Biotechnol. frog. 1992, 8, 240-243
240
Acoustic Resonances in Chicken Eggs Dipen N. Sinha,**+ Roger G. Johnston,: W. Kevin Grace,i and Cheryl L. Lemanskil Electronics Research, Physical Chemistry/ProcessTechnology, and Biochemistry/Biophysics Groups, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
We report the first acoustical vibration studies on chicken eggs. Measurements of the acoustic resonances offer the potential of simple, noninvasive, and rapid characterization of eggs. The technique appears at least partially sensitive to yolk puncture, shell cracks, and the presence of SaZmonelZa bacteria. There may be practical quality control applications.
Introduction
Experimental Procedures
We report in this paper the results of acoustical studies on chicken eggs. Measuring the acoustic resonances is a noninvasive, nondestructive, and rapid technique for characterization. There may be quality control applications for this novel technique. Such quality control and, in particular, possible detection of Salmonella bacterial contamination is of interest to the egg industry. Salmonella contamination of chicken eggs continues to be a serious economic and public health issue (Romanoff and Romanoff, 1949; St. Louis et al., 1988; Centers for Disease Control, 1990; Blimenthal, 1990). In addition to practical concerns, eggs are of interest because they provide a unique natural system to study the vibrational characteristics of thin shells of revolution. The mathematical study of vibrations in thin spherical shells began over 100 years ago with Lamb (1883) and Strutt Lord Rayleigh (1872). Their work forms the basis of all contemporary studies on vibrations of thin shells. An elastic thin shell of revolution possessesmany modes of vibrations that are commonly referred to as the resonant modes or resonant frequencies. If the shell is mechanically excited at frequencies that happen to coincide with these resonant frequencies, the large-amplitude vibrations of the shell body that result can be easily detected. At other frequencies (Le., away from resonance), the vibrational response of the shell is minimal. The prediction of these various modes of vibrations for aspherical shells is nontrivial (Rand and DiMaggio, 1967). Most experimental acoustical studies to date have relied on artificially made resonators. Eggs represent an interesting class of resonators in which a sturdy, but thin, shell completely surrounds the fluid contents. In this case, the vibrational modes of the eggshell get modified by the physical properties of the egg white and yolk. Our interest has been to study the natural vibrations of chicken eggs to see what meaningful information regarding the eggshell material and the condition of the egg contents (egg white and yolk) can be extracted from the vibrational characteristics of the eggs themselves. In this paper, we present our preliminary results of a study showing the empirical relationship between resonant vibrational characteristics of chicken eggs, puncturing of the yolk, cracks in the shell, and the effect of incubating Salmonella bacteria inside the egg.
Figure 1shows the apparatus used for this experiment. It consists of two piezoelectric (PZT) transducers (VIDEOSCAN V102-RB and V103-RB, Panametrics, Waltham, MA), a rigid support base, and a mechanical arrangement with which to adjust the height of one of the transducers. The apparatus was tested to ensure that it did not resonate during the measurements; no spurious vibrations were detected by the transducers. The transducers were wide-band, with a 1-MHz center frequency and diameters of 1 in. (V102-RB) and 0.75 in. (V103-RB). The bottom transducer (larger diameter) is used to excite the vibrations in the egg, while the smaller transducer on top of the egg is used to sense the vibrations. The electronics system consists of a custom, sweptfrequency spectrum analyzer, controlled by an IBM PC/ AT. We routinely used a frequency sweep range between 400 and 2000 Hz and a sweep duration of 6-10 s for most measurements. The sine-wave excitation amplitude ranged between 5 and 8 V. Complete details of the various techniques used and the electronics instrumentation will be published elsewhere. Before each measurement, the egg was gently shaken for a few seconds to dislodge the yolk from the shell membrane in case the yolk was partially attached. This permits good reproducibility of the data if the measurements are repeated on any given egg. After being shaken, the egg was centered between the two transducers. No coupling fluid was used. The top transducer rested very lightly on the egg; its weight was largely supported by an arm attached to a sliding rod. This sliding rod permits a very smooth movement of the transducer in the vertical direction. Two sets of measurements per egg were made: one with the egg’s long axis in the vertical position and the other with the long axis in the horizontal position (see Figure 1, panels a and b, respectively). The eggs used in this study were white, large Grade AA chicken eggs obtained from local food stores just prior to their use in our experiments. For some eggs, a small hole, typically 1.5 mm in diameter, was made in the eggshell using a sterile drill bit. This hole was placed along one spot on the minor axis of the egg, where the eggshell is flattest. With care, the hole could be drilled without causing any further damage such as cracking or puncturing of the eggshell or egg yolk. The hole was then sealed with a small amount of DUCO cement (Devcon Corp., Danvers, MA). The cement had no effect on the acoustic resonances. For some eggs, we deliberately placed a single puncture hole in the egg yolk using a 22-gauge syringe needle (0.7-
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* To whom correspondence should be addressed at MSD429, Los
Alamos National Laboratory, Los Alamos, NM 87545. Electronics Research Group. Physical Chemistry/Process Technology and Biochemistry/ Biophysics Groups. 8756-7938/92/3008-0240$03.00/0
0 1992 American Chemical Society and American Institute of Chemical Engineers
w.., 1992, Vol. 8, No. 3
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Figure 1. Experimentalsetup: (a) shows verticalpositioningof egg and (b) depicts horizontal measurement conditions. PZT, piezoelectric transducers;SR, sliding rod for height adjustment; PE, Plexiglas enclosure used to acoustically isolate the experiment. In the vertical position (a) the egg's air sac (curvy lines) is always at the bottom. mm diameter). The needle was inserted into the 1.5-mm hole in the eggshell, and the tip of the needle was inserted into the yolk once. (We believe these punctured eggs were sterile; we were unable to culture Salmonella or any other bacteria from them.) We carefully broke open the eggs after theacousticmeasurementstoexaminetheir contents. We found no evidence that any yellow yolk material had escaped into the egg white through the 0.7-mm hole in the yolk. For other eggs, viable Salmonella enteritidis bacteria wereplacedinside the eggandallowedtoincubateatroom temperature. The bacteria (ATCC13076) were obtained from the American Type Culture Collection. The introduction of the bacteria into the egg was accomplished by first dipping a small syringe needle into a culture of the bacteria and then wiping the needle on the edge of a sterile flask to remove the majority of the liquid. We estimate that approximately 10 000s.enteritidis bacteria remained on the syringe needle in less than 0.005 cm3 of liquid. The needle was then inserted just inside the eggshell into the egg white through the 1.5-mm hole in the shell. Care was takennottopunctureortouchtheeggyolkwiththeneedle. This careful insertion procedure allowed some or all of the bacteria to be dispensed into the egg white. A small dab of DUCOcementwasagainused toseal the 1.5-mmeggshell hole. The bacteria were then allowed to incubate in the egg at room temperature for 24-36 h before acoustic measurements were made. Soon after the acoustic measurements, the egg white and yolk were aseptically cultured on Hektoen enteric agar to obtain an estimate of the live Salmonella bacteria concentration. In the yolk, the concentrations were always in excess of 1million bacterial cellsig. Concentrations in the egg white were
