In the cold supply chain the big risk is thermal abuse. In order to properly ensure quality control of perishable products throughout the cold chain process, we need to combine good science with sound operational procedures, and then continuously measure temperature each and every step within the chain.
If we do not measure – we cannot manage.
The Science (for non-scientists)
Simply put; Warm always wins!!!!
Heat can spontaneously flow from a higher-temperature region to a lower-temperature region, but not the other way around. (The Second Law of Thermodynamics) We can see this in the diagram of the refrigerated trailer below which depicts the effect of heat from warm air outside penetrating and permeating from the trailer skin and the resulting internal airflows. The heat from warmer air outside flows to the cooler air – warming it.
As the forced chilled air moves across the top of the trailer from the nose to tail, the heat from the warm air spontaneously flows towards the cooler air. As the air returns along the base of the trailer, it continues to absorb heat from the floor and product, therefore the amount of cool air continues to decreases dramatically. The entropy increases – more warm air. The warmest air nests towards the tail and along the bottom of the trailer, especially towards the nose.
The example stated above is under optimal conditions of unobstructed airflows in an empty, pre-cooled, sealed trailer. As the trailer is loaded with pallets of product, the flow of chilled air is obstructed; slowing the airflow and making less air absorb the same amount of heat and raising the air temperature in that vicinity. This is made worse during the loading process when warmer air infiltrates the trailer.
Impact of Thermal Abuse & Dehydration
When temperature in the refrigerated trailer rises above the desired level for a given period of time, the product will be thermally abused. Some of the harm is spoilage caused just by the increase in temperature. But thermal abuse can also be more insidious when product partially thaws and then refreezes. This is more difficult to identify by inspection, and causes problems when the product is used at the restaurant. For example,
- A coffee creamer that is frozen and then thaws, will curdle your coffee;
- Ice crystals forming on the surface, causing freezer burn;
- Hamburger patties that stick together due to thawing and refreezing
- Product dehydration causing uneven cooking. (A patty properly cooked on the outside and under-cooked (pink) on the inside.
Dehydration is a very common problem if the temperature quality of the cold chain distribution is compromised. To prevent dehydration, we have to know first how the moisture is locked in the product, and then under what circumstances can that water be extracted (Dehydration).
We can use a frozen hamburger patty as an example, although the principle applies to other frozen products such as French fries. In the freezing process, raw beef parts are sent through a massive grinder, are blended and then shaped/formed into a hamburger patty. The patties are then flash frozen, typically with CO2, and deep frozen in a quick freeze chamber using liquid-nitrogen. This quick freeze process takes just minutes and it locks in the moisture (water) in the patty.
This locked in water cannot be extracted by additional chilling or freezing. Dehydration only occurs when the frozen patty is exposed to warmer temperatures which essentially unlock the water in the patty. The outside of the patty will defrost more quickly than the inside the patty due to the exposure to warmer, more chaotic air. The patty is losing moisture, thus it is dehydrating.
The defrosting patty now has water molecules forming on the surface of the patty. If that patty is then subjected to colder temperatures i.e. re-freezing, the hamburger patty appears frozen as normal, but will have less water content. This is “Thermal Abuse” caused by dehydration. When the burger is cooked, the surface will cook more quickly since it is in direct contact with the heat source. This raises the likelihood that the patty may be properly cooked on the outside and under-cooked (pink) on the inside due to the lack of moisture (water).
Under normal conditions, properly hydrated hamburger patties will cook evenly because the moisture (water) in the patty actually heats up more quickly than the meat (liquids vs. solids) – thus the water content actually acts as a heat agent and cooks the patty evenly.
Warm spots and natural heat nests in trailers during transport will increase the likelihood of dehydration problems.
The main focus for the study was temperature monitoring of the distributors trailers, both Procuro and our distribution customers were determined to use the PIMM™ TMC hardware to supply additional data metrics in order to provide a total system solution for trailers which in turn would provide an ROI to the DC for their investment.
Based upon our findings during this test, we believe that temperature monitoring and management is an essential component to ensure the highest level of food quality at the restaurants, and we believe that there is a significant need for temperature management on outbound trailers. Generally speaking distributors consider temperature monitoring of their outbound trailers to be a net additional operational cost. However, during the test Procuro developed a wide variety of trailer based solutions to operational problems, which individually and together offer an excellent return on the investment required for temperature monitoring.
Data obtained during the project from the PIMMified temperature sensors installed on the DC’s fleet highlighted a number of previously undetected operational issues. Having this data enabled Procuro to design and develop better technology to assist distributors manage a very difficult part of their operation – outbound deliveries.
Maintaining proper temperature control on outbound restaurant delivery trailers is difficult under normal conditions and nearly impossible if corporate policies and procedures are not strictly adhered to. As with every operational function, it takes a system or process to ensure satisfactory execution – outbound delivery is no different. Everything from trailer maintenance to delivery practice and everything in between will make a substantial difference in optimizing this part of our Restaurant/Store customer’s cold chain process.
The following section(s) describes a variety of identified operational issues that can lead to extensive thermal abuse conditions.
Reefer Settings and Maintenance
During this project, PIMM™ identified inconsistencies with the inside ambient air temperature and the actual temperature settings on the reefer units. The temperature variance was significant – 8˚-10˚ F and 15˚- 20˚ F in the cooler and freezer compartments respectively. Upon this discovery, the reefer settings for the trailers were adjusted accordingly. Procuro is in discussions with the reefer manufacturers in order to establish accepted guidelines for reefer settings. The reefer manufactures claim that the variance between the ambient air and the reefer settings is approximately 3˚-5˚ F. The findings during this project are consistent with the single compartment trailers that are used by suppliers where it is becoming a standard procedure to set the reefer units at -15˚ F to maintain an ambient air temperature of +10˚ F.
Maintenance of the reefer units also plays a significant role in the process. Typically, reefers are scheduled for preventive maintenance on a time-based schedule and therefore there is no way to validate or calibrate the temperature settings on a real-time basis. If there is a mechanical and electrical problem with the reefer it may go undetected – causing thermal abuse conditions as described in the pre-cooling and overnight storage sections of this document. PIMM™ monitoring can catch this as it happens, and make maintenance more effective.
Trailer Configurations and Maintenance
Unlike inbound supplier trailers which are typically 53 ft. single compartment configurations, the outbound distributor trailers are multiple compartment trailers. The configuration of these trailers can be vertically or horizontal sectioned off to support cooler, freezer and dry sections. Typically, the vertical configurations will have one reefer unit in the nose of the trailer with rubber divider walls separating the cooler and freezer compartments longitudinally from the nose to the rear of the trailer. The horizontal configuration uses the rubber divider walls to divide the trailer cabin into three sections – the freezer section is closest to the nose of the trailer; the cooler is located in the center section; and the dry section is located at the rear of the trailer. The horizontal configurations support two (2) reefer units per trailer – one in the freezer section and one in the cooler section. The temperature data was much more consistent in trailers with horizontal configurations. In trailers with vertical configurations it was virtually impossible to maintain proper temperatures in both cooler and freezer sections.
In addition to the trailer configurations, maintaining the trailer skin (body); the rubber diving walls; and door drapes is of critical importance to an efficient operation. As one can imagine, the wear and tear on a trailer is significant. The constant loading and unloading using fork lifts can cause damage from time to time. Any structural compromise can cause additional temperature loss.
Pre-cooling is a standard procedure for every distributor, however, without have onboard temperature sensors there is no guarantee that the cooler and/or freezer compartments are properly pre-cooled before the trailers are loaded. The day to day operations of a distribution center is dynamic. Trailers come and go throughout the day and the DC loading supervisors assign a properly configured trailer for each and every route as the empty trailers return to the DC yard. Due to the very nature of this operation it is almost impossible to know what trailer will be available and when it will arrive. It is a fairly routine occurrence for a trailer to be loaded as soon as it arrives in the DC yard. If the reefer on the empty trailer was turned off on the return trip to save fuel – the internal temperature of the trailer can rise to 60˚-70˚ F (higher in the summer).
It will take approximately an hour to properly pre-cool the trailer providing the trailer is empty; all doors are closed; and the reefer settings are adjusted as described the reefer setting section above.
Thermal abuse conditions are exacerbated when the cabin temperatures rise above 45˚-50˚ F. Proper pre-cooling is essential and is easily monitored and controlled with the PIMM™ real-time temperature reporting.
Overnight Storage (Staging)
Once a trailer is loaded with product it is often staged overnight in the DC yard or shuttled to a remote yard until the route drivers (reps) pick them up (3am – 5am) and are dispatched on their individual routes. This staging and shuttling process can be anywhere from a 1 – 15 hours in duration.
Unless the trailers are equipped with PIMM™ real-time temperature reporting, this storage time represents a gap in the chain of temperature monitoring from supplier to restaurant, which starts with delivery monitoring from the suppliers to DC, continues with the PIMM™ DC Facility Management system (which is installed in the majority of our Restaurant/Store customer’s DC’s) and concludes with the PIMM monitoring of outbound delivery from DC to restaurant.
Of course, properly maintained trailers and reefers are also crucial during this period.
DC Yard – Trailer Storage
During this project, we discovered that some of the trailers in the fleet were used for long term storage. Procuro at its own expense installed a PIMM™ TMC on those storage trailers in order to monitor temperature and to guard against unforeseen maintenance issues.
Once again we believe it is important to maintain the integrity of temperature monitoring continuously from supplier to restaurant.
Product Temperature vs. Ambient Air Temperature
Due to the variety of operational issues that can cause thermal abuse conditions during the distribution process, Procuro decided to expand the project and conduct additional tests to determine the effect that ambient air temperatures had on the product temperatures.
This has been an area of controversy in the food service industry for many years — so we conducted a series of tests to begin the process of building a correlation between air temperature and product temperature.
Burgers vs. Fries
The first series of tests that we conducted was to determine if there was a significant difference in product temperatures based upon the product itself — we chose beef patties and french fries. The test was simple – place both products in the same cold environment conditions and measure the respective product temperature over time.
As the chart below clearly shows, both burger and fries tracked within 1˚-2˚ F as we modified the ambient air conditions. We conducted additional tests using both burger patties and French fries with similar results each time. This chart is representative of the series of tests that we conducted.
Ambient Air vs. Product
The next two series of tests were conducted with burger patties and french fries and again the results were similar so we are using data charts for burger patties as a fair representation of the series of tests conducted. This series of tests were design to determine what affect changes to ambient air would have on the actual product temperatures.
We placed PIMM™ temperature sensors in the refrigeration compartment(s) and in side the box of burger patties to capture the ambient and product temperature respectively.
Series I Tests
We placed the frozen product (28˚ F) in the freezer with an air temperature range 6˚-10˚ F; then we exposed the product to room temperature; then back to the freezer; and finally to a cooler or refrigerated condition.
- The frozen product (28˚ F) took approx. 24 hours to reach the ambient air temperature range of 6˚-8˚ F.
- Once the frozen product reaches the desired temperature when stored in properly controlled ambient air temperature, the product holds its temperature.
- When frozen product is exposed to higher “cooler” temperatures, there is a lag time and temperature variance between the ambient air temperature and the product temperatures. i.e., air temp rises to 33˚ F, the product temp will increase from 6˚ F to 20˚ F in approx. 5 hours; and 6˚ F to 30˚ F in approx. 24 hours.
- Once a product is exposed to temps above 45˚-50˚ F, the product loses temperature at a significantly faster rate.
- Products that reach room temperature and are placed back in a 6˚-10˚ F freezer compartment can reach 30˚ F in approx. 8 hours and will require an additional 24 hours to reach the ambient air temp of 6˚-10˚ F.
Series II Tests
We placed the frozen product (5˚ F) in the freezer with an air temp of 6˚-10˚ F; then we exposed the product to room temperature; then back to the freezer; and finally to a cooler or refrigerated condition.
- The frozen product maintains its desired temperature when stored in properly controlled ambient air temperature; the product holds its temperature.
- When frozen product is exposed to higher “room” temperatures, there is a shorter lag time and temperature variance between the air temperature and the product temperatures. i.e., air temp rises to 65˚ F, the product temp will increase from 6˚ F to 40˚ F in approx. 3 hours.
- Products that reach “cooler” temperature (38˚-40˚ F) and are placed back in a 6˚-10˚ F freezer compartment can reach 6˚-10˚ F in approx. 8 hours.
- When frozen product is exposed to higher “cooler” temperatures, there is a lag time and temperature variance between the air temperature and the product temperatures. i.e., air temp rises to 40˚ F, the product temp will increase from 6˚ F to 30˚ F in approx. 24 hours; and additional 24 hours to reach 40˚ F.
Ambient Air vs. Product Temp Results
The results of our tests were both interesting and operationally significant. Measuring and correlating the effect of ambient air temperatures on product temperatures clearly shows that product temperatures lag air temperatures as changes (up or down) are introduced.
It is also evident that at approximately 45˚ F, the product temperatures rise more rapidly. When ambient air temperature reaches or exceeds room temperatures conditions – the product temperature rise dramatically and the quality of the product could be jeopardized during a routine outbound delivery run. Because of these findings, Procuro developed a wireless solution that manages and correlates both ambient air and product temperatures during the entire outbound delivery process.
We strongly believe that if the proper temperature management procedures for the maintenance trailers; the pre-cooling of trailers; the overnight staging and shuttling of trailers; and delivery procedures are in place – that there would not be enough elapsed time on a standard 8-10 hour restaurant delivery route to cause a measurable effect of the quality of the product. Using PIMM™ to manage the outbound delivery process protects our customers against minor lapses and/or discrepancies because problems are caught before a major issue develops.
Recommended Operational Procedures
Now that we understand the science of the trailer, we can incorporate sound operational procedures and practices that are designed to minimize the risk of thermal abuse discussed above.
But knowing that 100% compliance is impossible we need to continue to monitor temperature at all stages, to catch breakdowns immediately as they happen, minimizing loss and quickly identifying where and when the failure happened.
Golden Rule of Refrigerated Transport
Reefers are designed keep cold things cold – they do not chill or freeze!!!
One the biggest problems in the cold storage industry is that too many people are unaware of the “science” of cold chain process. Many people still believe that if they load a warm trailer with chilled/frozen products, the chilled/frozen products will cool off the trailer and assist the reefer units. To make matters worse, the reefer units are then expected to chill/freeze or re-freeze the contents of the trailer.
Remember – warm always wins! In order to avoid thermal abuse of a refrigerated load – it is ESSENTIAL that refrigerated vehicles are pre-cooled to the appropriate temperature before loading and that they maintain the proper temperatures throughout the cold chain process.
If we do not measure – we cannot manage.
The following is a quick guide to sound operational procedures:
- Pre-cool all trailers with door(s) closed
- Validate trailer temperatures before loading (do not use reefer settings)
- Stage products on pallets and load trailers as quickly as possible
- The use of pallets is highly recommended to allows for proper airflow
- Do not stack products too high on pallets – allow spacing for proper airflow
- Immediately seal compartment(s) with bulkheads and close door(s)
- Maintain proper refrigeration (reefer) settings (temperature and fan operation) during staging
- Monitor/record (measure) the actual temperature inside the trailer during staging
- Monitor/record (measure) the fuel levels and other vital elements on the reefer units during staging
- Maintain proper refrigeration (reefer) settings (temperature and fan operation) during transport
- Monitor/record (measure) the actual temperature inside the trailer during transport
- Monitor/record (measure) the fuel levels and other vital elements on the reefer units during transport
- Ensure that there are no unscheduled stops and door openings during transport
- Keep doors closed until receiver is ready to unload
- Immediately validate the quality of each load before unloading, based upon onboard temperature monitors
- Rate the quality of delivery of every shipment based upon temperature
- Accept or Reject load based upon quality rating
- Unload trailers as quickly as possible and store product in proper cold storage facility