Agitating – By vibration – Of stirrer
Reexamination Certificate
1997-07-03
2001-08-14
Cooley, Charles E. (Department: 1723)
Agitating
By vibration
Of stirrer
C366S197000, C366S204000
Reexamination Certificate
active
06273600
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to devices for suspending microbes or other analytes to allow the determination of the microbiological safety or hygienic quality of foods and other samples or to mix powders or liquids.
BACKGROUND OF THE INVENTION
In order to determine the existence and quantity of microbes or other analytes in foods and clinical samples it is usual to remove them from the sample and disperse them in water or other liquid. It is common to suspend the microbes from a 10 gram sample into 90 ml of liquid, a total volume of 100 ml. The suspension preferably contains a minimum of debris suspended from the sample, since it may interfere with the analysis. It should be noted that microbes attached to surfaces are relatively easily removed, whereas those situated internally in pores or folds or otherwise trapped in foods require a greater effort to disperse them. Many techniques and devices have been developed with the aim of obtaining representative removals of the microbes from test samples, for example, swabbing the surface or pressing agar contact plates on them. However, the variability of microbial adhesion makes swabs and contact plates very imprecise in their yield of microbes.
Techniques in which a portion of the test sample is excised and blended in liquid in a blender or other device which breaks up the sample and disperses the microbes generally produce microbial suspensions that reflects more accurately the microbial load of the sample. However, a blender is inconvenient because the suspensions it produces contain a great deal of debris, and because it must be cleaned and sterilized after use. An apparatus in which the test sample is sealed in a plastic bag and kneaded and crushed by electric-motor driven paddles to produce representative microbial suspensions is described in U.S. Pat. No. 3819158. The microbe-suspending action of this apparatus depends on the crushing action and on the motion of the liquid as it is driven from side to side in the bag. This apparatus is convenient because the sample-containing bag may be discarded after use, thus eliminating the need to clean and sterilize the apparatus. However, the crushing action on samples still produces suspended debris and prevents the apparatus being used for samples containing hard objects such as stones or shell because these pierce bags and cause them to leak. Also, such crushing devices cannot safely use transparent glass or plastic doors and it is thus not possible to see the state of the sample without switching off the apparatus and opening it up.
In experiments on microbe removal from foods I have shown that a crushing action is rarely necessary and that high rates of liquid shear alone are very effective at suspending microbes. For example, the microbe suspending action of devices known as “vortex stirrers” is quite good but these devices are inconvenient because they cannot be used with large volumes or with disposable plastic bags. Similarly, spraying samples with liquid by means of a spray gun for which the impacting of the liquid spray causes a strong shear is effective; however, it is difficult to collect the microbe-containing liquid when the test surface is at certain angles to the vertical.
SUMMARY OF THE INVENTION
The vibration of liquids when sound waves are passed through them causes liquid shear at the surfaces of objects immersed in liquids, mainly because the differing densities of object and liquid prevent them from following the pressure waves at identical rates. Ultrasound, that is, sound at frequencies higher than human hearing abilities and generally in the range 20-30 kHz, has been shown to remove microbes efficiently from foods in limited situations, producing microbial suspensions relatively free of debris. However, ultrasound is useful only where microbes exist on surfaces that are easily accessible to the sound energy because sound intensities high enough to agitate samples and release internally situated microbes cause a cavitation which quickly kills microbes. At lower sound frequencies the energy for a given amplitude is less and less cavitation is produced; however, a device operating solely by sound energy in the audible range, such as might be generated by an electronic frequency generator and speaker or similar transducer would be impractically noisy in operation.
It should be noted that shock waves and sound waves are essentially similar and that shock waves can be generated in a liquid contained within a plastic bag by striking the bag sharply with a solid object. I have shown that a microbe suspender combining the microbe suspending ability of shock waves with a mechanical stirring effect that exposes the internal surfaces of samples without crushing them, removes microbes from foods effectively and reduces the amount of sample debris passing into suspension. Such an action can be obtained by beating a liquid-containing plastic bag alternately on its opposite faces by a beater at a frequency which is high enough to prevent the bag completely following the movements of the beater so that in each cycle of the beater it hits the bag and sends shock waves into it, provided that said frequency also is low enough that during a relatively large proportion of the beater cycle the beater remains in contact with the bag and transfers its energy efficiently to the bag as stirring energy. This energetic beating action is also very effective for mixing dry powders or for suspending them in liquids.
Referring again to U.S. Pat. No. 3819158, while this patent described the use of a plastic bag to which is applied mechanical energy in order to suspend microorganisms it describes a microbe suspending apparatus comprising two hard paddles kneadingly acting on the sample against a backing plate which forms part of the kneading means. Such a device must operate at approximately 300 pummellings per minute or slower. Carrying out this type of operation at higher speeds such as might cause shock waves is impossible because the paddles produce pressures in the liquid that rupture the bag. My invention provides a novel and improved means of suspending microorganisms in a plastic bag by providing for the combined application of shock waves and intense mechanical agitation to a sample and liquid contained in the bag.
By avoiding the crushing action of earlier apparatuses which knead samples inside plastic bags, my invention provides two major improvements. Firstly it minimizes the amount of sample debris passing into suspension, and secondly it permits the use of a transparent door through which the progress of the sample can be viewed, which is very desirable and which previously was not safely possible.
It has also been a problem with available bag-kneading microbe suspenders that the plastic bags are releasably sealed between an operator-protecting door and the kneading paddles by toggle clamping means that do not easily allow removal of the door to give the operator complete access to the kneading area. Thus it is difficult to clean up the mess when bag leakage occurs and in the event of a leak contaminated liquid drips onto the bench below the apparatus.
My invention provides means for confining a test sample with suspending liquid in a plastic bag, and of transmitting suspending energy to said sample and suspending liquid by means of non-crushing impacts on the outside of the bag, said impacts being within a range of frequency and amplitude such that they produce a suspending energy composed of a combination of shock or sound waves and mechanical shaking. To achieve this the sample and suspending liquid is releasably sealed in a flexible bag so that the bag hangs freely with some air space above the liquid. Said bag is then beaten by a vibrating or reciprocating beater which beats the bag on alternate sides at a frequency high enough that the bag is only able to partially to follow the movements of the beater thus producing an action in which:
1 the shock or sound waves caused by impact of the beater with the bag have a suspending effect, and;
2 impulses imparted
Cooley Charles E.
Filtaflex Limited
Seaby George A.
LandOfFree
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