[Digestion] Orange Peels

[Hoysall Chanakya]

Dear List readers,

Orange peels have troubled biomethanation processes for long.  We have
tried orange peels (mix of orange and sweet lime, dominant is sweet lime)
both in a solid state fermenter and plug-flow type fermenters where these
are fed on an as-is-received basis with no pretreatment.

Fermenting them as a single feedstock brings about some form of inhibition
to the overall methanogenic process and the biogas produced even smells of
oranges.  The inhibition is not to methanogenesis alone but to the overall
process as intermediate VFA do not accumulate to toxic levels (c.250-1000
mg acetate equivalent /L).

When fed as a part of overall fruit wastes (<20% of total weight), these
two do not inhibit the overall process and gas production remains
sufficiently high.  However, when these fruit peels are dried and powdered
and tested for BMP at 0.25% TS, gas production does not seem to be
inhibited.  Our suspicion therefore is that the essential oils that
accompany the peel is the culprit and subjecting these to a steam
distillation step prior to digestion could recover the essential oils as a
saleable commodity, rendering the peels more digestible.

Some of my old friends use a fungal pectinase and a pectinase generating
fungus (CFTRI, Mysore) to liquefy these pectin bearing peels and then
subjected them to digestion in (the pulpy material remaining after fungal
/pectinase treatment) conventional floating drum Indian digesters with
much success.  I guess I have captured some of the South Indian experience
in this field.
best regards
chanakya





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