3.3. Fermentation of brown juice to

3.3. Fermentation of brown juice to VFA
Because of the unpredictability of the single stage fermentations
of green juice, additional experiments were conducted in
which the fermentation was carried out in two stages, the first a
green juice fermentation to lactic acid, followed by a fermentation
of this lactate-containing brown juice to VFA. For this second stage,
adjustment of pH to 6.8 rendered the brown juice fermentable by
M. elsdenii if the lactate concentration did not exceed 0.25 M
(22.5 g L1
; Table 3). However, some brown juices contained up
to 47 g lactate L1
, which exceeded the amount expected from
the sum of the initial sugar and starch in the green juice; this suggests
additional contributions from the fermentation of other juice
components (e.g., pectin or fiber). At these higher lactate concentrations,
dilution with water to achieve 98%) consumption of lactate and partial
(25–30%) consumption of the acetic acid, along with net
production of both even-chain and odd-chain VFA at an approximate
yield of 0.90–0.94 g VFA (g lactate consumed)1
. Calculations
of fermentation stoichiometry (Table 4) indicate that, on a molar
basis, 96–105 percent of the carbon in the lactate consumed was
recovered in VFA products. Additional experiments revealed that
pH adjustment to 5.2–6.5 prior to M. elsdenii inoculation was suffi-
cient to obtain complete fermentation of 0.2 M (18 g L1
) lactate
within 24 h, and similar proportions of individual VFA products
as were obtained at pH 6.8 (Table 5). Following adjustment of
brown juice to pH 6.5 and inoculation with M. elsdenii, similar fermentation
product concentrations were obtained with either CO2
or N2 as the headspace gas in the fermentation vessels (data not
shown).Time course incubations (Fig. 3) indicate that the M. elsdenii fermentation
of brown juice was marked by an extensive lag period
(nearly 12 h), after which nearly all of the lactate was removed
within the succeeding 12 h. Continued incubation maximized
formation of propionate, butyrate and valerate, whose production
was maintained by an abundance of the precursor acetate. Conversion
of lactic acid to VFA resulted in a gradual increase in pH of the
brown juice by 0.1–0.2 units as the fermentation proceeded, due to
the conversion of the lactate (pKa = 3.8) to the less acidic VFA
(pKa = 4.8)