Corn Starch Peanuts and Compostable Bags

In the past, polystyrene packing peanuts were popular as padding material for protecting delicate paraphernalia transported via parcel post. While cheap, convenient, and cost-effective for commercial companies, they were considered an anachronistic annoyance to anyone else.

Sustainable packing, in the form of cornstarch peanuts (other starch sources are available), started to appear about 25 years ago but even these are now losing ground to the ubiquitous paper/cardboard padding/packaging of Amazon fame.

Last week, I received a consignment of organic bread flour which did contain starch peanuts (see photo) though perhaps they would be better described as starch quavers after the well-known cheesy snack.

I also use compostable bags in my kitchen caddy where I collect food scraps before transferring them to the hot-composting bins. Typically, one compostable bag will be re-used 10-20 times (1-2 months) so I don't use that many. Cornstarch peanuts, when available, are added to the caddy in small amounts.

The simplest way to see if you have the cornstarch or polystyrene (PS) peanuts is to pour boiling water over them (see video). Cornstarch will dissolve whereas PS will just float.

So, how compostable are cornstarch peanuts and compostable bags?

The first thing to note is the difference between biodegradable and compostable bioplastic.

Biodegradable - simply means the material can be broken down by micro-organisms There is no time limit and the breakdown products may, or may not, contain toxic chemicals.

Compostable - specifically can be broken down by micro-organisms into water, carbon dioxide and biomass with NO toxic residue. The rate of decomposition has to be the same or faster than cellulose and the end product shall be indistinguishable from naturally-composted biomass.

Cornstarch Peanuts

Made from 100% cornstarch, these do disappear in the composting process either by biodegradation and/or dissolution. Being cellulose-based, they will take a while to break down; about 6 months in a  'cold' compost heap (25 - 40 oC) or 6 weeks in a 'hot' composter (50 - 60 oC). I did a quick literature search on Google Scholar to see whether the peanuts could be pretreated (e.g. acid hydrolysis) to speed up the composting process but it seems the natural enzymes in the compost heap do the job quicker and at lower temperatures. I am currently looking into the best way to add 'peanuts' to the composting process. Intuitively, if the compost heap is on the wet side (e.g. lots of lawn cuttings, kitchen scraps) then it may be best to add as whole peanuts as this will assist natural aeration of the heap. On the other hand, dissolving the peanuts in a minimum of hot water and mixing into the heap would maximise the cellulosic surface area and increase the composting rate.

Compostable Bags

These are definitely trickier and more difficult to compost.  Even after hot composting at 40 - 60 oC for 30 days and then further ambient composting for 6 months, there will usually still be remnants of the bags in the compost. I could pick out the bits and return them to the hot composter bin for another cycle but it is probably not worth it. Leaving the remnants exposed to sunshine and air on the soil will complete the biodegradation process.

The problem with compostable bags is that they are only 85-90% starch. Other stuff (co-polymers, plasticizers, etc) is added to give the bioplastic film the necessary physical and mechanical properties (e.g. water-solubility, tensile strength, permeability, etc.). So the added extras and the manufacturing process make biodegradation more difficult, especially in 'cold' composters. The video shows the effect of boiling water on a compostable BioBag...

...which illustrates the problem!

Nowadays, magazines are often delivered in compostable wrappers. When I next get one delivered, I will subject it to the hot water test. In the meantime, I shall continue to cut them up, add to the kitchen caddy and hope they eventually decompose.


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