Examining powder problems: Physical, chemical & microbial stability

However, most powders can be categorized by molecular structure: amorphous, crystalline, or a combination of the two. The ratio of crystalline to amorphous and how they interact will affect almost every characteristic of powder.

Additionally, particle size has a significant impact on powder characteristics (and the common problems associated with them). Wherever powder particles come into contact with one another, bridging – the first step in caking and clumping – can begin. The smaller the particle size, the higher the likelihood of bridging, which leads to sticking, then agglomeration, and further problems. Crystalline powders can be particularly tricky because (to a certain point) their ordered structure only allows moisture to adhere to the outside of the structure.

Research indicates that mixing two crystalline powders with different particle sizes can cause the mixture to deliquesce (change from a solid to a liquid) at lower water activity levels than either of the individual powders would. 

Amorphous powders tend to have crevices and irregular shapes, making it easier for water to bind to their particles.

Moisture content, water activity, and powder isotherms

There are two key water-related measurements: moisture content and water activity. Both are important to understand if one hopes to control physical, chemical, or microbial stability issues in powders. 

Most people in the food or pharmaceutical industries are familiar with moisture content. For some, water activity may be a new concept. Moisture content measures the amount of water. Water activity measures the energy of the water – as in, what is the water able to do? These two parameters are measured in completely different ways.

Moisture content is measured in percent of overall mass – essentially, how much of the sample is water, based on weight. 

Though moisture content is a popular method, it isn’t particularly precise. This can make it difficult to see and solve problems. Moisture content alone can’t provide a complete picture, especially in powders.

To check water activity, a device will measure vapor pressure. It can be helpful to think of water activity as the equilibrated humidity a sample releases.

Water activity is often misdefined as “water availability.” This isn’t quite right. Water activity is a thermodynamic principle – it’s the energy of the water. This is important to know because that energy can be used in chemical reactions, texture changes, and other reactions.

When the relationship between water activity and moisture content are graphed, the result is called an isotherm. In the right hands, an isotherm can provide a great deal of valuable information. Among other things, they can:

• Reveal the water activity levels where texture and structural changes begin (DDI isotherms)
• Show the point at which a product begins to absorb more moisture more quickly
• Identify the molecular structure (amorphous or crystalline)
• Determine how long specific changes or reactions take or how quickly they occur (DVS isotherms)

Key factors in physical stability

To understand the physical stability of powders, there are three major factors to consider: moisture, temperature and time.

Moisture

Moisture has a major impact on physical stability. Water can be a solvent or a reactant – it can even buffer chemical reactions. In general, more moisture means quicker reactions, but an isotherm can provide specific case-by-case information.

Temperature

Temperature’s effects are similar to water’s: Change happens faster when temperature increases (see figure above). Raising the temperature means adding energy into the system, which enables more physical changes at lower water activity levels.

Time

Given enough time, every process will come to completion. Some processes may happen so slowly that they’re imperceptible – panes warping in very old glass windows, for instance  – but they do still occur, even if factors like temperature and moisture are controlled. 

Physical stability case study: Caking and clumping in spice mixes

We know that water activity levels drive the movement of moisture between substances. But how much does it move, what equations and models can be used to predict movement, and how accurate is the prediction?

The METER Food R&D Lab performed the following research on six different spice blends to illustrate answers to the questions above.

Process overview:

1. Generated an isotherm for each individual ingredient
2. Mixed the ingredients in known mass ratios (see table below)
3. Predicted final water activity level for each blend using the isotherms, mathematical equations, and coefficients 
4. Measured each spice blend’s water activity level after equilibration
5. Compared the predictions to the measurements

Findings:

• The corn starch and onion salt prediction was extremely close to the final measured water activity. 
  • Both ingredients have fine particle sizes, which tends to mean more particle contact and quicker equilibration. 
• Predictions for other spice mixes were also highly accurate. 
• The least accurate prediction in the test came from the sage, cumin and oregano blend. However, it was still remarkably close, at 0.05 lower than the final measured water activity level.

The process described in this case study can be helpful to any food scientist, particularly those who are under pressure to formulate new products quickly. The models, tools and equations can provide insights about the final characteristics of dry ingredient mixes before they’re mixed. 

It can take time up front to build a library of isotherms. But once they’re created, formulators are free to experiment with recipe adjustments, predict final shelf life, equilibrium water activity levels, and make packaging decisions from their desk – with no need to conduct physical studies.

Key factors in chemical stability

Manufacturers need to be aware of how water activity can affect chemical reaction rates – and which reactions will end their product’s shelf life. Without a sound understanding of chemical stability, it’s easy to promise greater benefits than a product will actually deliver.

Tracking chemical reaction rates can be complicated, but it is possible. It’s often up to the manufacturer to decide when shelf life limits have been reached. Pinpointing that moment requires some of the same shelf life prediction information mentioned in the case study above. 

Chemical stability case study: Vitamin C degradation

How can a nutritional supplement manufacturer determine ideal storage conditions? At what rate will a particular ingredient degrade, and when will the product no longer match the claim on the labels?  

The research described below, as performed by the METER Food R&D Lab, can help answer these questions. The study was conducted on vitamin C (ascorbic acid), but the principles and techniques apply to any substance that might degrade or react over time.

During the study, ascorbic acid was exposed to two different water activity levels and three different temperatures. Degradation was tracked using UV-Vis spectroscopy, and rate of degradation was calculated. The goal was to find how temperature and water activity affect degradation rate. 

First, the team decided which temperatures (30˚, 40˚, and 50˚ C) and water activity levels (0.76 a_w and 0.948 a_w) to target. They then determined when shelf life would be considered ended – in this case, when 75% of the initial amount of vitamin C remained. They entered the necessary information into the Moisture Analysis Toolkit and ran an accelerated shelf life study, which produced the following:

Key factors in microbial stability

Water activity is an excellent way to limit microbial growth. At water activity levels below 0.6, nothing will grow. 

This fact gives many manufacturers a false sense of safety – they believe that if their product has a low water activity level, microbial contamination need not concern them. This is a dangerous perception that has led to many recalls and  outbreaks in foods such as peanut butter, flour, and baby formula. 

Water activity can prevent microbial growth, but it is not a kill step. Microbes at low water activity levels can survive in stasis. If they are exposed to a higher water activity environment – for instance, by mixing flour into cookie dough – they can begin to proliferate and become dangerous. 

A low water activity product may be safe, but it is not necessarily sterile.

While there are many microbial control hurdles and precautions that can be taken, the topic remains complicated and challenging. A great deal of research into sterilizing or pasteurizing low moisture foods is ongoing. For now, stringent sanitation policies are the most effective way to prevent contamination and ensure microbial stability.

Further resources

For more in-depth powder science, watch the free on-demand webinar below. In it, Dr. Zachary Cartwright and Mary Galloway further explore powder flow, caking, molecular structure, and isotherms.