Oxidation converts rather than adds
Once the leaf is rolled and its cells ruptured, enzymes act on its polyphenols in the presence of air, converting them into larger compounds that carry colour and weight. The liquor darkens from green through copper to red, and the flavour follows: fresh and grassy give way to malt, dried fruit, cocoa and the brisk astringency that stands up to milk.
Assamica gives more of it than sinensis
Large-leaf assamica is higher in the compounds that drive body and briskness, which is why Assam, Kenyan and Ceylon teas read as malty and full while sinensis-based blacks — Darjeeling, Keemun, Jiu Qu Hong Mei — read as lighter and more aromatic. The variety decides the ceiling; the processing decides how much of it is reached.
Manufacture changes it as much as origin
CTC cuts the leaf into granules, maximising surface area so oxidation runs fast and even and extraction is near-complete in two minutes. The result is strong, coloury and blunt. Orthodox rolling keeps the leaf intact, so extraction is gradual and a garden’s character survives. A CTC Assam and an orthodox Assam from the same district taste substantially different, and the machinery explains more of that than the terroir does.
Where black tea stops being malty
Bud-heavy Chinese blacks go elsewhere entirely. Jin Jun Mei, Yunnan Golden Bud and Bailin Gongfu are honeyed and cocoa-sweet with almost no grip, because bud material carries a different balance and is processed gently. Darjeeling first flush is barely oxidised at all and brews closer to a light oolong. 'Black tea is malty' describes the assamica mainstream and not the category.