An earlier work showed that when the bagasse content (BC) of the solid medi
um was decreased within a wide range of values, penicillin production by so
lid-state fermentation was always increased. Respiration studies were perfo
rmed to understand how BC controls the secondary metabolism in this culture
system. CO2 production of solid cultures with different compositions was m
onitored. In cultures of series A, the initial moisture content was increas
ed and this variation was compensated by decreasing the nutrient and BC of
the medium. in series B the initial moisture content was increased while BC
was decreased and the nutrient content increased. In addition, penicillin
production and respiration was also studied in extreme media (dry and conce
ntrated and humid and diluted), with high and low BC. Criteria for the inte
rpretation of respiration kinetics of the idiophase were established for th
e first time in this work. For the cumulative form (total CO2/g dry matter
vs t) as well as for derivative (CO2/g dm/h vs t) respiration kinetics, the
CO2 production rate (Q(CO2)) was determined by calculating the slope of th
e cumulative curve. Results indicate that eo, of the tropho- and idiophases
was directly related to the BC of the solid medium (and inversely related
to penicillin yields). These conclusions were confirmed by analysis of the
derivative form, the results of which indicate that a lower but stable meta
bolic activity is essential for obtaining high penicillin yields in solid-s
tate fermentation (SSF). The results indicate that the derivative CO2 produ
ction kinetics proved to be a more precise and sensitive indicator of the c
ulture metabolic activity during idiophase than the cumulative respiration
kinetics.