Reducing sugars are responsible for desirable brown color development in many foods through Non-Enzymatic Browning (NEB) reactions. One of the most important NEB reactions is the Maillard reaction, which significantly influences the quality, color, flavor, and safety of thermally processed or stored foods, including citrus products.
The Maillard reaction occurs between a reducing sugar (aldose or ketose) and a free amino group from an amino acid or protein. The reaction progresses through a sequence of well-defined stages, which can be better understood through a schematic representation.

Initially, the reducing sugar reacts reversibly with an amino group to form a glycosylamine, which undergoes the Amadori rearrangement to produce a more stable Amadori compound. Under acidic conditions (pH ≤ 5), further dehydration reactions lead to the formation of intermediates such as 5-hydroxymethylfurfural (HMF). Under less acidic or prolonged heating conditions, these reactive intermediates polymerize to form dark-colored, high molecular weight compounds known as melanoidins.
While Maillard browning contributes desirable flavors and colors in frying, roasting, and baking, excessive NEB during processing or storage may result in undesirable color changes, off-flavors, nutrient loss, and formation of compounds such as HMF that may impact food safety.
Therefore, monitoring the extent of NEB (e.g., by estimating HMF formation) is important for evaluating product quality and safety during processing and storage