Corn syrup is made by breaking down the starch in corn kernels into smaller sugar molecules using a combination of water, heat, and enzymes. The process starts with separating starch from the rest of the kernel, then converting that starch into a thick, sweet liquid. Depending on how far the conversion goes, manufacturers produce syrups with different levels of sweetness, thickness, and sugar composition.
Separating Starch From the Kernel
A corn kernel contains starch, protein, oil-rich germ, and a fibrous outer hull. To make corn syrup, you only need the starch, so the first stage is pulling the kernel apart and isolating that one component. This happens through a process called wet milling.
Whole corn kernels are soaked in warm water (around 50 to 55°C) containing small amounts of sulfur dioxide and lactic acid for 24 to 48 hours. This long soak softens the kernels, loosens the bonds between starch and protein, and prevents unwanted bacterial growth. After steeping, the softened corn goes through a coarse grind. The oil-containing germ floats and is skimmed off using hydrocyclones, which are essentially high-speed continuous centrifuges. The fibrous hull is filtered out on fine screens. What remains is a slurry of starch and protein. Because corn starch is slightly denser than corn protein, a second pass through hydrocyclones separates the two. The result is a nearly pure starch slurry, ready for conversion into syrup.
Turning Starch Into Sugar
Starch is a long chain of glucose molecules linked together. To create syrup, those chains need to be chopped into shorter pieces or individual glucose units. This happens in two enzymatic stages: liquefaction and saccharification.
In liquefaction, the starch slurry is heated to around 75°C and mixed with a heat-stable enzyme called alpha-amylase. The temperature is then raised to about 95°C for roughly 30 minutes. Alpha-amylase cuts the long starch chains into shorter fragments, turning the thick paste into a thinner liquid. The mixture is then cooled to around 63°C for the next step.
In saccharification, a second enzyme called glucoamylase goes to work. While alpha-amylase makes random cuts along the starch chain, glucoamylase works from the ends, snipping off one glucose molecule at a time. Other enzymes like pullulanase can be added to break apart branching points in the starch that the other enzymes can’t easily reach. This stage typically runs for several hours at temperatures between 55 and 65°C. The longer the process runs, the more glucose is freed, and the sweeter and thinner the syrup becomes.
What Dextrose Equivalent Means
Not all corn syrup is the same. Manufacturers control how far they break down the starch to produce syrups with different properties. The industry measures this using a scale called dextrose equivalent, or DE. Pure starch that hasn’t been broken down at all has a DE of zero. Pure glucose (also called dextrose) has a DE of 100. The higher the DE, the sweeter and less viscous the syrup.
Two grades dominate commercial use. A 42 DE syrup is moderately converted: about 19% of its dry weight is glucose, with the rest being maltose, larger sugar fragments, and residual dextrins (31%). It’s thick, mildly sweet, and prized for its body and moisture-retaining properties in baked goods. A 65 DE syrup is more thoroughly broken down, with about 55% glucose on a dry basis and only 6% dextrins. It’s noticeably sweeter and thinner. The larger sugar molecules that remain in lower-DE syrups are less soluble and less sweet, which is why a 42 DE syrup tastes much less sugary than a 65 DE version despite both being “corn syrup.”
How High-Fructose Corn Syrup Differs
Regular corn syrup is mostly glucose. High-fructose corn syrup (HFCS) takes the process one step further by converting some of that glucose into fructose, which tastes significantly sweeter. This extra step uses an enzyme called glucose isomerase, which rearranges the glucose molecule into fructose without adding or removing anything.
The conversion is reversible and reaches an equilibrium, so it doesn’t turn all the glucose into fructose. The initial product is HFCS 42, containing about 42% fructose. To make HFCS 55, the version used in most soft drinks, manufacturers pass the syrup through a separation column that concentrates the fructose fraction, then blend it back to hit the 55% fructose target. That small difference in fructose content makes HFCS 55 close to the sweetness of table sugar, which is why it became the dominant sweetener in beverages starting in the 1980s.
Refining and Purification
The raw syrup coming out of the enzymatic stages isn’t ready for food use. It contains residual proteins, minerals, and color compounds that affect taste, appearance, and shelf life. Purification typically involves two main steps.
First, the syrup is passed through activated carbon beds. These beds have a massive internal surface area, with microscopic pores that trap color-producing molecules and other impurities as the syrup flows through. Industrial systems pump syrup through these carbon beds continuously at high flow rates. Second, the syrup goes through ion exchange columns, which use charged resins to pull out dissolved minerals and remaining organic compounds. The result is a clear, clean-tasting syrup. After purification, the syrup is concentrated by evaporating excess water under vacuum until it reaches the desired thickness, typically around 77 to 84% solids by weight.
Why Corn Syrup Behaves Differently Than Sugar
Corn syrup shows up in so many recipes and processed foods not just because it’s sweet, but because of how it behaves physically. The mixture of different-sized sugar molecules in corn syrup interferes with crystallization. When you’re making candy, caramel, or ice cream, table sugar tends to form grainy crystals. Corn syrup prevents this, keeping textures smooth.
Lower-DE syrups are especially useful for their viscosity and ability to hold moisture, which keeps baked goods soft over time. Higher-DE syrups and HFCS are valued more for sweetness and their ability to blend easily into beverages. The specific sugar profile, whether the syrup contains mostly large starch fragments or mostly free glucose, determines which application it’s best suited for. This is why food manufacturers don’t just use one type of corn syrup. They select a specific DE grade the way a painter selects a specific brush.