Current advances in food freezing

There is a general view that fast freezing, and the formation of small ice crystals, offers some quality advantages. Although not true for all foods, minimising freezing times can be advantageous in also reducing energy consumption, increasing throughput, and improving yield.

Refrigeration systems and food freezing

There are two main types of freezing system: mechanical (which use a circulating refrigerant to reduce the temperature of air or a liquid which is passed over the food) and cryogenic (which use the direct application of liquid nitrogen or carbon dioxide onto the food). Both systems can freeze the food using various freezer designs, including tunnel, spiral, fluidised bed, impingement, spray, immersion, etc.

There has been a constant debate in the industry between the relative merits of mechanical versus cryogenic systems. In many ways this debate has been to the benefit of all sides leading to advances and improvements in all systems. Fast freezing rate, smaller footprint, and lower capital costs are usually cited as the main benefits of a cryogenic system, while high cryogen cost is often cited as the main disadvantage. However, advances in both technologies are reducing these distinctions. In practice, the food manufacturer must carefully balance the capital cost, operating cost, throughput and product quality, when deciding on which systems to install.

Freezer designs

There is no single type of freezer that is best for all food freezing applications. The selection and design is dependent upon operational requirements as well as product condition. Many parameters will influence the choice of system, including: product type, size, in-feed temperature and condition (any prior process such as cooking or marinating), appearance, production line capacity (kg/h), critical operating times and factory space limitations.

Blast air freezers

The most common way of freezing food is to use an air blast freezer. In many small operations products are batch frozen on, or in, racks of trays that are manually loaded into a freezing system. These freezers require double handling and it is difficult to achieve uniform freezing conditions. For larger operations it is better to use linear tunnels or spiral freezers. These can have single or multiple conveyors, the latter enabling freezing of more than one product at a time and at different throughputs. Linear tunnels are of simpler construction than spirals but can restrict the floor space available. Although this restriction can be solved by using stacked multiple passes. Flat belt tunnels are modular and thus can be extended many times over. The limitation to capacity is only set by the space available and the energy consumption costs of running straight tunnels with high fan loads. Spiral freezers are often a more viable alternative. The rise in the use of spiral freezers can be seen to be a response to the importance of reducing the footprint of freezers in order to fit more in existing factory plans.

There is a belief in industry that spiral freezers represent the peak of freezer technology. We have often heard talk of replacing existing tunnel systems with spirals under the mistaken belief that spiral are more effective and faster at freezing. This is a common fallacy. Spirals do offer footprint advantages over tunnels, but neither is more inherently effective at freezing, and spirals can be more expensive and complex.

The freezing time of the product is reduced as the air speed is increased. An optimum value exists between the decrease in freezing time and the increasing power required to drive the fans to produce higher air speeds. Up to 30 per cent of the total energy consumed in a blast freezer may be consumed by the fans and the extra refrigeration required to extract heat generated by the fans.

Fluidised bed freezers are used to prevent small products (such as peas and rice) sticking together and to the conveyor belt, thus ensuring the pieces are Individually Quick Frozen (IQF). Air is circulated up through the conveyor belt at high speed, lifting and agitating the produce to ensure it remains separated. Such agitation to produce IQF products can also be accomplished through mechanical means.

The use of impingement technology to increase the surface heat transfer in air freezing systems has been one of the few recent innovations to be truly commercially realised. The very high velocity impingement air jets increase heat transfer by breaking up the static surface boundary layer of gas that surrounds a food product. Impingement freezing is best suited for products with high surface area to weight ratios (i.e. products with one small dimension, such as burgers or fish fillets). The process is also very attractive for products that require very rapid surface freezing.

The big advantage of air systems is their versatility and flexibility, especially when there is a requirement to freeze a variety of irregularly shaped products. There are an almost infinite number of configurations available, enabling equipment manufacturers to ‘tailor make’ solutions that will fit within almost any facility.

Contact freezers

Contact freezers are extremely efficient and often faster than air systems. They can also be more energy efficient and well suited to soft, delicate, liquid or semi-liquid foods.

Plate freezers

Plate freezers are contact freezers where a refrigerant is passed through hollow metal horizontal or vertical plates that are pressed either side of the food being frozen. Plate freezers are best suited limited regularly shaped products with a maximum thickness of 50 to 70mm. Air spaces in packaging and fouling of the plates can have a significant effect on cooling time. While often overlooked as old technology, modern plate freezers are fast and energy efficient. Fully automated semi-continuous plate freezing systems are available and developments in single station opening plate freezer design allows the accommodation of multiple package sizes on the same line.

Belt freezers

One recent development is the renewed interest in contact belt freezers. Current belt freezers generally use a disposable plastic film that acts as the conveyor belt travelling through the freezing tunnel over refrigerated plates. These plates rapidly and effectively freeze the contact surface of the product, while the tunnel environment is cooled to reduce the entire product temperature. Using cryogenic or mechanical refrigeration systems these freezers are mainly designed for processing delicate, sticky and hard-to-handle products, even liquids can be frozen with ease. They may also be used for crust freezing, particularly of the underside of the product, prior to full freezing in an air blast tunnel or spiral. As well as rapid and efficient freezing, the use of flat belts delivers products free from belt marks and wrinkles. Since the belt is disposable, the freezer can be quickly and efficiently cleaned at the end of production, minimising expensive delays. Due to the single-use film, different products can be processed without cleaning steps.

Immersion/spray freezers

Immersion freezers utilise tanks of non-toxic salt, sugar or alcohol solution in water, or cryogen (liquid nitrogen). The product is immersed, either wrapped or unwrapped, in the solution whilst being conveyed through a tank. Alternatively the cooling medium can be sprayed on to the product. Some freezers, notably cryogenic systems, utilise both approaches, as well as often combining an aspect of air blast freezing. The same principle is used in drum, tumble or coating freezers, where the refrigerant (usually liquid nitrogen or solid carbon dioxide) is used to rapidly freeze products within a rotated drum. The product can also be simultaneously enrobed/coated by spraying a sauce onto the product at the same time.

Clearly if the food is unwrapped, the liquid has to be ‘food safe’. Any uptake of the cooling medium, whether food safe or not, by the product may present problems both in terms of flavour changes and the requirement for periodic replacement of the medium. This transfer can be minimised by packaging, although this may hinder heat transfer.

Spray/droplet freezers

In spray/droplet freezing, a liquid is injected in the form of a spray or droplets into a cold gaseous or liquid environment (usually liquid nitrogen) where they subsequently solidify into individual frozen droplets. Such freezers can produce novel products that would be difficult to manufacturer in any other way.

Key drivers to improvement and change

There appear to be a number of key drivers leading the improvement and development of current refrigeration systems. These include: Cost efficiency, hygiene, maintenance, space saving and energy efficiency. Some of the biggest developments have been in the practical running of these systems. Developments include designed for long-running, low maintenance and ease of hygiene, with an all stainless steel internal structure. The use of touchscreen computer interfaces that allow the user to monitor operation and maximise performance are now common. Equipment manufacturers are increasingly using direct drive technology, eliminating drive chains, sprockets and frequent oiling inside the freezer.

The build-up of snow across the evaporator in mechanical air blast freezing systems can prolong production downtime and lower output. Many food manufacturers demand extended production runs without requiring downtime for defrosting. Overall, a mechanical system can be custom designed to run for as long as needed to minimise production downtime and maximise output through the choice of the correct evaporator and sequential defrost coils that can be independently isolated and defrosted or automatic air defrost technology.

Equipment manufacturers are increasingly citing the hygiene and cleanability of their equipment, ease and speed of cleaning clearly being seen as important benefits. A number of manufacturers cite the use of run clear surfaces to enable quicker and simpler cleaning processes that minimise downtime between product line changeovers. Clean in Place (CIP) automated cleaning and sanitising system are also becoming more common. Reduced water usage is often cited as another benefit of such designs.

Few food manufacturers can now ignore energy use, from commercial and environmental points of view. Refrigeration makes up a large proportion of the energy used in food manufacturing and it estimated that the efficiency of most refrigeration systems could be improved by 20 per cent. Energy consumption can be reduced by optimising the heat loads on the plant and/or by operating refrigeration equipment more efficiently.

Equipment selection can have a large effect on overall efficiency. Although often initially more expensive, equipment such as inverter drives on compressors are beginning to be used more widely to allow more efficient use of compressors over a range of conditions. Evaporative, adiabatic and water cooled condensers can also save energy. Recent developments in electronically-commutated fans, electronic expansion valves and controls have also provided energy savings. Considerable energy savings are also possible by optimising defrosts.

Using modern insulating materials and protection can also reduce heat loads across insulation and through openings. With claimed thermal conductivities up to five times lower than typical polyurethane panels, vacuum insulation panels offer enhanced thermal resistance for the same or even reduced thickness.

The questionable long-term sustainability of HCFC and HFC refrigerants and implementation of the F-gas (fluorinated gasses) regulations has resulted in interest in new refrigerants and new refrigeration technologies. Many of these refrigerants are not actually new refrigerants but are ‘old’ refrigerants that have either come back into vogue or are now better able to compete due to improvements in equipment, such as ammonia, carbon dioxide and air. Other refrigerants such as hydrocarbons have also been successful in certain markets.

References

1. Food for Thought (fft.com)
2. New Food, Volume 15, Issue 4 (August 2012)

About the authors

Christian James is a Food Technology graduate and a Senior Research Fellow in the Food Refrigeration and Process Engineering Research Centre (FRPERC) at the Grimsby Institute. Chris joined FRPERC in 1993 and has now been active in food process engineering research for over 21 years. He has carried out studies on rapid cooling, initial freezing points and super-cooling, the use of heat pipes and tempering among others. He has also been very active in thermal processing of food including cooking and surface decontamination studies (particularly the application of steam and hot water interventions for treating meats and produce). He is author, or co-author, of over 150 publications on these subject areas. He routinely carries out industrial consultancies in all aspects of food processing.

Stephen J. James is the Founder and Director of the Food Refrigeration and Process Engineering Research Centre (FRPERC) at the Grimsby Institute. After working at the Meat Research Institute (MRI) and Institute of Food Research – Bristol Laboratory (IFR-BL), he joined the University of Bristol and formed FRPERC in January 1991. In 2009 FRPERC moved to the Grimsby Institute. Over the past 47 years he has carried out research into all aspects of food refrigeration ranging from primary chilling through to domestic handling. He is author, or co-author, of over 450 publications on these subject areas. He routinely carries out industrial consultancies in all aspects of food processing, including expert witness work.