DR. LUIS LUCHSINGER, POSTHARVEST AND COLD CHAIN MANAGEMENT SPECIALIST.
Among postharvest factors, one of the most decisive for maintaining the quality and condition of any perishable product —and particularly blueberries— is the cold chain. However, in practice, this concept is often mistakenly reduced to “being cold,” when, in reality, the cold chain is a complex system that must be properly managed throughout the entire process. When the cold chain fails, no agronomic handling, technology, or packaging can compensate for the loss of condition and weight of the product.
Generating cold is not the same as managing a cold chain. While the refrigeration industry focuses on producing low temperatures, the cold chain requires an understanding of how the fruit, packaging, refrigeration, cooling tunnels, and the process itself interact. Only from this integrated perspective it is possible to protect the quality and condition of blueberries until they reach the final consumer.
A Structural Mistake in the Industry: Delegating the Cold Chain
Historically, the focus in the agricultural industry has been on production, nutrition, and field management. However, one of the structural problems that has developed within the sector is the delegation of the cold chain to the refrigeration industry.
This approach is conceptually incorrect because the cold chain should be designed based on the product’s needs, not solely from the equipment perspective. In the design of a fruit processing center, it is the agroindustry that must define the process requirements, and based on these, the refrigeration industry should propose the appropriate technical solutions.
Rapid Cooling Reduces Dehydration and Costs
A deeply rooted misconception in the industry suggests that fruit dehydrates less when cooling is slow. However, the opposite is true: the slower the cooling process, the greater the dehydration of the product and, consequently, the weight loss.
Postharvest physiology has often misunderstood this concept, interpreting fruit dehydration as a physiological phenomenon, when it is actually a physical process associated with the vapor pressure deficit between the fruit and the surrounding environment. For this reason, it is essential to have rapid cooling tunnels that can reduce the fruit’s temperature within a maximum of 20 minutes. However, there are still industries that cool blueberries over periods of up to three hours, even though technology is now available to reduce the fruit’s temperature from 30 to 6°C in just seven minutes, as is the case with cherries and other species.
Contrary to common belief, designing faster cooling tunnels reduces the overall project cost. While each tunnel has a unit cost higher than traditional tunnels, fewer units are required, leading to a lower total project cost, while simultaneously minimizing fruit dehydration and optimizing energy consumption — a key factor in operations and a critical global issue.
Process Temperature: Impact on People and Fruit
In the blueberry industry, the operating temperature in processing rooms presents a problem with two clearly differentiated aspects. On one hand, the staff is not accustomed to working at excessively low temperatures, with operations typically occurring around 6°C air temperature. On the other hand, from the perspective of the fruit, it is recommended to raise the air temperature in the processing room to ranges of 16 to 18°C and, simultaneously, reduce the relative humidity of the environment (below 70%) to lower the dew point. This allows the raw material to cool at higher temperatures.
In practice, if a processing room operates at 18°C with a relative humidity close to 65%, it is possible to cool the fruit to temperatures around 12°C. This approach significantly improves the working conditions for staff and, at the same time, reduces mechanical damage to the fruit, which is the second aspect of this problem.
The anomaly of water explains that water is the only solid that floats, while any other liquid in a solid state sinks. In fruit handling, this translates to the fact that the colder the fruit is when processed, the more it is damaged and softened. In some cases, fruit is processed at 0°C due to the lack of cooling equipment capable of operating at higher temperatures in bags with 0% ventilated area, or the so-called Modified Atmosphere Packaging (MAP). Thus, fruit processed at 12°C is less damaged than fruit processed at 6°C, and this, in turn, is less damaged than fruit processed at 0°C.
The Ventilation Problem in the Refrigeration Industry
When fruit is processed at 0°C and packed directly in clamshells without bags, cooling is faster because cold air circulates freely around the packaging. However, when a bag is added, an additional resistance to airflow is created, which decreases the cooling speed, as it increases the pressure drop in the fan and significantly lowers the airflow in the system.
This is not an issue with the bag itself, but rather with the ventilation system’s capacity: the fans do not provide the necessary static pressure to overcome this additional resistance. As a result, air does not adequately pass through the load, and the cooling process becomes slower and less efficient.
It is essential to understand the type of packaging being used, particularly the percentage of ventilated area in the sleeve bag (in each box), as this parameter directly impacts the pressure drop the fan must overcome. As with an irrigation pump, as pressure increases in the system, the available flow decreases.
To achieve efficient cooling, a sufficient airflow (measured in m³/h) is required to pass through the fruit load uniformly. In many cases, the industry is forced to cool without the bag and then bag it, which means performing the operation twice, with the associated operational and logistical costs.
From an energy and process efficiency standpoint, the solution lies in improving ventilation systems by incorporating fans capable of delivering high airflow at greater pressures. These types of fans now exist and perform well; however, their performance depends on proper balancing with the evaporator, which must be carefully designed.
Similarly, and closely related to cooling efficiency and ventilation system design, another key aspect impacting fruit dehydration during storage and transportation is the ventilated area percentage of the bags used. In practice, much of the industry cannot work with bags of low ventilated area because they do not have ventilation systems capable of achieving efficient cooling under those conditions.
Faced with this limitation, the industry opts to cool the fruit to lower temperatures, which increases the risk of mechanical damage, and then add the bag or forgo it altogether due to the operational complexities. However, working without a bag during storage and transport leads to higher weight loss, resulting from accelerated dehydration.
Bags commonly referred to as modified atmosphere packaging
In the industry, certain plastic bags without ventilated areas or used in Flow Pack systems are commonly referred to as modified atmosphere bags. In this process, clamshells are packaged in a continuous line inside a plastic bag and then placed in boxes for handling and transportation.
The key point is that while these packages are commercially labeled as modified atmosphere bags, they do not always generate an atmosphere modification in the ranges that the fruit actually requires from a physiological and pathological standpoint. From a technical postharvest perspective, a modified atmosphere refers to the significant reduction in oxygen concentration—from the 21% present in air—and the increase in CO₂ levels, which in normal conditions is close to 0.03%.
Real Scope and Limitations of Modified Atmosphere
Systems such as controlled atmosphere storage are specifically designed to adjust gas concentrations to defined levels in order to achieve specific effects, such as pathogen control or the reduction of the fruit’s respiration rate. In the case of these bags, although a modification of the atmosphere does occur from a formal standpoint, the levels achieved are generally insufficient to meet the physiological requirements of the fruit. For example, effective fungal control typically requires CO₂ concentrations above 10%, whereas these systems usually reach values in the range of 4–6%. Similarly, the reduction in oxygen concentration required often falls below 8–10%, while in these packages oxygen decreases only from 21% to ranges between 14 and 18%.
That said, the positive impact of this type of bag is highly significant: because these packages have 0% ventilated area, they provide exceptional control over weight loss due to dehydration. However, in order to properly take advantage of this benefit, it is essential to correctly select the type of plastic used, thereby avoiding undesirable effects on fruit condition.
In general terms, bags referred to as modified atmosphere packaging are primarily manufactured using two types of plastic materials: polyethylene and polyamide. The main difference between them lies in their permeability to water vapor. Polyamide exhibits significantly higher permeability, which means that in the presence of condensation it facilitates the transfer of moisture from the inside of the package to the outside. Once this gradient is established, the system continues extracting water from the fruit, thereby increasing dehydration.
In practice, this behavior translates into considerably higher weight losses compared with polyethylene packaging, potentially even doubling them. For this reason, when the primary objective is to minimize dehydration and preserve fruit weight, polyethylene bags represent the most suitable option.
Decay: A Persistent Sanitary Challenge
Another aspect closely linked to postharvest management—and particularly relevant from a sanitary standpoint—is the control of fruit decay. In recent seasons, both Peru and Chile have reported recurring problems associated with these issues. The management of decay must be addressed comprehensively: on the one hand at the field level, through the reduction of inoculum, and on the other hand in postharvest through specific tools such as fumigation chambers, sulfur dioxide (SO₂) generators, or controlled atmosphere storage.
Fumigation plays a fundamental role, as it reduces the presence of exogenous Botrytis and Penicillium, eliminating fungal spores that come into contact with SO₂. In addition, it contributes to wound healing in the fruit, which translates into reduced weight loss during storage and transportation. In practical terms, this process can reduce weight loss by approximately 0.8%, a difference that is economically significant.
Taken together, these factors—cooling efficiency, packaging management, dehydration control, and decay management—represent some of the main challenges currently faced by the industry.
Pallet Stacking: A Critical and Often Overlooked Factor
Another frequently underestimated aspect with major implications for postharvest performance is container pallet stacking. The way pallets are arranged inside a container determines the distribution of cold air and, consequently, the uniformity of temperature during transport. In many cases, significant thermal differences can be observed between the engine end and the door end of the container, increasing the risk of dehydration, fruit softening, and decay.
Proper pallet stacking helps minimize these temperature differences and substantially improves fruit condition upon arrival at destination. Specific stacking configurations have been designed to reduce the thermal gradient within the container, thereby contributing to lower decay incidence and reduced weight loss. In this context, temperature once again emerges as a critical factor, transversal to the entire postharvest management process and decisive in determining final product quality.
Real Innovation and Remaining Gaps in Postharvest
In terms of innovation, one of the most relevant advances currently observed in the industry is the development of high-pressure, high-airflow ventilation systems. These fans, already implemented in projects in countries such as Peru, Chile, and Georgia, have proven to be a real and energy-efficient solution. Their main advantage lies in their ability to operate with virtually any type of packaging, including bags with 0% ventilated area, without the need to cool fruit to excessively low temperatures—provided that the system has been properly designed.
However, this type of technology requires a high level of technical expertise. System design must incorporate structural reinforcements in cooling tunnels, since the high pressures generated can lead to implosions or structural failures if not properly managed. The technology exists, works, and is highly efficient, but its success depends on integrated and well-executed system design.
Regarding fumigation chambers, although they are a well-known and validated tool within the industry, inefficient solutions still persist in the market. Many of these systems are excessively complex and involve costs that are difficult to justify from a technical standpoint. In many cases, attempts are made to overcomplicate processes that, in essence, have been simple and well understood for decades.
Beyond these specific innovations, many of the current challenges facing the blueberry industry involve returning to the fundamental principles of postharvest management. There is still a significant lack of understanding of basic concepts, particularly regarding the cold chain. In this regard, it is illustrative to compare the performance of different fruit industries: cherries stand out as the species with the most advanced postharvest management worldwide, followed at some distance by blueberries, then table grapes, and further behind apples and bananas.
For the blueberry industry to approach the standards achieved by cherries, it is essential to understand and apply the principles that underpin modern postharvest management. This is not only a technological challenge, but also an educational one There remains a clear shortage of education and technical training in cold chain management, and globally, postharvest physiologists and technicians often remain detached from aspects related to refrigeration systems.
In practice, much of what is currently described as a “cold chain” is limited to refrigeration systems, without a comprehensive understanding of the entire process. As long as the cold chain continues to be interpreted merely as refrigeration—rather than as an integrated system designed around the needs of the fruit—improvements will remain partial and results, predictable. The technology exists; the real challenge lies in understanding it and applying it correctly.
