The idea of using frass as a partial substrate component is not new. The nutrient profile is attractive on paper: nitrogen-rich, with a microbial community that can support larval development if the conditions are right. We had tried it twice before and abandoned it both times, once because of mould and once because of a contamination flag from our QC process. The third attempt, which we ran from January through May this year, finally worked. This post covers what changed and what we are still watching.
Why the first two attempts failed
The first failure was a moisture problem we have discussed elsewhere in these notes. Frass incorporated into fresh bran at more than about 15% by weight pushed the substrate toward the upper end of our moisture envelope, and during a warm spell it pushed past it. Mould appeared within four days across three trays. We pulled everything and treated it as a write-off.
The second failure was more instructive. Our QC process flagged elevated chitinase activity in finished product from a batch that had used frass substrate. The frass itself was fine. The issue was that we had not adequately controlled the age of the frass before incorporating it. Frass that has sat for more than 72 hours at ambient temperature develops a microbial profile that is different from fresh frass, and some of that microbial activity carries through to the larvae’s gut environment. The finished product result was not a safety failure but it was outside our specification, and we removed it from sale.
What the third attempt did differently
Three changes made the difference:
- Frass incorporation capped at 10% by weight of the substrate blend, down from the 20% we had tried previously
- Frass used within 36 hours of collection, stored at 4C after collection to arrest further microbial development
- A dedicated QC screen for the substrate blend before trays are loaded, using a rapid moisture meter and a visual mould check
Fresh frass is a different input from stored frass. Treat them as separate materials and label them accordingly.
The batches run under this protocol showed no mould incidents and passed all product QC screens. Feed conversion ratio improved by approximately 6% compared to our bran-only baseline, which is consistent with the nitrogen supplementation hypothesis.
The contamination limit we are still watching
Our concern now is heavy metal carryover. Frass concentrates trace elements from the feed substrate, and cadmium in particular accumulates in larval tissue when the diet has elevated levels. Our substrate ingredients test within normal ranges, but we are running a six-month monitoring series on frass samples from each batch to establish our own baseline before we scale the protocol. If that baseline looks clean, we will move frass incorporation into standard operating procedure. If it does not, we will know before the problem reaches product.