Although there is still much to learn about the process of mixing solid particles, designers of industrial plants often fail to take advantage of available knowledge so that the best possible results can be achieved. The operation of a mixer must be regarded as producing equilibrium between mixing and segregation; those mixers being used can be classified as either segregating or nonsegregating. Components—or mix ingredients—to be used can be similarly classified. In addition, the selection of the mixer for a particular duty should be based on how materials tend to segregate; those that do should not be put into a segregating mixer. Further, care must be taken in the handling of a mixture to avoid segregation. In regard to powdered foodstuffs, we can distinguish three main “actors”: ingredients, mixers and handling or materials management.

The Role of Quality Assurance
Quality assurance (QA) systems take a much wider view of what is involved in satisfying customer and production needs, focusing on the prevention of problems, not simply on their cure. Curing problems is expensive; quality cannot be “inspected into” a product.

A QA approach, therefore, includes the whole production and distribution system, from the suppliers of raw materials through management to the customer. QA systems should be documented in a simple way to show who has responsibility for doing what and when. The QA focus on prevention should mean that action is taken to meet a specification and prevent failures from occurring a second time. This is done by planning, management action and agreements with key suppliers and other people in the distribution chain.

QA requires that staff is well trained and motivated. Workers are normally well aware of the causes of most problems, and when QA is used properly, they can resolve most quality problems within their control. It is the responsibility of business owners to ensure that the QA system together with any necessary equipment and information are available to the workers to allow them to exercise this control. Then the importance of QA becomes obvious when food safety is concerned.

Why Use a Powder?
The major reason for production in powdered form is simply to prolong the shelf life of the ingredients by reducing water content; otherwise, the ingredient will be degraded. Thus, the major function of the powdered form is to maintain the stability of the ingredient’s functionality until it is required for use, which is eventually in some sort of wet formulation.

Food safety and QA must be concerned with those manufacturing hazards affecting foods (Table 1).

When dealing with powdered beverages, in most cases, the hazards—if the raw materials have been handled properly—are primarily physical in nature because such ingredients, due to their low water activity coefficients, do not allow for microbiological hazards.

The food industry still has processing problems and requires further basic research to understand processes and handle powdered foodstuffs, which means food safety and QA programs should be integrated into research for such developments.

For example, different product groups can be distinguished in the following ways:

Powdered products:

• Powdered beverages (no fat)
• Soups (powder and garnishes, low in fat)
• Sauces and gravies (powder; sometimes garnishes; low in fat and often high in starch)
• Recipe mixes (low in fat, spices)
• Ready-to-serve dishes (cup products, pasta and rice preparations, big pieces)
• Powdered bouillon (high in salt)
• Seasonings (high in salt and glutamate)

Agglomerated and granulated products:

• Vending soups (agglomerated powder)
• Foodservice sauces and gravies (agglomerated powder; high in starch)
• Granulated or agglomerated seasonings
• Instant bouillon (agglomerated or granulated powder, high in salt)

Tabletted and compacted powders:

• Bouillon tablets (hard and soft bouillon, high salt content, medium to high fat content)
• Sauces (tabletted sauces; high in salt, fat and starch)

Powdered beverages are a group within the powdered products category that have simpler formulations to control because they lack fat ingredients, similar to other powders like spices. This condition raises fewer concerns with regard to their manufacturing and distribution than those where fat is involved. Mixes containing powdered and coarse particles, which often have different densities and shapes, tend to segregate during production and transport; this phenomenon must be viewed differently if the powdered formulations include fat. In cases where no fatty ingredients are used, mixing time becomes an important issue, such that the added mixing time and further handling of the different dry components allow additional segregation, and very often the homogeneity of the mix decreases.

Powder Components and Characteristics
Powdered foods, including powder milks, dry-mix fruit drinks and juices, gelatin dessert mixes, starch-based pudding, tapioca-based desserts, “energy” mixes and other sugar- and starch-based powdered formulations, tend to have diverse components, many of which with substantially different particle sizes and flowability characteristics. Please note that the above examples are products that are essentially fat-free. The reason for this segregation is due to the different mobility of the particles in the bulk state. However, there is no standardized and accepted method to simulate this segregation behavior depending on the real movement of the powder during production and transportation, making any kind of measurement difficult. This situation creates a QA concern that must be approached individually in each product case, making the setting of quality control parameters—and testing methods—very difficult.

Measuring uniformity of a mix requires experience and must be specific to the product and manufacturing piece of equipment. Furthermore, a clear rule or processing procedure is needed for how to design the product and its production to avoid this segregation.

Experience has also demonstrated that external environmental conditions in the manufacturing area have important effects on the characteristics desired in the finished product. An example typical of this effect is sugar-based, fruit-flavored powdered products. When the product is sugar-sweetened, this ingredient will be the majority of the formulation, the rest being acid, flavor and other minor ingredients (color, stabilizer, etc.). In a moist climate and processing area without climate control, the humidity level will dramatically affect the time and order of addition of ingredients—not to mention the design and effectiveness of the mixer. Additionally, the time of mixing and the flowability of the finished product will suffer. In this case, the shelf life of the product is affected as well as its ease of packing.

When using visual examination for quality control and QA purposes, a variety of powders leads to a broad classification of free-flowing and cohesive categories. Free-flowing powders, such as granulated sugar, exhibit a smooth flow, an attractive nondusty appearance and little adhesion to the container. Differently cohesive powders, such as flour, have an erratic stick-slip flow, are often very dusty and stick to the container walls. Particle size is evidently an important determinant of flow type. If the mean particle size of a powder is greater than about 50 µm, it will tend to be free flowing, whereas below that size, it will tend to be cohesive. This boundary between flow types depends on more characteristics than just particle size, as it determines the philosophy of the processing of the powder and the type of mixer to be used. Particularly for large-tonnage industries, the attractions of a free-flowing powder from both processing and marketing points of view tend to outweigh the quality advantages of the cohesive powder. The attractions of free flow are often so strong that significant process costs will be incurred to aggregate an otherwise cohesive powder to achieve free flow. Only when product quality becomes dominant does the finer texture of the cohesive powder become attractive. This is often the case for high value-added applications, such as an intensely sweetened beverage, a mixture that uses agglomerated stabilizers and flavors, sophisticated flavored sauces that contain various types of chilies, etc., which, with a small amount of added ingredients, can produce a higher-value product that can command a higher price. A specific example would be a powdered beverage sweetened with aspartame instead of sugar; in this case, substantial savings in packaging materials and distribution costs allow for a relatively cost-competitive, sugar-free product.

Formulation Innovations
The powdered drink category is undergoing rapid changes, driven by exciting product innovations; technology exists to help overcome challenges in formulating with functional ingredients, and encapsulation technology is available to protect vital flavor components from degradation. Whether the products are powdered soft drinks or high-performance nutritional powders, they all must be hydrated prior to consumption and their presentation (to the consumer) must be taken into consideration in any QA program. It is easier if individual servings are packaged for consumer use; the instructions on the packet will indicate the need to use the total amount of powder in the package to prepare the drink. Thus, if actual segregation of the ingredients does occur, this will not affect the finished drink. Yet, if the powdered beverage is being sold in a canister or jar, and in order to prepare a drink, water or milk must be added to x number of teaspoons (or other solid measurement) of powder, then the homogeneity of the product in the canister or jar becomes paramount. These conditions require a much “tighter” QA program, which begins with tighter raw material specifications and mixing times, and involves additional ingredients, climate control and mixer designs, as well as manufacturing and packaging specifications. Only in this way will the proper ratios of powder to liquid be assured when and if instructions for preparation of the drink are followed. One other example that often occurs, especially if a child is preparing his/her own chocolate milk using a powdered mix: The excessive amount of the powder in a glass of milk can result in a lack of dissolution of the powder, an oversweetened drink, an intensive chocolate-flavored (or other flavor) drink, etc.

Food ingredient companies can tailor their ingredients to be used in powder formulations such that they can give a large variety of functionality; however, such ingredients must be used exactly in the process for which they were designed, and most applications have been developed empirically. This caveat also tends to limit research and development, since suppliers regard it as an added cost, and their customers are reluctant to change from established optimized processes even if they have only been developed empirically. Still, these prevalent issues concerning food safety and QA must be considered, even if it means more cost and time involvement with the supplier. The safety of the consumer is the responsibility of the manufacturer, not of the ingredient supplier.

Food ingredient powders will eventually be utilized in some sort of wet formulation. For powdered beverages, that functionality will depend on the powder particle and its interaction with water. Air is the cheapest food ingredient, followed by water, and many ingredients are applied to entrap more air and water by utilizing their aeration and gelation properties. Such characteristics have been appearing in new dry beverage powders to show foam and/or similar characteristics upon the final preparation of the product.

The use of complex mixtures of food products makes it difficult to establish optimal drying, flowability prediction, control of dust, strength, etc. of the final products. Food powders are not just powders. They have activity that can be microbial, enzymatic, flavor- and taste-enhancing, etc. This means that these aspects must be maintained as much as possible during manufacturing, creating extra problems during product development, production and distribution as well as a further need for QA. As a result, sometimes the products may contain active ingredients that are potentially harmful. Thus, a proper understanding of the responsible ingredients and a prediction of possible outcomes are needed, not only during product development, but also during manufacturing, quality control/QA and distribution. An example would be the distribution of an orange-flavored powdered beverage in a tropical country. The product was developed with a natural orange extract and a flavor that was tested in the laboratory for stability, yet after 6 months in the market, signs of rancidity appeared in the warmer and humid areas of the country, conditions that during testing were simulated but not tested in situ. The orange-flavored additive in the flavoring mix had an unpredictable reaction with the orange extract that caused the rancidity reaction.

Another different, but similar example of looking at the resulting product during its development is a nutritional powder product to be used as a weaning food in low-income communities. In this case, the powder must be diluted in a certain amount of water to deliver the expected nutrition; also, the product must be prepared with potable water. The experience was that after market testing and diarrhea in some of the children, it was concluded that the instructions were unclear, establishing the paramount need for proper training of the users.

What about Powdered Food Safety?
Food safety of dry powders is not given much attention because of their characteristic low water activity, yet requires much more attention, especially because of the sourcing of materials and complexity of dry product handling.

Finally, because of the complexity of the total powdered foodstuff manufacturing process and the lack of a formal education for practicing engineers and food scientists, college-level courses must be established. Meanwhile, some courses are available, conducted mainly by industry consultants and certain academic institutions. The final question is: If there is a need for more educational opportunities, when are the educational institutions going to meet it? There is a potential for a worldwide distance learning program dealing with aspects of powder science and technology, which should include specific food safety and QA concerns.

Establishing an adequate food safety and QA program for powdered food products requires it to be specific to the product (formulation considering available specifications of the ingredients), its manufacturing (handling, mixing and packaging equipment), its distribution and its ultimate use by the consumer. The needs and controls required cannot be generalized.

The complexity of the overall processes of manufacturing and distributing powdered foods is not typically science- or technology-based, mainly because of a lack of formal educational courses. The process has been conducted more as an art, with experience and hands-on learning. There thus exists an opportunity for more research and development with the objective of technologically improving the process. Consequently, inclusion and updating of food safety and QA procedures in manufacturing must constantly be stressed.

Powders in the Food Market
The powdered beverage market has grown beyond traditional sugar-based drinks, basic protein shakes and sports energy beverages. Functional beverage and food marketers see powders as a new opportunity to expand their delivery formats and offer convenience, better value and an environmentally friendly solution to ready-to-drink options. Supplement marketers offer increased value by adding flavor, function and hydration components, creating delicious powdered drinks for consumers to enjoy. However, crossing over to powders has its unique design and development considerations. Expert powder manufacturers use a host of technologies to meet the functional and sensory expectations of both clients and consumers. These technologies include unique food safety and QA considerations to ensure the uniformity of the finished product as it is consumed.

Conclusions
Powdered food products and, specifically, powdered beverages are mixtures of different ingredients that in all cases have different characteristics and specifications. These mixes as food products are rarely consumed as a powder, meaning that those ingredients require mixing to produce the food product. Experience has shown that those mixing procedures, at an industrial level, are tricky—homogeneity being the most important consideration—and in many instances require years of experience. In addition, in the development of the product as well as of its manufacturing procedure, simple instructions often do not contain sufficient detail to manufacture or use the product with a consistent quality result, resulting in a serious concern for QA and food safety. The recommendation is to be very aware of these conditions and product characteristics in order to have success in the quality and consistent use of the finished, consumable product.

Herbert Weinstein, Ph.D., earned his chemical engineering degree from the Universidad Nacional Autonoma de Mexico and his M.Sc. and Ph.D. in food science and technology from the Massachusetts Institute of Technology. He can be contacted at HERBWEIN@aol.com.

Resources
1. From proceedings of a workshop organized with the support of the European Commission, 5th Framework Program, Quality of Life and Management of Living Resources, Key Action 1—Food, Nutrition and Health at DSM Food Specialties, 544-0150, PO Box 1, NL 2600 MA Delft, The Netherlands.

Problems while handling and processing powdered convenience foods by Dr. Ing. Stefan Palzer from Nestlé PTC Kemptthal, Zürich, Switzerland.

Mixing of powders by N. Harnby from the University of Bradford, Bradford, West Yorkshire, UK.

Functional properties of food powders and particulates by Peter Lillford from the University of York, UK.

Powder technology by DSM Food Specialties.

Powders in applications by Gabrie M. H. Meesters.

2. Food powder handling and processing: Industry problems, knowledge barriers and research opportunities. Published in Chem Eng Process 2005; 44:209–214 by John J. Fitzpatrick from the Department of Process Engineering, University College Cork, Cork, Ireland, and Lilia Ahrné from The Swedish Institute for Food and Biotechnology, Gothenburg, Sweden.