For manufacturers of milk powders, protein powders, beverage mixes, starches, flours, seasonings, spices and other dry ingredients, water activity can provide information that moisture content alone cannot. It can help explain whether a powder is likely to absorb moisture from its environment, when flowability may begin to deteriorate, why components in a blend exchange moisture, how packaging influences stability and why a low-moisture product can still present a microbiological safety challenge.<\/p>\n\n\n\n
The important point is that there is no universal ideal water activity for powders. Different powders fail for different reasons. The objective is to identify the mechanism most likely to limit the stability of a specific product, determine the relevant water activity range and then keep the product within that range throughout processing, storage and distribution.<\/p>\n\n\n\n
Powder stability is not one single problem<\/h2>\n\n\n\n
\u201cPowder\u201d describes a physical form rather than one type of material. Spray-dried milk powder, crystalline sugar, a protein blend, flour, cocoa powder and ground spices can behave very differently when exposed to moisture and temperature.<\/p>\n\n\n\n
For many amorphous powders, moisture can increase molecular mobility and promote a transition from a rigid glassy state toward a softer, more mobile state. The result may be stickiness, agglomeration and eventually caking. Crystalline powders behave differently. They may remain stable until the surrounding humidity reaches conditions at which sufficient water is adsorbed or the material begins to deliquesce, after which dissolution and recrystallization can create strong bridges between particles. In powders containing significant surface fat, caking can also occur through fat-mediated interactions. Mechanical pressure, particle size, temperature and storage time can further influence all of these mechanisms.<\/p>\n\n\n\n
This distinction matters because a manufacturer cannot solve every powder problem simply by specifying \u201clow moisture.\u201d The relevant question is: What makes this particular powder lose its functionality, quality or safety \u2014 and how is water activity involved?<\/p>\n\n\n\n
What is water activity?<\/h2>\n\n\n\n
Water activity describes the thermodynamic state of water in a product. It is defined as the ratio between the vapor pressure of water above the product and the vapor pressure of pure water at the same temperature:<\/p>\n\n\n\n
aw<\/sub> = p \/ p\u2080<\/strong><\/p>\n\n\n\n where p<\/em> is the vapor pressure of water above the product and p\u2080<\/em> is the vapor pressure of pure water at the same temperature.<\/p>\n\n\n\n Water activity ranges from 0 to 1, with pure water having an aw<\/sub> close to 1.00. It is also related to equilibrium relative humidity:<\/p>\n\n\n\n ERH (%) = aw<\/sub> \u00d7 100<\/strong><\/p>\n\n\n\n A powder at 0.30 aw<\/sub> will therefore tend toward equilibrium with an atmosphere of approximately 30% relative humidity at the same temperature.<\/p>\n\n\n\n This relationship is especially useful for powders because it explains the direction in which moisture will tend to move. If a powder is exposed to an atmosphere whose relative humidity is higher than the equilibrium relative humidity corresponding to the powder\u2019s aw<\/sub>, the product tends to adsorb water. If the surrounding humidity is lower, it tends to lose water.<\/p>\n\n\n\n This does not<\/strong> mean that a powder automatically cakes whenever ambient relative humidity exceeds its ERH. Moisture uptake must first be sufficient, for a sufficient period of time and under the relevant temperature and mechanical conditions, to cause an undesirable physical transition. That distinction is important when designing realistic storage specifications.<\/p>\n\n\n\n Water activity is rooted in chemical potential and Gibbs free energy. For water, the difference in chemical potential relative to the reference state can be expressed as:<\/p>\n\n\n\n \u0394\u03bcw = RT ln(aw<\/sub>)<\/strong><\/p>\n\n\n\n where \u0394\u03bcw represents the change in chemical potential of water, R is the universal gas constant, T is absolute temperature and aw<\/sub> is water activity.<\/p>\n\n\n\n This relationship explains why water activity should not be interpreted simply as the percentage of \u201cfree water\u201d in a product. An aw<\/sub> of 0.50 does not mean that 50% of the water is free, nor does it mean that the product contains half as much water as another product at 1.00 aw<\/sub>.<\/p>\n\n\n\n Instead, water activity describes the thermodynamic state of the water and therefore its tendency to participate in processes such as moisture transfer, microbial growth and changes in molecular mobility. That is fundamentally different from moisture content<\/a>.<\/p>\n\n\n\n Moisture content tells us how much water is present. Water activity tells us how that water behaves thermodynamically. The two measurements are related, but they are not interchangeable.<\/p>\n\n\n\n Their relationship is described by the product\u2019s moisture sorption isotherm, which relates water activity to equilibrium moisture content at a defined temperature. Because every formulation interacts with water differently, every product has its own relationship.<\/p>\n\n\n\n A protein powder, starch, maltodextrin, milk powder and spice blend can therefore all contain the same percentage of moisture while having different water activities. Conversely, two powders at the same aw<\/sub> can contain very different quantities of water.<\/p>\n\n\n\n This has important practical consequences. A moisture specification transferred from one formulation to another does not necessarily provide the same physical stability. Even within one product family, changes in protein, carbohydrate, fat, salt, sugars or particle structure can alter the moisture\u2013aw<\/sub> relationship.<\/p>\n\n\n\n For powder stability, moisture content remains useful for mass balance, drying performance and compositional specifications. Water activity adds different information: it helps describe the driving force for moisture movement and the conditions under which the product may change.<\/p>\n\n\n\n Caking is the unwanted formation of aggregates or solid masses from initially free-flowing particles. It can reduce powder recovery, interfere with dosing and filling, block hoppers and conveying systems, make reconstitution more difficult and ultimately make a product unacceptable to the customer.<\/p>\n\n\n\n Water is often a major driver of caking, but the underlying mechanism depends on the material. This is where it is important to move beyond the simplified idea that \u201chigh aw<\/sub> causes all powders to cake.\u201d<\/p>\n\n\n\n Many spray-dried and freeze-dried ingredients contain substantial amorphous fractions. In an amorphous glassy state, molecular mobility is low and the powder can remain relatively rigid and free flowing.<\/p>\n\n\n\n Water is a powerful plasticizer. As an amorphous powder adsorbs moisture, its glass transition temperature, Tg<\/strong>, generally decreases. If storage temperature becomes sufficiently high relative to Tg, molecular mobility increases and particle surfaces can become sticky. Contact between particles can then develop into bridges, agglomerates and eventually a more consolidated cake.<\/p>\n\n\n\n Figure 1. Comparison between thermal methods that scan temperature while holding moisture content (% moisture content) constant to determine the glass transition temperature (Tg) and sorption isotherm methods that scan water activity to identify a critical water activity (RHc) while holding temperature constant. In theory, both methods should provide the same information.<\/p>\n\n\n\n Water activity, temperature and time therefore work together. A powder that remains stable at one temperature may become unstable at the same aw<\/sub> at a higher storage temperature. Similarly, brief exposure to humid air may have little effect while long exposure allows sufficient water uptake to produce major changes.<\/p>\n\n\n\n Recent research also reinforces an important nuance: glass transition is central to the stability of many amorphous powders, but it is not the only variable determining whether a cake develops. Water can also contribute directly to interparticle binding, and the kinetics remain product-specific.<\/p>\n\n\n\n This is why a scientifically meaningful powder specification should consider aw<\/sub> together with expected temperature and storage time.<\/p>\n\n\n\n For many powders, it is possible to identify a water activity region above which physical behavior changes much more rapidly. This is often called the critical water activity<\/strong>, sometimes expressed as RHc when related to critical relative humidity.<\/p>\n\n\n\n Figure 2. DD isotherm of spray dried milk powder showing an inflection point at a critical water activity of 0.43 aw<\/sub>. The high resolution of the DDI method makes it possible to visualize inflection points in the curve. In the amorphous glassy state, sorption is limited adsorption, but the glass transition causes sorption to switch from surface to bulk absorption and results in a drastic sharpening of the isotherm curve.<\/p>\n\n\n\n For an amorphous powder, the critical region may be associated with a moisture-induced glass transition. Below it, the material remains relatively rigid and stable. Above it, increased molecular mobility can accelerate stickiness and caking.<\/strong><\/p>\n\n\n\n A moisture sorption isotherm can help identify these transitions because changes in structure can produce characteristic changes in sorption behavior. High-resolution sorption studies can therefore be particularly useful during product development for identifying critical stability regions.<\/p>\n\n\n\n The important manufacturing step comes afterward. Once a critical region has been determined experimentally, routine production does not need to operate directly at that boundary. A manufacturer can define an appropriate aw<\/sub> operating range with sufficient process margin, then use routine direct water activity measurements to verify that production remains inside that stable region.<\/p>\n\n\n\n The objective is therefore not simply to measure aw<\/sub>. It is to connect: <\/p>\n\n\n\n product behavior \u2192 critical aw<\/sub> \u2192 process specification \u2192 routine QC<\/strong><\/p>\n\n\n\n Crystalline powders should not automatically be interpreted using the same glass-transition model as amorphous powders.<\/p>\n\n\n\n Materials such as crystalline salts and sugars may remain relatively stable over a range of humidity conditions and then undergo rapid changes when a critical humidity region is reached. At sufficiently high humidity, a soluble crystalline material may absorb enough moisture for a saturated solution to form at its surface. This process is known as deliquescence<\/strong>.<\/p>\n\n\n\n Once liquid bridges form between particles, subsequent changes in humidity or temperature can lead to recrystallization and the formation of strong solid bridges. The resulting cake can be difficult or impossible to reverse by simple agitation.<\/p>\n\n\n\n Particle size, impurities, mechanical load and humidity cycling can further modify this behavior. This means that \u201ccaking\u201d can look similar to the operator while being caused by very different physical mechanisms.<\/p>\n\n\n\n Understanding whether a powder is predominantly amorphous, crystalline or a mixture of both provides important context for interpreting water activity data.<\/p>\n\n\n\n A scientifically robust powder strategy also needs to recognize the limits of aw<\/sub>. Some powder systems can cake through mechanisms in which moisture is not the dominant driver. Fat-containing powders are one example. Research on cocoa powder has shown that liquid fat bridges at particle surfaces can cause caking when the fat is in an appropriate physical state, even where water activity itself does not explain the observed caking behavior.<\/p>\n\n\n\n Mechanical consolidation can also occur when powders are subjected to high pressure during bulk storage, transport or stacking. Particle shape and particle-size distribution influence contact points and packing behavior, while electrostatic forces can affect very fine powders.<\/p>\n\n\n\n Water activity should therefore be treated as a powerful diagnostic and control parameter, not as a claim that every powder failure is caused by water. This distinction increases the value of the measurement because it encourages the correct question: is moisture availability the dominant mechanism in this particular failure?<\/p>\n\n\n\n Flowability is one of the most commercially important characteristics of an ingredient powder. Poor flow can cause inconsistent dosing, slow filling lines, block pipes and hoppers, increase cleaning requirements and lead to rejected material. A powder that flows correctly at production may behave very differently after weeks of storage if it adsorbs moisture.<\/p>\n\n\n\n Research reviewing food powder flowability has shown that moisture strongly influences interparticle interactions and that water activity can provide useful information about changes in flow behavior. Reported relationships with moisture content are often highly product-specific, while aw<\/sub> has shown useful correlations with flowability across a range of powders.<\/p>\n\n\n\n Nevertheless, there is no universal aw<\/sub> value at which every powder stops flowing. Particle size, composition, surface characteristics, mechanical stress and storage history remain important.<\/p>\n\n\n\n For R&D, it is therefore useful to combine aw<\/sub> with physical flow measurements such as flow factor, angle of repose, Hausner ratio or other product-appropriate tests. The resulting data can establish a product-specific relationship between water activity and actual functional performance. Once this relationship is known, aw<\/sub> becomes much more powerful as a routine specification.<\/p>\n\n\n\n Many powders are hygroscopic, meaning they readily adsorb moisture from the surrounding atmosphere. This is particularly relevant during operations such as cooling, conveying, blending and filling, when the product may be temporarily exposed to factory air.<\/p>\n\n\n\n Consider a powder at 0.25 aw<\/sub>. At equilibrium, this corresponds to approximately 25% ERH at the measurement temperature. If that powder is exposed to air at 70% relative humidity, there is a strong thermodynamic driving force for moisture uptake.<\/p>\n\n\n\n The powder will not necessarily cake immediately. The speed and consequence of the moisture uptake depend on factors such as exposure time, temperature, air movement, particle characteristics and sorption behavior. But if the product absorbs enough moisture to approach its critical stability region, the risk of physical change can increase dramatically.<\/p>\n\n\n\n This explains why a powder can perform perfectly during production in one climate but experience stability problems during tropical transport, humid-season warehousing or filling in an insufficiently controlled room. Water activity connects the product specification to the environmental conditions surrounding the product.<\/p>\n\n\n\n Many commercial powders are mixtures rather than single ingredients. Seasoning blends, protein mixes, beverage powders, nutritional formulations and premixes can contain ingredients with significantly different initial water activities.<\/p>\n\n\n\n When ingredients with different water activities are mixed, water tends to move from regions of higher chemical potential \u2014 generally the higher-aw<\/sub> component \u2014 toward regions of lower chemical potential until the system moves toward equilibrium.<\/p>\n\n\n\n The final equilibrium aw<\/sub> cannot be predicted reliably from the starting aw<\/sub> values alone. It also depends on how much of each ingredient is present and on the moisture sorption behavior of each component.<\/p>\n\n\n\n This is where sorption isotherms become particularly useful. Combining ingredient mass ratios with their sorption behavior can support prediction of the equilibrium state of a mixture before large numbers of physical prototypes are produced.<\/p>\n\n\n\n The practical lesson is simple: when a previously stable powder suddenly begins to cake after reformulation, the problem may not be the total amount of added moisture. A new ingredient may have changed the water activity distribution and moisture migration within the blend.<\/p>\n\n\n\n Physical stability is not the only shelf-life concern. Powders and spices can also deteriorate through chemical reactions that change color, aroma, flavor, nutritional value or functionality. Relevant mechanisms may include Maillard browning, oxidation, hydrolysis, pigment degradation and loss of sensitive vitamins or flavor compounds.<\/p>\n\n\n\n Figure 3. The impact of water activity on various stability factors in food. <\/p>\n\n\n\n Water activity can influence these reaction rates because it affects molecular mobility and the thermodynamic environment of reactants. However, the relationship is not universal or necessarily linear.<\/p>\n\n\n\n Lipid oxidation is a particularly important example. Reducing aw<\/sub> does not always continuously reduce oxidation. Very dry lipid-containing systems can still oxidize rapidly, which means that \u201clower aw<\/sub>\u201d should not automatically be interpreted as \u201clonger chemical shelf life.\u201d<\/p>\n\n\n\n Milk powders may be influenced by both lactose-related physical transitions and oxidation of lipids. High-sugar powders may be susceptible to physical changes and non-enzymatic browning. Spices may remain physically free flowing while gradually losing aroma or developing undesirable colors and odors.<\/p>\n\n\n\n The optimum water activity should therefore be selected according to the dominant failure mechanism of the actual product, not by applying a generic number.<\/p>\n\n\n\n Spices<\/a> are dry ingredients, but \u201cdry\u201d does not mean chemically or microbiologically inert. Their commercial quality depends on aroma, volatile compounds, color, flowability and microbiological safety. The limiting shelf-life mechanism may differ substantially among pepper, paprika, garlic powder, onion powder, cinnamon, herbs and complex seasoning blends.<\/p>\n\n\n\n For some spices and seasoning powders, caking may be important. For others, changes in aroma, oxidation or pigment stability can determine shelf life much earlier than physical caking.<\/p>\n\n\n\n This is another reason to avoid treating all powders as one category. A useful water activity program begins by establishing what the customer would notice first when the product fails and then determining whether and how aw<\/sub> influences that change.<\/p>\n\n\n\n One of the most important principles for low-moisture foods is the difference between microbial growth and microbial survival. At sufficiently low water activity, vegetative microorganisms cannot proliferate. Below approximately 0.60 aw<\/sub>, microbial growth is generally considered unable to occur.<\/p>\n\n\n\n But microorganisms that cannot grow may still survive. This is particularly relevant to pathogens such as Salmonella<\/em>, which can persist for long periods in low-moisture foods. A contaminated powder can therefore remain low in aw<\/sub> and still transfer viable pathogens into another product or to a consumer.<\/p>\n\n\n\n A low aw<\/sub> result is consequently not a microbiological clearance test and not a lethality step<\/strong>.<\/p>\n\n\n\n This issue has received significant regulatory attention. The U.S. FDA specifically includes products such as powdered infant formula, powdered drink mixes, milk powders and powdered spices within its current work on low-moisture ready-to-eat foods. The regulatory approach is risk based and includes appropriate CGMPs, hazard analysis, preventive controls, sanitation, environmental monitoring and corrective actions where relevant.<\/p>\n\n\n\n The correct conclusion is therefore not that every powder automatically requires the same lethality process. It is that manufacturers must evaluate the hazards relevant to their specific product and process and implement appropriate validated controls.<\/p>\n\n\n\n Water activity also influences the thermal resistance of microorganisms. Research on low-moisture foods has repeatedly shown that pathogens such as Salmonella<\/em> can become more resistant to heat under low-moisture conditions. Water activity at the actual processing temperature can therefore be an important factor when developing and validating thermal treatments.<\/p>\n\n\n\n This has an important practical implication. The same characteristic that helps prevent microbial growth during storage \u2014 low available water \u2014 can make an existing microbial population more difficult to inactivate during thermal processing.<\/p>\n\n\n\n Lethality validation for low-moisture foods should therefore be based on the actual product matrix and process conditions rather than assuming that conventional high-moisture processing data can be transferred directly. Water activity is one relevant parameter in understanding that matrix.<\/p>\n\n\n\n Powder production often involves energy-intensive drying. It is tempting to assume that a lower final moisture level or lower aw<\/sub> will always provide better stability. In reality, once the required physical, chemical and microbiological stability range has been reached, additional drying may provide limited benefit and can create other disadvantages.<\/p>\n\n\n\n Excessive drying can consume more energy, reduce dryer throughput and remove additional product mass. Depending on the product, it may also influence particle structure, reconstitution behavior or susceptibility to other degradation mechanisms.<\/p>\n\n\n\n The better objective is to define a scientifically justified final water activity range based on the powder\u2019s actual stability requirements.<\/p>\n\n\n\n The production question then changes from:<\/p>\n\n\n\n \u201cHow dry can we make this powder?\u201d<\/strong><\/p>\n\n\n\n to:<\/p>\n\n\n\n \u201cHow dry does this powder need to be to remain stable throughout its intended life?\u201d<\/strong><\/p>\n\n\n\n For high-volume drying operations, that distinction can have meaningful implications for energy use, capacity and product yield.<\/p>\n\n\n\n Reaching the correct aw<\/sub> at the end of production is only useful if the product remains within its stable range afterward.<\/p>\n\n\n\n Powders can gain moisture during cooling, transfer, filling, storage and transportation. Temperature changes can alter sorption behavior and can also influence the permeability of packaging materials. Repeated temperature and humidity cycling can be particularly demanding.<\/p>\n\n\n\n Once the relevant critical water activity has been established, packaging can be designed to slow the rate at which the product approaches that limit.<\/p>\n\n\n\n For moisture-sensitive powders, an important packaging parameter is WVTR \u2014 Water Vapor Transmission Rate<\/strong>. A suitable moisture barrier slows the transfer of water vapor between the external environment and the product.<\/p>\n\n\n\n The required barrier performance depends on the product\u2019s starting aw<\/sub>, critical aw<\/sub>, sorption behavior, package geometry, expected temperature and humidity, product mass and required shelf life. This allows a more useful packaging question than simply asking for a \u201chigh-barrier film\u201d:<\/p>\n\n\n\nThe thermodynamics behind water activity<\/h2>\n\n\n\n
Water activity vs. moisture content in powders<\/h2>\n\n\n\n
Why powders cake and clump<\/h2>\n\n\n\n
Amorphous powders and glass transition<\/h2>\n\n\n\n
![\"\"](\"https:\/\/novasina.ch\/wp-content\/uploads\/2026\/03\/rubber-or-glassy.png\")
<\/figure>\n\n\n\nCritical water activity: finding the point where stability changes<\/h2>\n\n\n\n
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<\/figure>\n\n\n\nCrystalline powders behave differently<\/h2>\n\n\n\n
Not every caking problem is controlled by water activity alone<\/h2>\n\n\n\n
Water activity and powder flowability<\/h2>\n\n\n\n
Hygroscopicity: what happens when the powder meets the real environment?<\/h2>\n\n\n\n
Moisture migration in powder blends<\/h2>\n\n\n\n
Chemical stability: a powder can remain free flowing and still lose quality<\/h2>\n\n\n\n
![\"\"](\"https:\/\/novasina.ch\/wp-content\/uploads\/2026\/06\/AppNote-Stability-Diagram-2-997x1024.png\")
<\/figure>\n\n\n\nSpices require special attention<\/h2>\n\n\n\n
Low water activity prevents growth \u2014 but it does not prove microbiological safety<\/h2>\n\n\n\n
Why low water activity can make thermal processing more difficult<\/h2>\n\n\n\n
Drying endpoint: lower is not automatically better<\/h2>\n\n\n\n
From dryer to packaging: stability must be maintained<\/h2>\n\n\n\n