Ingredients as Mixtures of Functional Components

Extrusion deals with a variety of ingredients. The most frequently used ingredients are grain flours. The grain flours may be refined or they may be whole grain.

One approach to understand the how ingredients will function in a system is to compare how one ingredient compares to another. There are any number of resources that will give guidance on where rice flour may be the “best” choice for the ingredient and other situations where corn flour may be the “best choice”.

An alternative approach is to look at what the ingredients are composed of and use that knowledge to guide how to use an ingredient or set of ingredients to get the desired product from them. This chapter will use the composition to understand how ingredients will behave in the process. This approach is built on the assumption that similar compositions will have similar extrusion characteristics.

Broad Classification: Plasticizable and Non-Plasticizable Ingredients

A broad and very simplistic classification would to classify the ingredients as

• Liquids

• Plasticizeable Components

• Non-Plasticizable Components

Liquids:

These are materials we typically think of as liquids. Examples can include water, glycerin, alcohol, oils, and other items we see as liquids in everyday experiences.

Plasticizable Components

There are materials we tend to think of as solids that are able to flow like a liquid under the extrusion conditions being used.

Plasticizable ingredients are ingredients that will transition from a solid to a flowable mass due to exposure to a combination of moisture, temperature and shear. The term “plasticizable” is being used to avoid the complexity of using the term “melt”, which may or may not strictly apply to the various components within the extrudate.

Non-Plasticizable Components

There are materials we typically think of as solids that will continue to behave as solids under extrusion conditions.

There are a number of components used in food extrusion that are not able to be plasticized under extrusion conditions typically used. These ingredients will maintain their solid nature and act as discrete particles within the extrudate . One example to demonstrate this would be to consider the non-plasticizable ingredients to be sand within the remaining extrudate: the non-plasticizable ingredients exist within the overall mass, but act as a discrete particle in the overall fluid. Examples include bran, calcium carbonate, and other minerals added to extruded products.

Application:

This approach gives enough information to begin predicting how an ingredient will function.

Using whole grain cereal extrusion as an example:

• Water will be added to the process. This is easily classified as a liquid component.

• Starch and protein will be able to be plasticized by water and the extusion conditions. These components can be classified as plasticizeable components.

• There is also bran in the incoming flour. This bran will absorb some water, but will not become a flowable material. The bran will come out of the extruder with the same size and shape as the incoming bran, other than any bran that is physically torn or cut into smaller pieces by the screws and barrel. The bran can be classified as a non-plasticizable component.

The ingredients that are not able to be plasticized can cause problems. If a small die opening is used and there are large bran particles present, the bran can plug the die opening. The bran particles are also areas that interrupt the plasticized material, potentially interfering with the ability of the main mass to foam or changing bubble structure in the foamed product by acting as a nucleation point (a point where a bubble exists or can form).

This approach will suggest that if you take a different grain flour with a similar amount of fiber, you are likely to get similar results. There are details in the composition of the ingredients that will be missed. Examples include amylose and amylopectin composition of the starch, molecular weights of the starch,and amount of starch branching.

More Detailed Classification based on Composition:

Proximate analysis is a way of looking at food materials to begin to understand what they are and how they may function. This approach was used in early nutritional studies to understand how various foods may behave in the digestive system. Proximate analysis would include: moisture, ash, protein, fat, and fiber.

Building on the proximate analysis, a useful breakdown of the ingredient composition of may include:

• Moisture (water)

• Ash (minerals)

  • Soluble minerals

  • Insoluble minerals

• Proteins

  • Protein classes

• Lipids (fats)

• Fiber

  • Insoluble fiber

  • Soluble fiber

• Other carbohydrates

  • Starch

  • Sugar

  • Non-starch storage carbohydrates

Looking at each component in each ingredient in a blend to be extruded allows some guidance on how the material will perform in an extruder.

Water will generally be the main plasticizer in a food extrusion system. Water is a material we are familiar with from everyday interactions: It will make grains or flours become more pliable as more water is added. Under the right conditions, water flashing to steam will cause foods to foam. This foam forming process is frequently called puffing in extrusion. The experience many people will have with water flashing to steam to foam a starchy base is popcorn.

Ash is the mineral content in the ingredient. Ash may remain as a solid and not interact water, but there are some mineral compounds that are soluble in water. Salt (sodium chloride) may be the most common material that would analyze as ash that will dissolve in water.

Protein is the nitrogenous material in the ingredient. Protein is able to be plasticized by water. Proteins like milk protein, soy protein, egg protein, and wheat protein are able to changed from a solid to a pliable material by water. Proteins can be grouped by compatibility with solvents, such as with the Osborne fractionation method. This information can be quite useful if phase separation of the components in the extrudate is important.

Fat is the lipid in the system. Fat is typically incompatible with water and will maintain a separate phase in a solution or in a plasticized mass. These components may exist either as a plasticizer within a material is it compatible with or as a separate phase within the extrudate. Oil and water will generally form distinctly separate phases in common experience. In a highly agitated system, such as a vinegar and oil salad dressing or homogenized milk, the oil will break into smaller droplets within the continuous water phase.

Fiber is the portion of the ingredient that is not easily digestible in the ingredient. Bran is a common example of this. Bran is an insoluble fiber and will generally not become pliable with water. Bran will absorb water, but will not become plasticized. One way to think of this is to consider cotton. A washcloth can absorb a fairly large amount of water, but the washcloth will not reach conditions where it can flow through a small opening and take on a new shape, it will always be able to unfold into the original washcloth shape. There are also soluble fibers, such as gums, that will plasticize, allowing the material to flow like a fluid.

Carbohydrates include fiber, but there are other carbohydrates. These include sugars, starch (a long-chain sugar polymer), and short-chain sugar polymers (such as maltodextrins). Sugars are unable to build a foamed structure in extrusion systems, but it will plasticize the starch in the system – it will make the starch more fluid. Maltodextrins should also plasticize starch.

Starch is a carbohydrate that is the typical structure builder in extruded food systems and is able to be used to create and maintain a foam structure. Starch has a wide range of molecular weights and has both a linear form (amylose) and a branched form (amylopectin). The molecular weight and amount of branching will drive extrusion characteristics of the starch. A higher molecular weight starch will have a higher viscosity (at a given moisture content) than a lower molecular weight starch. A more branched starch will have a higher viscosity (at a given moisture content) than a less branched starch.

Using this approach can give useful insights for how one formulation will perform when compared to another. It can also allow building analogous structures and properties when starting with different ingredients.

Interactions of Ingredients.

Components within ingredients may be able to be fully incorporated within another material or may result in a secondary phase of material within the extrudate. The easiest material to visualize this with is oil and water: the two materials are not compatible (miscible) with each other and separate phases can be seen in the overall mixture. These same types of interactions exist in other food materials, with starch and protein in some meat analogs being an example.

Miscibility of extrudate components:

Pasta can be used as a simple model system. The incoming materials are simply semolina (coarse flour from durum wheat) and water. The semolina at incoming moisture content is a solid in any meaningful sense of the word. When water is added to the semolina, the mixture has the appearance of a collection of solid particles. Once the hydrated semolina is in the screw of the pasta press, the transition from a solid to a plasticized mass becomes apparent. The pasta extrudate behaves as a dough, more specifically, it most likely could be described as a non-Newtonian fluid that has a yield stress, perhaps being described as a Bingham plastic.

Composition of the ingredients can be analyzed, but it is typically adequate to find typical compositions in resources. The USDA Food Composition database (https://ndb.nal.usda.gov/ndb/) is one potentially useful resource. For example, data for “Wheat flour, whole-grain” will give water (moisture content), protein content, lipid content, carbohydrates (including fiber and sugar as separate lines of data). Ash is not specifically called out. Not all materials in the USDA Food Composition Database have enough data to be useful, so it may be necessary to look at several materials to identify one with enough composition information to be useful.