Washing vegetables: what cuts residues and germs, and what to avoid

Washing vegetables does two jobs at once: it reduces chemical residues on the surface, and it removes soil, dust and germs. The good news is that what works is simple and cheap. The thing to know is that no method removes everything. This article gives an order that works for most produce, the amount of baking soda that research supports, and what not to do.

This is general food-preparation information, not medical advice.

How much can washing do?

Some pesticides sit on the surface and wash off well. Others are systemic: they move into the plant’s tissues, and no amount of washing gets them all out.

Washing reduces residues; it does not bring them to zero. The most effective step is a source you trust, or growing your own, and then washing properly every time.

An order that works for most produce

1. Wash your hands first, with soap and water, for about 20 seconds.
2. Remove damaged leaves and the outer layers. The outermost leaves of cabbage and Chinese cabbage get the most spray and dust. Peel off two or three.
3. Separate leaves and open up stems. Crops with layered stems, like Chinese kale, mustard greens and spring onion, hold a lot of soil at the base.
4. Soak in clean water or a baking soda solution for about 10-15 minutes.
5. Rinse under running water, leaf by leaf or a small bunch at a time, rubbing gently with your hands for about 1-2 minutes. This is the most important step. Don’t skip it.
6. Drain and dry in a colander, a salad spinner or a clean cloth.

Baking soda: how much?

A study published in 2017 tested apples and found that soaking them in a 1 per cent solution of baking soda (sodium bicarbonate) for about 12-15 minutes removed surface residues of the two pesticides tested better than tap water. The same study also found some of the residue had already moved into the fruit, where washing could not reach it. It covered apples and two chemicals, so treat it as a guide, not a guarantee for every vegetable.

A 1 per cent solution is 10 grams of baking soda per litre of water. One teaspoon of baking soda weighs about 4-5 grams.

Example: one basin of soaking water

Say you use 4 litres of water.

1. Baking soda needed = 10 g × 4 = 40 g
2. One teaspoon is about 5 g, so 40 ÷ 5 = 8 teaspoons
3. 3 teaspoons = 1 tablespoon, so that is about 2 tablespoons plus 2 teaspoons

You don’t need to be exact to the gram. Stir until it dissolves before adding the produce, always rinse under running water after soaking, and pour the soaking water away rather than reusing it.

Different produce, slightly different handling

Produce	Do this
Leafy greens: Chinese kale, water spinach, lettuce	separate leaves, soak, rinse leaf by leaf or in small bunches
Heads: cabbage, Chinese cabbage	peel 2-3 outer leaves, halve, then follow the steps
Fruiting: tomato, chilli, cucumber	rub by hand or with a soft brush under running water, before cutting
Roots: carrot, ginger, galangal	brush off soil under running water; peel if it suits the dish
Leafy herbs: holy basil, Thai basil, coriander	shorter soak, gentle rinse, shake dry; they bruise easily

Wash fruiting vegetables before you cut or peel them. A knife carries whatever is on the skin into the flesh.

What not to do

• Dish soap or hand soap. Many vegetables absorb it and it does not rinse off fully, so you end up eating it instead.
• Potassium permanganate. The wrong strength can irritate, and it is not needed for washing produce at home.
• Bleach or strong chlorine meant for floors and bathrooms. They are not for food.
• Soaking for an hour. Produce bruises, and more of the water-soluble vitamins such as vitamin C leach out.
• Soaking without rinsing. What came off is still floating in the water and can settle back on.

Kitchen hygiene, the other half

Wash vegetables perfectly and then cut them on the board you used for raw meat, and the germs are back.

• Keep separate boards and knives for produce eaten raw and for raw meat.
• Wash produce just before eating or cooking, not as soon as you get home if you will store it for days.
• Keep washed produce in a clean box lined with paper towel, in the fridge.
• Throw away parts that are rotten, slimy or smell wrong. Don’t try to save them.

Pesticide-free vegetables from your own bed still need washing. Soil, bird droppings and insect eggs don’t care that you never sprayed.

If you have questions about food safety for someone with weak immunity, someone pregnant, or young children, ask a doctor or dietitian, as they may need to be more careful with raw produce than most people.