Module 3 · Processing

Processing is everything that happens to food between the farm and your plate. This lesson follows processing aids that never reach the label; the compounds that form when food is browned, grilled, smoked or cured (acrylamide, AGEs, HCAs, PAHs and nitrosamines); and methods that change the food itself: fermenting, pasteurizing, hydrogenating, refining and nixtamalizing. Each molecule is shown in ball-and-stick form with what it does, the risks, and where it is found. The figure is on the left and the lesson script on the right; the quiz is at the end.

Previous lesson: Module 3 · Antinutrients

Figures1 2 3 4 5 6 7 8 9 10

Figure 1

The Processing Family Tree

What happens to food between farm and plate, in three groups.

What the tree shows

Three kinds of changeAlmost every food is processed somehow: washed, cut, heated, fermented, pressed or milled. This lesson follows three kinds of change: helpers used along the way that never reach the label, new compounds that form when food is heated hard, and methods that change the food itself.
Good, bad and bothProcessing is not good or bad as a whole. Pasteurizing and fermenting make food safer; nixtamalizing makes corn more nutritious; charring and partial hydrogenation make compounds worth limiting; refining trades nutrients for shelf life and texture.
Mostly not on the labelProcessing aids and the compounds that form during cooking are not listed as ingredients. A few methods show up in the wording, such as “partially hydrogenated”, “enriched”, “pasteurized”, “cured” or “nixtamalized corn”.

Hexane: a straight chain of six carbons with fourteen hydrogens; every hydrogen shown

Processing aid	What it does	Ends up in the food?
Hexane	dissolves oil out of seeds	evaporated off; tiny traces
Enzymes such as pectinase	clear fruit juice; break down starch in baking and brewing	inactivated by heat or filtered out
Fining agents (gelatin, bentonite clay)	pull haze out of wine, beer and juice	settle out and are removed
Anti-foaming agents	stop foam in fryers and tanks	trace amounts
Carryover additives	a preservative in an ingredient (say, the dried fruit in a cereal) that does nothing in the final food	small amounts

Asparagine: an amino acid with an amide group on its side chain; every hydrogen shown

Acrylamide: a carbon-carbon double bond next to an amide group; every hydrogen shown

PhIP: two fused rings containing nitrogen, with an NH2 group, a methyl group and a benzene ring attached; every hydrogen shown

Benzo[a]pyrene: five benzene rings fused into a flat sheet, with twelve hydrogens around the edge; every hydrogen shown

Nitrite ion: a blue nitrogen between two red oxygens, carrying a negative charge

Dimethylamine: a nitrogen holding two methyl groups and one hydrogen; every hydrogen shown

Lactic acid: a three-carbon acid with an OH on the middle carbon; every hydrogen shown

Ethanol: two carbons and an OH group; every hydrogen shown

Method	Temperature and time	Used for
Low-temperature, long time (vat)	63°C (145°F) for 30 minutes	small dairies
High-temperature, short time (HTST)	72°C (161°F) for 15 seconds	most milk and juice
Ultra-high temperature (UHT)	about 135–150°C for a few seconds	shelf-stable milk and cartons; sealed sterile

cis-2-Butene: four carbons with a double bond in the middle and both end carbons on the same side; every hydrogen shown

	Whole grain	Refined (white)	Enriched refined
Parts kept	bran, germ and endosperm	endosperm only	endosperm only
Fiber	high	low	low (not added back)
B vitamins and iron	natural amounts	mostly lost	thiamin, riboflavin, niacin, folic acid and iron added back
Magnesium, zinc, vitamin E, B6	natural amounts	mostly lost	not added back
Shelf life	shorter (oily germ)	long	long

Hydroxide ion, OH-: an oxygen with one hydrogen

Figure 9

Where processed foods act in the body

Tap a number on the cell to see what that part does; the list below it gives every other place in the body.

What cooking and processing make or change in food reaches the gut, liver and blood vessels.

Inside a cell

A typical animal cell, not to scale. Real cells hold thousands of ribosomes and hundreds of mitochondria.

Cooking and processing change molecules that later reach your cells.

AGEs (browning products) can bind the RAGE receptor and trigger inflammation signals.
Some cooking byproducts, such as heterocyclic amines from charred meat, become DNA-damaging once liver enzymes activate them.
Liver enzymes activate or break down cooking byproducts such as acrylamide and heterocyclic amines.
Oxidized fats from reused frying oil add to oxidant load that the cell's antioxidant enzymes must clear.
Refined starches are digested fast, so more glucose arrives at once and is stored as glycogen or fat.
Nutrients lost in processing, such as B vitamins from refined grains, are the ones these energy machines need; fortification adds some back.

Everywhere else in the body

Each item, the places outside the cell where it is found or used, and what it does there.

Cooking in general

Stomach and small intestine. Cooked starch and protein are digested faster; heat also kills germs.

Acrylamide

Small intestine. Absorbs it.
Liver. Turns some into glycidamide, which can damage DNA, and clears the rest with glutathione.
Blood. Binds hemoglobin, which is how exposure is measured.
Nerves. High exposure at work damages nerves.
Placenta and breast milk. Pass small amounts.

AGEs (browning products)

Small intestine. Absorbs a share of those in food.
Blood vessels. Link collagen fibers so vessels stiffen, and trigger inflammation through the RAGE receptor.
Kidneys. Clear AGEs; they build up in kidney disease.
Eye lens and skin. Build up with age in long-lived proteins.
Blood. The body makes its own when sugar is high; HbA1c, the diabetes test, measures an early step of the same reaction.

HCAs and PAHs (charring and smoke)

Liver. Activates them, and some products can damage DNA.
Colon, prostate and breast. Where HCA damage has been studied in animals.
Lungs. PAHs from smoke reach them directly.

Nitrite and nitrosamines

Stomach. Nitrite and amines can form nitrosamines in its acid.
Liver. Activates nitrosamines.
Large intestine. Eating a lot of processed meat is linked to colon cancer.

Trans fats

Blood. Raise LDL and lower HDL cholesterol.
Artery walls and heart. Raise the risk of plaque and heart disease.

Fermentation and pasteurization

Large intestine. Live cultures from yogurt and other fermented foods meet the gut bacteria.
Small intestine. Yogurt bacteria break down part of the lactose.
Whole body. Pasteurization kills germs such as Listeria and Salmonella.

Refining, fortification and nixtamalization

Skin, gut and brain. Nixtamalized corn frees niacin, preventing pellagra.
Bones. Lime-treated corn adds calcium.
Bone marrow and embryo. Fortified flour adds iron for red blood cells and folic acid for the neural tube.

Figure 10

Putting It All Together

The whole lesson in one table, plus a few extras.

Processing at a glance

Process	What it does	What forms or changes	Main foods	Worth doing
Processing aids	help make the food, then are removed	tiny traces; not labeled	vegetable oils, juice, wine	choose expeller-pressed oil if you prefer
Browning (dry heat)	Maillard reaction of sugar with amino acids	acrylamide, AGEs, flavor and color	fries, chips, toast, coffee, seared meat	cook to golden, not dark brown
Grilling, smoking	burns fat and drips; smoke coats food	HCAs, PAHs, smoky flavor	charred meat and fish, smoked foods	avoid flare-ups; trim char
Curing	nitrite preserves and colors meat	nitrosamines when fried hot	bacon, ham, hot dogs, salami	eat processed meat sparingly
Fermenting	microbes turn sugar into acid, alcohol, gas	lactic acid, ethanol, CO₂, vinegar	yogurt, sauerkraut, bread, wine	a useful way to preserve and digest
Pasteurizing	brief heat kills germs	few changes to nutrients	milk, juice, eggs	choose pasteurized dairy
Partial hydrogenation	adds hydrogen to oil	trans fat	old shortening and margarine	avoid; banned in the US
Refining	removes bran, germ or impurities	loses fiber, vitamins, minerals	white flour, white rice, refined oils	choose whole grains often
Nixtamalizing	alkaline lime cook of corn	frees niacin; adds calcium	tortillas, hominy, masa	a traditional gain

Worth knowing

Ultra-processed food

A popular classification called NOVA groups foods by how much they are processed. Diets high in ultra-processed foods, made mostly from refined ingredients and additives, are linked in many studies to obesity and chronic disease, though researchers still debate which features are responsible.

Frying oil

Oil reused many times for deep-frying breaks down into polar compounds and aldehydes; restaurants are told to change it when it darkens, smokes or foams.

Microwaving and pressure-cooking

Fast, moist methods often keep more water-soluble vitamins than long boiling, and form few of the heat compounds in this lesson.

General education, not medical advice. Facts follow mainstream sources (US FDA and USDA, EU regulations, IARC, US National Cancer Institute, NIH Office of Dietary Supplements, standard food-science texts).

Nutrition reads this on your call. Tap Next as we go, or tell Nutrition the line number.

1.1
Processing
This lesson is about processing: everything that happens to food between the farm and your plate. Almost every food is processed somehow, and this tree sorts what we'll cover into three groups.
Title, then the root box
1.2
Unlisted Helpers
On the left are processing aids, like the solvent hexane, which help make a food and then are removed, so they never reach the label.
The left branch
1.3
Made by Heat
In the middle are compounds that form when food is heated hard. Browning makes acrylamide and AGEs, grilling and smoking make HCAs and PAHs, and curing plus frying makes nitrosamines.
The middle branch
1.4
Changing the Food
On the right are methods that change the food itself: fermenting, pasteurizing, hydrogenating, refining and nixtamalizing. Each box links to its figure.
The right branch
1.5
Good, Bad and Both
Processing isn't good or bad as a whole. Pasteurizing and fermenting make food safer, nixtamalizing makes corn more nutritious, charring and partial hydrogenation make things worth limiting, and refining trades nutrients for shelf life.
The three boxes

2.1
Hexane
Here's hexane: six carbons in a row with nothing but hydrogens around them. With no oxygen, it mixes with oil, not water.
Marker 1
2.2
Pulling Out Oil
That's why it's used to make most soybean, canola, corn and sunflower oil. Crushed seeds are washed with hexane, which dissolves the oil out, and then the hexane is boiled off and recycled.
2.3
How Much Is Left
Hexane isn't a nutrient. Breathing high levels at work can damage nerves, but the traces left in oil are tiny; the EU caps them at one milligram per kilogram. Expeller-pressed, cold-pressed and USDA organic oils are made without it.
2.4
Other Aids
Other processing aids include enzymes that clear fruit juice, fining agents like gelatin or clay that pull haze out of wine and beer, anti-foaming agents, and carryover additives that came in with an ingredient.
The table, row by row
2.5
Why Not Listed
US and EU rules let manufacturers leave these off the label when they're removed or do nothing in the final food. The exception is allergens: if an aid comes from fish, milk or another major allergen, it must still be declared.
The two boxes

3.1
Asparagine
Browning starts with amino acids. Asparagine is plentiful in potatoes and grains. Heated above about 120 degrees Celsius with a sugar like glucose, it loses its amino-acid end.
Markers 1 and 2
3.2
Acrylamide
What's left is acrylamide: a reactive double bond next to the same amide group. It's part of the Maillard reaction, the browning that colors crusts, chips and coffee, and it was found in food in 2002.
Markers 3 and 4
3.3
The Risk
The body turns acrylamide into a compound that can damage DNA. It causes cancer in animals at high doses, and IARC calls it probably carcinogenic to humans. Studies in people haven't shown a clear link, but the advice is to fry, bake and toast to golden, not dark brown.
In your body
3.4
Found In
It's in fries, chips, crackers, cookies, toast, cereals and coffee. Boiled and steamed foods have essentially none.
3.5
AGEs
Sugars can also stick to proteins. Lysine's side chain ends in a free NH2 group, and after several steps it can carry this small acid group. That's carboxymethyl-lysine, the best-known AGE, or advanced glycation end product.
Markers 5 and 6
3.6
AGEs in Food
AGEs build up in the body with age and in diabetes, and food adds some, mostly from meat grilled, roasted or fried. How much food AGEs matter is still debated. Stewing, poaching and steaming make far fewer.

4.1
HCAs
This is PhIP, a heterocyclic amine, or HCA: rings containing nitrogen with an amino group. HCAs form when creatine, amino acids and sugar in muscle meat react at very high heat.
Markers 1 and 2
4.2
HCA Risk
The liver activates HCAs into forms that damage DNA, and IARC lists PhIP as possibly carcinogenic. The National Cancer Institute suggests avoiding direct flame, turning meat often, and cutting off charred parts.
In your body
4.3
PAHs
Benzo a pyrene is a PAH: five benzene rings fused into one flat sheet. PAHs form when fat drips onto flames and the smoke coats the food.
Marker 3
4.4
PAH Risk
IARC classifies benzo a pyrene as carcinogenic to humans, and the EU limits it in smoked foods. Trim fat, avoid flare-ups, and scrape off char. Cigarette smoke is a far bigger source than food.
In your body
4.5
Smoke Flavor
Guaiacol is a benzene ring with an OH and a methoxy group side by side. It forms when wood burns and gives smoked foods their aroma, and phenols like it slow spoilage.
Markers 4 and 5
4.6
Smoked Foods
Guaiacol itself isn't a concern; the concern with smoking is the PAHs that come with the smoke. Liquid smoke is filtered, so most PAHs are removed.

5.1
Nitrite
Nitrite is the curing salt from the Additives lesson. It keeps cured meat pink, adds flavor and stops the bacterium that causes botulism.
Marker 1
5.2
Making a Nitrosamine
Amines like dimethylamine come from protein breakdown. In frying heat or stomach acid, nitrite puts an N=O group on the amine's nitrogen, making a nitrosamine. NDMA is the most common one.
Markers 2 and 3
5.3
Nitrosamine Risk
Nitrosamines cause cancer in many animal species, and IARC calls NDMA probably carcinogenic. They're thought to be one reason processed meat is classed as carcinogenic to humans, mainly for colorectal cancer. Crisp-fried bacon forms the most.
In your body
5.4
Vitamin C Blocks It
Vitamin C, as ascorbate or erythorbate, reacts with nitrite first and blocks the reaction. US rules for bacon require it alongside the nitrite.
First box
5.5
Uncured and Vegetables
Meat labeled uncured is usually cured with celery powder, which bacteria turn into nitrite, so the chemistry is the same. Vegetables are different: they supply most dietary nitrate, along with vitamin C, and are linked to lower cancer risk.
The last two boxes

6.1
Lactic Acid
Fermentation uses microbes to change food. Lactic acid bacteria turn sugar into lactic acid, which sours milk into yogurt and preserves cabbage as sauerkraut and kimchi.
Marker 1
6.2
Alcohol, Gas and Vinegar
Yeast turns sugar into ethanol and carbon dioxide; in bread the gas raises the dough and the alcohol bakes off. Then Acetobacter bacteria can turn ethanol into acetic acid: vinegar.
Markers 2 to 4
6.3
Why Ferment
The acid keeps harmful bacteria from growing, and fermentation can predigest food: yogurt has less lactose, and sourdough less phytate. The things to watch are alcohol and the salt in some ferments.
In your body
6.4
Pasteurization
Pasteurization uses just enough heat to kill germs. Most milk is heated to 72 degrees Celsius for 15 seconds; ultra-high temperature milk gets about 135 to 150 degrees for a few seconds and keeps without refrigeration until opened.
The table, row by row
6.5
Why Pasteurize
It kills Listeria, Salmonella, E. coli and Campylobacter while barely changing the nutrients. US health agencies warn that raw milk is riskier, especially for children, pregnant women and older adults.
The two boxes

7.1
Adding Hydrogen
Hydrogenation bubbles hydrogen gas through hot oil with a nickel catalyst. A four-carbon piece stands in for a fatty acid here, with a natural cis double bond.
The first cell, then marker 1
7.2
Full or Partial
Add both hydrogens and the double bond becomes single: a straight, solid, saturated chain. Stop partway and some double bonds flip to trans instead. That's trans fat, from Figure 7 of the Fats lesson.
Markers 2 and 3
7.3
Trans Fat Rules
Industrial trans fat raises LDL and lowers HDL. In 2015 the FDA ruled partially hydrogenated oils no longer generally recognized as safe, and they've been removed from US foods. Fully hydrogenated oil is saturated fat with almost no trans.
7.4
Refining Grain
Refining grain removes the bran and germ, leaving the starchy endosperm. That loses fiber, B vitamins, iron, magnesium, zinc and vitamin E. Enriched flour gets some B vitamins, folic acid and iron back, but not the fiber or the other minerals.
The table, column by column
7.5
Refining Oil
Most oils are also refined, bleached and deodorized. That makes them clear, mild and stable for frying, but strips much of their vitamin E and polyphenols. Extra-virgin olive oil and cold-pressed oils skip these steps.

8.1
Lime
Nixtamalization starts with slaked lime, calcium hydroxide. In water it breaks into calcium ions and hydroxide, making a strongly alkaline soak.
The first two cells
8.2
Freeing Niacin
In raw corn, niacin, vitamin B3, is tied to other molecules and can't be absorbed. The alkaline soak cuts it free.
Markers 2 and 3
8.3
Other Gains
It also loosens the hull, adds calcium, improves the balance of available amino acids, lowers some mold toxins, and lets the dough hold together for tortillas.
In your body
8.4
Pellagra
Where corn became a staple without it, in parts of Europe and the US South, pellagra was common: niacin deficiency, with dermatitis, diarrhea and dementia.
8.5
Corn to Tortilla
The steps are simple: cook dried corn with lime, steep it overnight, rinse, and grind into masa. Corn tortillas, tamales and hominy are nixtamalized; cornmeal, grits and polenta usually aren't.
The steps, left to right

9.1
Where Processed Foods Act
Here's where compounds from cooking and processing act: first inside a single cell, then everywhere else in the body. Some are made by heat, some by curing, and some by refining.
The cell drawing and the list of places
9.2
Inside a Cell
Processing changes what arrives at your cells. High-heat browning makes compounds like acrylamide and AGEs, and charring meat makes heterocyclic amines. The liver's smooth ER processes them, sometimes turning them into forms that can damage DNA. Refining strips some vitamins the mitochondria rely on, which is why many flours are fortified.
In food and in the body
Acrylamide. Forms when starchy foods brown at high heat (fries, toast, chips).
AGEs. Form with browning, grilling and roasting; the body also makes them when blood sugar is high.
Fortification. Refined flour is often enriched with iron, B vitamins and folic acid.
The cell drawing and its numbered parts
9.3
Digestion
Cooking softens starch and protein so the small intestine digests them faster, and nixtamalization frees niacin in corn and adds calcium from the lime. In the stomach, nitrite from cured meat can form nitrosamines.
In food and in the body
Nitrosamines. Formed from nitrite and amines in cured meats, especially when fried, and in the stomach. Some cause cancer in animals.
In the list: stomach, small intestine and bones
9.4
Liver and Blood Vessels
The liver activates and clears acrylamide, HCAs and PAHs, and some of the products can damage DNA, which is why charring and very browned starchy food are best kept moderate. Trans fat from partial hydrogenation raises LDL cholesterol and harms blood vessels. The kidneys clear what's left in urine.
In food and in the body
Acrylamide. Formed when starchy foods such as fries, chips and toast brown at high heat. The liver turns it into glycidamide, which can damage DNA.
HCAs and PAHs. HCAs form in meat cooked very hot, PAHs in smoke and charring. The liver activates and clears both.
In the list: liver, blood vessels and kidneys
9.5
Fermentation and the Gut
Live cultures from yogurt and other fermented foods meet the gut bacteria in the large intestine. Eating a lot of processed meat is linked to colon cancer.
In food and in the body
Live cultures. Bacteria such as Lactobacillus in yogurt and kefir. Many die in the stomach, and those that survive pass through the large intestine; most don’t stay there for long.
Processed meat. Bacon, ham and sausages, preserved with salt and nitrite. Eating a lot of it is linked to colon cancer.
In the list: the large intestine
9.6
Simplified
This is simplified. The amounts in normal diets are small, and the links to disease come from large studies of people, so the list shows where these compounds act, not a measure of harm.
NoteThe note under the list

10.1
One Table
Here's the whole lesson in one table: each process, what it does, what forms or changes, the main foods, and what's worth doing.
The table, row by row
10.2
Worth Knowing
A few extras. Diets high in ultra-processed foods are linked to obesity and chronic disease, though which features matter is still debated. Frying oil reused many times breaks down. And fast, moist methods like microwaving and pressure-cooking keep more vitamins and form few of these heat compounds.
The three notes

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Quiz

Tap a question to see the answer

1. What is a processing aid, and why is it usually not on the label?

A substance used during manufacturing that is removed or has no function in the finished food; US and EU rules exempt it, unless it comes from a major allergen (Figure 2).

2. What is hexane used for?

Dissolving oil out of crushed seeds such as soybeans and canola; it is then evaporated off (Figure 2).

3. Which two things react to form acrylamide?

The amino acid asparagine and a sugar, heated above about 120°C (Figure 3).

4. How can you form less acrylamide when making fries or toast?

Cook to golden yellow rather than dark brown (Figure 3).

5. What is an AGE?

A protein or fat permanently altered by a sugar, such as carboxymethyl-lysine; they form in browned food and in the body (Figure 3).

6. Where do HCAs and PAHs come from?

HCAs form in muscle meat cooked very hot; PAHs form when fat drips on flames and the smoke coats the food (Figure 4).

7. What gives smoked foods their smoky aroma?

Phenols such as guaiacol, made when wood lignin burns (Figure 4).

8. What does nitrite react with to form a nitrosamine?

An amine from protein breakdown, especially in frying heat or stomach acid (Figure 5).

9. Why is ascorbate or erythorbate added to cured bacon?

It reacts with nitrite first and blocks nitrosamine formation (Figure 5).

10. What do lactic acid bacteria and yeast each make from sugar?

Lactic acid bacteria make lactic acid; yeast makes ethanol and carbon dioxide (Figure 6).

11. What does partial hydrogenation create that full hydrogenation does not?

Trans fat, when some double bonds flip from cis to trans instead of being filled (Figure 7).

12. Why did pellagra appear where corn was eaten without nixtamalization?

Corn’s niacin is bound and cannot be absorbed unless the corn is cooked with lime (Figure 8).

Next lesson: Module 3 · Bioactives →