Do you need to rinse before the dishwasher?

ENERGY STAR is blunt about the water arithmetic: rinsing dishes before loading them in the dishwasher increases the total water and energy used, and the advice is to just scrape food off dishes, because certified dishwashers and today’s detergents are designed to do the cleaning1.

The reason a rinse costs so much relative to the wash is the machine’s own budget. To be certified, a standard-capacity dishwasher must use no more than 240 kWh a year and no more than 3.2 gallons a cycle, with compact models held to 155 kWh and 2.0 gallons, under a specification effective from 19/07/20232. A machine allowed 3.2 gallons for an entire load is being asked to compete with a tap that a person leaves running while they think about something else.

Miele’s US operating instructions carry the instruction with an exclamation mark, which is as animated as an appliance manual gets: remove coarse food residues from the dishware, and there is no need to rinse items under running water3.

The same manual is precise about what must come off, and it is not food. Items soiled with ash, sand, wax, grease or paint are not to be washed in the dishwasher at all, because they will damage it3. So the scrape has a target: solid debris that would clog the filters or survive the wash, rather than the residue a detergent is formulated to deal with.

The manual also prices the alternative. Miele sells a Pre-Rinse option as an extra for removing easily soluble soiling, and states that adding it increases the programme duration and increases energy and water consumption3. When the manufacturer of the machine sells you a pre-rinse and warns you what it costs on the same page, that is about as disinterested as the advice gets.

Cascade answers the question in the title of a page and gives the mechanism underneath it. Its detergent is formulated with enzymes that latch onto food left on dishes and break it into particles small enough to flow down the drain, and the page states in as many words that the enzymes work better if you do not pre-wash, because without food to attack their cleaning power is underutilised4.

This is the part of the answer people find hardest to believe, and it is the part with the clearest chemistry behind it. An enzyme is a catalyst with a substrate: no substrate, no reaction. A rinsed plate does not give a protease anything to cut, and the detergent goes to work on the plate itself. Cascade adds a water figure to the same page, claiming a saving of up to 20 gallons a load by skipping the pre-rinse4, which is a manufacturer’s figure on a manufacturer’s page and should be read as one.

Modern machines choose their own cycle length by looking at the water. GE describes its Clean Sensor as sending a beam of light through the wash water, measuring the clarity of that water, and determining the amount of water, temperature and time needed for the load, monitoring soil level, temperature and water level throughout5. On the normal selection that means between four and six fills and between 36 and 73 minutes of cycle time, and on pots and pans between five and eight fills and 43 to 77 minutes5.

So a pre-rinse does not only add tap water to the total. It also changes what the machine sees at the start, and a cycle chosen from clearer water is a shorter cycle than the load may have wanted. The habit that feels like helping is arriving before the only measurement the machine takes.

The nearest thing to an outside estimate comes from a life cycle study by Porras, Keoleian, Lewis and Seeba, published in Environmental Research Communications in 2020. It found that avoiding pre-rinsing reduces life cycle emissions by about 3 per cent, and recorded that around 75 per cent of machine dishwasher owners pre-rinse even though the practice is not recommended6.

Two things about that paper belong in the same paragraph as its result. The research was supported by funding from Whirlpool Corporation alongside University of Michigan sources, and Whirlpool provided the machines and facilitated the observational laboratory study6. And a 3 per cent reduction in life cycle emissions is a modest number: it is the sum over ten years of use, in which manufacturing and electricity dominate, so it should not be read as the whole saving of a habit whose water cost lands entirely on the tap.

• Scrape properly. Coarse residues are the thing to remove, and the manual’s reason is the filter rather than the wash3.
• Keep ash, sand, wax, grease and paint out of the machine entirely. That is a damage warning, not a cleaning preference3.
• If the load will sit for a day or two before it runs, the dried-on soil is the problem, and the machine’s own rinse or soak option is the tool for it, at a stated cost in time, water and energy3.
• Run a full load. Dishwashers use about the same amount of energy and water regardless of how many dishes are inside1.
• Judge a change by the glassware. A film across the glass rather than discrete spots is a dosing question, and that argument is set out in our rinse aid and salt guide.

Nothing on our dishwasher tablet table is ranked on how clean the dishes come out, because we have washed none. What this page adds is the part of the outcome that has nothing to do with which tablet is in the drawer: the habit at the sink is worth more water than the difference between two detergents, and it is free to change.

The gap worth naming is that no field measurement exists here from a party with no stake. Every figure above comes from an appliance maker, a detergent maker, an efficiency programme or a study funded in part by an appliance maker, and all four point the same way, which is suggestive rather than conclusive. We would rather say that than round the agreement up into proof.