Cleaning

First, let’s consider how food and the environment in which it’s handled are cleaned. Virtually all food is subjected to cleaning before it reaches us, even the most seemingly minimally processed kinds. Remember, food is grown outside! Therefore, we want our food and anything touching our food to be clean. What is the definition of clean and what needs to be cleaned?

To help you appreciate the level of cleaning that is necessary, consider the hand print in Figure 9.9. This is the impression of a child’s hand onto a petri dish. After the petri dish was incubated, all these microorganisms grew! This means that even though your hand may look clean, it’s actually covered with microorganisms, some of which may be capable of causing disease!

By clean we mean on an invisible level, on a microorganism level. Clean from dirt but also from contaminating microorganisms.

Now, you may be saying to yourself, but that was a child’s hand and everyone knows kids aren’t as careful about washing their hands. Look at Figure 9.10. What is this a picture of? Run your tongue over your teeth. Do you feel that fuzzy layer that’s been building up since the last time you brushed your teeth? Those are microorganisms growing in your mouth! And this is a picture of those microorganisms from someone’s mouth!

Companies have to follow Good Manufacturing Practices (GMPs) for cleaning machines and other equipment. They might clean them by hand or they might use another machine to clean. Either way, the important factors are the same as the ones you employ when washing your hands:

• Time
• Activity
• Concentration
• Temperature
• Waste disposal

Food processors go one step beyond cleaning. After a food facility is cleaned such that it looks clean to the naked eye, then it is sanitized to kill microorganisms that cannot be seen. Sort of like when you use bleach to sanitize your kitchen at home.

Separating

Let’s move on to another unit operation. Separation is a step done in the processing of many foods. It can be done based on a variety of factors related to the particular food, including

• Composition
• Size
• Shape
• Density

Some easy to understand examples are removing the husk from grain in order to make flour and separating the cream from milk.

Look at this kernel of wheat, similar to all grains. The kernels have outer coatings that are not digestible (fiber, called husk and bran), a seed (called germ, that would go on to grow a new plant), and the interior endosperm (the white part, full of starch).

During separation in flour milling, rollers are used to squeeze, but not crush, the grains. The friction applied helps to lose and remove the husk.

Video 9.1 shows how the same principles are applied to peel garlic cloves.

Cream is separated from milk based on density. If, after milking the cow, the milk is left to sit, it will naturally separate. The fat in milk (called cream) is less dense than the water and protein in milk, which sink to the bottom.

Size Reduction

Size reduction is another common unit operation. You do this at home when you’re cooking, think of cutting up vegetables into bite size pieces or grinding coffee beans. Size reduction is the principle behind milk homogenization, which prevents the separation that we just talked about.

Sometimes something as simple as processing via size reduction is absolutely vital for the final product. Take, for example, tea. Green tea is essentially the whole tea plant’s leaves simply dried, while black tea is derived from those same leaves but cut up into small pieces and then dried.

There is also size reduction in the flour manufacture process. The endosperm, or starch, has to be crushed into a fine powder in order to make flour.

In the case of milk, size reduction of the fat bubbles in milk will make them stable and no longer float to the top. In this image, milk is pumped at high speed from the left. The large globules of fat are all rushing to try and pass through the small space, so they are forced to form smaller globules which are then stably suspended in the watery part of the milk (See Figure 9.12).

Size reduction is also very common, practically universal, when it comes to meat. Think about how you buy your meat, very few of us buy the entire animal not to mention cook the entire carcass all at once. Rather, we tend to buy different cuts, individual portions, or multiple servings, such as chops or roasts. The pinkish and red that you see in Figure 9.13 is meat or muscle tissue. The white is fat and connective tissue, like ligaments and tendons.

Mixing

When it comes to our concern over food processing, things get sticky when many processes are combined and the final product seems to have little in common with the starting material. Think of a hot dog. Where did an all-beef hot dog come from in relation to the original cow? There is no one muscle or part of the cow that makes up a hot dog. This image of a hot dog versus the cow brings us to a discussion of pink slime. Have you heard of it? It’s the result of a combination of unit operations on meat products.

You see all sorts of commentary on what is in our food circulating the internet. Look at this description of pink slime from one internet site. No wonder there’s concern over food processing!

Technically speaking pink slime is actually lean, finely textured beef (and beef only, not chicken or pork). What this means is that it’s meat, muscle, that could not be easily cut away from the carcass and so the meat was removed using heat and a machine to do the separation. This results is small bits of meat being recovered. The process of mechanical separation is much more efficient than a human trying to do it. The result, however, are very small pieces of meat that are so small they form a paste, hence the name pink slime. That is why this is only used in ground meat type products (think frozen hamburger patties).

Because pink slime, though perhaps gross, is nothing more than beef and food labeling focuses on food composition, lean, finely textured beef does not have to be declared on food labels. However, given the recent controversy, companies that use lean, finely textured beef, like Cargill, have said they’ll start clarifying it on food labels. Please note that ground beef you get at your local grocery store is usually ground in store from whole muscle cuts and therefore does not contain lean, finely textured beef.

In contrast, mechanically-separated chicken, fish, and turkey have always had to be labeled, and still are, despite confusing messages put out by celebrity chefs such as Jamie Oliver. However you may feel about pink slime and related products, you can avoid consuming it if you wish but it’s a great example of how we try to minimize the amount of food wasted.

Heat Transfer

The transfer of heat is another very commonly used unit operation. We do this at home when cooking all the time, transfer heat from our stoves into our food. In the case of pasteurization, this is a relatively mild heat treatment that comes in the form of various time and temperature combinations. As you know, in any heat exchange, heat flows from the warmer to the colder. In the case of many pasteurizers (See Figure 9.14), such as for milk, the warm pasteurized product is used to warm the incoming cold product (raw milk). This also results in a cooling of the already pasteurized product/milk. This heat recovery is a critical to energy conservation. The heat kills of many microorganisms and thus makes the food safer and last longer. Many foods are required to be pasteurized:

• Milk
• Juice
• Beverages

Canning is a stronger heat treatment, going to a higher temperature for a longer time and including a vacuum seal. The result of this process is a shelf stable food in which all the microorganisms have been killed. These products must be packaged in special containers in order to maintain their sterility, such as cans or pouches.

Concentration and Drying

Removing water from food (drying) is another common process. Water is often removed because it’s heavy, and therefore is expensive to transport. Water removal also increases foods’ shelf-life. Water can be removed in a variety of ways:

• Freeze-drying, for example, works by freezing the food and then reducing the surrounding pressure to make the water frozen inside the food sublimate into gas and drift away.
• Spray drying works by spraying liquid foods into a fine mist, where the water can quickly evaporate away from the tiny droplets leaving behind the solid remnant.

Instant coffee, powdered milk, potato flakes, frozen juice, and canned soup are examples of foods that have had water removed.

Forming

Forming is the second to last unit operation that we’ll discuss. It includes various subcategories, such as compaction, molding, and extrusion. In extrusion, a food dough is forced through a mold to produce often complex shapes. Think of that Play-Doh spaghetti maker you had as a kid! This process may be done with or without a simultaneous heating step. Many foods are extruded, such as the pasta in Figure 9.16.

What other foods can you think of that are formed by extrusion? Click through the image carousel to see examples of common extruded foods.

Packaging

Lastly, packaging. Packaging has a variety of goals:

• Maintain food quality/safety and minimize waste.
• Communicate information to the food consumer.
• Maximize food convenience.

Packaging materials are many and include

• Paper for milk cartons, crackers, cereals
• Metal for canned foods
• Glass for acidic foods like jelly, pickles, juices
• Plastics for milk jugs, cellophane wrapped sandwiches, salads

Paper was once a common method of packaging but it is limited in that it cannot get wet. So paper packages are usually coated in wax. Metal (mostly cans) is another popular choice, especially for shelf-stable foods. See Figure 9.17 to examine a cross section showing how metal cans are sealed. Metal is also relatively easily recycled.

Glass is a wonderful, inert material, so it doesn’t react with acidic foods. But, because it is prone to breaking and is therefore dangerous, it has largely been supplanted by plastic. Plastic is the dominant food packaging material due to its desirable physical properties (light, flexible shape, can be colored, low cost.) Unfortunately, plastics are not easy to recycle as compared to the glass and metal.