When every drop counts: How rising prices are driving micro businesses to reuse cooking oil and the health concerns

At 4 am, Sophia Wanjiku and her employee are already at work, standing over a hot jiko in Nairobi as they prepare mandazis for the morning rush.

For Wanjiku, however, frying mandazis is no longer just about getting the temperature right. It is also about making every litre of cooking oil last.

At an average of more than KSh358 a litre, cooking oil is among the most expensive kitchen essentials for households and small food businesses.

Data from the Kenya National Bureau of Statistics (KNBS) shows that the average retail price of one litre of cooking salad oil increased by 3.3 per cent, from KSh347.17 in June 2025 to KSh358.63 in June 2026 year on year.

For consumers already dealing with rising food costs, the increase is changing not only shopping habits but also how food is prepared.

“Sometimes you cannot just throw it away because you have used it only once. You have to think about the money you spent buying it,” Wanjiku says.

Like many households and small food vendors, she stores leftover oil for later use. At times, she mixes fresh oil with oil that has already been used.

For people operating on tight budgets, reusing cooking oil is less a matter of preference than a way of cutting costs.

But food scientists and toxicologists warn that repeatedly heating cooking oil causes chemical changes that can potentially increase health risks with prolonged exposure.

The concern is compounded by what experts describe as a policy gap: Kenya does not currently have a published national guideline specifying when repeatedly heated cooking oil should be discarded in homes, restaurants or small food businesses.

That leaves consumers and food vendors to make decisions about the safety of reused oil largely on their own.

What happens to oil when it is repeatedly heated?

Cooking oil starts undergoing chemical changes when it is exposed to heat.

Dr Sheila Kilonzi, a lecturer in Food Science and Nutrition at Jomo Kenyatta University of Agriculture and Technology (JKUAT), says three key processes occur: oxidation, hydrolysis and polymerisation.

Oxidation occurs when oxygen reacts with the oil. Hydrolysis happens when moisture from food breaks down fat molecules, while polymerisation causes degraded molecules to combine and form larger compounds.

These reactions gradually affect the oil's colour, smell and texture.

But experts say visible changes are not necessarily a reliable indicator of the extent of chemical breakdown.

Repeated heating can produce compounds such as peroxides and aldehydes while reducing naturally occurring antioxidants, including vitamin E.

Toxicologist Joseph Bundotich says repeated heating can also result in the formation of free fatty acids and aldehydes, including acrolein, as well as other chemical compounds.

“Cooking oil doesn't simply lose flavour after repeated heating. It changes chemically,” he says.

The rate at which this happens depends on several factors, including the type of oil, cooking temperature, duration of heating and the type and moisture content of food being fried.

What does that mean for health?

The presence of breakdown products in repeatedly heated oil is a concern because some compounds can contribute to oxidative stress and other biological effects.

Bundotich says some chemicals produced during heating can be harmful at sufficient levels of exposure. Benzene, for example, is a recognised carcinogen and prolonged exposure is associated with serious health effects, including damage to bone marrow and an increased risk of certain blood cancers.

Other compounds, such as acrolein, can irritate the eyes and respiratory system.

Dr Mercy Maina, a toxicologist and pharmacologist, says the concern should not be interpreted as meaning that eating food prepared in reused oil once will cause illness.

Rather, the issue is repeated exposure.

“A small bite once in a while is different from daily consumption of food prepared in heavily reused oil,” she says.

She compares the principle to exposure to cigarette smoke: the potential harm is linked to repeated exposure rather than a single isolated incident.

The risk can also vary depending on how often the oil is reused, how hot it becomes, how long it is heated and how frequently a person consumes deep-fried food.

Not all oils deteriorate at the same rate.

The type of oil also influences how it responds to repeated heating.

Polyunsaturated oils, such as sunflower and soybean oil, are more susceptible to oxidation because of their chemical structure. Monounsaturated oils, including olive and canola oil, are generally more resistant to oxidation.

Saturated fats are more resistant to oxidation but excessive consumption is associated with other health concerns, including elevated cholesterol.

Experts caution, however, that no cooking oil should be considered immune to breakdown when it is repeatedly exposed to high temperatures.

“Heat, the duration of cooking and the number of times oil is reused all influence how quickly it deteriorates,” Kilonzi says.

Dieticians have also cautioned that the health qualities of a cooking oil cannot be considered separately from how it is used.

During an online nutrition discussion hosted by dieticians from Kenyatta National Hospital, dietician Eric Ngereso noted that frequent deep-frying can undermine an otherwise healthy choice of cooking oil.

For households and food vendors trying to make their oil last longer, mixing fresh oil with previously used oil is a common practice.

But experts say adding fresh oil does not reverse the chemical changes that have already taken place.

“Adding new oil to one that has already been reused does not make it fresh again,” Kilonzi says.

Filtering the oil can remove food particles and other visible impurities, but it does not remove all the chemical compounds that may have built up during heating.

The implication is that topping up old oil with fresh oil may dilute it, but it does not restore it to its original condition.

Kenya has standards governing edible cooking oil before it reaches consumers.

The Kenya Bureau of Standards (KEBS) requires manufacturers to comply with KS EAS 321:2018, while vitamin A fortification is covered under KS EAS 769:2019.

However, these standards mainly focus on production and quality of edible oils and do not set a simple household or commercial rule for how many times cooking oil can be reused.

KEBS has confirmed that Kenya does not currently have a published standard specifying a fixed number of times cooking oil can safely be reused.

Instead, the bureau is working towards guidance based on Total Polar Content (TPC), a measure used to assess how far cooking oil has broken down.

TPC can provide a more meaningful measure than simply counting the number of times oil has been used because breakdown depends on factors such as temperature, heating duration, food moisture and storage conditions.

Until such guidance is adopted, consumers and food businesses have limited clear information to determine when reused oil should be discarded.

For Wanjiku, however, the science has to compete with the economics of running a small food business.

Every litre of oil represents money spent on ingredients. Throwing it away after a single use can directly affect her costs and, ultimately, her income.

Her dilemma is shared by households and food vendors across the country who are trying to keep meals affordable as food prices rise.

Experts acknowledge that simply telling consumers to discard oil after every use may not be realistic.

Instead, they recommend avoiding overheating oil to the point where it smokes, limiting repeated heating, storing oil properly and reducing reliance on deep-fried foods by choosing methods such as boiling, steaming or baking where practical.

For Wanjiku and thousands of others, however, the pressure to stretch household and business budgets remains.

As cooking oil becomes more expensive, the question is no longer just how many times oil should be reused.

It is whether consumers have enough information to make safe decisions when economic realities make throwing away a seemingly usable litre of oil difficult.

The debate, for now, is shaped by food prices, consumer safety and public health.

And for the people preparing food before sunrise, it is a decision made one frying pan at a time.