Drying is a method of food preservation that works by removing water from food. This reduction in water inhibits the growth of microorganisms and minimizes some natural food decay. In ancient times the sun and wind would have naturally dried foods. Evidence shows that Middle East and oriental cultures actively dried foods as early as 12,000 B.C. in the hot sun. Later cultures left more evidence and each would have methods and materials to reflect their food supplies—fish, wild game, or domestic animal flesh. Vegetables and fruits were also dried from the earliest times. The ancient Romans were particularly fond of any dried fruit they could make. In the Middle Ages purposely built “still houses” were created to dry fruits, vegetables and herbs in areas that did not have enough strong sunlight for drying. A fire was used to create the heat needed to dry foods and in some cases smoking them as well.
The early traditional methods to dry foods included sun drying, wind drying, and drying over a fire. Most likely the earliest dried foods were simply fruits that naturally dried on the vine. For example, grapes will become natural raisins after drying in the sun. Humans wanting to leave nothing to waste probably ate these and noticed that they were quite good to eat. It is probably a similar eureka moment when a fishing culture left their catch to hang on a line. The sun and wind dried the fish so that it didn’t spoil. Dried fish certainly didn’t taste as good as raisins, but it was lighter and equally nutritious. It is no doubt that some of the most ancient cultures understood drying could preserve foods. They then would apply this knowledge to drying game meats and drying simple grains or legumes.
Modern foods are dried because they are safe from pathogens, convenient and have a long shelf life. Examples are milk powder and egg powder. The key to safety is to have a moisture content below the level at which molds can grow. The crudest drying techniques may leave 20‐30% moisture. Better processes leave only 10‐15%. It is difficult to remove all moisture. The lower the residual moisture, the more difficult to re‐dissolve the powder in liquids for use.
Modern foods are dried because they are safe from pathogens, convenient and have a long shelf life. Some very common dried foods includes dried powdered milk and dried powdered infant formula. Trail mix, soft chewy cookies, dried ethnic products like coconut powder and dry dog foods are also foods preserved with a low water activity. These latter products are not traditionally dried like milk, but are more complex.
Utah has a history of storing foods. Natural dried foods will store best. An entire industry has been built around this market. Here is just one company’s products including dried whole egg powder, dehydrated carrots, dried cheese sauce, dried textured vegetable protein (TVP), dried butter, and freeze dried blueberries.
Lets look at powdered milk. It has been around since the middle 1800’s. Today, powdered milk is usually made by spray drying nonfat skim milk, whole milk, buttermilk or whey. Pasteurized milk is first concentrated in an evaporator to approximately 50% milk solids. The resulting concentrated milk is then sprayed into a heated chamber where the water almost instantly evaporates, leaving fine particles of powdered milk solids.
A older method of drying foods is drum drying. Compared to spray drying, drum drying is a more intense heat treatment which results in more denatured proteins and a possible cooked flavor. Denatured proteins also result in a less soluble powder. Drum drying is still used for some products because it is very economical.
A fluidized bed dryer is a more modern drying system. Basically, wet materials are deposited on a porous bed. Heated air from below the bed flows up through the product. The heated air both dries the food and propels the food as a liquid through the machinery. Think of how an air hockey puck moves along. This fluidized bed is advantageous when product transitions from wet to dry. Picture trying to convey a sticky non‐flowing food through ordinary equipment.
During World War II research was being conducted on how to dry penicillin and other antibiotics. The scientists developed a method of removing moisture by freezing items under a vacuum. This technology became freeze drying. After the war the company working on this technology formed the Minute Maid Corporation to freeze dry orange juice. Freeze drying works on the principle of sublimation of water. Basically at very low pressure and a low temperature solid (frozen) water will sublimate directly to gaseous (water vapor). On the chart water solid will sublimate at any point where the red and green portions meet. The important factor here is that the food is never heated much above freezing. Without heat this process preserves vitamins, preserves enzymes and their activity, and will not denature any proteins. Nearly all food volatile compounds will be retained, preserving the delicate flavor profile of foods. Approximately 95% of water is removed from a freeze dried food leaving the final water activity between 0.1 and 0.2.
What affect do you think freeze drying will have on microorganisms? Is this process lethal to them? What will happen to the microbes when the food is rehydrated?
Drying is bacteriostatic and bactericidal. Cell death is based on heat intensity. Drying has little sporocidal activity against yeast, mold, or bacterial spores. Most spores dry in a viable state. If drying was via high heat some death may occur in yeast and molds. However, dry heat is far less lethal than wet heat. This is mostly due to the ability of water to transfer heat.
No microorganisms, even xerophiles, cannot grow at a moisture content below 15%. At this level, all spoilage will be chemical including oxidative rancidity, non‐enzymatic browning, and degradation of vitamins such as vitamin C. To minimize chemical spoilage there are several things food product developers can do. It includes reducing the moisture level below 15%, minimizing reducing sugars, blanching foods to destroy enzymes, using SO2 if possible, and packaging foods to minimize moisture pick up.
The extrinsic properties of water activity and moisture were discussed previously. Moisture is a good predictor of microbial growth only at its lowest end of the scale. What happens if there is more than 15% moisture? Will that automatically mean that microbes can grow? Honey has around 20% moisture, yet no microorganisms can grow. The reason is all of the sugars in honey lowers the water activity to 0.6. Water activity has been known and understood since the 1950’s.
This is an interactive chart that can be used in dried food product development. Note that once we get to a water activity of approximately 0.65 we have precluded the possibility of microbial growth. Further reductions can be used to preserve product quality. For a product high in fats a water activity of 0.2 to 0.5 would be optimum. For a product high in reducing sugars, non‐enzymatic browning can be reduced by reducing Aw from 0.6 to 0.2.
An expression of the amount of water present in a food or ingredient that is available to support microbial growth. As Aw is reduced, the rate of growth of microorganisms declines. A key food preservation principle is based on the reduction of water activity by removing water or by adding solutes such as sugar or salt.
Water activity is based on the ability of the water molecule to participate in chemical reactions or to be transported across a cell membrane. In the diagram free water molecules can pass through the cells membrane and participate in chemical reactions for the cell. Chemically bound water cannot pass through the cell’s membrane. If bound water cannot get into the cell, it cannot be used.
Water activity is measured in an Aw meter. Basically, the meter is reading the relative humidity around a sample to determine the Aw. Aw meters are expensive and can cost from 2000‐4000 dollars. The Aw values obtained go from 0 meaning no moisture present to 1.0 which is equal to pure water. To manipulate Aw food product developers would need to either reduce mositure or add more humectants such as sugar, salt, or proteins.
What water activity is needed to prevent all bacteria from growing? Click on the appropriate row.
Dried foods are classified into three categories. The first is high moisture with an Aw of 0.85 or greater. At that water activity Staphylococcus aureus can start to grow. At Aw’s just a little higher we see other bacteria growing. The opposite end is low moisture foods. These have a water activity below 0.6. At that level, no microorganisms can grow unless the food picks up moisture from the environment. In the middle is the intermediate moisture foods with an Aw from 0.6 to 0.84. Pathogenic bacteria cannot grow, but yeasts and molds can. Some intermediate moisture foods are quite interesting.
Here are some examples of intermediate moisture foods. Dried fruits have an Aw from 0.6‐0.7. Cakes and pastries have an Aw from 0.6‐0.9. Sugar syrups have an Aw from 0.6‐0.75. jams and jellies have an Aw from 0.8‐0.9. Some fermented and dried sausages can have an Aw from 0.83‐0.87.
To formulate an intermediate moisture food we have to first ensure that the water activity is at or below 0.85. Then we must design a process that eliminates yeast and mold or prevents their growth.
Now lets look a little closer. What water activity is safe for a cheese spread that is pasteurized in the jar? Why?
Let’s say we have a soft chew cookie at Aw 0.78. How many potential organisms can survive? How must that cookie be packaged?
Yeast and molds can be killed using pasteurization heat or propylene oxide gas. They can also be inhibited from growth using antifungals such as sorbates, benzoates, or propionates. It is important to know that some osmophiles are resistant to these antifungals. Modified atmosphere packaging can be used. Removing oxygen will prevent mold growth. Adding carbon dioxide as a MAP gas will inhibit both yeast and mold growth.
The last method to formulate intermediate moisture foods is to further lower the water activity. As the Aw gets closer to 0.6 fewer osmophiles or xerophiles exist. For example at Aw 0.74 only two spoilage organisms can potentially spoil our product. For example the soft chewy cookie that had a water activity of 0.78 in our example earlier could be reformulated with glycerol and propylene glycol until the Aw gets to 0.74. Using good sanitation after baking, these cookies are placed into wrappers in a manner that prevents any possible environmental cross contamination by Zygosaccharomyces or Monascus.
Proper drying or water activity levels are required to prevent growth of pathogenic bacteria. As long as the entre food product maintains an Aw of 0.85 or less, there is no concern. However, it is possible to raise the water activity of complex foods. For example if a cake has an Aw of 0.84. It is frosted with a high moisture frosting. If water were to migrate from the frosting to the cake, the Aw could be raised above 0.85. moisture could also be picked up from the environment. That same cake stored open in a Louisiana summer could pick up quite a bit of moisture. The second area of concern for pathogens in dried foods is the ability of Salmonella, E. coli and Cronobacter to survive the drying process. Salmonella has caused numerous outbreaks in low Aw chocolate and Cronobacter has caused illnesses in dried milk formulas.
Cronobacter sakazakii is a Gram‐negative, rod‐shaped, pathogenic Enterobacteriaceae. It is related to both Salmonella and E. coli. It is more of an opportunistic pathogen affecting the immunocompromised including neonatals. Some neonatal Cronobacter sakazakii infections have been associated with the use of powdered infant formula with some strains able to survive in a desiccated state for more than two years. In infants it can cause bacteremia, meningitis and necrotising enterocolitis. Infant illness has a 20‐50% mortality. The natural habitat of Cronobacter sakazakii is not well understood. The bacterium can be detected in the gut of healthy humans, most probably as an intermittent guest. It can also be found in the gut of animals as well as in the environment.
The slide depicts four different strains’ ability to survive desiccation. Cells were air dried in either infant formula (solid circles) or tryptic soy broth (open circles). Note how in most cases the infant formula dried samples had higher survival rates.
Over the last several decades, a number of outbreaks of salmonellosis have been associated with the consumption of ready‐to‐eat low‐moisture products, including chocolate, powdered infant formula, raw almonds, toasted oats breakfast cereal, dry seasonings, paprika‐ seasoned potato chips, dried coconut, infant cereals and, more recently, peanut butter and children’s snacks made of puffed rice and corn with a vegetable seasoning.
There is a common misconception that low numbers of Salmonella are not a problem in low moisture foods because these products do not support Salmonella growth. However, low numbers of Salmonella in foods can cause illness, and the presence of the organism in low moisture ready‐to‐eat foods must be prevented.
Here is a quick graph done by my lab of the survival of Salmonella in a dried meat paste. The meat paste is primary preserved with low moisture and high salt. The graph shows that we could get a 5.5 log reduction after 28 days and a 7 log reduction after 5 weeks, but we were never able to completely eliminate all Salmonella. Not shown on this graph was another dried meat paste product. There was no log reduction at all during the 120 days it was monitored.