Investigating Infusion
December 01, 1995 - Article
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Investigating Infusion

December 1995 -- ApplicationsBy: Lynn A. Kuntz
Associate Editor*

*(Editor since August 1996)   Fruits and vegetables as Mother Nature designed them present a number of challenges in processed foods because of their high water content. Shelf life, functionality and compatibility issues often arise.  Humankind has countered with several solutions to the problems. One of them, a process known as infusion, solves the problems associated with high moisture while imparting some unique characteristics, as well.  Infusion takes advantage of the concept that in a multi-phase food system both solutes and water seek equilibrium. When the water activity and concentration of all components equalize, the movement toward equilibrium stops. By controlling this diffusion, water can be encouraged to migrate out of a piece of fruit or vegetable and be replaced by solutes, usually some type of soluble carbohydrate such as sugar.  Infusion resembles a process known as osmotic dehydration -- in fact the line is somewhat blurred. Osmotic dehydration removes substantial amounts of water from a product while adding minimal solids. Instead of merely taking water out, infusion maximizes the osmotic movement in both directions so solutes move into the food. This yields a different set of characteristics in the finished product.  "The difference between the two has always been the end goal," explains Kristen Barner, director of R&D, Graceland Fruit Cooperative Inc., Frankfort, MI. "The end goal of osmotic dehydration is the removal of water to make the product stable. In infusion, we want water removal but we also look at how much sugar we can put into the product."

Inducing infusion

  We can take several different approaches to the infusion process. They all begin with the immersion of the fruit or vegetable in a high-solids matrix. This may consist of a high-solids solution or dry solids that are added directly on top of the fruit or vegetable. Liquid from the food dissolves the solids and allows them to migrate inside. Typically the infused piece undergoes a draining step, and the process often includes a washing step. Further drying reduces the moisture to the desired level.  "Just putting a piece of fruit into a high-sugar medium is what I call passive infusion," says Scott Summers, director of R&D, Tree Top Inc., Selah, WA. "It is just setting up a concentration gradient with a high concentration of soluble solids on one side of the fruit membrane. You allow the mixture to equilibrate, usually at room temperature. in some ways it is the least damaging to the subject material. It's a very gentle method, and it precludes some of the things that actually deteriorate your fruit -- elevated temperatures, pumping."  The secret to infusion is the rate of equilibration. If the water comes out too fast, the structure of the fruit or vegetable may collapse. Several factors affect the rate and degree of the infusion process, including time, temperature, pressure, and the type and concentration of the solutes, or solids gradient. The higher the temperature and the greater the pressure, the faster the transfer occurs. However, raising the temperature too high creates problems. Volatiles flash off, and color and flavor degradation can occur. Vacuum processing helps drive solids into the product. Vacuum infusion tends to maintain the shape of a piece better, especially when combined with vacuum drying in the final stage.  Infusion can be performed in a single steeping step or several infusion steps. Because of the transfer of water and solids, the percent solids in the solution drops as equilibrium occurs. To maintain a solids gradient, new solids must be introduced into the system. Sugars can be added to the system, or the solution can be drained off and fresh high-solids solution can be added.  The process may use a simple batch system or more elaborate continuous systems. Passive infusion, because of the length of time needed to reach equilibrium, generally is performed in a batch system. Active infusion usually puts heat and/or vacuum to the system to accelerate the process. Most commercial systems use various permutations of these elements.  "If you look at all of the existing patents on infused fruit, you'll find there is a tremendous range of things that are covered," says Summers. "Most of the processes in commercial operation are usually based on existing inventions with sufficient variations to avoid patent infringement. But essentially there are a lot of similarities between inventions."  After infusion, the solids content in the product rises from the original level to 50% or higher. Draining the product leaves two fractions: the modified fruit or vegetable, plus the infusion syrup. Some infused products can be used at this point, especially those designed for high-moisture applications such as ice cream.  For many applications, however, more water must be removed to lower the water activity (Aw) and to facilitate handling.  Because syrup coats the outside of the pieces, they generally require washing and drying. Washing creates potential problems because it removes some of the solids and can put water back into the product. However, without washing, crystallization could occur on the surface. This could serve as a seed for undesirable crystallization throughout the product.  Due to their high sugar content, infused products will tend to stick together or block if handled improperly, especially if subjected to high temperatures. This can be minimized via some type of coating -- a light oil spray or non-humectant powder such as apple fiber -- or by the addition of other flow agents.  The drying processes resemble those used for dehydration. However, special attention must be paid to the conditions because of the high solids content.  "Drying can be difficult," points out Mike Augustine, manager of food ingredient applications, A.E. Staley Manufacturing Co., Decatur, IL. "What you've done is add high levels of ingredients that want to hold on to water. Because the ingredients added may be thermoplastic, it makes drying even more difficult."

Material issues

  The ingredients used for infusion affect both the process and the finished product. Not all fruits and vegetables can be successfully infused on a commercial scale. Some, such as raspberries, are too delicate. Soft products tend to come apart under mechanized handling and form a puree rather than infused pieces. Softer products usually require passive infusion. Others, like carrots, although they can be infused with some difficulty, show no advantages over those made with simpler dehydration processes.  The characteristics of the raw materials influence how easily and quickly they can be infused. Products with a skin or tough surface such as peas, corn, cherries and blueberries resist infusion unless the skin is broken or scarified to facilitate the movement of water and soluble solids.  "The fruit affects the process in some ways, although it's hard to quantify," says Frederick Robinson, director, R&D, Jasper Wyman & Son, Milbridge, ME. "Some blueberries will be infusible and some won't. We don't always know the reasons. It could be related to growing conditions, or it may be something else. Blueberry skin may be variably porous, and it can be scarified or cut slightly to assist infusion. A piece of cut fruit like an apple would have open pores all the way through. Size definitely would be a factor. It determines how fast the infusion process takes place and how fast the sugars get to the center."  Notes Summers: "In our experience, the firmness of the fruit is a direct function of how well the product picks up the solution. The firmer it is, the longer it takes and the less readily it picks up solutions. In apples, unripe fruit has a greater amount of starch and as fruit ripens those starches convert to sugars. A truly ripe piece of fruit has virtually no starch. That would affect the infusion rate because those starches are less soluble than sugar."  While it may be tempting to use very ripe fruit in infusion to facilitate the process, overripe fruit has a softer texture. This means more difficulty in handling -- that is, the fruit will tend to fall apart.

Ingredients inside

  The type of solids used to infuse the raw materials also affects the process and the finished product. In most cases, the solids used for infusion are smaller molecular weight carbohydrates; the smaller the molecule, the better for infusion. Smaller molecules penetrate more easily and replace the water more quickly. A disaccharide infuses more slowly than a monosaccharide, for example. A higher molecular weight compound goes into the product more slowly, increasing the tendency toward osmotic dehydration rather than infusion.  The smaller molecular weight ingredients don't always give the right finished product results. They may affect flavor. Glycerin can produce bitter off-notes at high levels. Corn syrups can impart their characteristic flavor to delicately flavored products. Sugars can be too sweet, especially in vegetable infusions. If sweetness is an issue, ingredients such as maltodextrins, corn syrup solids or rice syrup solids can be infused into the product instead of sugar. Because of their molecular size, these may not infuse as readily as simple sugars, so the design of the process must account for the speed of the transfer.  Another characteristic that affects the process is solubility. The infusion ingredients must dissolve in high-solids systems used at the appropriate temperatures. The systems typically range from about 70% up to about 90% solids.  "There are a couple of things that determine how well a particular sweetener will work," explains Augustine. "First of all, molecular weight, transfer and permeability. But the second thing is the effect on the finished product attributes."  Fructose is very soluble; it's a very good plasticizer or softening agent. Glycerol is half the molecular weight so it is more effective at reducing water activity. However, there are some off-flavors associated with glycerin. Other issues include sweetness, flavor and texture. Dextrose has the same molecular weight as fructose but is less sweet and less soluble, which causes crystallization problems. Designers will face trade-offs in terms of solubility, flavor, size and rate of transfer.  "One company put higher molecular weight solids in the product before drying it to prevent the structure from collapsing and to make it readily rehydrated when it was dried," continues Augustine. "It's slower and more difficult with higher molecular weight carbohydrates, but it gives you a different kind of functionality. It depends on what you are trying to accomplish. Is it texture, sweetness, or the amount of water you want to drive out? Is it the ratio of the sugars inside or the water activity?"  Because infusion is an equilibrium process, not only do the percent solids equilibrate, but the ingredients themselves try to equilibrate, also. Therefore, infusion changes the proportions of sugar present. For example, dextrose makes up about 50% of the sugar in most fruits. Using fructose drives out dextrose along with the water, so dextrose content goes down inside as it migrates into the syrup. Reducing the dextrose can help maintain a soft texture, especially in cases where the packaging may be deficient.  Some solutes control water activity better than others. Humectant ingredients such as sugar alcohols, glycerol, propylene glycol, and similar ingredients have a marked effect on water activity and can change the product texture. Altering the types of solids and their ratios can create a product that is very plastic, or rubbery, or something that is more firm.  "Glycerin allows us to keep more moisture in the product at a lower water activity or, conversely, to get a lower water activity at the same moisture," Barner notes. "It also helps to keep the fruit soft. If everything else stays the same and you just add glycerin to the infusion process, you will see a lower water activity in the finished product. It contributes to the texture, resulting in a softer product."  Soft texture is not always desirable. It works for most fruits, but not most vegetables. Higher molecular weight solutes create a firmer texture. Rather then being chewy or pliable, the finished product will have a more intact shape and structure, especially after processing.  Shelf life also will help determine what types of solids are used. "One important thing is the characteristics of the sugars in terms of recrystallization," advises Robinson. "You don't want recrystallizing in the finished products. You would see a grayness or change in color like you often see on raisins. That's due to crystallization. Fructose and dextrose have less tendency to recrystallize over time than sucrose has."  Even labeling issues contribute to the selection process. Some manufacturers, especially those in the health or natural food area, do not want sugar on the label. Infusing with fruit juices prevents this. Others prefer to see fructose instead of sucrose because they believe fructose has a more positive connotation. Some just like to avoid a long ingredient list.  "You can select solutes based on the customer's ingredient mix," says Lisa Westing, manager of new product development, Basic Vegetable Products, Suisun, CA. "If the infused product is going into a dry mix, for example, and that dish contains some simple solutes in it already, you can work with some of the same ones to avoid additions to the ingredient statement...if there is maltodextrin in the seasoning mix, for example. Often you can work with a similar ingredient, provided you get the functional properties that you are looking for."  Components other than solutes designed to increase solids and water activity can be infused into food products. Among these are antimicrobial agents, colors and flavors. Traditionally, candied fruit retained enough moisture so that microbial spoilage was not eliminated. During the infusion process, adding a soluble antimicrobial compound such as sodium benzoate allowed it to migrate throughout the product, providing protection. Other ingredients such as alcohol can act in the same manner.  The same process allows manufacturers to add colors and flavors. During infusion, a significant amount of the flavor leaches out of the product. Using flavored fruit juices or any water-soluble flavor in the solution increases the flavor impact of the finished product as the flavor level equilibrates. This can be used to add other flavors to a piece -- for example, to create a peach-flavored infused apple.  Color can be improved by infusing the product with additional color. This concept is readily apparent in infused cranberries. Although the berries may have widely differing colors going into the infusion process, the color will equilibrate during soaking. Certain pigments, such as those that produce the red color in strawberries, will not withstand heating. They tend to brown. Either artificial or natural water-soluble coloring agents can be used, depending on the desired label impact.  "If you look at our label on infused strawberries, you will see cranberry juice concentrate," says Barner. "It not only contributes color, but sweetness. In R&D we have looked at artificial colors and they work well, but that moves us away from the concept of a natural piece. We could do it if a specific customer needed that type of product."

Increasing solids

  Low moisture and low water activity are two of the goals for an infused product. Water activity indicates several things -- most importantly, microbial stability and moisture transfer. A low water activity limits the movement of water from one region to another. For dry applications such as cereal, bringing the Aw of the fruit and the cereal as close as possible to each other keeps the driving force of moisture migration down so the cereal does not pick up moisture and become soggy.  The two factors are related, but not directly. Most infused products use moisture or percent solids as a specification or test method, but often the characteristic of concern is water activity. Because water activity is a function of the number of molecules, those solutes such as glycerin or fructose will lower the Aw to a greater degree than those with a higher molecular weight. Percent solids can only be a reliable measure of water activity if the same solutes are used in the same ratios.  Water activity relates to a number of finished product characteristics, some directly, some indirectly. Along with being indicative of microbial stability and the tendency of water to migrate in food systems, Aw is related to other properties. For example, fruits destined for ice cream must contain a high level of solids to depress the freezing point so the fruit pieces remain soft.  "Freezing point depression is a colligative property, as is water activity, meaning it is dependent on the number of particles," Augustine says. "A given weight of glycerol contains twice the number of molecules than an equivalent weight of fructose. Freezing point depression is driven the same way as water activity -- the lower the molecular weight, the lower the freezing point. You would formulate with the lowest molecular weight component if you wanted to optimize either one."  Different solutes create different textures in the same type of fruit. A soft texture appears to be related to water activity.  "Some of the things we've done for cereals in the 0.30 to 0.35 Aw range seem tough and chewy," notes Summers. "But we've had some interesting results by putting glycerin into the system in combination with sugars. The textural characteristics are much better at lower water activities. We've looked at a wide range of glycerin levels. We can use a relatively high level, but apparently there is a point in addition with other ingredients where the bitter glycerin aftertaste goes away."  Softer may not always be better. Some people believe that if a food system contains significant levels of a plasticizer, it increases the opportunity for things to go wrong because of the increased mobility of the compounds in the system.  "If there is some type of undesirable reaction that could occur as the result of the system being very soft and mobile, then the lower water activity could conceivably be bad if you've used very plasticizing ingredients," suggests Augustine. "One of our concerns is that if you take fruit and put in low molecular weight sugars that make the product soft and pliable, will you have problems with the color. With too much mobility, you may get a color change with time. Or an off-flavor may develop. It may not occur when something has the same water activity but less mobility because you've just removed water. Typically the more mobility comes from the lower molecular weight compounds. After all, water is the ultimate plasticizer."

Incorporating infusion

  Infusion actually creates two products. The primary product is the infused piece. The secondary one is the infusion liquid.  "One of the biggest problems with this technology is what to do with the liquid that is left," remarks Augustine. "You have 50% solids or more, a lot of flavor and color, and different types of sugars. You can recycle it to a certain extent but heating may result in flavor problems and the build-up of the things you have taken out of the fruit."  Re-concentrating and reusing is one of the most common applications for the spent syrup. Concentration requires heat which can darken or brown the color, as well as drive off flavor volatiles. Filtration removes most of the particulates and foreign matter. In general, the syrup will impart some of the characteristics it gained for prior infusions to the product. That may not be all bad, especially if it replaces some of the flavor lost during the infusion process. Other options exist -- syrups for fruit fillings or beverage bases, for example.  Finding uses for the infused fruits and vegetables is much easier. Fruits can be added to cereal, granola bars, baked goods and mixes, ice cream and other dairy products, and candies, and they can even be eaten out of hand. Vegetables work well in boxed mixes, dry soups, or as a substitute for fresh ingredients in prepared salads.  The main consideration is that the process must result in a finished product that fits the specific application. Water activity and texture are probably two of the biggest concerns. In most cases, the piece should maintain its texture and not change the surrounding product.  Other characteristics can become important depending on the application. Rehydration rates can be important in applications such as batters, or in cooked vegetable applications such as rice or stuffing mixes. Infused vegetables rehydrate in 15 to 20 minutes, times that are compatible with many prepared mixes.  "Our infused apple has different textural characteristics, but more than that, the way it behaves in the finished product is different from normal apples," Summers says. "Typically apples have a tendency to float. These have a different bulk density, so they don't float to the top in a cake batter or other liquid product and more readily distribute throughout. If you are making batter, you don't want the product to look like a pineapple upside down cake; you want uniform distribution."  Infusion can not only improve texture and functionality over conventional drying, it can bring improvements in color and flavor, too.  "Infused vegetables are used as garnishes in a number of food products," says Westing. "One of the important attributes is color. A lot of the pigments are sensitive to oxidation. Infusion helps stabilize the natural colors in the vegetable. It stabilizes the color by reducing oxidation. It helps seal off some of the pores, as well as restrict mobility."  Says Ray Turner, Basic Vegetable's vice president of marketing: "Because of infusion we are able to dry more gently, so fewer off-flavors develop. For instance, conventional air-dried celery can get kind of a hay-type flavor, and the infusion process helps avoid those sorts of off-flavors."  Infused fruits and vegetables can bring several advantages to the design of a food product. Just keep in mind that the improvements to the original must meet the needs of the application.


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