Preventing Non-enzymatic Browning in Foods by Controlling Water Activity

Every processed food you pick up from a store shelf has been engineered to look, taste, and feel a certain way. But behind the scenes, one of the biggest threats to that consistency is a chemical reaction that quietly darkens colour, degrades flavour, and reduces nutritional value – all without a single enzyme being involved. This reaction is called non-enzymatic browning, and controlling it is one of the most important challenges in food science. The good news? A single measurable parameter – water activity (a_w) – gives food manufacturers a powerful lever to manage it.

What is non-enzymatic browning? 🔗

Non-enzymatic browning is a chemical process that turns food brown without any enzyme playing a role. Unlike the browning you see on a sliced apple (which is enzyme-driven), non-enzymatic browning is purely a result of chemical reactions between naturally occurring food compounds – primarily reducing sugars and amino acids. The two main types are caramelization (heat-driven breakdown of sugars) and the Maillard reaction (a reaction between sugars and amino compounds). Of these, the Maillard reaction is far more relevant to food storage and processing because it can happen even at relatively low temperatures over time.

Sometimes, non-enzymatic browning is exactly what you want. The golden crust on baked bread, the deep colour of roasted coffee, and the rich flavour of seared steak all depend on Maillard reactions. But in many processed and stored foods – dried milk powder, dehydrated fruits, protein bars, breakfast cereals – this browning is an unwanted defect that signals quality loss to consumers.

The Maillard reaction: a closer look 🔗

The Maillard reaction was first described in 1912 by French chemist Louis-Camille Maillard, who observed browning when he heated a mixture of sugar and amino acid. The reaction is not a single step but a cascade of chemical transformations that produces hundreds of different compounds, including flavour volatiles, aroma compounds, and brown-coloured pigments called melanoidins.

Three stages of the Maillard reaction 🔗

The Maillard reaction proceeds in three broadly defined stages:

Initial stage: A reducing sugar (such as glucose or fructose) reacts with the amino group of an amino acid or protein. This forms an unstable compound called a Schiff base, which quickly rearranges into a more stable product known as the Amadori compound. No browning is visible at this point, but the nutritive value of the protein can already start to decline.

Intermediate stage: The Amadori compounds break down through multiple pathways to yield reactive intermediates like reductones, dicarbonyl compounds, and furfurals. Strecker degradation also occurs here, where amino acids are broken down by dicarbonyls, producing aroma-active aldehydes. This stage is where many of the characteristic flavour and aroma compounds form.

Final stage: The highly reactive intermediates from the previous stage undergo polymerization and condensation to form melanoidins – large brown-coloured polymers responsible for the visible browning. These pigments are what make the reaction a concern for food manufacturers aiming to keep products light-coloured and visually appealing.

Understanding water activity (aw) 🔗

Before we can understand how to control browning, we need to understand water activity. Water activity is not the same thing as moisture content. Two foods can contain the same total amount of water but have very different water activities. Water activity measures how “available” the water in a food is for participating in chemical and microbial processes.

It is expressed on a scale from 0 to 1, where pure water has an a_w of 1.0. According to the UC Master Food Preserver Program, water activity can be understood as the relative humidity inside a sealed jar containing a food sample after it reaches equilibrium – divide that percentage by 100, and you get a_w.

Fresh fruits and vegetables typically have a_w values between 0.95 and 0.99. Dried foods like crackers and powdered milk range from 0.1 to 0.5. Intermediate moisture foods – jerky, soft cookies, dried fruits – fall in the 0.6 to 0.85 range. This intermediate zone is critical for browning, as we will see.

How water activity influences non-enzymatic browning 🔗

Here is the core concept: the rate of non-enzymatic browning does not simply increase or decrease with water content. Instead, it follows a bell-shaped curve that peaks at intermediate water activity levels.

Research consistently shows that the Maillard reaction rate peaks at a_w values around 0.6 to 0.8. This happens because water plays two opposing roles in the reaction.

At very low water activity (below 0.3) 🔗

When a_w is very low, there is almost no free water available. The reactant molecules – sugars and amino acids – cannot dissolve and move around to encounter each other. Molecular mobility is extremely restricted, so the reaction rate is very slow. Foods in this range, such as bone-dry powders and crackers, are relatively safe from Maillard browning.

At intermediate water activity (0.5-0.8) 🔗

This is the danger zone. There is enough water to dissolve the reactant molecules, allow them to move freely, and facilitate the initial condensation reaction between the sugar and amino acid. But the water concentration is not high enough to significantly dilute the reactants. The result is a maximum browning rate. According to a ScienceDirect review, the Maillard reaction peaks at approximately a_w 0.6-0.7 in most food systems.

At high water activity (above 0.8-0.9) 🔗

When a_w is high, there is plenty of free water. While this promotes reactant mobility, the excess water now dilutes the concentration of sugars and amino acids, reducing the likelihood that they will come into contact and react. Additionally, some early-stage Maillard products are water-soluble, and high water content can inhibit their further progression. So, paradoxically, very wet foods tend to brown more slowly than moderately dry ones.

This bell-shaped relationship is one of the most well-established findings in food chemistry. A classic study published in PubMed confirmed that browning rate rises with increasing a_w up to a maximum, after which reactant dilution causes the rate to drop.

Why this matters for food products 🔗

Many popular processed foods sit right in the intermediate moisture range where browning is most active. Dried fruits such as raisins and apricots, protein bars, soft-baked cookies, breakfast cereals, and powdered dairy products all have a_w values between 0.5 and 0.8. If not managed carefully, these products can develop unwanted colour changes, off-flavours, and reduced nutritional value during storage.

The nutritional impact is particularly concerning. The Maillard reaction consumes lysine, an essential amino acid, making it biologically unavailable. Over extended storage, this can lead to measurable losses in protein quality – a significant problem for products marketed as high-protein or nutritionally complete.

Beyond nutrition, browning also produces compounds that consumers may perceive negatively. Off-flavours described as bitter, stale, or burnt can develop. Colour shifts from light to dark brown signal reduced freshness in the consumer’s mind, even when the product is still safe to eat.

Practical strategies for controlling browning through water activity 🔗

Food manufacturers use several approaches to keep water activity outside the danger zone – or to combine a_w control with other methods for more effective browning prevention.

Drying and dehydration 🔗

Reducing a_w well below 0.5 through drying is one of the oldest and most effective strategies. As the FAO notes, at a_w around 0.3, food products reach their highest stability against non-enzymatic browning, lipid oxidation, and enzyme activity. Spray-dried milk powder, freeze-dried coffee, and dehydrated vegetable flakes are all formulated to remain at very low a_w during storage.

Use of humectants 🔗

Humectants are substances that bind water molecules, reducing their availability for chemical reactions. Common food humectants include glycerol, sorbitol, propylene glycol, salt, and sugar. Adding humectants lowers a_w without necessarily removing water from the product. Research has shown that liquid humectants in high-moisture foods increase viscosity and shift the peak browning a_w lower, effectively acting as browning inhibitors at higher moisture levels. Sorbitol, for example, slows the Maillard reaction partly through its viscosity-increasing effect, which reduces the mobility of reactants.

Ingredient selection 🔗

The choice of sugars and proteins can make a big difference. Sucrose, a non-reducing sugar, does not participate directly in the Maillard reaction and is far less reactive than glucose or fructose. Manufacturers can substitute reducing sugars with sucrose in formulations where browning is a concern. Similarly, selecting proteins with fewer available free amino groups reduces the pool of reactants. AQUALAB notes that water activity influences browning, lipid oxidation, vitamin degradation, and protein denaturation – so ingredient reformulation must consider all these factors together.

pH adjustment 🔗

The Maillard reaction proceeds faster under alkaline conditions. Lowering pH through acidification (using citric acid, for example) can significantly slow browning. At pH values below 6, the Maillard reaction rate drops considerably because the nucleophilic amino groups needed for the initial condensation are largely protonated and unreactive.

Temperature control 🔗

Since the Maillard reaction is temperature-dependent, maintaining lower storage temperatures slows down browning during the product’s shelf life. This is especially important for products in the intermediate moisture range that cannot be reformulated easily. Keeping storage environments cool and consistent helps keep browning within acceptable limits.

Sulphite treatment 🔗

Sulphites have traditionally been used to inhibit non-enzymatic browning, particularly in dried fruits and vegetables. According to the FAO, sulphites work by reacting with carbonyl intermediates in the browning pathway, blocking further progression. However, their use is regulated due to health concerns, especially for people with sulphite sensitivities.

Hurdle technology 🔗

In practice, the most effective approach is usually a combination of methods – often called hurdle technology. A manufacturer might lower a_w to the 0.4-0.5 range, use sucrose instead of glucose, reduce pH slightly, add a small amount of humectant, and maintain cool storage temperatures. Each “hurdle” contributes to keeping the overall browning rate well below the level that would affect product quality.

Monitoring water activity in food production 🔗

Regular water activity measurement is essential for any manufacturer dealing with products susceptible to browning. Modern instruments – typically based on chilled-mirror dew point or capacitive sensor technology – can measure a_w with an accuracy of ±0.003 in as little as five minutes. These measurements are commonly integrated into HACCP (Hazard Analysis Critical Control Point) plans, where a_w serves as a critical control point for both microbial safety and chemical quality.

It is worth noting that a_w can change during storage due to moisture migration – especially in multi-component products where ingredients with different a_w values are in contact. A cereal bar with a dry crunchy layer and a soft fruit filling, for example, will experience moisture movement from the high-a_w filling to the low-a_w cereal over time. This shift can push the cereal component into the browning-prone a_w zone, leading to quality issues. Proper packaging, moisture barriers between components, and careful formulation help manage this challenge.

The bigger picture: browning is not always the enemy 🔗

It is important to remember that the Maillard reaction is not inherently undesirable. The flavour of freshly baked bread, the aroma of roasted peanuts, the deep colour of dark beer – these are all products of controlled Maillard browning. In these cases, manufacturers deliberately target the intermediate a_w range and apply heat to promote the reaction. The same chemistry that ruins a dried milk powder is what makes a toasted marshmallow irresistible.

The key is control. Understanding the relationship between water activity and browning rate gives food scientists the ability to either accelerate or suppress these reactions, depending on the product goal.

What do you think? Given that so many everyday foods – from protein bars to dried fruit – fall in the water activity range most prone to browning, how might climate change and warmer storage conditions make this problem harder to manage in the future? And have you noticed colour or flavour changes in foods stored in your kitchen that might now make more sense in light of this water activity-browning connection?

References

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC12154226/
2. https://en.wikipedia.org/wiki/Maillard_reaction
3. https://pmc.ncbi.nlm.nih.gov/articles/PMC4745522/
4. https://ucanr.edu/program/uc-master-food-preserver-program/article/water-activity-and-its-role-food-preservation
5. https://www.mdpi.com/2304-8158/14/11/1881
6. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/maillard-reaction
7. https://pubmed.ncbi.nlm.nih.gov/906925/
8. https://www.fao.org/4/y4358e/y4358e06.htm
9. https://aqualab.com/en/knowledge-base/expertise-library/water-activity-food-safety-and-quality