For many years frozen foods had been perceived, and often treated, as the poor relations of the fresh or chilled sector. The emphasis has tended to be towards cheap and convenient products. Over the last decade, however, this emphasis has changed strongly. Due in part to the availability of new technologies and also to increasing expectations from consumers, the emphasis has shifted to higher quality, value added food products.
The production and distribution of frozen foods involves a long and complex chain. The guarantee of delivering a high quality product requires an understanding of all the stages of that chain. This commences with the initial produce production on the farm and continues through to retail display and home transport. In this article I will outline the areas where developments have been most rapid and where control is most critical.
On the farm
The quality of frozen fruits and vegetables depends both on the technological processing and on the suitability of the original material for freezing. Each year many new cultivars (cultivated varieties) are developed by the agricultural industry. For example there are now over 2000 cultivars of strawberry and many hundreds of cultivars of apples, green beans etc.. New cultivars are developed by cloning and variety crossing and characteristics such as disease resistance, simultaneous ripening and suitability for mechanical harvesting have provided much of the impetus for this work. The same characteristics are important for produce destined frozen food products but additionally the selection of cultivars for their suitability to freezing will have a noticeable effect on final product quality. For example figure 1 shows measurements of drip loss after thawing for a range of strawberry cultivars¹. The final values range from 8% to 38%. The varieties Miss and Don both have good colour flavour and harvesting properties. However the increased resistance to drip loss and retention of turgor after a freeze thaw cycle make the latter a far more suitable selection for frozen food production.
Once the cultivar is selected a number of agricultural factors further effect the quality of the plant product. These include soil type, sunlight, wind, rain and altitude. For example prolonged exposure to sunlight or high temperatures has been shown to block lycopene (an antioxidant) formation in tomatoes and can effect colour in tomatoes and peppers. High winds, apart from the obvious buffeting damage can effect pollination and fruit set. Apples, particularly golden delicious, have been demonstrated to have a firmer pulp and better aroma profile when grown on mountains than in otherwise similar conditions in the valley.
Our understanding of all these factors is progressing and, together with sensory and mechanical testing of crops, is leading to an improvement in the specifications set for frozen food ingredients.
As with vegetables, so the selection and handling of meat and poultry contributes significantly to the final eating quality. Eating quality is judged primarily in terms of toughness, juiciness and flavour. Although animal to animal variation makes the understanding of individual influences harder than in fruit and vegetable production, some factors are now well established. As well as differences between breeds, the effects of diet, particularly antioxidants, handling during transport and the effects of chilling and ageing processes have all been demonstrated to contribute to the final meat quality. For many frozen meat products the shelf life determining factor is the onset of rancidity through the oxidation of fats. This is because lipid oxidation is effected adversely by the removal of water from saturated fatty acids. The formation of ice crystals can remove water from the side chains of unsaturated fatty acids leading to a relative increase in the rate of oxidation. The rate of rancidity development and hence eventual shelf life can therefore be affected by factors as diverse as the original fat content to the use of vitamin E supplements in the diet which act as antioxidants in the processed meat.
Given the use of high quality ingredients the next step in the production of quality products is the pretreatment processes, which can prevent deterioration during storage. Unlike in the chill chain, at frozen storage temperatures microbiological deterioration is not a problem. Rather, a combination of chemical and physical processes is responsible for loss of quality. In particular enzymatic activity and structural damage due to ice formation and growth.
As a food is frozen, the concentration of solutes in the unfrozen phase will increase. This will lead towards faster reaction rates. Simultaneously the drop in temperature will tend to slow reactions. A further complication may occur if viscosity changes in the unfrozen phase limit reactant mobility and therefore also tend to a slowing of reaction kinetics. Different types of reaction are affected by these processes to varying degrees, and not surprisingly reaction kinetics in frozen foods are complex.
Traditional pre-treatments have included the blanching of products to inactivate enzymes and the soaking of products in antioxidant solutions to reduce the effects of browning. Recent developments have refined drying soaking and osmotic dehydration processes driven by a greater understanding of the balance of chemical reactions and physical change as a function of the balance of different solutes.
Simply partially drying fruits and vegetables before freezing can reduce the amount of water, which leads to less internal damage from the expansion of water to form ice, and to a lower energy cost. However, if the same reduction in water content is achieved by an osmotic process ² , then the reduction in water content can also be combined with the addition of salts, sugars or other cryoprotectants. The addition of solutes by osmosis can allow a lowering of the water activity of the food with only a minimal change in the consistency of the product. The consistency is usually associated with the swelling effect of water on the cellulose and pectin content of fruits and vegetables. Controlling water activity in this way gives an extra variable to the freezing parameters of the product. So, for example, it is possible to produce fruit pieces for use in ice cream products which maintain a soft consistency rather than an icy crunch.
A wide selection of solutes has been investigated for osmotic soaking processes and suitable choice will depend often on the products taste requirements. Typical solutes are sucrose, corn starches, concentrated fruit juices and binary salt mixtures.
Similar soaking techniques have been applied to add cryoprotective solutes to meat and fish products. In particular, sugars, amino acids, polyols, methyl amines, carbohydrates and some proteins have all been shown to impart some cryoprotective properties in frozen fish. Probably the most abundant usage has been the addition of various carbohydrates including sugars to surimi in order to minimise loss of protein functionality.
Altering the solute content will clearly increase the viscosity of the unfrozen matrix which surrounds the ice crystals in a frozen food. This may increase stability by bringing dynamics closer to glass transition.
A final step in this process has been the combination of soaking with immersion freezing techniques. Binary solutions using salts and ethanol to lower the freezing temperature aim to balance the rates of heat and mass transfer to provide a surface formulation and freezing process.
For many years the design of freezing tunnels had been rather static. The choice facing food manufacturers was largely between cryogenic and mechanical equivalents of standard freezing tunnels. The choice was usually made on the basis of capital and running costs and the value of water lost during dehydration of the freezing product. Both cryogenic and air blast tunnels achieved similar heat transfer coefficients (The ratio of heat transfer to temperature difference for unit area of product) of around 50W/Km 2 , but cryogenic freezers were somewhat faster because of the colder refrigerant temperature.
The last decade has seen rapid improvement in the design of freezing tunnels from a point of view of efficiency and in the rate of heat transfer achievable.
Both of these factors affect final product quality and cost. For example the Cryo Quick VT tunnel developed by Air Products removes the heat load associated with fans by locating these under the belt. It maximises heat transfer by directing the air through vortex forming channels leading the airflow directly downwards onto the food, and finally allows input of liquid refrigerant to be controlled at a number of points along the freezer. Such design improvements have lead to heat transfer coefficients as high as 120W/Km² being attained. Similar improvements have been achieved in air blast freezers through a process called impingement. An example of this is the flat bed freezer developed by Frigoscandia. This again controls speed and direction of the airflow and can achieve heat transfer coefficients of 175W/Km². But improvements have not all been about increased freezing rates. Many refrigeration companies are now supplying complex freezers which can combine the advantages of cryogenic and mechanical refrigeration. So, for example, products such as cooked chicken pieces can be rapidly crust frozen by a nitrogen immersion technique to minimise moisture loss and then the remainder of the heat can be removed by a more economic air blast tunnel.
There has also been great concern in recent years about the effects of refrigerants used for mechanical freezing on the environment in general and the ozone layer in particular. This has provided an impetus for a return to an early form of refrigeration cycle where air is used as the primary refrigerant gas. The Air Cycle was first utilised in the last century but has not been as efficient as cycles that employ, for example, ammonia as their refrigerant. However developments in turbine technology and operation at -60ºC are beginning to make this cycle practical for use again.
One final novel freezing technique that bears mention is the use of low adhesion technology. Early experiments on the adhesion of ice showed that adhesive forces increased as the temperature decreased and for many years this was the excepted wisdom. In fact this is not the case below -80ºC where adhesive forces in fact become very small and Air Products have developed technology for freezing liquids, and ice cream in particular, where products can be moulded and removed without the need for a surface re-melting stage. This has lead to the possibility of a range of novel ice cream products with imaginative shapes and intricate surface design features.
In order to maintain the quality of frozen foods during distribution it is important to have good temperature control. This is not simply to prevent temperatures rising above the melting point but also to limit moisture migration and ice crystal growth which are accelerated by temperature fluctuations and high sub zero temperatures. For example it takes 48 days for the average size of ice crystals to double in beef samples at -25ºC. The same effect can be achieved in just 24 hours at -5ºC. Fortunately it is possible to attain good control and monitoring of temperatures along most of the distribution chain provided good management and training systems are in place. Once we get to retail display however the situation changes.
Retail display cabinets need to satisfy two conflicting requirements. One is to provide high visibility and easy access to products. The other is to maintain products at the desired temperature. For this reason sophisticated modelling and design techniques are now being applied to retail cabinet design. From a sales and access point of view the most suitable display cases are open front vertical cabinets. Such cabinets provide the easiest access for consumers and maintain temperature control by passing a double or triple layer of cold air in front of the shelves to form a curtain between the products and the shop floor. Increasingly, computational fluid dynamics (CFD) is being applied to retail cabinet design. A large number of factors must be taken into consideration. The airflow must be sufficient to maintain food temperature but not so fast as to create turbulence. Heat loads will come from lighting and heating in the store, defrost cycles and customer handling. Effective operation of retail cabinets therefore requires an integrated approach to store design, packaging and cabinet design and operation to achieve the optimum balance between product display, temperature control and energy efficiency.
Retail display is another area where new refrigeration cycles are under consideration. CFC’s are being replaced by HCFC’s which have less ozone depleting effects but these in turn will have to be phased out in the coming years. For this reason new systems which have remote ammonia refrigeration are being considered for larger retail outlets as well as the possibilities of employing the Air Cycle as discussed above.
In this article I have tried to outline briefly some of the rapid changes which are taking place in the frozen food industry. These developments are occurring along the entire food chain from the development of new cultivars on the farm through to improvements in retail display cabinet design and operation. The theme linking these developments is an increasing awareness that to produce and deliver high quality frozen product sections of the chain can no longer be seen in isolation but must be seen as an integrated system. The network of partners who took part in our project reflected the diverse inputs required to understand such a system. To aid those who work with frozen foods we have recently published a book, “Managing Frozen Foods”, which provides a more in depth insight to many of the topics discussed above.
1 Managing Frozen Foods. Editor C.J.Kennedy. Woodhead Publishing, Cambridge U.K. 2000
By C.J.Kennedy NutriFreeze Ltd