A comprehensive review of cold chain logistics for fresh agricultural products: Current status, challenges, and future trends

1 The stages of the cold chain

The cold chain is an organic combination of four linked systems: precooling, warehouse refrigeration, refrigerated transport, and marketing, which differ in sequence and timing (Fig. 1). Each link in the cold chain is correlated and interdependent, and problems in any link will increase the loss and waste of food and related human and material resources and render irrelevant any subsequent links in the cold chain. The following review presents in detail the current state of research in this field as well as the challenges and future trends to expect from these four linked systems. In addition, current research on domestic refrigerators is also discussed.

1.1 Precooling

Before entering the cold chain low-temperature environment, the first step for fresh agricultural products is precooling, which is critical to ensure food safety and extend shelf life. The main purpose of precooling is to rapidly remove field heat (or carcass temperature) of fresh produce after harvest, slaughtering, or fishing. This step slows the physiochemical activities in the produce, slows the development of disease, and minimizes the destruction of the sensory properties and nutrients. In addition, precooling reduces the demand for cooling capacity and prevents large temperature fluctuations in the subsequent cold chain. Several studies claim that precooling is likely the most important and critical of all operations for the successful maintenance and storage of fresh and perishable produce (Brosnan amp; Sun, 2001). Precooling can be divided into heat-conduction cooling and phase-transition cooling;Table 1 lists the applicable scope, advantages, disadvantages, and future trends of various precooling methods. No single precooling method is appropriate for all produce, and the choice of the most appropriate precooling method depends mainly on the specific circumstances, such as produce species, packaging type, refrigeration temperature, cooling rate, sensitivity to water, and maturity (for fruits and vegetables) (Duan et al., 2020).

1.2 Refrigerated warehouse

A refrigerated warehouse (RW) serves mainly to provide a stable, suitable, and long-term low-temperature environment to conserve the quality of fresh agricultural products after precooling. As such, RWs are a crucial link in the food supply chain and provide centralized storage and management of goods, maintain a balance between supply and demand, and regulate the transport capacity of goods. At present, the common storage methods for fresh agricultural products are cold air storage (CAS) and modified-atmosphere storage (MAS) (Table 2). CAS uses air as the cooling medium, and its effectiveness (i.e., cooling rate and uniformity, energy consumption, produce water loss, rate of chilling injury) depends directly on packaging and stacking patterns, the thermophysical properties of produce, air velocity, air temperature, and humidity (Bideau et al., 2018). MAS is based mainly on CAS but also adjusts the composition of the storage atmosphere (e.g., high carbon dioxide and low oxygen) to inhibit the physiological and biochemical processes of food deterioration or the microbial activity in the food product. MAS thus extends the food shelf life compared with CAS.

Fig. 2 shows the different cold storage categories and the corresponding products that can be suitably stored at various temperatures. Fresh or frozen products are most often stored in high-temperature (HiT), medium-temperature (MeT), or low-temperature (LoT) facilities. Quick-freezing (QuF) and ultralow-temperature (UT) cold storage may also be used, although the extremely low temperature may irreversibly damage meat products (e.g., degrade taste and sensory characteristics, reduce water retention, decrease thawing efficiency). In addition, maintaining an ultralow temperature requires significant electric power, which increases stakeholder risk and lengthens the profit-recovery cycle.

In recent years, an increasing effort has been devoted to balance the electric-power demand of cold storage with the conservation of product quality (Table 3). These studies can be grouped into three research topics: The first group focuses primarily on the details of airflow (including buoyancy-driven infiltration airflow) and the heat and mass transfer in cold rooms (e.g., the effect of opening and closing the coldroom doors). These studies mainly focus on optimizing the uniformity and stability of the cold-room temperature and humidity by applying energy-saving regulations, with the goal being to satisfy the requirements for a low-temperature, stable storage environment for the given food product (Tian et al., 2018).

The second group of studies investigates how different storage atmospheres and packaging materials affect the physicochemical quality of the food product (e.g., color and texture, the concentration of bioactive compounds, or the molecular response of produce when exposed to specific atmospheres), which provides a reliable theoretical and experimental basis to better understand product quality and the associated molecular processes. This approach allows a multi-objective optimization of product quality and energy conservation in lowtemperature storage (Akerma et al., 2020; Mditshwa et al., 2018; Zudaire et al., 2017).

Finally, the third group of studies focuses on the development and optimization of refrigerating systems, refrigeration storage technology, and refrigerants in an effort to reduce energy consumption. These studies are motivated by the global environmental and energy crisis and seek to promote energy conservation and the environmentally friendly development of RWs (Azmi et al., 2017). However, this research is sti

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