Investigations into the chemistry of the flue-curing of tobacco

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Abstract
During the yellowing stage there is a steady fall both in the rate of O2 consumption by the tissue and in the R.Q., corresponding to a gradual exhaustion of respiratory substrates and an increasing call on the protein matter for metabolic purposes. The cause of "red leaf" is now clear. If the leaf is still moist when the critical temp. is reached at which the semi-permeability of the protoplasmic membranes breaks down, polyphenols diffuse into the cytoplasm where they are oxidized to form brownish red pigments. To avoid this the leaf must be sufficiently dried out by the time the critical temp. is reached to prevent diffusion of the vacuole contents into the cytoplasm. In the fixing stage chemical changes probably do not take place to any great extent owing to the dryness of the tissue. Enzymes, however, are still active and if the tissue is moistened, as when suspended in water in Warburg vessels, both protein and polyphenol oxidation can take place. Only after the temp. has been maintained at 160[degree] F for some time, during which period the mid-rib is dried out, is it safe to allow the tissue to pick up moisture, for by then the enzymes are sufficiently destroyed and further marked chemical changes in the leaf are unlikely.

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