Candy products are typically safe from mould and microbial growth, but measuring and controlling product moisture can be critical for maintaining the right taste and texture, and preventing quality issues during storage.

Historically, manufacturers have understood that water content is an important parameter in candy making. Soft, chewy candies become harder as water content decreases. Hard candies lose flavour and become sticky as water content increases. Glassy confections can become rubbery with the wrong water content.

In fact, all kinds of problems can be caused as candy loses or gains moisture. The nature of these changes is not well explained by water content measurements. In order to better understand these effects, we need to measure the energy of water, not simply the amount.

Water activity is the most powerful way to measure and monitor moisture in confectionery, because it describes the energy status of water in a system. Many people are not accustomed to thinking of water in terms of energy. However, most of us have an intuitive understanding of this concept.

Try this thought experiment: imagine dipping a dry sponge into a glass of water. What is the difference between water in the glass and water in the sponge? The water in the glass is free water, while the water in the sponge is, to some extent, bound. It has a lower energy state than the water in the glass. We know that, because to get the water out of the sponge, we would have to do some work by squeezing the sponge.

The reduction of energy in the water within the sponge reduces its vapour pressure, increases its boiling point, and reduces its freezing point. In other words, the water in the sponge is different from the water in the glass in ways we can quantitatively measure.

DEFINITION OF WATER ACTIVITY

We can measure the change in energy by putting the sponge into a closed chamber and waiting while the liquid phase water in the sponge comes to equilibrium with the vapour phase water in the headspace above the sponge. The percentage of relative humidity of the air above the sample divided by 100 is the water activity of the sponge. So if the relative humidity was 56 percent, the water activity of the sponge would be 0.56 aw.

The definition of water activity is the ratio of partial pressure of water above a product to that of pure water at the same temperature. Pure water has a water activity of 1.00 aw. The range of all water activities goes from 0 to 1.00.

WATER ACTIVITY SENSORS

Three types of sensors are commonly used to measure relative humidity in the headspace above the sample: capacitance sensors, chilled mirror dewpoint sensors, and tunable diode lasers.

Capacitance sensors are made up of two charged plates separated by a hygroscopic polymer membrane. As the membrane adsorbs water, its ability to hold a charge changes. That change is approximately proportional to the change in relative humidity.

When calibrated against known salt standards, this can be translated into a water activity reading. Because they require calibration, capacitance sensors are a secondary method.

Chilled mirror and tunable diode laser methods are both primary ways of determining water activity. The chilled mirror uses an infrared beam focused on a tiny mirror to determine the precise dewpoint temperature of the sample. That dewpoint temperature is then translated into water activity.

The tunable diode laser emits a finely-tuned laser beam across the headspace above the sample. Because water vapour has strong absorption bands in the near infrared, the sensor can measure the presence of water vapour in the headspace very precisely. The beam of the laser, less than one nanometre wide, is specific for the commonly occurring isotope of water.

WATER ACTIVITY AND CONFECTIONERY

In the 1950s, William James Scott showed that water activity, not moisture content, was the best predictor of stability in foods. He used water activity as a predictor of microbial stability, but later research has shown that water activity is also the best indicator of physical and chemical stability.

A candy’s texture and flavour are critical to its identity and appeal. Moisture plays a key role in both texture and flavour. The relationship between water activity and moisture content at a given temperature is called the moisture sorption isotherm. Each product has a unique isotherm, and these isotherms can be used to determine specifications to optimise important texture characteristics and maximises quality and taste.

PRECISION IN MEASUREMENT

Water activity can also bring needed precision to moisture measurements. Most candies tend to have relatively low moisture. A type of marshmallow candy for example, has only about five percent water by weight. According to the company which produces this confection, the product moisture must be between 4.4 and 6.3 percent to be acceptable. That is a range of just 1.9 percent—it would be difficult to measure this accurately with a moisture meter.

Fortunately, when there are even subtle moisture content changes in low moisture products, the water activity reading changes a lot. In this particular confection for example, 4.4 percent moisture is a water activity of 0.450, and 6.3 percent is 0.600. That makes the range a total of 150 water activity units. If you use a water activity meter with an accuracy of ± 0.003 aw, it is easy to get the accuracy you need to meet the specification.

Water activity is about 15 times more precise than moisture content in this instance. Though this relationship varies from product to product, the effect will be similar for most low moisture candies and confectionery.

MOISTURE MIGRATION

The best production techniques can be sabotaged by water movement as the product sits on the shelf. Even in a product at rest, water keeps moving from high to low energy. Picture a pot of boiling water. Steam rising from the pot represents water molecules moving from high energy (the boiling water) to lower energy (the atmosphere).

Energy differences between the components in confectioneries are not as extreme, but over time, water will move from the filling into the coating (or vice versa), and the originally acceptable product may become gummy, hard, cracked, stale, or otherwise unacceptable.

Water activity is the best way to predict whether and wher water will move between components in a product. Relying on moisture measurements will only leave you in frustration because ingredients with similar moisture contents can have very different water activities; energy differences (measured by water activity) is what determines the direction of moisture migration.

To avoid surprises, adjust and equalise the water activity of different components during formulation to make a product that will maintain high quality over the course of its shelf life.

MOISTURE BARRIERS

Equalising water activities can solve moisture migration problems, but sometimes it is difficult to manipulate the water activity of a product component without changing its texture.

In a chocolate covered caramel and cookie product, for example, it would be difficult to formulate a soft caramel with a low enough water activity or a crispy cookie with a high enough water activity to prevent moisture from migrating between these components. In this case, a moisture barrier may be needed.

This may be a type of packaging that protects each component, or they can be edible films used to slow down moisture migration. Chocolate is often used as an edible barrier, but lipid-, protein-, and cellulose-based coatings are also available.

SHELF LIFE

Moisture migration is not the only factor that affects shelf life. Case in point: single serving packages of taffy candy, which over time become stuck to the wrapper. As the taffy ages, it loses moisture through the wrapper, eventually becoming rock solid. We can understand and predict what will happen over this products shelf life by measuring the taffys water activity and the wrappers vapour transmission rates.

In fact, because water activity is tied to many of the factors that end shelf life, it is an excellent predictor of shelf life for most candy and confectionery products. It can also be used to formulate a product to meet a specific shelf life target, and can be used to selec appropriate packaging.

The best packaging decisions are made using models that predict packaging performance under various storage conditions and a correct critical water activity. The critical water activity can be determined using sorption isotherms to find glass transition and phase changes, or it can be based on sensory analysis or microbial limits.

WATER ACTIVITY: POWERFUL TOOL

Though moisture content is frequently measured in candy and confectionery products, water activity is the most powerful way to measure and monitor moisture. Because it is a measure of the energy of water in a product, it is closely related to product texture and many of the other attributes that determine quality. It is much easier to identify potential problems early in development using water activity models than it is to reformulate in order to correct a problem.

Water activity can be used to predict and resolve problems with moisture migration and shelf stability. It can identify whether or not microbial growth will be a concern, and it is a key parameter in choosing the right packaging and performing accelerated shelf life studies. It can be used on the receiving dock to test incoming raw ingredients, at the line to guarantee consistency and quality, and in the quality check lab as a release specification.

In fact, water activity is a powerful tool that can and should be used at all stages of candy making.