Method 84 Extraction method for obtaining silage juice for analysis for VFAs


• Extractor - the extractor is shown in Fig. 8.7, and the means of attaching the small torque wrench in Fig. 8.8. Ideally it should be constructed from stainless steel, but we only used a stainless steel threaded rod or studding (10 x 58 in, 11 t.p.i., or 254 x 15.9 mm), and the rest was machined from brass. The silage juice should not remain in contact with the brass for long, however, because the brass will be attacked by the acids. The dimensions


Butyl rubber sealing ring

Fig. 8.7. Silage juice extractor.

Fig. 8.8. Attachment of torque wrench to silage juice extractor by means of a socket.

of the cylindrical body are 2 in ID x 21/ in O.D. x 9 in long (approx. 50 mm ID x 63.5 OD x 230 mm long). The piston or plunger contains a groove, which is fitted with a butyl rubber (oil-seal) ring. The top and bottom of the cylinder are threaded for easy removal and cleaning. The bottom measures 23A in (70 mm) diameter and is 1 in (25 mm) in depth. It is perforated with 75 holes of 3 mm diameter, and 20 holes of 2.5 mm diameter. The top is the same diameter as the bottom, but is 11A in (32 mm) in depth to allow sufficient thickness for the portion threaded for the studding.

• Torque wrench and socket - a small size torque wrench capable of being set at 8.0 Nm, and a socket having two slots milled opposite to each other to fit over the 3/e in (9.5 mm) diameter T-bar at the end of the threaded rod.

Procedure. Load about 40 ml of fresh (or thawed) silage into the extractor and screw on the top, with the plunger piston located at the top end. Attach the torque wrench, fitted with the socket, over the T-bar. Slowly increase the torque to about 5.4 Nm over 0.5-1 min and collect the juice in a beaker. About 70-80% of the silage moisture content will be obtained. Then increase the torque to 8.0 Nm, allow the few extra millilitres of juice to drain into the beaker, then re-apply the torque to obtain the remaining drops. Next centrifuge the juice in polypropylene tubes at 20,000 g for 10 min at a temperature of approx 4°C to prevent loss of volatiles. Analyse immediately, but if this is not possible, store the extracts in capped polythene sample tubes in a deep freeze. On thawing, the solutions may appear cloudy, and a fine brown residue will inevitably be deposited. Therefore, filter a sufficient quantity of the solutions using a 25 mm x 0.45 pm polyether sulphone (PES) membrane syringe filter (e.g. Syrtec 0.45 pm PES sterile, Cat. No. 8670180142, Techmate: for the analysis, which should be carried out immediately. Note: PES is not the same as polysulphone (PSul). A porosity as fine as 0.45 pm is essential to remove particles that could seriously reduce the life of an HPLC column or guard cartridge.

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