Managing moisture variability in dried fruits and nuts

Dried fruits and nuts are popular among food producers, whether used as standalone products or as ingredients in other foods. 

There are good reasons for that. Fruits and nuts are natural foods that are widely loved. They are delicious on their own or as ingredients in protein bars, salads, baked goods, and more. Furthermore, today’s consumers are ingredient-savvy. They read labels and like to see familiar foods listed as ingredients in their favorite manufactured snacks.

But there are also good reasons for food manufacturers to avoid them. Both fruits and nuts present food companies with challenges due to their wide variability in both water activity and moisture content. Here, we will discuss these variables and offer solutions for food manufacturers who work with fruit and nuts in their production processes.

Variability in Fruits

Fruit is a large category of food. Each fruit has a unique combination of water activity, moisture content, fiber, sugar, and other variables.

Much variability exists in fibrous fruit like mangoes, both in water activity and moisture content. Other fruits, like blueberries, have less variability between the two. However, even in these more predictable fruits, water activity can vary depending on drying methods, processing methods, added sugars, pH levels, and even growing seasons.

Variability in Nuts

While not as variable as fruits, nuts are also difficult to keep stable in manufactured foods. Figure 3 below illustrates the water activity variability among almonds, peanuts, cashews, pecans, pistachios, a snack mix, and walnuts.

As is the case with fruit, focusing on one specific nut can be helpful in analyzing trends. There is less variability among almonds, for example, than there is among blueberries—but there is some.

Like fruits, nuts can be influenced by their growing season. But processing is also an essential variable in nuts’ water activity. If they’re intended to be consumed raw, the water activity will be different than if they are dried. Adding salt or other flavorings also influence a nut’s water activity and moisture content. Significantly, nuts also have a lower water activity range than fruits, which means they are more susceptible to rancidity.

Implications of Variability: Taste and Texture

A theoretical understanding of the relationship between water activity and moisture content is interesting, but how these variables affect the taste and texture of food is much more helpful from a business perspective.

An Experiment: Over- and Under-Hydrating Fruits and Nuts

For these purposes, consider the results of an experiment conducted by METER Group’s R&D lab manager, Mary Galloway. Galloway set two conditions to demonstrate what happens to fruits and nuts when they are over-hydrated or over-dried. 

Galloway presented a sample of blueberries and almonds, both with a relatively high water activity level of 0.7.

Over-hydrated Fruit

The blueberry is very soft. The texture is ideal for eating the blueberry on its own—although, with a water activity rate of 0.7, it is at risk of becoming moldy. This experiment shows that over-hydrating fruit may be acceptable in certain situations, though it is risky.

Over-hydrated Nuts

However, a water activity level of 0.7 is far too high for the almond. As a result, the almond has become too moist and has a soft texture. Biting into the nut will not provide a satisfying crunch.

Over-dried Fruit

Blueberries have a naturally high water activity level, so the berries become hard and dry when the water activity level is lowered. They rattled when Galloway put the berries in a jar and shook it. The over-dried blueberries would not make a good snack—though they might be perfect as an ingredient in breakfast cereal.

Over-dried Nuts

As one might expect, over-dried nuts were better for snacking than over-dried fruit. The manipulated almonds have a lower water activity have a nice crunch that is perfect for snacking. 

Finding the balance between over- and under-hydrating foods—especially when using them as ingredients in the same food—can be difficult for food scientists. But measuring and accounting for variability in fruits and nuts can help them find delicious solutions.

Measuring Moisture Variability and Successfully Mixing Fruits and Nuts

In the current food manufacturing industry, most companies use moisture content to measure the moisture variability in foods. They often use loss on drying as a measurement method for determining moisture content, which requires heating. If the ingredient has an undesirable reaction to high temperatures—and fruits and nuts both do—the sample may become browned or toasted, changing the food’s moisture content. Some food scientists also measure a change in electroconductivity when determining the moisture content of fruit pastes. This, too, is less than ideal. 

These traditional methods are widely used, mainly because they have been around for a long time. But a more modern measurement technique can benefit food manufacturers—water activity. Measuring water activity is an easier, more precise way to measure water in food. It’s also relatively easy to understand how a food’s water activity relates to its quality and safety. 

The benefits of measuring water activity become apparent when scientists perform complex food calculations—like those required when food manufacturers mix fruits and nuts in the same product. 

Some people wrongly assume that moisture moves from one food to another due to the amount of moisture in the first food. However, food with more water isn’t necessarily more likely to move that moisture into another food. Instead, a food’s energy determines its likelihood of moving water into nearby foods. In other words, the food with the higher water activity will be the one that loses water. 

Take, for example, a box of cereal with crisp, dry flakes and moist fruit, like a raisin. To successfully mix the two, food scientists add a humectant—like sugar or salt—to the outside of the raisin. These humectants lower the raisin’s water activity level, which prevents water from moving from the soft raisin into the crispy flakes.

In decades past, food scientists relied on the scientific method to determine how much humectant to add to a food to achieve the optimal water activity in each ingredient. Unfortunately, this usually involved a lot of trial and error, which cost a lot in both time and money. 

However, with METER Group’s water activity measurement tools and accompanying software, food scientists can take samples of foods and then run these “experiments” from their computers. These sophisticated water activity measurement tools save companies time and resources as they help with calculations that previously took significantly longer. This allows companies to make decisions regarding manufacturing, warehouse conditions, packaging, and more with more confidence and more quickly than ever before.