Defrosting and Tempering of food products
PHT Masterclass: Defrosting and Tempering of food products
A change of state: The science of defrosting
At school we learn that a liquid change to a solid during freezing, and that this is a reversible change of state. Simple. But when it comes to freezing and defrosting food, we realise that there’s a bit more to the science behind the process. During a recent master class hosted by PHT, authoritative voices in the industry shared with delegates what proper defrosting involves, why it is important to get it right and what solutions are available.
Thwarting microbes during thawing
‘Freezing and thawing are inextricably linked,’ says Prof. Elna Buys of the Department of Consumer and Food Sciences at the University of Pretoria. ‘Thawing is not simply the reverse of freezing; it is a substantially longer process.’ And therein lies the key for understanding the freeze–thaw process to ensure that we handle frozen food safely.
During freezing, the water in food turns into a solid. The size of the ice crystals depends on the rate of freezing, and ultimately determines the quality of the food product upon thawing. Rapid freezing leads to fairly small ice crystals forming, whereas slower freezing allows larger crystals to form. Large ice crystals cause more damage to cells and so increase drip losses during thawing.
Because freezing merely makes microorganisms dormant and slows down enzyme activity, rather than stopping microbes in their tracks altogether, excessive dehydration due to an inappropriate freezing process can create favourable conditions for microbial action upon thawing. Add to that an undesirable microbial load before freezing, and the scene is set for a food safety risk being introduced.
Fast freezing coupled with controlled thawing at temperatures below 4 C is ideal. Maintaining defrost temperatures below 4 C keeps microbial action at bay and so allows food to defrost fully in a safe time window of two to three days. However, if the product is overheated and allowed to reach temperatures above 5 C during defrosting, the lag phase before microbial growth resumes is reduced and the period for safe defrosting is substantially reduced.
The effect of the food matrix is also an important consideration for safe defrosting, as substances such as sugars, amino acids and fats can act as cryoprotectants during freezing, and so facilitate renewed microbial action when the product thaws. In fact, a recent modelling study showed that although the safe defrosting period reduces for all the food products included in the model as the defrosting temperature increases, the effect differs across food products. The takeaway message therefore is that the type of food product being defrosted – whether it is meat, fruit, fish or poultry – should be considered when choosing your defrosting approach.
Heat in, heat out
The most widely used defrosting and tempering solutions either induce heat transfer at the surface of a product (through the process of conduction, convection or condensation) or relies on heat generated within the product spreading outwards, such as is achieved with microwave technology.
As Jens Wittig of Schröter Technologies explains, an application such as the CLIMATjet uses surface heat transfer under carefully monitored conditions to allow defrosting to proceed slowly and safely while drip losses are minimised. With this application, steam is introduced into the defrost chamber under controlled conditions, which drives the initial stage of the defrost process.
The product’s core temperature increases fairly rapidly, but remains below –5 C. Feedback monitoring of the surface temperature carefully controls the external temperature in the chamber to prevent overheating and so brings about a gradual increase in core temperature until the target temperature is reached and the product is fully thawed.
In some industrial settings, however, a conventional defrost approach may not be suitable and microwave applications can be considered as an alternative. As Mathieu Deschamps of Sairem explains, microwave energy is able to penetrate a frozen product homogenously. This results in successful tempering being achieved within as little as 10 minutes. Depending on the client’s specific operational needs, microwave-based tempering can translate to substantial cost and energy savings.
(De)Frost forward
A master class like this one is an opportunity for the food industry to learn and explore new directions, knowing that they can tap into the collective expertise of industry leaders. And as Josef Messmer, Managing Director if the PHT Group, has said many times before, ‘you don’t have to know everything yourself, you simply need to know who to ask.’ PHT invites you to get in touch and will be happy to advise you on the best approach for your specific defrosting and tempering needs, supported by their network of leading industry partners.
Please contact PHT South Africa at [email protected]
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This article was written by Linda Pretorius
