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Keep your cool with liquid nitrogen

28 July 2019

Neil Hansford discusses an alternative to the use of carbon dioxide in food mixing and blending operations which can also help to speed up and increase reliability of the process. 

Last summer’s shortage is still fresh in the memory of the UK food and drink industry. The reliance of the food industry on carbon dioxide was brought into sharp relief by a well-publicised shortage last summer. 

Finding viable alternatives, strengthened by a secure supply channel is a key part of the longer-term solution to this problem, which resulted in Air Products looking at whether liquid nitrogen (LIN) could be used as a replacement gas in food mixing and blending to cool ingredients. The subsequent development of its Freshline LIN-IS system, convinced the company that it had a solution that could deliver a range of additional benefits to food manufacturers – speeding up and increasing the reliability of the process, with the added advantage of using less gas.  

One benefit is a reduction in food waste which is becoming an increasing concern. The production process itself has its part to play here. The benefit of LIN links to its temperature, which is more than twice as cold as when CO2 is used. At -196°C, rather than CO2’s -78°C, much less of the product sticks to the surface of the mixer blades, reducing food waste in the manufacturing process.  In particular, this helps to reduce waste when mixing products with high fat content – such as pork or dough – which usually creates a sticky mixture. 

The use of LIN can also make the overall process of cooling much quicker and more efficient.  Partially, this is because the Freshline LIN-IS system injects the gas underneath ingredients, rather than above it.  Injecting from the bottom of the mixer ensures that the gaseous phase of the LIN is used to cool the product, giving us a larger amount of cooling energy, 373kJ as opposed to 311kJ with CO2. This means it takes less time for it to mix into the product, reducing the time it takes to sufficiently cool the mix and therefore decreasing production timescales.  Naturally, the speed of the process also means that amount of gas injected drops considerably when compared to using CO2 – on average by between 20 and 30%. 

There are wide ranging benefits linked to product consistency. Food mixtures must remain at the optimum texture before the forming process to keep the final product consistent. The speed with which LIN can cause a drop in temperature assists this process – the longer mixing times of CO2 can cause over-mixing of the product, resulting in texture issues.

Another point to note is that LIN doesn’t solidify during the mixing process, further increasing the likelihood of a consistent end product.  This solves another barrier to consistency which can be a factor in CO2 cooling, as some of the gas can solidify during the mixing, creating a solid, snow-like substance. 
A change in process can often mean a major change of equipment which can be costly, both in terms of investment and downtime.  While the change to a different gas will require changes to the injection system, as the copper pipes which are used to inject CO2 into mixers are not suitable for LIN due  to the significantly lower temperature.  However, this can be done through retrofitting the existing system to introduce vacuum insulated pipework, making the change from CO2 to LIN possible without incurring the considerable cost of replacing a mixing vessel. 

While the industry may be over-reliant on CO2, the example of LIN shows that, in some areas, there are viable alternatives and more manufacturers are now looking at this option – not only easing concerns of a repeat of last summer’s standstill, but allowing for a more streamlined operation which also helps to tackle broader issues such as food waste.
Case study
One food producer needed to find a way to reduce waste in its manufacturing process by rapidly cooling its vegetarian dough mixture, prior to it being formed.  A dough that is too hot is sloppy, fails to stay in shape and can stick to the forming plate, resulting in unnecessary waste. Without the time spare to store the dough in a cold chamber prior to forming, and the addition of ice water or frozen product proving ineffective, a new solution was needed.

The company’s director of operations explained that mixing in nitrogen offered the company a good way of quickly chilling mixtures to the ideal processing temperature of approximately -2°C.

By fitting Freshline LIN-IS to its mixers, the company is now able to set its mixture to the ideal temperature and texture, quickly and easily. This accurate and repeatable temperature control of the dough ensures the forming step is successful – increasing the rate at which products can be made and reducing waste by 2%.  

Neil Hansford is an industrial cryogenic and food expert at Air Products.

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