Cold chains are extremely important to safeguard any perishable product, be perishable food or temperature-sensitive pharmaceuticals. Sadly, even though there are technologies and knowledge out there that could avoid loss of temperature control during storage and transport of goods, we continue to see spoilage and food safety issues that could have been avoided with better temperature control solutions.
I wrote these top ten reasons for cold chain failure 8 years ago. Do you think that these have changed?
1. High initial product temperature. The extra-heat load generated by warm product entering a truck, container or coldstore can significantly impair the performance of the refrigerated equipment. During transport, temperature differences in boxes of product before truck loading can persist until the end of the voyage. This is particularly true for mixed loads of products: suppose that a truck is half loaded with boxes of green salads with leafy vegetables that have been kept in a cold store at 0 deg C and the rest of the truck is loaded with salad dressing that has just been pasteurised and cooled to 15 deg C. The overall internal temperature of the truck will be closer to that of the bulk containers and the salads, which are items highly responsive to air temperature changes, will warm as a consequence. And remember: not all coldstores are designed to cool product! Some coldstores are used to maintain the temperatures of products that have been previously cooled in chillers, freezers or precoolers.
2. Warm loading conditions. Imagine that a refrigerated container or truck is being loaded with chilled goods during a summer day in Sydney. Even though the cargo space was previously cooled to the recommended transport temperature, the simple act of opening the doors to load the product lets warm, humid air in at the expense of cold, dry air being lost. Additionally, the product being loaded will increase its temperature during loading. If the product happens to be fruit or vegetables, the extra degrees will accelerate the respiration rate, which in turn will demand more cooling capacity during the voyage. A refrigerated loading dock should be used to minimise the rise of product temperature and the entrance of warm air and humidity.
3. Haphazard/inadequate loading patterns. The loading pattern should ensure an even distribution of air within the cargo. Good loading practices include uniform stowage, filling gaps between the end of the load and the doors and gaps between pallets, leaving sufficient air space between the top of the load and the ceiling and the use of vertical separations (dunnage) for respiring cargo. For frozen cargo, the pattern should be as compact as possible, although spaces between ceiling and top boxes and sidewalls and the boxes closest to the walls should be allowed to allow airflow around the load.
4. Excessive product handling. In 1995, the Food Safety and Inspection Service and other US departments established a group that identified primary hazards associated with the transport of perishable products and provided recommendations to ensure food safety during transport. The team found the following handling issues:
Handling problems were more numerous with partial loads and mixed loads
Handling problems were more frequently encountered with small carriers, which were more likely to haul smaller, mixed cargoes
Loading and unloading of cargo was also more frequent with mixed loads, thus contributing to temperature deviation.
The recommendations that followed included: hauling products with similar temperature specifications and maintaining the temperature of 0 deg C to 4.4 deg C during loading and unloading operations.
5. Inefficient airflow management. The most common air circulation pattern in refrigerated trucks is top-air delivery, lengthwise, front-to-rear. In reefers, the cool air is delivered through floor channels towards the door, and then circulated back to the evaporator in the space created by the uppermost boxes and the ceiling. The performance of refrigerated transport can be greatly improved by generating adequate air movement within the cargo space. Recommended measures to achieve effective air circulation include: installing solid return air bulkheads, providing uniform stowage, securing the load away from doors and sidewalls, and providing space underneath the load to create effective return paths for the air. Multiple compartments or partitions between very different types of products allow independent airflow patterns and temperature control within the cargo space. Plastic or air curtains for doors can greatly decrease the entrance of warm air during deliveries.
6. Poorly designed packaging. The design of the package will also determine the uniformity of temperatures within the cargo. Air circulation will be more effective with ventilated cartons, especially for fruits and vegetables. However, aspects such as packaging structural integrity and heat transmission from product through the package, carton and pallet stack need to be considered.
7. Leaky door seals, damaged insulation and old insulated bodies. The possibilities of water-damaged insulation and/or faulty door seals increase in old trucks and containers for which maintenance has been insufficient. High air leakage rates and high heat infiltration will inevitably lead to loss of temperature control, unless the refrigeration unit has been sized to take these into account. The deterioration of the insulation materials due to aging is estimated to be about 5% of the insulating quality per year, also contributing to the uncertainty in the thermal performance of refrigerated trucks that have been in service for a number of years.
8. Inadequate selection of refrigeration capacity or control systems. The sizing of the refrigeration unit is critical to ensure that the truck, container or coldstore can cope with extra heat loads usually encountered during the transport of fresh produce, such as the initial warming of both product and truck occurring during loading (in the absence of refrigerated loading facilities), product respiration (considering the higher rate found for fresh cuts), air exchange, infiltration through the insulated walls in diverse ambient conditions, defrosting, fans and door openings. Mechanical refrigeration systems coupled with thermostats sensing the air return or air delivery temperatures are the most common means of temperature control for the transport of chilled products. The control system can vary from simple ON/OFF strategies to Proportional-Integral-Derivative controls. Regardless of the system, air temperature fluctuations and significant temperature differentials lengthwise and along the height of the truck will occur. The extent of these variations will depend on design features such as the location of the temperature sensor and the control strategy.
9. Gaps in education and training. Operators in the cold chain require an understanding of the effect of temperature on foods, correct loading and unloading practices, quality assurance systems such as hazard plans and risk analyses, and a general knowledge of refrigeration systems, among other aspects. In the wider context, modern supply chains now underpin international perishable food distribution. Chain participants may come from a range of industries, roles and location in the value chain. Quite understandably, there will be varying levels of understanding on important cold chain aspects and collaboration with multiple supply chain players. Knowledge and skills gaps in the handling and transport of perishable products throughout the chain need to be attended.
10. Lack of supply chain vision and strategy. An integrated cold chain is the result of a careful selection of technology, the implementation of education and training required to raise the bar in quality over competitors and an analysis of the market and trends in the distribution of perishable goods. An example described by Jude Barry, from Cornell University, is Earthbound Farm - the US largest grower shipper of organic produce. Amongst the external factors to the business which are critical in the decision making of their distribution strategy, they include the changing models of distribution and transportation and the need to get business closer to the customer to ensure high quality product can be economically distributed. These drivers have implications on the selection of distribution routes and the selection of storage and transport technologies. Other global trends that have far-reaching effects on the design of cold chains are the rise of the foodservice and convenience sectors as a market power to be reckoned with, and the consolidation of growers/shippers at the marketing level, to increase economies of scale and to structure year-round supply chains.
Perhaps you have stumbled upon other common reasons for cold chain failure that I have not discussed in this article?
Reprinted from my newsletter 'Chain of Thought', Food Chain Intelligence Jan 2008.