Grape Pomace After the Harvest: A Seasonal Methane Problem With a Local Fix

Every vintage ends the same way. The press goes quiet, the tanks are full of new wine, and behind the cellar a heap of skins, seeds and stems is left steaming in the autumn air. For the global wine industry this is the unglamorous end of a glamorous product, and it is a textbook case of an organic waste stream that is concentrated in time, concentrated in place, and poorly served by the centralized infrastructure built for everything else. This article looks at grape pomace through the lens of ESG reporting, circular economy design and organic waste diversion, and explains why decentralized Black Soldier Fly (BSF) bioconversion deserves a place in the winery's waste plan.

How much pomace are we talking about?

The International Organisation of Vine and Wine (OIV) put its first estimate of 2025 world wine production at about 232 million hectolitres, roughly 3 percent above the historically low 2024 harvest but around 7 percent below the five-year average. Italy (47.3 million hl), France (35.9 million hl) and Spain (29.4 million hl) led the table. Even in a short year, that is an enormous volume of fruit moving through presses.

Pressing leaves a solid residue. A 2025 review published by the Royal Society of Chemistry notes that roughly 20 to 25 percent of the processed fruit mass remains as pomace, and that more than 50 million tonnes of grapes processed into wine in 2017 yielded over 12.5 million tonnes of it. Other estimates cited in the trade press, attributed to FAO statistics, put annual global pomace at around 14 million tonnes. The exact figure varies by year and method, but the order of magnitude is clear: well over ten million tonnes of organic waste a year, produced in a few intense weeks.

Why pomace is harder than it looks

Pomace is not a simple food scrap. It is a mixture of skins, seeds and stems that is high in fibre (grape seeds alone are reported at around 35 percent fibre, and dried pomace in a range of roughly 32 to 52 percent), and rich in polyphenols such as anthocyanins and stilbenes. Those compounds are valuable to extractors and a headache for biological treatment. The same review notes that pomace is typically sent to landfill or used as animal feed, and that pomace left to decompose anaerobically releases methane and carbon dioxide.

That final point matters for climate change accounting. Methane emissions from decomposing organic waste in landfill and in unmanaged heaps are among the most important levers in near-term global warming policy, and they sit squarely in the corporate carbon footprint of any wine group that reports on Scope 3 emissions. Leaving pomace in a pile is not a neutral act: it is an unmeasured emission source.

The usual alternatives each carry limits:

• Landfill wastes a resource and generates methane emissions, and in many jurisdictions organic waste diversion rules are tightening.

• Composting works at modest scale but is slow, land-hungry and seasonal, and the heap itself can be a methane and odour source if it goes anaerobic.

• Anaerobic digestion and biomethanisation require a minimum, steady feedstock flow. A winery's pomace arrives in a burst of a few weeks and then stops, which is a poor match for a digester that wants to run all year. Fibrous, lignified seeds and stems are also slow to break down.

• Centralized hauling moves a heavy, wet material over road distance, adding Scope 3 transport emissions to a waste that was already a carbon problem.

• Feed and extraction routes are real and worth using where they exist, but they depend on drying, transport and specialist buyers, and many smaller estates simply cannot access them.

What BSF bioconversion adds

Black Soldier Fly larvae (BSFL, Hermetia illucens) convert organic residues into larval biomass and a soil-ready residue known as frass fertilizer. The question for pomace is whether the larvae can do it on such a fibrous, polyphenol-rich substrate. The early evidence is encouraging, though honest about its limits.

A 2024 study in the journal Animal reared BSFL on grape pomace from two varieties, Becuet and Moscato, using six replicates of 1,000 larvae each and harvesting on day 27, when 30 percent of larvae had reached the prepupal stage. The larvae grew on both, at 4.4 mg per day on Becuet versus 3.2 mg per day on Moscato, and the substrate shaped the lipid profile of the larvae: Moscato-fed larvae carried more total lipid, while Becuet-fed larvae contained far more conjugated linoleic acid. The authors suggest the larvae could supply fat-rich meal and oil with interesting fatty acid profiles. In October 2025 Australian press reported on a trial by an insect farm in the Barossa wine region using grape marc as a feedstock, with early results described as suggesting insects could help close the loop on winery waste.

These are early signals rather than a finished commercial case, and a fair reading adds two caveats. Pomace on its own is fibrous and relatively low in the easily digestible nutrients that BSFL prefer, so practical systems will usually blend it with more digestible co-substrates and manage moisture. And growth rates on pomace appear modest compared with softer feedstocks. This is exactly why on-site trials matter, and why a flexible, modular system is more useful than a fixed-recipe megafacility.

Why decentralization fits the vineyard

The winery's problem is a logistics problem. Pomace is generated at the press, in volume, on a deadline. Decentralized BSF units can sit at or near the winery, so the waste is treated where it arises, within days, without a truck. That has several consequences for an ESG team:

1. Lower transport emissions. Treatment at source removes the haul leg from the Scope 3 inventory.

2. Methane avoidance. Moving pomace from heap to controlled bioconversion within days cuts the window in which it can ferment anaerobically.

3. Modular scale. Units can be sized to a single estate or shared across a cooperative, rather than waiting for a regional plant that may never be built.

4. Co-products that stay local. Frass fertilizer returns nutrients to vineyard soil, supporting soil health, regenerative agriculture and biodiversity, while larvae can feed into insect protein and sustainable animal feed value chains where regulation permits.

5. A story that reports well. For wine brands facing SBTi-aligned targets, buyer sustainability questionnaires and circular economy claims, a measured, auditable waste-to-value pathway is more credible than an assertion that pomace is "used as compost".

How this compares fairly

Wineries with an on-site distillery, a nearby extractor or a reliable animal-feed buyer already have good routes, and BSF bioconversion should be seen as complementary there, taking the residual fractions or the surge volumes that other routes cannot absorb. Where a large digester sits within easy reach and runs year-round, co-digestion can handle some fraction of the material. But for the very many estates that are small, remote, seasonal or all three, the choice has usually been the heap. For them, a compact bioconversion unit that treats waste in days and produces saleable outputs is a step change, not a marginal improvement.

What to do next

A sensible first step is a pilot: characterize the winery's pomace, stems and lees by moisture and fibre, trial blends with a digestible co-substrate, and measure waste reduction, larval yield and frass quality over a full vintage. A simple life cycle assessment (LCA) comparing the pilot to the current baseline, with methane emissions and transport included, gives the sustainability team numbers it can put into disclosure. If the pilot works, it can be scaled estate by estate.

The wine industry sells terroir, provenance and care for the land. Its waste plan should say the same thing. Treating the harvest's leftovers close to where they grow, and returning the nutrients to the soil, is a very natural fit for a decentralized, circular approach, and it is the kind of practical, local zero waste step that the sector can begin before the next vintage arrives.

B-BOX develops decentralized Black Soldier Fly bioconversion systems for organic waste. A Traditional Chinese version of this article is also available on this blog.

Sources: OIV, "OIV Releases 2025 World Wine Production First Estimates" (Nov 2025); RSC review on grape pomace valorisation (2025), pubs.rsc.org/en/content/articlepdf/2025/fb/d5fb00040h; Renna et al., "Animal" (2024), PubMed 39079311; Eco-Business, "The grapes of trash"; Barossa Leader, "Insects may help to close the loop after trial" (22 Oct 2025).