The other weekend, my husband came in from the backyard with a basket full of juicy Meyer lemons off our tree. I’d been meaning to make this video for ages, but seeing those beautiful, ripe lemons right there made it the perfect time to share this fresh lemon soap recipe with you all.

Please refer to the full recipe tutorial on Youtube for the full demonstration of this recipe 👇

Why use citric acid in handmade soap recipes?

The citric acid in soap acts as a chelating agent. This means it binds with the metal ions present in hard water, which in turn reduces soap scum and helps slow down the oxidation of soap-making oils. This ultimately extends the shelf life of your soap.

I find this really useful because I’m often using up old bottles of oil that I’ve been given or that I’ve had for a while. Ideally, we’d all use super fresh oil in our soap recipes, but I know that’s not the reality for many of us, and I’d rather make the most of what I have than waste precious resources.

The fresh lemon juice is frozen to make the lye solution.

How to calculate the lemon juice/citric acid amount for your own soap recipes

Before we get to the recipe shared in the video, it’s really important to know that when you add citric acid (as powder or as a percentage of fresh juice) to a soap recipe, the lye amount needs to be adjusted. The lye will react with the citric acid first, before it starts turning your oils into soap. So, you need to add extra lye to your recipe to account for this.

For those of you who want to dive deeper into soap chemistry and adjust this recipe for your own oils or a different batch size, it's absolutely crucial to do the calculations yourself. It might seem daunting, but it’s just a few simple steps.

Step 1: Determine your total oil weight

The amount of citric acid you add is based on your total oil weight. Let's say, for example, your total oil weight is 1260g (like in the recipe below).

Step 2: Choose your desired citric acid percentage

Decide if you want to use 1% or 2% of your total oils as citric acid (only a small percentage is required!). For this example, let's use 1%.

Step 3: Calculate the grams of citric acid you need

Multiply your total oil weight by your desired percentage (as a decimal):

1260g (total oils) x 0.01 (1%) = 12.6g
You need a total of 12.6g of citric acid.

Step 4: Calculate the grams of lemon juice needed

Now, use the average citric acid content of your lemons to figure out the juice amount:

For Meyer lemons (average 4% citric acid):
12.6g (needed) ÷ 0.04 (4%) = 315g
You would need 315g of Meyer lemon juice.
For regular lemons (around 5% citric acid):
12.6g (needed) ÷ 0.05 (5%) = 252g (I round down to 250g)
You would need 250g of regular lemon juice.
The lemon juice is then combined with water to bring it up to your chosen liquid amount for the recipe. I recommend 2:1 liquid to lye solution for this recipe.

Step 5: Calculate the Extra Lye Needed

For every 1g of citric acid, you need 0.624g of extra sodium hydroxide (NaOH).

12.6g (citric acid) x 0.624g = 7.86g

You need to add an extra 7.86g of sodium hydroxide to your recipe.

Step 6: Final Lye and Water Adjustment

Total Lye: Add the extra lye (7.86g) to the amount of lye your soap calculator recommends for your oils. This gives you your new, final lye amount.
Total Water: The amount of lemon juice you calculated (315ml or 210ml in this example) now becomes part of your total water for the recipe. Simply subtract this amount from the total water your recipe calls for.

A quick reminder: While these calculations are essential, the exact amount of citric acid in fresh fruit can vary. By incorporating a decent superfat (I recommend at least 5-8%), you build in a good safety margin.

Using powdered citric acid is often a safer and more precise option for beginners, but if you are keen to try this, and know how to evaluate the safety of your recipe after making it, then go for it!

Fresh Lemon Juice Soap Recipe - Elly’s video recipe

Recipe specifications:

2:1 liquid to lye ratio
5% superfat
1260g oil batch (to make 17-18 flower soaps)
1% citric acid of total base soap oils, assuming my Meyer lemon juice has around 4% citric acid (which may not be totally accurate, this is guesswork based on average lemon juice CA values!)
Essential oils were calculated based on dermal limits of each and keeping total EO under 3% of the total base soap oils
All measurements are by weight in grams - please use a digital scale.

Ingredients:

441g Palm Oil (or substitute with tallow or lard)
315g Coconut Oil (regular, not fractionated)
441g Canola Oil
63g Castor Oil
315g fresh Meyer lemon juice (strained and frozen)
40g filtered or distilled water (to add to the juice)
185g Sodium Hydroxide (Lye)
Optional: 9g Lemon Myrtle essential oil, 10g Litsea Cubeba essential oil, 12g Cedar Atlas essential oil
Optional: 1 heaped teaspoon of White Kaolin Clay

Note: If using regular lemon juice (5% citric acid), use 250g fresh juice and 105g of distilled water instead of the amounts listed above. The lye amount (185g) remains the same for this recipe as it has been adjusted to account for the total citric acid of the juice.

Method:

Before you start, please review my soap safety essentials video if you haven’t already seen it.
Prepare the Lemon Juice: Wash your lemons well with a good scrub under fresh water. Squeeze out the juice and strain out any pulp. Measure 315g of Meyer lemon juice and pour it into a jug. Add 40g of water to bring your total liquid to 355g. Pour this into an ice cube tray and freeze it for a few hours until solid. The lemon juice is frozen to prevent the lye solution from overheating and scorching, to keep the colour nice and to keep you safe. Lemon juice is full of natural sugars!
Prepare the Oils: While the juice freezes, measure out your palm oil and coconut oil into one jug and gently melt them in the microwave on a low power setting. Stop frequently to stir as you go. In a separate container, measure out your canola and castor oils.
Safety First - Prepare the Lye: Put on your safety goggles and gloves. In a well-ventilated area, carefully weigh out 185g of sodium hydroxide into a container. Put the lid on immediately and set it aside securely.
Prepare Optional Additives: If you're using essential oils, measure them into a small dish and set aside. I also used a heaped teaspoon of white kaolin clay, which I blended into the oils after they were combined.
Make the Lye Solution: Once your lemon juice is frozen into cubes, put them into a heatproof jug. Making sure your safety gear is still on, carefully pour the sodium hydroxide granules over the ice and begin mixing. The ice will melt and react with the lye, eventually forming a golden orange-coloured lemon juice lye solution.
Combine Oils and Blend: Once your lye solution has cooled to around 40°C/102°F and your oils are a similar temperature, carefully pour the lye solution into the oils. Use a strainer to filter out any small beads of undissolved lye.
Bring to Trace: Because of the palm and coconut oil and the sugars in the lemon juice, this batter can thicken very quickly. Blend for a couple of short bursts with your stick blender to begin emulsification, then hand stir the rest of the way. Keep a close eye on it.
Add Essential Oils: As soon as you can feel the mixture thickening, quickly add your essential oils and mix them through thoroughly.
Pour the Soap: Carefully pour the soap batter into your prepared moulds. I poured the batter into a larger jug first to make it easier to pour into my small flower moulds.
Insulate and Wait: Spray the tops with alcohol to reduce soda ash, then put them somewhere warm to go through saponification. I used my oven (turned off, but still warm from turning on for 1 minute then off again) for a while, and then moved them to a cooler wrapped in a blanket overnight.
Curing and storage: Like most cold process soap recipes, this soap will need to dry and cure in a cool, dry place for 4-6 weeks before using. It’s also a great idea to test your soap with pH test strips or the Zap Test before first use to ensure complete saponification and that it’s safe to use.

A note on the saponification process: I was quite dismayed when I first took these out a day later and they hadn't fully saponified. They were still caustic! But, a quick test showed that the recipe was fine, it just needed more time (I hot processed one of the bars in a pyrex bowl in my microwave - shall I do a video on that?).

It was a cold winter's day and the individual moulds didn't insulate the soap as well as a log mould would have. So, if yours don't saponify right away, don't panic—just give them a couple more days! I ended up putting my moulds back into the oven over the weekend and warmed it up for a minute once or twice, to encourage saponification.

I was so happy when they came out perfectly smooth and fully saponified three days later. This soap smells great, looks wonderful, and has a lovely lather.

Unmoulding fresh lemon juice cold processed soap — Unmoulding the soap after a few days, it took a while to fully saponify!

Have you ever made soap with fresh citrus juice?

I'd love to hear your thoughts in the comments below!

Thanks for reading and watching my videos, I hope you find them useful :)

If you would like to support my work making these free resources, you can ‘buy me a coffee’ here (big thanks to all of you who do!🌼).

See you next time,
Elly

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