 0
';:FA 50 25 0
0
20
40
60
0
20
r
0
20
40
60
3
20 0
*.
10
8
0 40
60
TIME (days) +
FIG. 1. QUALITY CONTROL CHARTS FOR HAUGH UNITS Experimental data are shown in symbols (m). Solid lines describe the lower and the upper control limits. (a) Untreated eggs, (b) Oil coated eggs.
Haugh Units and the Yolk Indices The range of the Haugh units and the yolk indices neither increased nor decreased consistently during storage; therefore the range chart was constructed with Eq. (7) - (9). The average Haugh unit and the yolk index values decreased
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with time and CL, was calculated as given in Eq. (12). The parameters LCL, and UCL, were calculated from Eq. (5) and (6) after substituting equations (12) and (3) for CL, and R, respectively. Since the range of the sample sets did not change with trend during the storage period the means chats parameters, i.e., CL,, LCL, and UCL, were parallel (Tables 2 and 3). Two sets of typical plots were given in Fig. 1 to exemplify the modified Shewhart charts. Scatter of the experimental data among the control limits given in Tables 1-3 were similar to those of Fig 1.
CONCLUSIONS Weight loss, Haugh units and the yolk index values were used to construct modified Shewhart charts with all the control and shell treated egg groups to maintain the inevitable quality loss within acceptable limits. Violation of the quality control limits, especially in the early stages of storage, warns the operators that the eggs will deteriorate if the storage conditions should not be improved. Then the operators may readjust the storage conditions, i.e., may lower the store room temperature, etc., to slow down further quality loss, or send the eggs to the market earlier.
ACKNOWLEDGMENT This study was supported by the applied research fund No. 91-03-1402 of the Middle East Technical University.
REFERENCES KAHRAMAN-DOGAN, H. 1993. Shelf life improvement and quality retention of shell-eggs. MS. Thesis, Middle East Technical University, Turkey. MILLER, I. and FREUND, J.E. 1985. Probability and Statisticsfor Engineers, 3rd Ed., pp. 426-43 1, Prentice-Hall, Inc., Englewood Cliffs, NJ. STADELMAN, W.J. 1977. Egg Science and Technology. Van Nostrand Reinhold/AVI, New York. USDA. 1972. Egg Grading Manual, USDA consumer and marketing Service, Agriculture Handbook No: 75, Washington, D.C.
