This in turn could result in a decrease in the actual accumulated lethality which could potentially result in an unsafe product. Another technique to determine temperature within a fluid is the introduction of tracer capsules or data tracers in the flow, retrieving it at the exit, and downloading the temperature data to a computer.
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Infrared imaging is a technique that can be used to obtain surface temperature information. Thermochromic dyes that change color with time and melting point indicators that melt at a specific temperature are some of the other techniques that can be used to determine the temperature within fluids and also within particles. Thermoluminescent markers that emit a certain wavelength of light depending on the temperature can be used to determine the temperature within clear fluids on-line.
First and foremost comes the safety of the process and compliance with regulatory requirements. Other factors that come into picture are the extent of enzymatic inactivation and nutrient retention.
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Thus, the process is designed such that it is safe and results in maximum nutrient retention and the appropriate level of enzymatic inactivation. In the U. For example, UHT milk processing is covered under title 21 parts , , of the code of federal regulations CFR. The process should also adhere to the pasteurized milk ordinance PMO. When meat is involved, the regulations are imposed by the USDA.
In addition to these regulations, certain states have state regulations imposed on certain processes. During the past few years, HACCP has gained tremendous importance and its implementation has been extended by the FDA to various products after its initial application to certain acidified and low-acid canned foods. The most direct technique to do this is by measuring the temperature at the desired locations.
However, this is not easy in a continuous flow situation. Thus, indirect mechanisms, such as the change in color of a dye or the extent of sucrose inversion are used. As far as microorganisms go, one of the techniques to ascertain the extent of microbial destruction is by using an alginate particle with spores of Bacillus stearothermophillus embedded in it. The gel ensures that the microorganisms do not leak out and result in inaccurate degree of microbial destruction. When vegetative cells of bacteria are subjected to harsh conditions high heat or lack of nutrients , they form a hard proteinaceous coating outside the cell that can withstand the harsh conditions, and go into a passive stage and the organisms in this state are called spores.
Other factors affecting heat resistance are presence of ionic species, oxygen content, water activity moist heat is generally more effective than dry heat , pH acid medium is usually more effective than alkaline medium which is usually more effective than neutral medium , salts and sugars high concentrations are effective in reducing their resistance , and proteins and fats the presence of these materials increases the heat resistance.
Thus, it is important to determine the heat resistance of the organisms of concern in the substrate of interest and under the appropriate processing condition. It should also be noted that bacteria which tend to clump together are generally more resistant to heat and care has to be exercised when dealing with them. The slope of this line is equal to the negative reciprocal of the decimal reduction time. This graph is also referred to as the survivor curve, thermal death curve, inactivation curve or the thermal death time TDT curve. However, there are other models that do not make this assumption.
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Another commonly used technique to express the dependence of the rate of reaction on temperature is the quotient indicator method which defines a quotient indicator as the ratio of the reaction rates at two temperatures. Lethality is the ratio of the F0 value of the process to the F0 value required for commercial sterility. Thus, process lethality must be at least unity for commercial sterility. It can be thus seen that many combinations of time and temperature can yield an F0 value of 5 minutes.
The appropriate combination of time and temperature that is used for processing is based on other factors such as nutrient retention and enzyme destruction. This is where the cook value C of a process comes into picture. Graphical methods or optimization models are then used to determine the optimum time-temperature combination that renders the product safe and also retains the maximum possible amount of nutrients. The conservative approach involves the assumption that particles neither receive lethality nor any heat treatment in the heat exchanger.
The reason for not including lethality accumulated in the cooling section is that it is possible for particulates to break in the cooling section and thereby get cooled rapidly and hence not receive the assumed heat treatment and hence lethality. It all begins with an idea for the product. This is the product that we would like to aseptically process, package, and market as a high quality shelf-stable product.
The next step is to determine the tentative process layout. This includes the choice of pump, heat exchanger, holding tube, cooling unit, and packaging equipment. Let us begin by determining the appropriate pump to be used. This type of pump will not only result in uniform product flow rate, but also minimal damage to the particles. The next step is to identify the appropriate heating system. In situations where there are large particles, a scraped surface heat exchanger or a volumetric heating system is usually employed.
The SSHE serves to bring up the temperature of the product to room temperature and a certain elevated temperature and the RF heater is the finisher which has the effect of minimizing the difference in temperature between the fluid and particle since a low frequency The holding tube is one of the most important parts of the aseptic processing system as this is where the product receives its heat treatment from a commercial sterility standpoint.
A stainless steel helical holding tube assembly coil diameter 1" is used. A helical holding tube results in the development of secondary flow and hence causes mixing of the solid—liquid mixture and hence translates to a relatively uniform heat treatment of the product. A hydrocoil cooling unit will result not only in rapid cooling of the product, but will also be gentle enough to the product so as to not cause disintegration of the particulates.
The cooling medium is chilled water flowing through the system at a high flow rate 20 gpm of chilled water constantly flowing in and out of the system and gpm of water continuously circulating through the system. The package used was a oz cup with an aluminum foil used to heat-seal the top. The temperatures of the fluid are monitored at the entrance and exit of the pump, SSHE, RF heater, holding tube, and cooling section using type T thermocouples Omega Engineering, Stamford, CT installed with sanitary fittings.
Back pressure is provided to the system by means of a lobe pump Model 45U2, Waukesha-Cherry Burrell, Delavan, WI along with a T-junction with a manual control back pressure valve in the vertical section and a wire mesh screen that ensured that only the fluid portion passed through the back pressure valve and the particles passed through to the lobe pump. For the heat exchanger and the holding tube sections, the theory of the mathematical modeling has been presented in previous sections of this chapter. However, for modeling heat transfer in the RF heater, another model has to be used.
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Based on the modeling studies, the length of the holding tube is appropriately chosen conservative estimate. During the modeling, care should be taken to account for change in viscosity of the suspension as a function of time. To aid this, benchtop studies should be conducted to determine the rheological behavior of CMC as a function of shear rate, time, and the high temperatures encountered during processing.
During the modeling, conservative estimates of convective heat transfer coefficients and thermal diffusivities are used. Thermal conductivity is measured using the line heat source probe and specific heat is measured using a mixing calorimeter. The first phase in experimental studies is to incorporate tiny magnets of different magnetic strengths into several cube-shaped tracer particles in order to determine the residence times of the particles in various sections of the aseptic processing system especially the holding tube section.
Care is taken to compensate for the higher density of the magnets than the tracer particle since particle density is a major factor affecting the fastest particle residence time. Magnetic coils situated outside the tubes of the processing line picked up the signals produced by the motion of the magnets throughout the system, and this enables us to determine the residence times and hence the residence time distribution and also the fastest particle residence time.
The magnets are of low enough strength to not affect the electromagnetic field created by the RF system. The next step is to perform biological validation tests.
These tests are performed at various stages of the process—just after start-up, during the middle of the run, and just before shut-down. These tests account for variations during the process and also for factors such as fouling. This will aid in determining the minimum allowable process temperature that will result in a safe process. Microbiological validation tests are done using PA inoculated within alginate particles.
Care should be taken to ensure that the spores do not leach out into the fluid. The final step in commercialization of the product involves process filing with the FDA using form C. A comprehensive overview of the procedures and processes involved in process filing for a product such as the one discussed above has been given in a report elsewhere . With the advent of new technologies to inactivate microorganisms, some of the existing problems, such as slow heating of particles, can be overcome.
Nevertheless, new technologies, such as the use of high pressure or pulsed electric field, have to be carefully studied, since the target microorganism, extent of enzymatic inactivation and other factors might change. Despite the hurdles posed by aseptic processing, the high quality of the end-product will make this technology more prevalent in the US market as consumers are becoming more conscious about the nutritive value of foods and leading a healthy lifestyle.
David, J. Aseptic processing and packaging of food: A food industry perspective. Dean, W. Motion of fluid in a curved pipe. Philosophical Magazine Series 7 4 20 : — The stream-line motion of fluid in a curved pipe. Philosophical Magazine Series 7 5: — Truesdell, L. Numerical treatment of fully developed laminar flow in helically coiled tubes. AIChE Journal — Taylor, G.
The criterion for turbulence in curved pipes. Proceedings of the Royal Society of London A — Koutsky, J. Minimization of axial dispersion by use of secondary flow in helical tubes.