Kōji’d lemon skins

This is part of our series on upcycling fruit & veg by-products.

Table of Contents

i. Introduction

Did you know that almost all of the world’s citric acid doesn’t actually come from citrus fruits, but instead from microbes? Aspergillus spp. (particularly A. niger and A. luchuensis), Candida spp., Saccharomycopsis spp., Yarrowia lipolytica and others have all been used to produce citric acid at an industrial scale for decades.¹

In this versatile base recipe, we grow A. luchuensis on spent lemon skins (the lemons after being juiced) to return some of the citric acid. They can then be used as an ingredient in many other recipes, like marmalade and lemon curd, as well as perhaps more surprising applications like miso and tamari (recipes coming soon). Though lemons, and some other citrus fruits, are usually used just for their juice, with this method, the skins can be re-acidified and thus made delicious and useful for other applications.

The best way of initially preparing the lemon skins depends on how you will use the kōji’d lemon skins. For our marmalade and curd recipes, and younger, sweeter misos, it's best to cold-blanch the lemons first to draw out some of the bitter and astringent compounds and denature the enzymes that amplify them (see ‘Method’). This isn’t necessary for longer ferments like older misos and tamaris, where these bitter compounds will be broken down anyway. 

We refer to these lemon skins as ‘kōji’d’ rather than as true kōji. Whilst they do use a kōji mould to return acidity to the lemon skins, it isn’t used for the same umami/sweetness/enzymatic breakdown function that kōji is typically used for. We think that this is an interesting example of why naming novel fermentations with care matters and how the intended function of a particular microbe is one determinant to be considered in that naming. We will explore this idea further in a future essay. 

ii. Recipe

Ingredients

• Lemon skins (lemons after being juiced)

• Sugar, 1% by weight of lemon skins 

• Kōji spores (A. luchuensis, see note below)², 0.2% by weight of lemon skins

Method

1. The first step depends on how you plan to use your kōji’d lemon skins:

• If making lemon curd, marmalade, or a young, sweet miso, then cold-blanch the lemon skins (boil for 5 minutes, then shock in cold water to halt the cooking process)

• If making a longer-aged miso or tamari, instead steam the lemon skins for 5 minutes.

2. Immediately after blanching or steaming the lemon skins, add the sugar and toss to coat. 

3. Allow the lemons to cool to 35℃. 

4. Add kōji spores, making sure to distribute them evenly over the lemon skins, inside and out. Do not do this step if the lemons have not yet cooled, as it will kill the spores. 

5. Place the lemons cut side down on a tray with a damp towel wrapped around it. Incubate at 35℃ and 70% humidity for 24 hours, then at 28℃ and 70% humidity for a final 12-24 hours, depending on the kōji growth. Redampen the towel, if necessary. 

6. Remove from the towel. The lemon skins are now ready to use.

iii. Adaptations

A. luchuensis is primarily used for its citric acid-producing properties. It’s also possible to grow A. oryzae, the most common form of kōji, on lemon skins, but it won’t produce citric acid and will have a different flavour profile. Though we haven’t tried it yet, we wonder if this could work on other citrus fruit skins too. 

Don’t be tempted to skip the sugar, as it helps the kōji grow on the skins, which are otherwise a natural protective layer for the plant and inhibit the growth of most microbes. 

We use this base recipe for several different applications, which are linked above. We encourage you to try other applications too. 

Contributions & acknowledgements

Kim performed the original culinary R&D. Aly reproduced Kim’s recipe and documented the process with additional notes, which Eliot used to help write the article following further discussions with Kim. Josh contributed editorial feedback. Aly photographed the final product in our food lab.

This recipe has roots in Kim’s previous work as Head of R&D at the former Amass Restaurant in Copenhagen. Thank you Amass for facilitating such trailblazing work in culinary upcycling and holistic sustainability.

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Endnotes

[1] Ewelina Książek (2023), ‘Citric Acid: Properties, Microbial Production, and Applications in Industries’, Molecules.; Bikash Chandra Behera, Rashmiranjan Mishra, Sonali Mohapatra (2021) ‘Microbial citric acid: Production, properties, application, and future perspectives’, Food Frontiers.

[2] It’s a common misconception that ‘kōji’ is synonymous with Aspergillus oryzae—whilst this is the most well-known and widely used form of kōji, there are many others, including A. luchuensis that we use here.