What is Reducing Sugar in Potatoes? An Essential Guide

November 14, 2025 •

4 min read

When potatoes are stored at low temperatures, they undergo a natural process called cold-induced sweetening, which leads to a significant accumulation of reducing sugars. This physiological change has a major impact on the culinary and nutritional quality of potatoes, particularly when they are used for frying or baking.

Quick Summary

This article explains reducing sugars in potatoes, focusing on how cold storage converts starch into these simple sugars. It details the Maillard reaction, which is responsible for the browning and flavor changes during cooking, and discusses the formation of acrylamide. Different factors influencing sugar content and methods for managing them are also covered.

Key Points

Definition: Reducing sugars in potatoes are simple carbohydrates, primarily glucose and fructose, that can act as reducing agents in chemical reactions like the Maillard reaction.
Formation (Cold-Induced Sweetening): The most common cause is storing potatoes at low temperatures (below 10°C), which activates enzymes that break down starch into reducing sugars.
Browning Effect: High levels of reducing sugars react with amino acids during high-heat cooking (frying, baking) to cause rapid, and often excessive, browning through the Maillard reaction.
Acrylamide Risk: This same reaction involving reducing sugars and asparagine at high temperatures can form acrylamide, a potential human carcinogen.
Management: To control reducing sugar content, select cold-resistant varieties, store potatoes at warmer temperatures (above 10°C), and consider reconditioning or blanching.

The Science of Reducing Sugars in Potatoes

Potatoes, the starchy tubers consumed worldwide, undergo a complex biochemical transformation after harvest. A key component of this change is the accumulation of reducing sugars, primarily glucose and fructose. The potato's high starch content makes it a crucial subject in food science, particularly for manufacturers of fried products like chips and fries. Understanding the dynamics of reducing sugars is essential for controlling the color, flavor, and safety of these products.

The Process of Cold-Induced Sweetening

When potatoes are stored in cold temperatures, typically between 2-8°C (36-46°F), their natural metabolic activity shifts. This process, known as cold-induced sweetening, is a defense mechanism to prevent cellular damage from freezing.

Starch Breakdown: The primary mechanism involves the breakdown of complex starch molecules. Starch is a large, non-reducing carbohydrate, meaning it does not have the free aldehyde or ketone groups necessary for the Maillard reaction.
Enzymatic Conversion: Enzymes, particularly vacuolar invertase, become active in the cold and catalyze the hydrolysis of sucrose (a non-reducing disaccharide) into glucose and fructose, both of which are reducing sugars.
Accumulation and Effect: The result is a build-up of glucose and fructose within the potato tuber. The concentration of these simple sugars can rise dramatically, leading to a cascade of effects during high-temperature cooking.

The Impact of Reducing Sugars on Cooked Potatoes

When a potato with a high concentration of reducing sugars is exposed to high heat, a powerful chemical reaction called the Maillard reaction occurs. This reaction is responsible for many of the desirable and undesirable characteristics of fried potatoes.

Maillard Reaction: This non-enzymatic browning reaction takes place between the carbonyl group of reducing sugars and the amino groups of amino acids or proteins. It is responsible for the distinct golden-brown color and complex, savory flavors of many cooked foods. In potatoes, high levels of reducing sugars can cause the Maillard reaction to proceed too quickly and intensely, leading to an overly dark, burnt appearance and a bitter taste.
Acrylamide Formation: A significant concern arises from the Maillard reaction at high temperatures (above 120°C or 248°F). The reducing sugars react with the amino acid asparagine to form acrylamide, a potential carcinogen. Controlling the level of reducing sugars in raw potatoes is a key strategy for mitigating acrylamide formation in fried and baked products.

Factors Influencing Reducing Sugar Levels

The level of reducing sugars in potatoes is not static and is affected by several variables, including both pre- and post-harvest conditions.


Comparison of Pre-Harvest and Post-Harvest Factors Affecting Potato Sugar Content	Factor	Pre-Harvest Influence	Post-Harvest Influence
Genetics	Different potato varieties have varying genetic predispositions to accumulate sugars during cold storage.	Manufacturers select specific cultivars (e.g., cold-resistant vs. cold-sensitive) for different products.
Temperature	Low growing temperatures can stress the plant and increase sugar accumulation in the tubers.	The colder the storage temperature (e.g., 2-4°C), the more rapidly and intensely cold-induced sweetening occurs.
Maturity	Immature potatoes harvested from green vines tend to accumulate more reducing sugars in storage.	Properly matured tubers, after a curing period, are less prone to sugar accumulation.
Moisture	Water-stressed plants produce tubers that are more susceptible to sugar development.	High humidity during initial storage (curing) can help stabilize sugar levels.
Atmosphere	n/a	High carbon dioxide levels in storage can increase starch-to-sugar conversion.

Managing Reducing Sugar Levels for Optimal Results

For commercial processors and home cooks alike, managing the sugar content of potatoes is crucial for achieving desirable cooking outcomes. Manufacturers specifically select varieties and control storage conditions to minimize reducing sugars in potatoes intended for frying.

Variety Selection: Some potato cultivars are naturally more resistant to cold-induced sweetening, while others are more sensitive. Processors use near-infrared spectroscopy to rapidly assess reducing sugar content in batches of potatoes before processing.
Storage Temperature Management: The most critical post-harvest factor is temperature. Potatoes meant for frying should be stored at warmer temperatures, ideally above 10°C (50°F), to prevent or slow down the conversion of starch to sugar. For potatoes that have accumulated sugars in cold storage, a process called 'reconditioning'—storing them at a warmer temperature for a period—can sometimes reconvert sugars back to starch.
Blanching: Home cooks can use a technique called blanching, or soaking cut potatoes in hot water before frying. This process leaches out some of the excess sugars from the surface of the potato, reducing the potential for excessive browning.

Understanding the Chemical Changes in Stored Potatoes

At a cellular level, the potato is constantly in a state of flux, with starches and sugars being interconverted. Storage conditions tip the balance towards either starch synthesis or sugar accumulation. The conversion is regulated by enzymes like vacuolar invertase, which breaks down sucrose, and sucrose synthase, which can either synthesize or break down sucrose. At cold temperatures, the activity of invertase increases, driving the formation of glucose and fructose.

Conclusion

Reducing sugar in potatoes is not a single compound but rather a classification of simple carbohydrates—specifically glucose and fructose—that accumulate primarily during cold storage. This accumulation is the result of a natural defense mechanism that converts the potato's primary energy source, starch, into simple sugars. The presence of these reducing sugars is the central factor driving the Maillard reaction during cooking, which in turn determines the final product's color, flavor, and texture. For commercial producers and home cooks, careful management of potato variety, storage temperature, and pre-cooking preparation is essential for controlling the effects of reducing sugars and ensuring a high-quality culinary outcome. Understanding this basic food chemistry allows for more predictable and desirable results when preparing potato-based dishes.

[Authoritative Outbound Link]: Eliminating protein interference when quantifying potato reducing sugars - ScienceDirect.com

Frequently Asked Questions

Your potato chips likely turn dark because the potato tubers contained high levels of reducing sugars, such as glucose and fructose, which is a common result of storing potatoes in a refrigerator or other cold place. During high-temperature frying, these sugars rapidly react with amino acids through the Maillard reaction, causing excessive browning.

The presence of reducing sugars in potatoes is not inherently bad. However, when cooked at high temperatures, these sugars can participate in the formation of acrylamide, a compound considered a potential human carcinogen. For fried or baked products, it is generally safer to use potatoes with low reducing sugar content.

No, cold storage does not affect all potatoes equally. Genetic and environmental factors influence how a potato variety responds to cold temperatures. Some varieties are considered 'cold-resistant' and accumulate fewer reducing sugars, while 'cold-sensitive' varieties accumulate them more readily.

Yes, you can. A process called 'reconditioning' can be used to reduce sugar levels. This involves bringing the potatoes to a warmer temperature (around 10°C or 50°F) for several weeks, which can cause some of the accumulated sugars to convert back to starch.

Starch is a complex, non-reducing carbohydrate that acts as the potato's energy store. Reducing sugars, like glucose and fructose, are simple carbohydrates formed when starch is broken down. Unlike starch, reducing sugars possess a chemical structure that allows them to participate in the Maillard browning reaction.

For frying or baking, it is generally better to store potatoes in a cool, dark pantry rather than the refrigerator. Refrigeration temperatures (below 10°C/50°F) accelerate the accumulation of reducing sugars through cold-induced sweetening. For boiling or steaming, this sugar increase is less of an issue, but it can still affect flavor.

Blanching involves partially cooking potato slices or strips in hot water before frying them. This process washes away excess surface sugars, which helps to limit the Maillard reaction during the final high-temperature frying. The result is lighter, more evenly colored fries.