Sea fish is a very perishable food material and needs to be cared for from the moment of catch till cooked for the table or processed in the factory. Spoilage begins as soon as fish dies and affects its appearance odor and texture. Transport plays an important part in dealing with a perishable commodity like fish. When many indigenous and mechanized boats bring their catches ashore into numerous coastal villages, link roads between the villages and the nearest highways become essential for quick transport. Shellfish and fish destined to reach processing places are transported by trucks, trains, and sea vessels.
The commercial food sector, including agriculture, food manufacture transport, and retailing are responsible for 22% of the total greenhouse gas emissions. Food distribution and retail accounts for approximately one-third of this, with food transport which includes motive power and refrigeration estimated to be responsible for 1.8 % of total emissions. Road transport refrigeration equipment is required to operate reliably in much harsher environments than stationary refrigeration equipment. Due to the wide range of operating conditions and constraints imposed by available space and weight, transport refrigeration equipment has lower efficiencies than stationary systems. This coupled to rapidly increasing use of refrigerated transport arising from the much wider range of transported goods, home delivery, and greater quality expectations and is placing considerable pressures on the food industry to reduce the energy consumption of refrigerated transport.
Regulations for Refrigerated Transportation of Sea Fish
Various regulatory legislation covers temperature control requirements during the storage and transport of seafood items that require manufacturers to have suitable temperature-controlled handling and storage facilities that can maintain food at appropriate temperatures and enable these temperatures to be monitored controlled and recorded. Frozen seafood products are required to be maintained at -18°C or below and fresh fish, shellfish, and their products are required to be transported at temperatures close to 0°C. It is recommended to use freezer bags, ice slurry, chilled seawater, or refrigerated seawater wherever possible. Fresh whole fish should be kept in shallow layers of melting ice and maintained at a temperature between 0-4°C.
Components of Transportation System
Most of the refrigerated road transportation is conducted with semitrailer insulated rigid boxes. Many factors are considered in the design of the envelope of a refrigerated transportation unit: extremes of exterior weather conditions, desired interior conditions, insulation properties, infiltration of air and moisture, tradeoffs between construction cost and operating costs, and physical deterioration from shocks and vibrations. A rigid semi-trailer box normally consists of expanded foam insulation sandwiched between two external skins. Each skin consists of a few millimeters of plywood covered with a glass-reinforced polyester, steel, or aluminum skin. The most popular insulation is expanded polyurethane (PU) foam with cyclopentane as the blowing agent. This construction achieves a thermal conductivity in the region of 0.022 W/ (m K). Another popular insulation material is extruded polystyrene. The thermal conductivity of this insulation is higher than PU foam but in floor and roof construction where there are fewer constraints for overall thickness, bodybuilders can offset thermal losses by using thicker panels. Roofs and floors often have 100 mm or more insulation. Inside walls, the constraints mean the insulation is rarely more than 45-50 mm thick. The performance of insulation materials deteriorates with time due to the inherent foam characteristics. Recent data show a typical loss of insulation value of between 3% and 5% per year which can lead to considerable rises in the thermal conductivity after a few years.
The most common refrigeration system in use for refrigerated food transport applications today is the vapor compression system. Mechanical refrigeration with the vapor compression cycle offers a wide range of options for compressor drive methods. The choice may be based on duty required, weight, noise requirements, maintenance requirements, installation cost, environmental considerations, and fuel taxation. The most common drive systems for refrigerated transport vapor compression systems are:
Vehicle Alternator Unit: This method, which is commonly used in small delivery vans, the vehicle engine crankshaft drives an upgraded single alternator and a 70 Ah battery. The alternator charges the vehicle battery which feeds a small refrigeration system with a 12 V dc supply. The system can also be driven with a 230V mains electric supply during standby.
Direct Belt Drive: This system is used in most van-sized vehicles; the compressor of the refrigeration unit is directly driven from the vehicle engine through a belt.
Auxiliary Alternator Unit: This system uses a dedicated large alternator driven by a belt from the main traction engine, generating power to drive an electric motor in the refrigeration unit. Fan motors for the heat exchangers and the control system are also fed from the alternator output. An alternative arrangement for an alternator system is to use a diesel generator system. Using the ‘Genset drive gives the option of using red tax-free diesel to power the unit unlike running an alternator from the vehicle.
Auxiliary Diesel Unit: This system uses an engine built into the refrigeration unit which can be powered either by red diesel (cheaper) or white diesel (lower environmental impact). An optional particle filter and catalyst in new engine technologies can clean the exhaust emissions from an engine run on white diesel, leading to an over 90% reduction in emissions.
Air Delivery Systems
Top air delivery system is predominately used for air distribution in a refrigerated semi-trailer Refrigeration unit fans cause temperature-controlled air to circulate around the inside of the vehicle roof, walls, floor, and doors to remove heat, which is conducted and infiltrated from the outside, returning to the cooling coil via the floor or space under pallets. In recent years considerable effort has been devoted to the optimization of airflow and temperature distribution in refrigerated cargoes using a variety of modeling techniques including computational fluid dynamics.
In multi-compartment vehicles, the refrigerated space is subdivided into several compartments with individual temperature set-points to provide logistics flexibility for many business operations. It is common practice for supermarket chains to deliver produce inside multi-compartment semi-trailers. The different temperatures in each compartment are achieved by using distributed evaporator coils fed from a single condensing unit. In general practice, the coldest compartment is located at the front and the warmest is at the rear, but any temperature configuration is possible. actors specific to the design of multi-compartment refrigeration systems are the heat transfer between the compartments, product loading patterns and door openings, and method of temperature control.
Eutectic systems: Eutectic systems consist of hollow tubes, beams, or plates filled with a eutectic solution (phase change material) to store energy and produce a cooling effect whenever necessary to maintain the correct temperature in the refrigerated container. The Eutectic concept is different from conventional refrigeration systems in that a cold source (heat absorption) is provided by phase change material rather than direct expansion of refrigerant gas. The plates or beams that contain the eutectic are charged (frozen down) at night on mains power. Once the beams are frozen they operate silently and provide reliable, rapid cooling for a specific duration of time. Systems for food transport applications can be based solely on eutectic thermal energy storage or can be a combination of eutectic and vapor compression systems. Such systems can be cost-efficient for small deliveries where the heat loss through frequently opening doors can be a major problem.
Cryogenic Cooling Systems
As an alternative to mechanical refrigeration, total loss systems using liquid nitrogen or carbon dioxide injection may be used. Advantages are rapid pull-down of temperature and relative silence. For longer journeys, these systems are expensive to operate; so cryo-mechanical systems exist which combine the rapid pulldown of a total loss refrigerant with the more economical steady running of a mechanical system.
Good Transportation Practices
- Avoid unnecessary exposure to elevated temperature during the loading and unloading of fish and fish products.
- Loading to be done in such a way to ensure good airflow between product and walls, roof, and floor.
- During transportation, all precautions should be taken to maintain the appropriate temperatures so that discoloration, foul smell, and microbial spoilage could be avoided.
- Conveyances and/or containers or tankers used for transporting Fish and Fish products shall be kept clean, hygienic, and maintained in good repair condition.
- Appropriate measures should be applied to minimize damage to products and to ensure packaging integrity.
- Facilities for recording temperature or data loggers should be present.
- The transport vehicle should be examined for possible cross-contamination of ready-to-eat fish and fish products by raw fish and fish products.
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