The Four Stages of Food Processing

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  • 0:00 Food Processing
  • 0:30 Ingestion and Digestion
  • 2:22 Absorption and Elimination
  • 3:57 Lesson Summary
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Lesson Transcript
Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

Expert Contributor
Maria Airth

Maria has a Doctorate of Education and over 20 years of experience teaching psychology and math related courses at the university level.

Did you know you are a walking, breathing, processing plant? In this lesson, we'll explore the four stages of food processing in your body: ingestion, digestion, absorption, and elimination.

Food Processing

Processed foods have a bad rap. Some experts say they are bad for us, and maybe so. However, I'm going to let you in on a little secret. And that is: all food is processed. All of it. It's processed by your body. Otherwise you wouldn't be able to derive any benefit from it nor be alive! There's more than one stage to the food processing performed by your body, and this lesson covers all of them.

Ingestion and Digestion

Let's walk into a restaurant together to learn about how our body, our processing plant, processes food.

I order a smoked salmon. Not having had any breakfast or lunch, I'm super hungry and so begin to ingest, or eat, the salmon very quickly. Ingestion is the first stage of food processing by the body; the act of taking a substance, such as a food or liquid, into the body by mouth.

Great, now I don't feel as hungry. But of course, I can't chow down on all of the salmon at once. I need to eat it one bite at a time.

But my body doesn't care. As soon as I put that first piece of food into my mouth, the process of digestion begins without waiting for me to finish the entire salmon.

Digestion is the second stage of food processing, the physical and biochemical breakdown of food by the body. By physical, I mean chewing and churning. As I chew the food, I physically or mechanically break it down into smaller pieces much like I would break food apart with a knife before putting the food into my mouth. My stomach and intestines also churn and squeeze this food with their muscles to further break it down physically.

This mechanical digestion breaks apart the food in order to increase its surface area for biochemical digestion. The salivary glands as well as the stomach and intestines secrete biochemicals that act like little scissors that break down the food at a molecular level into even smaller pieces of nutrients. This biochemical digestion allows for large particles like proteins, carbohydrates, and fats to be broken down into smaller constituents, small enough to be absorbed by the body.

Absorption and Elimination

At this point, after all the biochemical and mechanical digestion, the once solid piece of pink salmon on my plate is actually a gooey white slime in my digestive tract.

It is this gooey white mess of very small particles like simple sugars and amino acids that is then absorbed by the body. Absorption is the third stage of food processing; the taking up of end products of digestion from the lumen of the intestines and into the body. The lumen is the empty space within the intestines where food and feces can pass through. So when I say we absorb something, I mean we take it in from this cavity and into the body's cells, blood, or lymph.

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Additional Activities

Mini-Book Related to the Four Stages of Food Processing

Students will compose an instructional book detailing the four stages of food processing based on what they have learned from the lesson as well as new information gained through research.


  • Card stock paper (printer paper will work as well)
  • Art supplies (colored pencils, markers, etc)
  • Staple
  • Pen/pencil
  • Access to research material
    • Library
    • Internet


  • After reviewing the lesson on the four stages of food processing, students will compose a mini-book related to this topic.
  • Each student's book should:
    • Be at least six chapters (or pages depending on the skill level of the student).
      • An introduction to food processing, a chapter for each stage of food processing and a summary which ties all the stages together.
    • Include creative attributes like annotated diagrams.
    • Include all information on ingestion, digestion, absorption and elimination gleaned from the lesson.
    • Include at least one (or more) additional facts for each stage of food processing.
  • Students are encouraged to design an interesting, related cover page and title for their mini-books.
  • The information in the mini-book should be geared toward students of a similar age to the student completing this lesson.
    • The mini-book should assume the reader has no prior knowledge of the stages of food processing.

Guidance and Examples

  • You could create a logical story-line to help guide your readers through the four stages of food processing.
    • Have you ever wondered what happens to food when you eat it? Let's follow this apple through a hypothetical body as it is processed.
  • You could add a fun slant to your book using a fictional story to guide your readers.
    • Oh no, Dr. Shouldnthave has accidentally shrunk himself into the size of a grape! Here comes Ann, hungry for grapes. Poor Dr. Shouldnthave has been eaten. Lucky for him he has his protective suit on. Hey, let's see what happens to him.
  • You could write your story in first person journal form as if you are the food being processed.
    • Day 1 in the life of an apple: The teeth tore at my flesh as the human began to ingest me.

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