Why Hunter L and a/b Move in Opposite Directions in Tomato Paste: A Review and a New Colour Score.

A clear negative association between Hunter L (lightness) and the a/b redness ratio is frequently observed in tomato paste at production time. This technical review assembles mechanistic and process-level explanations for that pattern, focusing on hot-break vs cold-break, evaporation regimes (vacuum vs atmospheric), and measurement practice on the Hunter Lab scale. Optically, higher concentrations of red carotenoids (principally lycopene) increase a and thus a/b, while reducing reflectance and therefore L. Thermal severity during breaking and concentration further darkens the matrix (Maillard browning) and can degrade or isomerise lycopene, often depressing a/b when heat load is excessive. Building on factory data, a composite Process Score (PS) is proposed to complement L, a, b and a/b. PS integrates redness (a), yellowness (b) and darkness (L) via the CIE-x chromatic coordinate, providing a single KPI aligned with processing effects and pigment retention. In 2021–2025 operations (n = 7,564), a linear model accounts for 67.5% of PS variance. Iso-PS boundaries on the L–a/b plane separate acceptable deep-red lots (low L, high a/b) from genuine browning (concurrent decreases in L and a/b). The report concludes with practical guidance for colour-preserving processing (moderate hot-break or cold-break; vacuum evaporation) and rigorous HunterLab methodology.