2.4. Alfalfa juice fermentations
Fermentation experiments were typically conducted in serum
vials (appx volume 60 or 160 ml, depending on the experiment)
or Balch tubes (28 ml). Green or brown juice samples (9.8–
39.2 ml) were added to the tubes or vials using a volumetric pipette
modified by removing the tip to provide a wider bore for handling
liquid suspensions containing small plant particles. Culture
vessels were sealed with flanged butyl rubber stoppers secured
by aluminum crimp seals. Except where indicated, the only additions
to the vials were the juice and the bacterial inoculant (i.e.,
no additional nutrients or reducing agents were added to the vials).
The gas phase was exchanged with CO2 on a gassing manifold (15
evacuation and CO2 flushing cycles of 10 s ea, hereafter termed
‘‘CO2 gassed’’) to establish anaerobic conditions. Most experiments
were conducted without sterilization of vessels or juice, although
one experiment compared the fermentation with versus without
autoclaving (15 min, 121 C) of the sealed, juice-containing vessels
prior to inoculation (Table 1). Vials and tubes were incubated at
39 C without agitation for 12–72 h, depending on the experiment
(see Sections 3.2 and 3.3).
Two-stage fermentations were conducted on 60 mL of green
juice using different inocula, in triplicate. After 12–48 h incubation,
the vials were decapped, the pH measured and 50 ml of the resulting
brown juice was transferred to centrifuge tubes immersed in
ice. After removal of 1.5 ml for analysis of organic acids and sugars,
pH of the remaining suspension was adjusted with 10 N NaOH to
various pH values (5.0–6.8). Subsamples (5 ml) of these brown
juices were dispensed into quadruplicate Balch tubes, sealed with
butyl stoppers and aluminum crimp seals, and then gassed with
CO2. For each set of four tubes, two were inoculated with 0.10 ml
of M. elsdenii culture and the other two left as uninoculated controls
(but containing the microbial population of the original juice).
All tubes were incubated for 48 h.
For time course experiments, 2.8 ml samples were withdrawn
at each time interval (including zero-time) using a hypodermic syringe
and immediately added to ice-cold 20 mL glass vials while
measuring pH. Samples were divided between paired ice-cold
microcentrifuge tubes and after centrifugation (12,000g, 10 min,
4 C) supernatants were used for duplicate analysis of organic acids
and sugars.