Equilibrium Moisture Content: Key to Food Preservation

Every food product, whether it’s a bag of rice in your pantry or a packet of biscuits on a store shelf, is in a constant tug-of-war with the surrounding air over moisture. The point at which this tug-of-war reaches a standstill – where the food neither gains nor loses water – is called the equilibrium moisture content (EMC). This single concept plays a decisive role in how food is dried, packaged, stored, and ultimately preserved. If you work in food processing or simply want to understand why certain foods last longer than others, EMC is the place to start.

What is equilibrium moisture content? 🔗

Equilibrium moisture content is the moisture level at which a hygroscopic material – in this case, food – stops exchanging water with the air around it. At this point, the vapour pressure of the water within the food equals the vapour pressure of moisture in the surrounding atmosphere. There is no net movement of water molecules in either direction.

EMC is typically expressed as a percentage on a dry weight basis. For instance, if stored wheat reaches an EMC of 14% at a particular temperature and relative humidity (RH), it means the moisture present equals 14% of the grain’s dry weight, and the grain will neither absorb nor release moisture under those specific conditions.

Two key environmental factors govern EMC: temperature and relative humidity. As a general rule, EMC rises when relative humidity increases and when temperature decreases. This relationship is not random – it follows predictable patterns captured in curves called moisture sorption isotherms.

Moisture sorption isotherms: the EMC roadmap 🔗

A moisture sorption isotherm is a graph that plots the final moisture content a food will reach when it comes into equilibrium with atmospheres of different relative humidities, all measured at a constant temperature. In practical terms, it shows you exactly how “wet” or “dry” your food will become when exposed to a given set of conditions.

Most food products display a characteristic sigmoid (S-shaped) curve. This shape reflects three broad zones of water within the food:

Tightly bound water (monolayer) – occupies the initial, relatively flat region of the curve at low RH values. This water is strongly attached to specific sites on proteins, starches, and other food constituents through hydrogen and ion-dipole bonds. It is not available for microbial or chemical activity.

Multilayer water – found in the mid-section of the curve. This water is held less tightly and forms additional layers over the bound water. Some biochemical reactions can occur in this zone.

Free water – represented in the steep, upper portion of the curve. This water sits in capillaries and pores and is readily available for microbial growth and enzymatic reactions. Small changes in moisture content here cause large shifts in water activity (a_w).

The link between EMC and water activity 🔗

Water activity and EMC are directly connected. When a food reaches equilibrium with its environment, its water activity equals the ambient relative humidity divided by 100 (a_w = RH/100). So a food stored at 75% RH, once it reaches EMC, has a water activity of 0.75. This connection is critical because water activity – not total moisture content – determines microbial safety and chemical stability.

According to Food Safety Magazine, the lowest water activity at which most food spoilage bacteria can grow is around 0.90, while certain moulds can survive at levels as low as 0.61. The U.S. FDA considers foods with a water activity of 0.85 or below exempt from certain canned food safety regulations because such products present minimal risk for pathogenic growth.

Why EMC matters for food preservation 🔗

Understanding EMC is not just an academic exercise. It has direct, practical applications across the food industry – from drying and packaging to long-term storage.

Designing efficient drying processes 🔗

Drying food below its expected storage EMC is a waste of energy. If rice is going to be stored in conditions where its EMC is 14%, there is no benefit to drying it down to 10% – it will simply reabsorb moisture until it returns to 14%. EMC data tells food processors the optimal drying endpoint, saving time, fuel, and processing costs.

This principle is well established in grain management. As noted by the Prairie Agricultural Machinery Institute (PAMI), if the goal is to dry wheat to 14.4%, the most effective strategy is to run aeration fans when the EMC of the ambient air is below 14.4%. The grain will eventually equilibrate to the moisture content dictated by the air conditions.

Selecting the right packaging 🔗

Foods with high sensitivity to moisture changes demand packaging with strong moisture-barrier properties. EMC data guides packaging engineers in choosing the right materials and container sizes. A product with a low target EMC – say, a crispy snack – needs packaging that tightly restricts water vapour transmission. Without this, the product absorbs ambient moisture, becomes soggy, and its shelf life drops sharply.

On the other hand, some products require a degree of breathability to avoid condensation. EMC data helps strike the right balance.

Predicting behaviour during storage and transport 🔗

When food moves between climates – from a dry warehouse to a humid port, for example – its moisture content will shift toward the new EMC dictated by local temperature and humidity. By consulting sorption isotherm data, processors and logistics managers can predict how much moisture gain or loss to expect, and plan accordingly with desiccants, climate-controlled containers, or adjusted packaging.

The hysteresis effect: why the path matters 🔗

One of the more interesting – and practically important – features of EMC is the hysteresis effect. This refers to the fact that a food’s EMC differs depending on whether it is gaining moisture (adsorption) or losing moisture (desorption).

When you plot both the adsorption and desorption isotherms on the same graph, the desorption curve sits above the adsorption curve. This means that a food drying down from a higher moisture level will retain more moisture at a given RH than the same food absorbing moisture up from a dry state at that same RH.

What causes hysteresis? 🔗

Several structural and chemical changes contribute. During drying, cells collapse, proteins denature, and the pore structure of the food changes. These alterations create different pathways for moisture movement when the food is rewetted. As explained in research published in the International Journal of Food Science & Technology, the desorption isotherm exists at a higher energy level than the adsorption isotherm, and the energy difference arises from physical changes in the food’s matrix during drying.

Another contributing factor is the reduction in available sorption sites. After drying, shrinkage reduces the number of places where water molecules can bind, which lowers the food’s capacity to reabsorb water to its original level.

Practical implications of hysteresis 🔗

Hysteresis has real consequences for food quality and process design:

Drying targets must account for history. A food dried from a high-moisture state will have a different EMC at a given RH than one that was previously dried and then exposed to the same RH. Processors need to know which isotherm – adsorption or desorption – applies to their product’s history when setting drying targets.

Storage stability varies. Products on the desorption curve (recently dried) tend to retain more moisture than identical products on the adsorption curve at the same ambient conditions. This can affect microbial stability and the rate of chemical reactions like non-enzymatic browning and lipid oxidation.

Temperature reduces hysteresis. Research on various agricultural commodities shows that increasing temperature tends to decrease or even eliminate the hysteresis gap for some foods. This is why products stored in warmer conditions may show less discrepancy between adsorption and desorption behaviour.

EMC and microbial safety 🔗

The core reason food processors care about EMC is its direct connection to microbial safety. By controlling the conditions under which food reaches equilibrium, you effectively control the water activity – and therefore the potential for microbial growth.

Most bacteria need water activity above 0.90 to grow. Yeasts generally need above 0.88, and moulds can persist down to around 0.65. Below a water activity of 0.60, no microbial growth occurs. This hierarchy is the foundation of preservation strategies like dehydration, salting, and sugar concentration – all of which work by pulling the food’s EMC (and therefore its water activity) below the threshold required by spoilage organisms.

However, microbial safety is only part of the picture. Even at water activity levels too low for microbial growth (0.40-0.70 range), chemical degradation – including Maillard browning, lipid oxidation, and vitamin breakdown – can still occur and limit shelf life. This is why achieving the right EMC is a balancing act: low enough to block microbes, but also managed to minimise undesirable chemical changes.

Factors affecting EMC in food systems 🔗

Several variables influence the equilibrium moisture content a food will reach in a given environment.

Relative humidity is the dominant factor. Higher ambient RH means a higher EMC, because there is more moisture in the air available for the food to absorb.

Temperature also plays a role but in a more nuanced way. At a constant RH, most foods show a decrease in EMC as temperature rises. However, the effect of temperature can vary by food type, and for some products, the relationship is not straightforward.

Food composition matters significantly. Starchy and protein-rich foods, with their abundant polar groups (hydroxyl, amino, carbonyl), have stronger water-binding capacity and thus higher EMC values at a given RH than fatty or oily foods. This is why whole grains and legumes are more hygroscopic than nuts or oilseeds.

Physical structure – including particle size, porosity, and surface area – affects how quickly and to what extent a food reaches equilibrium. Smaller particles with larger surface areas respond faster to changes in ambient humidity.

Applying EMC knowledge in real-world food processing 🔗

Bringing all these concepts together, here is how EMC knowledge translates into everyday food processing and preservation decisions:

Setting drying endpoints: Processors consult the sorption isotherm for their specific product at the expected storage temperature and RH to determine the target moisture content at the end of drying.

Conditioning dried products: After drying, individual pieces within a batch may have slightly different moisture levels. Conditioning – holding the batch in a sealed environment for a period – allows the pieces to exchange moisture and equilibrate, ensuring uniform water activity throughout the batch.

Selecting storage environments: Warehouses, silos, and cold stores are designed with target temperature and RH ranges that keep the stored food at or near its optimal EMC, preventing both overdrying (which affects texture and weight) and moisture gain (which promotes spoilage).

Multi-component product design: Products like muesli bars, which combine dry cereals with moist fruit pieces, present a unique challenge. Moisture migrates from the high-a_w component (fruit) to the low-a_w component (cereal) until equilibrium is reached. Sorption isotherm data for each ingredient helps product developers predict this migration and design appropriate moisture barriers between components.

Mathematical models for predicting EMC 🔗

Food scientists don’t rely on trial-and-error alone. Several well-established mathematical models describe the relationship between EMC, temperature, and relative humidity for different food types. Common models include the Henderson equation, the Chung-Pfost equation, the Halsey equation, the Oswin model, and the widely used GAB (Guggenheim-Anderson-de Boer) model.

The GAB model is particularly popular in food science because it provides a good fit across a wide range of water activities and accounts for multilayer moisture sorption. These models are used to generate EMC charts and tables that processors and grain handlers reference daily. For instance, the PAMI EMC charts for grain storage use modified Henderson, Chung-Pfost, Halsey, and Oswin equations depending on the crop type.

It is worth noting that plant breeding over the decades has changed the starch and oil content of many crops, potentially affecting their EMC behaviour. Published EMC charts should therefore be used as guidelines, and site-specific verification is always a good practice.

EMC in the broader context of food safety regulation 🔗

The concept of EMC feeds directly into food safety frameworks. Water activity – derived from EMC under equilibrium conditions – is a recognised critical control point (CCP) in many HACCP plans. Regulatory bodies like the FDA and USDA incorporate water activity thresholds into their food safety standards.

As noted in Food Safety Magazine, water activity measurement has been incorporated into FDA and USDA regulations, GMP requirements, and HACCP plans, serving as a key tool for ensuring product stability and consumer safety. Proper control of EMC is therefore not just a quality consideration – it’s a compliance requirement.

What do you think? How might a deeper understanding of EMC and the hysteresis effect change the way dried foods are packaged and labelled in tropical climates with high humidity? Could better use of sorption isotherm data help reduce food waste during storage and transportation?

References

1. https://en.wikipedia.org/wiki/Equilibrium_moisture_content
2. https://www.fao.org/4/v5030e/v5030e0b.htm
3. https://www.fda.gov/inspections-compliance-enforcement-and-criminal-investigations/inspection-technical-guides/water-activity-aw-foods
4. https://www.food-safety.com/articles/4420-water-activitye28099s-role-in-food-safety-and-quality
5. https://pami.ca/resource-library/equilibrium-moisture-content-charts-grain-storage/
6. https://www.intechopen.com/chapters/68496
7. https://ifst.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2621.2006.01203.x
8. https://nutritionmeetsfoodscience.com/2022/08/29/water-activity-and-food-preservation/
9. https://ucanr.edu/program/uc-master-food-preserver-program/article/water-activity-and-its-role-food-preservation