Above temperature monitoring was discussed in terms of displaying temperature readings of ambient air, food products, or their models, but you can also use changes in physicochemical properties that lead to changes in the indicator readings to display
a) changing the temperature,
b) crossing the temperature threshold or ...
c) at the same time temperature and time of its impact after activation. Devices based on this principle are in the first two cases called thermal indicators (temperature indicators, TI) or, in the latter case, time and temperature indicators (TTI).
Indicators are usually connected to packaging material, which may be attached to the product packaging, or placed outside the packaging or unpacked product. Thus, the indicator can accompany the product along the entire route of the cold chain and provide the following information (one indicator or several):
- failure or reception based on a change of color;
- temperature transition through a predetermined threshold;
- partial history of temperature changes over time when the temperature passes through a given threshold;
- complete history of temperature changes over time, related to shelf life.
For devices to be used commercially in the monitoring, they must have the following characteristics, and supplied with the following information from the manufacturer :
- methods of attachment to the packaging of the product;
- instructions for activating TTI prior to use, including the temperature at which the device must be stored prior to use after release at low temperatures;
- threshold temperature and permissible limits of its change (3 standard deviations) in ° С and time of inertia per minute (for TI);
- maximum and minimum temperature limits in ° C, in which the device will function (for TTI), as well as the time to reach the temperature at the end point with tolerances at a sufficient number of temperatures in the range specified by the manufacturer (above the critical reference point in the case of TTI with partial temperature history ); the number of combinations of temperature and time to the end point must be at least five;
- admission time to reach the temperature to the end point, in accordance with BS 7908: 1999:
- category A (up to ± 2,5%), category B (up to ± 5%), category C (up to ± 10%), category D (up to ± 20%);
- for a TTI with a partial temperature history, the critical reference temperature, that is, the temperature (° C) at which the physicochemical change is activated, leading to an irreversible change;
- storage conditions of devices must be specified so that their performance does not change; in addition, any other possible effects (other than temperature) that may affect the operation of the devices (for example, the degree of light) must be indicated;
- devices must exclude the possibility of fraud.
It is important to understand that the effect of TTI is based on physical changes, chemical or biochemical reactions. Their work usually simulates not microbiological changes in the food product, but biochemical or chemical reactions that cause the deterioration of the organoleptic properties of the food product. Usually, biochemical reactions are faster than chemical ones, but each product has its own combination of reactions, and, therefore, a different rate of deterioration. When constructing an indicator, it may be important that it has the same activation energy and speed of deterioration as that of a food product [30,31].
Processes that can be used as the basis for the operation of indicators are granted more than 100 patents based on melting point, enzymatic reaction, polymerization, electrochemical corrosion, etc. The result of a change is usually a color change, which can be represented as a static change or a moving strip . Industrial devices for controlling temperature and temperature over time are described in the literature [32-34]. Over the past 10-15 years, serial production of many temperature and time-temperature indicators has been started, but very few continue to be produced. Therefore, below we will focus on only a few of the most successful devices.
3M MonitorMark ™ Indicators
These TTIs consist of filter paper and a guide device, separated by a layer of polyester film. The paper is saturated with reagents with a certain melting point and blue paint. When a predetermined threshold temperature is reached, the reagents melt, and therefore such indicators are TTI with a partial temperature history. The indicator is activated by removing the film. The device has five windows, which, becoming blue, allow you to assess the impact of temperature. The diffusion rate increases with increasing temperature above the melting point. Various indicators are available, corresponding to different melting points (in the range of -15 ° Сdo ± 31 ° С).
Lifelines has developed several indicators that show a complete time-temperature history. The indicator part is based on polymer compounds that change color as a result of the total effect of temperature. The color change is based on the polymerization of acetylene monomers, which at higher temperatures move faster, which leads to a more rapid darkening of the indicator.
The first type of indicators consists of two parts - a standard bar code and an indicator strip. To read the indicator, a portable computer with a light pencil is used, giving reflectivity readings. At first, reflectivity is high (95-100%), and during operation it decreases as the reaction progresses, and the color becomes darker (reflectivity 50%). The computer associates the change in reflectivity with the time-temperature characteristics, which are compared with the product information in the bar code, on the basis of which predictions can be made about the shelf life with proper quality.
The development of Lifelines technology has led to the release of the Fresh-Check indicator, intended for use by consumers. This indicator consists of two circles — a small inner circle containing a polymer and a printed dark or black outer ring. The inner circle darkens over time from the effects of temperature at a rate given depending on the safety of the food product. The consumer is advised not to eat the product when the inner circle becomes darker than the printed outer ring (Fig. 5.11).
Fig. 5.11. Lifelines Fresh-Check Indicator - Examples of Stages of Change
In order to link the indication with both deterioration and microbiological safety, the next step was to introduce a second polymer system into the central circle. If the indicator remains at a temperature below the specified maximum level, the polymer changes color, and this change is related to the shelf life of the product. If the temperature rises above the maximum, the second system begins to polymerize, and the center darkens sharply after a specified time interval. Lifelines labels are not physically activated and, after fabrication, react to any temperatures to which they are exposed, therefore the indicators should always be stored at -18 ° C or below prior to use.
The TTI series is produced by the Swedish company Visual Indicator Tab Systems. These time-temperature indicators are based on the enzymatic release of protons, which changes the color of the pH indicator from green to yellow. The rate of release, depending on the temperature, can be changed so that it corresponds to the shelf life and the temperature of the cooled and frozen products. The indicator can be stored at room temperature and activated by the pressure destroying the inner bag, allowing the components to mix. The ring indicator can be printed on a flexible or semi-rigid package, inserted into a hermetic seam or placed on it. Activation can be done during sealing. There are also TTIs on a card that can be placed between packages on a pallet or inside a bulk product.
The use of time-temperature indicators (TTI)
Compared to other methods of monitoring, the use of TTI is associated with some difficulties. The fact that most of them are attached to the outer surface of the package means that changes in the indicator occur under the influence of the temperature of the outer surface. When food packaging is in boxes, this is probably a good indicator of the temperature of a product with a certain tolerance, but for products laid out on the shop window, it can give an erroneous indication of the shelf life due to the absorption of infrared radiation (if its action is possible or not compensated). Therefore, as a means of maintaining the continuity of the cold chain from manufacturing to the TTI showcase, it can have a practical advantage over some other types of monitoring, as they provide a simple and individual indication of the temperature violation.
The 511 consumer survey  showed that almost all respondents (95%) believe that using TTI is a good idea, but they understood their meaning only after some clarification. This indicates that a significant amount of advertising or a special educational campaign is needed. TTIs should be used along with the expiration date and clear instructions on what to do when the indicator changes color.
A certain problem was the ratio and the possible contradiction between the TTI readings and the expiration date indicated on the label. At retail, if the display has not changed, and the specified shelf life of the product has passed, almost half of the respondents would trust TTI. If the indication had changed before the expiration date when stored at home, the majority of respondents (57%) would themselves judge the safety of the product, and at least 25% would place some blame on the supplier of the product. At the same time, the value of TTI was recognized as a means of increasing retail confidence and improving hygiene and sanitation when transporting it home and storing it in refrigerators. Consumers expressed concern about technical quality (accuracy and reproducibility), and also raised the issue of the possibility of falsification or disruption of the indicators. This concern is shared by the food industry, and the technical characteristics of time-temperature indicators have been published [29-36] to clarify this issue.
The reluctance of retailers to use indicators on retail packages for consumer use is understandable and due to the problems that the use of indicators causes. Currently, in the UK, TTI indicators on retail packaging for mass production of food products have not found permanent use. In France and Spain, supermarket chains used Lifelines Fresh-Check indicators on some refrigerated products for a very long time, but it was decided not to use them in the future. TTIs have found wider application in medicine to ensure the proper transportation and storage of vaccines and drugs. In addition, the use of indicators to ensure continuity of the cold chain prior to retail sales using indicators on the outer packaging or on pallets for additional control is studied by manufacturers of chilled and frozen products and retail. The advantage of 777 over other types of monitoring equipment is that they provide simple and clear answers to the question of whether there has been a temperature violation. This makes such indicators an attractive additional guarantee of product safety and quality for the consumer.
Temperature modeling and regulation
The use of computer simulation as a means of integrated prediction of processes is widely used in the production of chilled products. This made it possible to predict the temperature of products under known operating conditions of refrigeration systems.
Trucks for delivery over short distances
The problems of monitoring and maintaining the temperature of food products in vans for small batches of a product that have a route with many stops at retail stores have been investigated at the University of Bristol . A commercial computer program (Soo1Uap) was developed to help design and operate such vans. This program analyzes the changes in air temperature inside the van, taking into account the heat input from the insulation from outside air, solar radiation, and air entry through the rear door (when the van moves and when it is standing with the door open). The thermal properties of the new thermal insulation of the van and its age make it possible to predict a decrease in the heat transfer coefficient of the walls, with each side of the van being considered separately. Air intake when opening doors is one of the main factors of heat influx, and transparent air curtains made of plastic strips are recommended to reduce air intake. Measurements showed that the air supply from the cooling system directly to the curtain prevented the flow of warm air into the slots in the upper part of the curtain.
At all stages of the development of the program, it was checked according to measurements. The program made it possible to predict at any moment of the voyage the average temperature of the product in the van with an error less than 1 ° C, however, the temperature in the van actually changed by more than 5 ° C due to differences in temperature inside the van.
Displays for retailers
Programs have been developed for computer modeling of conditions in retail windows for the purpose of improving their design. To see which effects give the best results before checking for full-scale measurements, computational gas and hydrodynamics allows you to change the computer model. This method was used to study the effect of refrigerated display cases on the environment in a supermarket, in particular, the leakage of cold air from a display case with frozen food into the aisles . Using the model, a prediction was made of a temperature at the floor level between 5 and 15 ° С, while the measured values were in the range from 13 to 22 ° С. It turned out that it is better to use this model to indicate the direction of change than to predict the actual temperature.
Obviously, there are great prospects for using computer models to improve the design of all refrigeration equipment in the product chain and increase energy efficiency while maintaining proper food temperature.
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