The science of food freezing

Following on from our recent article on frozen food, which explained the benefits of freezing and why it could be an exciting opportunity for Africa, we thought we would follow up by explaining a bit more about the science behind the technology. So, how is food frozen and what is the science underpinning its use as a preservation technique?

A brief history on freezing

Freezing is not a new technology. As far back as Palaeolithic and Neolithic times, people used snow and ice to preserve food. More recently, cooling using salts and ice was widely known about in the 1500s and reported by Robert Boyle in 1662. Shallow lakes were also often used to create ice by exploiting radiant cooling during the night. However, all of these methods use naturally occurring materials or phenomena and depend on their temporal availability.

Artificial cooling on demand started to develop in 1775, when William Cullen made ice by vaporising water at low pressure, and it was the mid-late 1800s when freezing food started to really take off, with cold storage warehouses and ships capable of transporting frozen products (mainly meat) across the globe. In the modern era, as freezing and storage systems began to become more reliable and available, the 1920s and 1930s saw the widespread adoption of frozen food. Clarence Birdseye was instrumental in developing this market, as he understood the need for rapid freezing to maintain the quality of frozen products.

Why freeze food?

Freezing greatly reduces the rate of deterioration in food, meaning that frozen products can be stored for considerably longer periods than fresh items. Apart from a few examples such as ice cream, that fundamentally require freezing for their structure, the quality of food cannot however be improved by freezing and it is therefore important that only good quality products are frozen. Freezing locks in nutrients and there is evidence that if produce is frozen rapidly after harvest, it can have significant nutrient benefits over its fresh counterpart¹.

Many perishable products need refrigeration to maintain their quality and safety, but at chilled temperatures (generally 0-10°C) this can be affected by the continued growth of bacteria, moulds and yeasts. When freezing to below -12°C, the latter can no longer grow and so shelf life is extended to lengths significantly greater than those for chilled food. Generally, freezing does not kill the bacteria (in most cases, what is present before freezing will likely still be there after thawing), there are, however, a number of exceptions. For example, freezing can kill some protozoan parasites and has been shown to reduce Campylobacter spp. on chicken² (Campylobacter is one of the most common causes of food poisoning in developed countries).

Most foods have a high-water content and this aids biochemical deterioration. However, frozen food has what is termed a low available water activity (a_w). Freezing reduces a_w because the ice crystals remove water from the food matrix. For high water content foods, freezing is therefore one of the most viable methods to extend shelf life.

Despite this benefit, freezing is not a means to extend storage life indefinitely. Chemical and enzymic reactions do still occur unless temperatures are greatly reduced to a point at which no reactions can occur. This point is called the glass transition temperature (the temperature at which no further water can be frozen). For most foods this temperature is below –30°C.

The freezing process

Equipment used for freezing food varies according to the food type, as well as the size of the item being frozen, and most freezers aim to remove the heat from products as rapidly as possible. For small thin products (e.g. beef burgers) this can be achieved though high air velocities and low air temperatures. For larger thicker products (e.g. a beef carcass) the process is technically trickier. In this case, once the surface temperature is reduced to close to the ambient freezer temperature there is limited benefit in high air velocities, as conduction cooling predominates. Therefore, for larger products a 2 or 3 stage freezing system with varied temperatures and air velocities is likely to prove the most efficient.

Freezing fast produces smaller crystals in the food and minimises weight loss (as water at the surface of a product is rapidly converted to ice). Slow freezing results in larger ice crystals that potentially impact on the food texture through damage to cell walls. Small ice crystals are quite unstable and over time they migrate to become larger crystals (called Ostwald ripening). This is exacerbated by temperature fluctuations. On thawing, extracellular ice does not re-enter the cells and may cause extensive drip loss.

Whether the ice crystal size has significant impact on the final product that is discernible by a consumer depends on the food type. For example, in the case of ice cream, crystal size must be small to ensure customers cannot detect ice (consumers can detect ice crystals of greater than 40–50 μm)³, whereas in meat the impact of freezing rate is much more unclear - with some researchers finding no impact of freezing rate and others finding a small negative impact⁴. Such lack of agreement may be caused by variations in storage time or how the food was stored.

Factors affecting storage life

The storage life of foods can vary considerably and are related to a number of factors. The temperature of the store has an impact, with lower values nearly always resulting in longer life. This can, however, be significantly affected by what happens before freezing.

Pre-freezing product, processing and packaging (PPP) factors have a substantial influence on storage life. Of particular significance is the time between harvest/slaughter and freezing, as well as how the product is handled post-harvest/slaughter (product factors). Processing factors include whether and how a product is heated, cut, or used in manufacturing. Packaging is extremely important for frozen food as it can reduce moisture loss and associated quality deterioration that may result in "burn" or the development of rancidity.

It should also be noted that how storage life is assessed varies considerably and it is often difficult to accurately compare similar products from different sources. An assessment may depend on a variety of factors, such as sensory assessment, chemical or instrumental tests, or a combination of these. Even sensory assessment may use different scales, have different levels of training for the sensory panel, or use different sensory testing methodologies.

Where can ACES contribute?

Although the science of freezing is reasonably mature and well understood, there are still many challenges that need to be addressed. Freezing processes can be further optimised to ensure that food is frozen economically and efficiently. Many freezing and storage plants remain in operation for 40-50 years, so designs need to be future-proofed in terms of operation (to cope, for example, with climate change) and refrigerants (to enable future phase down and phase out of high global warming potential refrigerants). In Africa, there are potential opportunities for products not previously frozen to be frozen and these will need research, development and assessment work to ensure optimised storage life. ACES is well placed to support the development and adoption of a well-designed, optimised, sustainable cold-chain for frozen food in Africa. Moreover, by being present locally on-the-ground at the inception of new ideas, concepts and businesses, we can help support an efficient, viable, low carbon, Africa-centric frozen food market across the continent.

If you would like to know more about frozen food and freezing, or would like to engage more widely with the ACES team, please contact us at the Clean Cooling Network (CCN).