Chilling of Food
Chilling is one of the most common methods for preserving foods. Carried out correctly, it can provide a high-quality, nutritious and safe product for consumption with a long storage life. The principal factor controlling the safety and quality of a refrigerated (chilled) food is its temperature. In many cases, the time taken to reach the desired temperature is also important…
World agriculture faces a daunting array of apparently contradictory challenges. The sector must massively increase output by the middle of the century to reduce hunger and satisfy the demand of a global population that is predicted to swell to over 9 billion. At the same time farmers need to slash carbon emissions, which currently account for 30% of the global total, if countries have a hope of complying with their Paris Climate Change Agreement obligations.
Until now, governments and companies alike have largely focused upon increasing the amount of food we grow, increasing intensity, boosting yields and investing in bio-technology. However, one approach that would help to achieve all goals simultaneously is simply to reduce the amount of food that is wasted. The statistics are shocking. According to estimates from the UN Food and Agriculture Organization, 800 million people live in hunger. At the same time, up to a third of all the food produced worldwide, more than a billion tonnes, is wasted every year. One study has indicated that if we could only halve food wastage, then we could feed another one billion people without needing to plant any more crops, catch any more fish or feed any more animals.
“The amount of food wasted and lost globally is shameful”, says World Bank President Jim Yong Kim. “Millions of people around the world go to bed hungry every night and yet millions of tons of food end up in trash cans or spoiled on the way to market. We have to tackle this problem in every country in order to improve food security and to end poverty.”
Food wastage also has enormous implications for the sustainability of agriculture and its carbon footprint. The FAO estimates that to grow the same amount of food that we currently waste would require a land area the size of Mexico; it would consume 250 km3 of water per year, which is three times the volume of Lake Geneva; and it would account for 3.3 billion tonnes of CO2 emissions, making it the third largest carbon emitter after the US and China. So, any effort to reduce food wastage will not only help to reduce hunger but also improve resource efficiency and help to tackle climate change.
Food is wasted at all stages of the supply chain, from the farm gate to the back of our fridge. The balance of these losses varies by crop and region, but across the world the greatest proportion of food is lost between farm and retailer.
Addressing this challenge will require a multi-faceted approach. Like all thorny issues, there can be no quick fix. However, in order to ensure that more of the food we grow gets to be eaten, keeping it cool from the point where it is harvested to the shops can play an enormously beneficial role. In the most simplistic sense, this means introducing more refrigeration to the way we harvest, store, transport and retail food in order to extend shelf life.
However, conventional ‘cold chains’ of refrigerated warehouses and vehicles can be carbon intensive, highly polluting and damaging to human health. The task of developing and deploying innovative clean cooling technologies is, therefore, absolutely central to the future of civilization.
Two thirds of the world’s food wastage happens in Asia and Africa, and these are also the regions where cold chain capacity is often rudimentary or non-existent. The International Institute of Refrigeration has estimated that if developing countries had same level of cold chain as developed nations, they could save 200 million tonnes of perishable food each year.
Reducing wastage by building cold chains in developing countries would not only increase the food supply, but also help tackle poverty. Food prices can be reduced for consumers, because if markets are better supplied, prices fall, and farmers’ incomes increase, because more of what they produce can be sold rather than discarded.
In fact, National Centre for Cold-chain Development (NCCD) has concluded that developing temperature controlled logistics in rural areas will be a critical factor in achieving the government’s target of doubling farmers’ income over the next five years. While various efforts are underway to incrementally reduce farmers’ costs, a transformative effect is expected to come from improving transport from farm-to-consumer. The NCCD points out that cold chain give farmers access to distant and potentially higher value markets, so increasing the incentives to raise production.
Demand for cold chain services is already beginning to surge in countries such as China and India as they become wealthier and more urbanized. However, emerging cold chains still rely on conventional and all too often highly polluting, diesel-powered transport refrigeration units (TRUs). These TRUs are not only carbon intensive, but also emit grossly disproportionate amounts of nitrogen oxides (NOx) and particulate matter (PM), the toxic pollutants that cause 3.7 million deaths worldwide each year. If cold chains continue to be built using conventional technologies and demand continues to boom as predicted, these countries will simply swap one set of problems for another.
New clean cooling technologies are, however, now being commercialized, which could resolve this dilemma. This means it is finally possible to secure all the benefits of reducing food waste – for food security, emissions and wider sustainability – without the downside. The challenge is now to deploy the new technologies urgently to prevent developing countries from adopting the old dirty technologies by default – locking in emissions for years to come.
Growth of cold chain in India, for example, has around 31 million tonnes of cold store capacity and perhaps 9,000 refrigerated trucks, whereas in France the balance is 5 million tonnes and 140,000 (India also has 27,000 milk tankers, but these are insulated and not refrigerated). The chief executive of India’s National Centre for Cold-chain Development, Pawanexh Kohli, says that this causes a ‘major breach of the cold chain’ and that there is ‘in effect no integration’. Kohli has also calculated that to make proper use of just 10% of India’s existing cold storage capacity, the country needs to build 30,000 new pack-houses with pre-cooling facilities, and 60,000 refrigerated trucks. By extension, making proper use of all of India’s cold storage capacity would require 600,000 refrigerated trucks. Taking a broader international perspective, if India had the same ratio of refrigerated vehicles to the value of its grocery market ($375 billion in 2012) as Britain ($243 billion), it would have 139,000 refrigerated vehicles, 18 times more than at present. And if it had the same ratio of refrigerated trucks to population as Britain, its fleet would number more than 1.5 million. The picture in China is similarly skewed. Demand for cold chain services has been building for longer in China than in India – fridge ownership among urban households, for example, rose from 7% in 1995 to 95% in 2007 – and yet the country still has much more cold storage than refrigerated vehicle capacity.
Chilling of food is a preservation technique in which the food either in raw form or in processed form is cooled to a temperature of 0ºto -5ºC. Chilling differs from freezing, in fact, that the temperature of food does not fall below one where ice is formed in the food. Many commercial chillers operate at higher temperatures of 10-12ºC. So, both time and temperature of holding the chilled food determines the storage life of food.
The objective of chilling food is to reduce or maintain the temperature of food so that changes occurring in food should be retarded or stopped. The changes include microbiological (microbial growth), physiological (ripening, senescence and respiration), biochemical (lipid oxidation and browning) and physical (moisture loss and weight loss).
So, in order to preserve the quality and achieve desired shelf life of fresh raw or processed product, a chill chain is introduced. Also the quality parameters like taste, smell, appearance and texture are also preserved to some extent by chilling of food.
Rates of chilling are governed by the laws of heat transfer. It is an example of unsteady-state heat transfer by convection to the surface of food and by conduction within food itself. The medium of heat exchange is generally air, which extracts heat from food and then gives it up to refrigerant in the evaporator. The rates of convection heat transfer from the surface of food and to the evaporator are much greater if the air is in movement, being roughly proportional to v0.8.
To calculate chilling rates, it is therefore necessary to evaluate:
a) Surface heat transfer coefficient,
b) Resistance offered to heat flow by any packaging material that may be placed round the food,
c) Appropriate unsteady state heat conduction equation.
Although the shapes of most food stuffs are not regular, they often approximate the shapes of slabs, bricks, spheres and cylinders.
Chilling Systems for Food
There are a large number of different chilling systems for food based on moving air, wet air, direct contact, immersion, ice, cryogenics, vacuum and pressure shift. For the majority of chilled foods, air systems are used primarily because of their flexibility and ease of use. However, other systems such as immersion, contact and cryogenics can offer much faster and more controlled chilling.
Air Chilling Systems
Air systems are most common type of systems as they are economical, hygienic and consist of relatively non-corrosive equipment. It consists of a fan which draws air through a refrigerated coil and blows the cooled air around an insulated room with a purpose-built conveyorized air-blast tunnels or spirals. The rates of heat transfer are relatively lower in air-cooled systems. The big advantages of air systems are their lower cost and versatility compared to immersion, contact and cryogens, especially, when there is a requirement to cool a variety of irregularly shaped products.
AIR CHILLING SYSTEM
One of the major disadvantages of air-cooling systems is their tendency to dehydrate unwrapped products. The alternate to this problem is to saturate the air with water. Wet-air cooling systems recirculate air over ice cold water so that the air leaving the cooler is cold (0–1 ºC) and virtually saturated with water vapor (100% relative humidity, RH). An ice-bank chiller uses a refrigeration plant with an evaporator (plate or coil) immersed in a tank of water that chills the water to 0 ºC.
Contact Chilling Systems
Contact refrigeration methods are based on heat transfer by the direct contact between products and metal surfaces, which in turn are cooled by either primary or secondary refrigerants. Contact cooling offers several advantages over air cooling, such as much better heat transfer and significant energy savings. Contact cooling systems include plate coolers, jacketed heat exchangers, belt coolers and falling film systems. Good heat transfer is dependent on product thickness, good contact and the conductivity of the product. Plate freezers are often limited to a maximum thickness of 50–70 mm.
Immersion/Spray Chilling Systems
Immersion/spray systems involve dipping products into a cold liquid or spraying a cold liquid on food. Cooling using ice or direct contact with a cryogenic substance is essentially an immersion/spray process. This produces high rates of heat transfer due to the intimate contact between product and cooling medium. Both immersion and spray methods offer several inherent advantages over air cooling in terms of reduced dehydration and coil frosting problems.
It includes two systems:
a) Ice systems: Chilling with crushed ice or an ice/water mixture is one of the simple, effective and commonly used methods for the cooling of food products. Ice has the advantage of being able to deliver a large amount of refrigeration in a short time as well as maintaining a very constant temperature, 0 ºC to −0.5 ºC (where sea water is present).
b) Cryogenic systems: Direct spraying of liquid nitrogen on a food product whilst it is conveyed through an insulated tunnel is one of the most commonly used methods of applying cryogens. Due to very low operating temperatures and high surface heat transfer coefficients between product and medium, cooling rates of cryogenic systems are often substantially higher than other cooling systems.
Benefits of Chilled Foods
• Offers consumers the choice of a wide range of tasty and nutritious foods that are quick and easy to prepare.
• Provides convenience in buying, preparing and cooking food helping to reduce the amount of time and energy.
• Chilled foods also reduce waste by cutting down on the need for consumers to buy unnecessary or large amounts of ingredients.
Chilling is one of the most common methods for preserving foods. Carried out correctly, it can provide a high-quality, nutritious and safe product for consumption with a long storage life. The principal factor controlling the safety and quality of a refrigerated (chilled) food is its temperature. In many cases, the time taken to reach the desired temperature is also important. To provide safe, high-quality refrigerated food products, attention must be paid to every aspect of the cold chain from initial chilling or freezing of raw ingredients, through storage and transport, to retail display.
AUTHORS CREDIT & PHOTOGRAPH
Department of Processing
and Food Engineering
Punjab Agricultural University,
Department of Processing
and Food Engineering
Punjab Agricultural University,