The chocolate industry is valued at over $129 billion and is a dynamic and influential industry that is recognized not only for its economic impact, but also for its cultural significance and continuous innovation. From handmade chocolate producers to multinational candiescompanyChocolate manufacturing covers a wide range of processes, raw materials, and product forms. With the increasing demand from consumers for high-quality, clean label formulas, low sugar content, and longer shelf life, manufacturers are facing increasing pressure to optimize every aspect of production. Among the many factors that affect the quality of chocolate, water activity plays a crucial role in ensuring product stability, safety, and sensory appeal, especially in complex formulas containing fillings, fillings, or layered structures.

Chocolate comes in various types and is often mixed with other ingredients to create unique candies. Chocolate originates from cocoa beans and is rich in flavanols, which have a natural preservative effect^[1]Chocolate itself can be classified based on its form and cocoa powder content:

Cocoa powder is ground from roasted cocoa beans without any additional additives, with a bitter taste and a shelf life of up to 3 years.

Baking pure solid chocolate liquid, without added sugar or milk, with a shelf life of about 2 years.

Dark chocolate with added sugar chocolate has a higher cocoa content and varies. The higher the cocoa content, the more bitter the taste and the longer the shelf life.

Milk chocolate contains about 10% chocolate liquid, plus milk and sugar, with a shelf life of about 1 year, depending on the product form.

White chocolate does not contain any chocolate solids and is only made by mixing cocoa butter with milk and sugar. Its shelf life is relatively short, about 6 months.

Chocolate liqueurs are mixed with different types of liqueurs to give them a unique flavor. The shelf life of such products is usually short, about 2 months or less.

Water activity is defined as the energy state of water in a system, which is associated with the fundamental laws of thermodynamics through the Gibbs free energy equation. It represents the relative chemical potential energy of water in the matrix determined by surface, number dependent, and capillary interactions. When water interacts with other molecules through various interactions, a portion of the energy contained in the water molecule bonds is transferred to these interactions, thereby reducing the energy of the water molecule itself. The more interaction between product ingredients and water, the lower the energy of water. The decrease in water energy also reduces its ability to escape into the gas phase, resulting in a decrease in vapor pressure. A water activity of 0.50 indicates that the water in the product has 50% of the pure water energy at the same temperature. The lower the water activity, the less the water in the system behaves like pure water. Please note that the definition provided here never mentions the term 'free water' as it is often mistakenly used to define water activity and has no scientific significance.

For chocolate products, the method of measuring water activity is to achieve equilibrium between the liquid phase water in the sample and the gas phase water in the top space of the sealed chamber, and then use a sensor to measure the equilibrium relative humidity (ERH) of the top space. Resistance electrolytic sensors, cold mirror sensors, tunable diode lasers, or capacitive hygroscopic polymer sensors can be used to determine equilibrium relative humidity (water activity). Novasina's instruments, such as Labmaster aw neo, use resistive electrolytic sensors to measure equilibrium relative humidity. Track changes in equilibrium relative humidity through changes in electrolyte sensor resistance.

Chocolate candy usually does not contain volatile substances unless it is a formula based on alcohol. However, a key challenge in measuring the water activity of chocolate products is the high fat content of chocolate candies, which results in a slower entry of water molecules into the gas phase, thus requiring a longer equilibrium time. Reading before reaching true equilibrium may result in inaccurate measurements, potentially affecting the safety and quality of the product.

In order to demonstrate the potential differences caused by different balance mode settings, Novasina scientists used the Novasina LabMaster aw neo water activity meter to conduct water activity tests on a series of products, using the fast balance mode with lower requirements and the slow balance mode with higher requirements (see Table 1). The fast mode aims to obtain results faster by requiring that the difference in water activity readings within 2 consecutive minutes does not exceed 0.001 aw. In contrast, the slow mode requires achieving the same level of stability within 6 minutes to ensure a more reliable equilibrium state.

Table 1 shows the test results of average water activity and test time (in minutes) of common chocolate candy products tested on Novasina's LabMaster aw neo device using a less stringent "fast balance" mode and a more stringent "slow balance" mode.

Although the fast mode can significantly shorten testing time, its accuracy may be reduced. If the difference between the results of fast mode and slow mode is less than 0.01 aw, it is usually acceptable to use fast mode to speed up the testing process. However, for critical applications that require high precision, slow mode can more reliably reflect the true water activity values.

Loose test endpoint settings should never be used arbitrarily without assessing potential differences in results. This is only feasible when the instrument allows users to adjust relevant parameters. Novasina suggests that when conducting water activity testing on new products, the fast stable mode should be used first to run the test. Afterwards, keep the sample in the instrument (without opening the lid) and let it stand for up to one hour. During this period, LabMaster aw neo will update the water activity readings in real-time and display the measured values in fast mode. Through this method, users can determine whether the differences are significant enough to require switching to a more stringent stable mode.

As shown in the previous data (Table 1), the difference in average water activity between some chocolate products (such as chocolate syrup) in fast and slow modes is extremely small (less than 0.01 aw), although slow mode requires longer testing time; However, some products (such as chocolate hazelnut sauce) have significant differences, reaching 0.09 aw. These results indicate that quality control laboratories can use this to determine which products are suitable for rapid testing and which require longer equilibration time to ensure measurement accuracy.

Water activity and microbial growth

Microbial safety is the primary concern for all food products. Proper processing is crucial for reducing the number of microorganisms and preventing the proliferation of harmful microorganisms. Water activity plays a crucial role in controlling microbial growth as it directly affects the reproductive capacity of organisms. When the water activity in the environment where microorganisms are located is lower than their internal level, they will experience osmotic stress. As they transition to a low-energy state, water will be lost to the environment^[2]This water loss will reduce turgor pressure and slow down metabolic activity.

In order to continue reproduction, microorganisms must lower their internal water activity to a level lower than the surrounding environment, allowing water to re-enter the cell. It will attempt to achieve this by concentrating solutes within the cell. The effectiveness of this strategy varies by organism, which means that each organism has a unique minimum water activity threshold below which it cannot grow^[2,3]Table 2 lists the minimum water activity limits for the growth of common spoilage microorganisms.

Table 2. Water Activity (aw) Limits for Common Microbial Growth

Microbial issues with chocolate syrup

Although the water activity of the hard chocolate candies listed in Table 1 is lower than the microbial growth limit, the water activity of chocolate syrup products is significantly higher. The water activity of both syrups exceeds the threshold for mold growth, and if exposed to spores in the air, it may breed mold. It is worth noting that the water activity of zero sugar syrup is much higher than that of sugar syrup. The water activity of sugar syrup is 0.827 aw, which is lower than the growth limit of all pathogenic bacteria and is considered stable during the shelf life. In contrast, the water activity of zero sugar syrup is 0.937 aw, exceeding the growth limits of Staphylococcus aureus and Listeria monocytogenes. According to the food regulations of the US Food and Drug Administration (FDA), products with water activity at this level can only be considered stable during shelf life if their pH value is below 5.6 (see Figure 1).

The increase in water activity highlights the challenges faced by formulators in developing healthier alternatives. Although reducing sugar content is beneficial for health, it may also affect product safety. Sugar is a natural moisturizer that controls water activity by providing multiple binding sites for water, thereby reducing the energy of water in products. Replacing sugars with substitutes with different chemical properties will reduce the number of binding sites for water. Unless the total moisture content is simultaneously reduced, which is usually impractical due to viscosity requirements for products such as syrup, the water activity will increase. If this change in water activity is not properly recorded and adjusted to ensure product safety, the re formulated product may pose a risk to health. Therefore, water activity must always be monitored during the formulation process, especially when removing polar components such as sugar or salt.

Table 3. Adapted from Table A in the definition of food safety time/temperature control in the 2017 Food Regulations of the United States FDA

*PHF stands for 'potentially hazardous feed'

**TCS stands for 'Time/Temperature Control for Safe Food'

***PA refers to the need for product evaluation

Water activity and chocolate flavor

Flavor is one of the most important quality attributes of chocolate candies. Consumers expect the flavor characteristics of a product to match its type, and any deviation may make the product unpopular, which actually means the end of its shelf life. The natural low water activity of most chocolate candies helps extend their shelf life by slowing down chemical reactions that may alter their flavor. However, fat oxidation remains a key issue as it can lead to the production of odors associated with sourness.

The water activity can affect the rate of lipid oxidation in chocolate products. Unlike most food reactions, lipid oxidation intensifies at both low and high levels of water activity, as shown in Figure 1. This means that there exists a range of water activity, known as the monolayer value, within which the oxidation rate is the lowest. For most chocolate products, this value is approximately 0.35 aw, which is consistent with the water activity levels of many products listed in Table 1. To ensure stable flavor and extend shelf life, chocolate products should be processed to achieve this ideal range of water activity. Incorporating water activity testing into chocolate manufacturers' quality assurance procedures is crucial for maintaining product integrity and consumer satisfaction.

Figure 1. General relationship between water activity and lipid oxidation rate.

Water activity and chocolate frosting

In addition to flavor, texture and appearance are also key quality attributes of chocolate candies. Consumers expect chocolate to have an attractive appearance and, more importantly, a specific taste. Dark chocolate usually has a crispy texture and a harder texture, while milk chocolate should have a silky smooth and delicate mouthfeel.

The most common degradation reaction that affects the appearance and texture of chocolate is chocolate frosting, which can be divided into frosting and cream. The water activity is most closely related to the formation of frosting. When chocolate is exposed to high humidity, its surface water activity increases and then returns to its original level, resulting in frosting. When the water activity increases, sugar will dissolve into a solution, and as the water activity decreases, sugar will crystallize again on the surface. This will cause a layer of white, powdery sugar crystals to form on the surface of chocolate, making the texture rough and affecting its visual appeal. In addition, this will also alter the melting characteristics of chocolate during consumption, affecting the expected silky texture.

To prevent frosting formation, it is necessary to determine the ideal water activity of chocolate products through water activity testing and process them according to this standard. In addition, maintaining stable storage conditions is also crucial to prevent frost formation caused by fluctuations in water activity.

Water activity and water migration

Chocolate candies are usually made by mixing solid chocolate with other ingredients such as nougat, peanut butter, nuts, or fruits. These combinations enhance their appeal to consumers by offering a variety of flavors and tastes beyond pure chocolate. However, the stability of multi-component products depends on preventing moisture migration between layers, as moisture migration may lead to undesirable taste changes and, more importantly, affect microbial safety.

Although chocolate coating can serve as a natural moisture barrier due to its high fat content, changes in the moisture activity of chocolate itself can affect its protective properties and overall quality. The most effective strategy to prevent water migration is to ensure that all components ultimately have the same water activity. Water moves from areas with higher water activity (energy) to areas with lower water activity, rather than based on water content. This is a key difference: it is water activity, not water concentration, that drives water migration.

For example, in the chocolate peanut butter stick shown in Table 1, the water activity of chocolate was measured to be 0.455 aw, and the water activity of peanut butter was 0.450 aw. These almost identical values indicate that the product has reached a good balance and is unlikely to undergo water migration over time, which contributes to its long-term stability.

The role of packaging

As mentioned in the previous sections, extending the shelf life of chocolate products depends on three key steps: producing the product to the desired range of water activity, verifying this through water activity release specifications and quality assurance testing, and preventing changes in water activity due to water migration or exposure to different storage conditions. Although the issue of water migration has been discussed earlier, the main means to prevent changes in water activity caused by environmental exposure is packaging.

Packaging serves as a moisture-proof barrier, protecting chocolate products from absorbing or losing moisture when the surrounding humidity is different from the product's water activity. The effectiveness of packaging materials is usually measured by their water vapor transmission rate (WVTR), and the lower the WVTR, the better the moisture barrier effect. A high WVTR means that water is more likely to pass through the packaging, which may alter the water activity of the product and affect its quality.

Choosing appropriate packaging materials requires a balance between performance and cost. Although materials with lower WVTR values have better protective effects, they are often more expensive. Choosing packaging with WVTR lower than actual demand will result in unnecessary cost increases and reduce profitability. The goal is to find a packaging that is both cost-effective and can maintain product stability under expected storage conditions.

By utilizing knowledge of the product's water activity, moisture absorption characteristics, and mathematical models, it is possible to predict the ideal packaging suitable for the product. Although this white paper does not cover the detailed steps of this modeling, Novartis can provide support and guidance for manufacturers interested in adopting this method.

Summary

The key to extending the shelf life of chocolate products is to determine and maintain an ideal range of water activity, within which the texture, flavor, appearance, and taste of the product can all be achieved. Researchers and formulators have a responsibility to determine this ideal range through experimentation and water activity testing, especially when developing healthier alternatives that may alter the composition controlling water activity. Once the ideal water activity is determined, it becomes the specification standard for product release, and routine quality assurance testing can ensure that each batch of products meets the target requirements. Finally, it is crucial to maintain the moisture activity of the product during transportation and storage. This can be achieved by selecting packaging materials that can effectively block moisture, preventing changes caused by environmental humidity or moisture migration. By controlling water activity throughout the entire process of formulation, production, and distribution, manufacturers can ensure product quality, safety, and extend shelf life. Novasina can provide knowledge on the application of water activity in chocolate candies, as well as the instrument performance required to maintain product quality and safety.

References

1. Geiger, M. 2023. Chocolate – Shelf Life, Storage, and Bloom. Iowa State University Extension.

2. Beuchat, L. 1983. Influence of water activity on growth, metabolic activities and survival of yeasts and molds. Journal of Food Protection 46(2):135-141.

3. Scott, W. 1957. Water relations of food spoilage microorganisms. Advances in Food Research 7:83-127.

4. Sutar, A.A., Dashpute, R.S., Shinde, Y.D. et al. A Systemic Review on Fitness and Survival of Salmonella in Dynamic Environment and Conceivable Ways of Its Mitigation. Indian J Microbiol 64, 267–286 (2024).