Measuring water activity in food production - a case study

Dr Brady Carter explains why water activity, not moisture content is the key for safer food production

Moisture content is commonly measured on food products for the purpose of making sure it will have the desired safety and quality. However, water activity, not moisture content, determines the safety of food products. Recently, a major food manufacturer became painfully aware of the importance of releasing product based on water activity and not moisture content. The company has been producing high quality product for over 20 years with little to no complaints. Then, out of nowhere, it started receiving multiple complaints about mouldy product. How could this happen? It had released product based on a set moisture content for 20 years with no problems. Without making any changes to its moisture content specification, it suddenly had mould. The cost of the subsequent recalls, public relations efforts, and causal discovery activities were in the millions of dollars.

How a water activity-based release specification would have prevented the problem

The first mistake made by this company was using a moisture content specification to control mould growth instead of water activity. Water activity is the energy status of water in the food and microorganisms don’t care how much water is present, only if the water activity (energy) is high enough for them to access it. Mould growth will be prevented if the water activity is less than 0.70 and any problems with mould growth would have been avoided if the company was using 0.65 aw as their release specification instead of moisture content.

The other contributing factor that led to the sudden appearance of mould was a change in the instrument being used to measure moisture content. This new method was unknowingly giving lower moisture content readings than the actual moisture content (Fig. 1). This means that although the company thought it was making product to the same moisture content specification; the actual moisture content was 3-4% higher. This higher moisture level now corresponded with a water activity higher than the growth limit for mould. If the company had been tracking water activity, it would have known immediately that something had changed, and that the product was now being made to water activities higher than 0.70 aw. A water activity-based release specification would have prevented any problems and saved millions in lost revenue.

How a water activity-based release specification proved to be the solution

The first change to be implemented was to immediately implement a water activity release specification below 0.70 aw and reject any product that does not meet this specification, regardless of its moisture content. This immediately stopped all complaints about mould. The second change was to correct the moisture content measurement to make sure it was giving results consistent with historical values. In so doing, the company realised that current formulations limit the amount of moisture that can be in its products and still maintain a safe water activity level. Consequently, the next step was to adjust the formulations to maximise the achievable moisture level while still meeting the newly implemented water activity release specification (Fig. 2). This will allow the food manufacturer to maximise profitability while having the security of knowing that the painful days of mouldy product are behind it.

Dr Brady Carter i9s application scientist with Neutec Greoup

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