Drying of Food Ingredients

Foodstuffs are very often available as an aqueous solution, a slurry or a paste, having a limited shelf life due to bacterial activity destroying the nutritive value of the product, unless it is kept at low temperatures or preservatives are added, sugar or salt for example, to reduce the bacterial activity.

Removal of water from the product will likewise reduce the bacterial activity in the final product and thus ensure an almost infinite shelf life, if the product is dry enough and kept in a dry, cold place.

Animal or vegetable food products or combinations hereof, are characterized by their content of:

• Proteins
• Carbohydrates
• Fat
• Starch
• Other ingredients

Proteins

Generally speaking, the more protein, the easier becomes the drying. With increased protein content, the viscosity of the concentrate will increase and, to maintain good atomization, it is necessary to reduce the solids content. Drying economy is thus reduced, as a pure protein product (egg white, Na- or K-caseinate) is dried from a feed of 20-24% solids, and the final product consists of single particles.

Proteins are difficult to agglomerate (no binding material) in conventional dryers. The Multi-Stage Dryer (MSD™) is therefore selected, if agglomerated powders are aimed at. Hydrolyzed proteins are generally more difficult to dry, as the short-chain molecules become thermoplastic and hygroscopic, which makes the product stick to the drying chamber walls.

Carbohydrates

Carbohydrates of different origins and forms are found in most food products. Generally speaking, the more carbohydrate, the more difficult becomes the drying process, as carbohydrates, if dried from a non-crystallized feed, are thermoplastic and hygroscopic. Carbohydrates will reduce the viscosity of the concentrate.

One large group of carbohydrates is lactose, which is found only in milk and by-products from milk processing, for example in whey from cheese production. The advantage of lactose from a drying point of view is that it can be pre-crystallized if found in a supersaturated solution (high solids, low temperature). The sticking point temperature is increased by pre-crystallization, and drying becomes easier.

Sucrose is used in food products as a sweetener, and also to add calories in e.g. baby foods and other powdered beverages. If the final sucrose content in a product must be larger than what is possible to incorporate in liquid form in the concentrate to be dried, it is possible to add it in dry form into the spray dryer during the drying of the liquid feed and thus obtain a blended, agglomerated, homogeneous final product.

Fructose as found in all fruits is very thermoplastic and hygroscopic. Products like fruits therefore cannot be spray dried unless a filler like skim milk solids or maltodextrine is used as carrier.

Maltodextrine is another carbohydrate, which is frequently used as a filler, carrier, or sweetener. With a DE of 10-40 it can be used in many food products, as it is easy to dry. Carbohydrates are excellent as binding material, and agglomerated products can easily be produced, if the spray drying plant is designed for this purpose.

Fat

If fat is not found in the original food product, it may be added to reach a certain composition. The more fat in a product, the lower the viscosity and the more difficult becomes the spray drying (powder deposits). If the fat is found in free form (unprotected by proteins) in the final product, the drying operation becomes even more difficult. Homogenization of the concentrate prior to spray drying is therefore recommended. Homogenization will increase the viscosity of a concentrate slightly.

Starch

If the starch is pre-cooled, it increases the viscosity. It is therefore necessary to lower the solids content in the concentrate to be dried.

Other Ingredients

There is a wide variety of other ingredients that are typically added in small quantities, such as:

• Vitamins
• Flavorings
• Emulsifiers
• Stabilizers
• Colors
  In general, these do not cause too many problems during spray drying.
• Salts and other chemicals
  The higher salt and chemical content the more difficult becomes the drying due to increased stickiness.