There are many excellent resources for incorporating fruits and vegetables into soap. WholesaleSuppliesPlus.com hosts Handmade 101, a searchable collection of recipes and instructional videos. Specific recipes and a wealth of tips and tricks can be found by searching these online. So with that said, what's left for me to talk about?

I will address those properties of fruits and vegetables that impact the chemistry of soaps that contain them. The first issue that needs attention is that of spoilage. Uncooked, unpreserved food might be expected to suffer the effects of time. Spoilage is caused primarily by microorganisms such as bacteria, yeasts and molds. Fruits and vegetables are most commonly preserved by cooking them to kill these microbes, and then sealing them to prevent re-contamination. While we could imagine treating soap in this fashion, sterile conditions could not be maintained once the soap was put into service.

Alternatively, vegetables can be pickled, meat salted, and fruit candied. These methods share the same preservational strategy: to create storage conditions that are hostile to the growth of microbes. Fortunately for us, the natural alkalinity and low moisture content of bar soap is sufficiently hostile to the growth of microbes that other preservatives are seldom needed.

The chief concern for incorporating fruits and vegetables in soap is to ensure that they are intimately mixed so that the alkalinity can perform its preservative function. If the food particles are large, the interior of these particles may harbor mold or bacteria. In practice, this means that fresh fruits and vegetables should be prepared as juice or puree. Since these contain significant amounts of water, it is appropriate to consider them as a replacement in whole or in part for the water portion of a soap formula.

Using juice could be as simple as directly replacing the water portion. If, for example, a soap formula calls for 12 oz. of sodium hydroxide and 29 oz. of water, you could substitute 29 oz. of juice for the water. There are three problems with this direct substitution. First, it results in loading the soap with a lot of juice, perhaps as much as 20% of the total soap weight, and depending on the concentration of the juice, this may overwhelm the ability of the soap to act as a preservative. Second, an acidic juice like citrus or tomato will neutralize part of your sodium hydroxide, resulting in an unintended lye discount. Finally, the lye solution will get hot as it is mixed, and this may cook or degrade the juice components. The situation is even worse with puree since it contains less water than juice does.

A more practical approach is to replace only part of the water portion. There is no single, correct proportion, but letting the juice weight be equal to half the weight of sodium hydroxide would be a good starting point. So using our previous example, we would need 12. oz of sodium hydroxide, 6 oz. of juice and 23. oz of water (29 - 6). Note that 6 oz. of juice plus 23 oz. of water adds up to our original 29 oz. water portion. Limiting the proportion of juice or puree mitigates two of our problems, but the lye solution will still get very hot when sodium hydroxide is added to our juice and water.

Makers of milk soaps often use frozen milk to prevent it from scorching, and I have seen the same technique used for making lye from juice. In our example, we would weigh out 6 oz of frozen juice or puree and 23 oz of ice cubes. When the sodium hydroxide dissolves, the ice will melt, but the resulting lye solution will not get too hot. While this is a reasonable approach for small test batches, it would become impractical for making soap on a large scale.

Instead, I would like to suggest an approach that will work for large or small batches of soap. Begin by making up a master batch of lye solution at exactly 50% concentration. The math here is ridiculously easy. Add 12 oz. of sodium hydroxide to 12 oz. of water. Or 1 kg of sodium hydroxide to 1 kg of water. Or 10 lb of sodium hydroxide to 10 lb of water. Make it up in whatever batch size is convenient, let it cool, and store it in a sealed, labeled container for the next time you want to make soap.

Returning to our previous example, let's imagine that your soap formula calls for 12 oz of sodium hydroxide and 29 oz. of water. Instead, we will use 24 oz. of our master-batched lye solution, 6 oz. of juice or puree, and 11 oz. of water. Since its concentration is exactly 50%, 24 oz. of master-batched lye contains exactly 12 oz. of sodium hydroxide. But because most of the heat was given off during the master batch step, the solution will get warm, but not hot. This procedure is practical for large and small batches alike.

To calculate what is needed for any size batch, you only need two numbers from your original recipe or lye calculator: the weight of sodium hydroxide (S) and the original weight of the water portion (W). To make lye from juice or puree, let the amount of master-batched lye be 2 times S, and the amount of juice or puree be half S. The extra water needed is then the original water portion, W, minus 1½ S (or 1.5 S). In our example, S = 12 and W = 29. The weight of master-batched lye is then 24, the weight of juice is 6, and the weight of water is 29 - 18 = 11.

If a test batch is too soft, it might be because acidic juice has neutralized some of your sodium hydroxide. In this case, you can increase S in the next batch. If it is too hard, it might be because your puree contains too little moisture. In this case, you can increase W. Within a few batches, you can optimize your formula for any fruits or vegetables you like. Now that's keepin' it fresh.