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Too thick, too thin or just right – why temperature is so important

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Monday, 8 April 2019

Too thick, too thin or just right – why temperature is so important

The March 2019 IDDSI newsletter discusses the issue of fluid thickness changing at different temperatures.

Thick liquids change thickness levels depending on their temperature

It has always been the case that the thickness of liquids change depending on their temperature. Generally speaking, a thick liquid that is served cold/chilled will be thicker than if it is served at room temperature, or heated. We see this effect in our daily lives as well. Think about a tube of toothpaste. During winter, more force is needed to push the toothpaste out of the tube, as the toothpaste is firmer. During the summer months, it is very easy to push the toothpaste out of the tube as it flows more easily due to the ambient heat. Think now also of a rich pumpkin soup. Served warm it behaves more like a liquid, but when it cools off, it becomes thicker. This is physics at work. Thick liquids have always behaved this way – thicker when cool, and thinner when warm. In research this is why it is critical to include the temperature for testing because the effect is well known. 

How does this affect individuals with swallowing difficulties?

Thickened milk is typically served warm to babies. The teat/nipple needs to be carefully selected so that the thick milk will flow through it when the infant sucks. If the baby is fussy, the thick milk cools over time and the thick milk becomes thicker and harder for the infant to suck through the teat/nipple. This can cause the baby to fatigue more quickly as they become inefficient with their feeding.

How can the IDDSI flow test help?

The IDDSI Flow Test can be used to accurately capture the change in thickness level as the fluid cools.

What about thickener products? 

Due to the known and scientifically proven variation in thickness with temperature, IDDSI is aware that a liquid that tests ‘in range’ at ‘room temperature’, may be considered too thick when chilled, or too thin when heated. We have asked manufacturers to state what temperature their products should be served at. For people with swallowing problems, we need the thick fluid to be the correct thickness at the time they are drinking it. So, if the person needs Level 2 Mildly Thick, then the drink should be that thickness level when they drink it (warm, chilled or room temperature). Because the same thick fluid is very unlikely to be ‘in range’ at all three temperatures, we ask manufacturers to nominate the temperature for optimal serving. This then is the temperature that ‘audits’ should be conducted at. If clinicians or patients choose to use the drink at other temperature, then they do this at their own risk or ‘off label’. 

The concept of ‘off label’ use is well understood in the pharmaceutical industry. For example, pills and solid dose forms are designed to be taken ‘whole’. Some people cut or crush the pills; however, the pharmaceutical company does not warrant its use under those conditions, only when it is ‘taken as directed’. Thick fluid companies may wish to conduct IDDSI testing for their liquids to give the IDDSI level for other temperatures as well as the optimal serving temperature, however, the product only needs to be ‘in range’ for what the company chooses to be the optimal serving temperature. IDDSI is working with clinicians to help them understand that the variability has always existed, and to understand that it is highly unlikely that manufacturers could produce thick liquids that will be ‘in range’ for temperatures spanning chilled to heated. Clinicians need to audit thick fluids at the intended serving temperature.