Supermarket shelves are full of oils of various provenances. But do you know which ones are best to cook with? Obvious, isn't it?! Vegetable fats are good and animal fats are bad. Well, you may need to think again. There are three main types of fat: monounsaturated, polyunsaturated and satu-rated. When fats and oils are exposed to temperatures close to 180 °C, they undergo an oxidation process, which leads to the formation of aldehydes and lipid peroxides. The same happens at room temperature, albeit far more slowly, leading to lipids going rancid. It so happens that consuming aldehydes, even at small doses, is linked to an increase in the risk of heart disease and cancer.
Corn oil and sunflower oil are rich in polyunsaturates and therefore gener-ate substantial levels of aldehydes when heated, at a level 20 times higher than recommended by the World Health Organization (WHO). Butter, goose fat, olive oil and cold-pressed rapeseed oil produce far fewer aldehydes because they are richer in monounsaturated and saturated fatty acids, which makes them more stable when heated. In fact, saturated fats hardly undergo this oxidation reaction at all. When it comes to cooking, it doesn't matter whether the olive oil is extra virgin or not; the antioxidant levels present in the extra-virgin products are insufficient to protect us against heat-induced oxidation. We have known for a while that saturated fat may, after all, not be so bad. Previously scientists had examined the links between eating saturated fat, such as butter, and heart disease. Despite looking at the results of nearly 80 studies involving more than half a million people, they were unable to find convincing evidence that eating saturated fats leads to greater risk of heart disease. One prominent cardiologist was quoted as saying: "Satu-rated fat makes you less hungry. There is certainly a strong argument that an over-reliance in public health on saturated fat as the main dietary villain for cardiovascular disease has distracted from the risks posed by other nu-trients, such as carbohydrates (e.g. bread, potatoes, pasta), which people started to consume in larger quantities to compensate for a lower intake in saturated fats. A high consumption of carbohydrates is associated with changes linked to diabetes and heart disease."
Q18.1 Which of these statements can be concluded from the passage?
A. Aldehydes form at a temperature of 180 °C.
B. Rancid oil does not contain aldehydes.
C. Heated saturated fats contain high levels of aldehydes.
D. Aldehydes tend to be generated from polyunsaturated fats.
Q18.2 Which one of the following oils would the author most likely con-clude is not good for use in a cold salad dressing?
A. Sunflower oil.
B. Extra-virgin olive oil.
C. Rapeseed oil.
D. None of the above.
Q18.3 Which of these statements cannot be concluded from the passage?
A. Butter is healthier when heated up than when cold.
B. Hot butter is healthier than hot sunflower oil.
C. Cold butter is healthy to eat in small quantities.
D. Eating butter makes you less likely to eat other potentially unhealthy foods.
Q18.4 Which of these statements is the author most likely to agree with?
A. Potatoes fried in goose fat cannot cause heart disease.
B. The public was misled about the dangers of saturated fats.
C. Pasta with extra-virgin olive oil is healthier than pasta alone
D. At room temperature, olive oil goes rancid more quickly than sunflower oil.
Q18.5 Which of the following options best ranks the three types of fat in increasing level of oxidation when heated?
A. Polyunsaturated < Monounsaturated < Saturated.
B. Saturated < Monounsaturated < Polyunsaturated.
C. Monounsaturated < Polyunsaturated < Saturated.
D. Saturated < Polyunsaturated < Monounsaturated.
Answer and Explanation
Q18.1 — D: Aldehydes tend to be generated from polyunsaturated fats.
The second paragraph states that corn oil and sunflower oil are rich in pol-yunsaturates and therefore generate substantial levels of aldehydes. This shows that polyunsaturates produce aldehydes. However, in order to con-clude that aldehydes tend to be generated from polyunsaturates we also need to show that nothing else generates substantial amounts of alde-hydes. We get the answer from the third paragraph, which states that mon-ounsaturated and saturated fats produce far less aldehydes. Therefore we can conclude that aldehydes tend to be generated from polyunsaturates.
Note that it is very possible that another type of fat not described in the text may also produce aldehydes; however, the question is asking what the au-thor is most likely to conclude as opposed to which sentence can be defi-nitely concluded from the text. This is where this format of question differs from the "true/false/can't tell" format. Looking at the other options:
• A: Aldehydes form at a temperature of 180 °C. We can actually con-clude from the text that this is false because, though it does state that
an oxidation process producing aldehydes takes place around 180 °C, it also states that this could equally happen at room temperature if the oil is left around for a long period of time. Therefore aldehydes can also form at much lower temperatures.
• B: Rancid oil does not contain aldehydes. Paragraph 1 is clear about the fact that the oxidation process leading to the formation of aldehydes can also happen at room temperature, leading to the lipids going rancid. Therefore we can logically conclude that rancid oil does contain aldehydes.
• C: Heated saturated fats contain high levels of aldehydes. The text states that saturated fats hardly undergo the oxidation reaction; there-fore we can reasonably conclude that heated saturated fats would in fact contain very low levels of aldehydes.
Q18.2 — D: None of the above.
From the text, we can only conclude that there is an issue when the oils are heated (in which case one would want to avoid sunflower oil). There is nothing in the text to indicate that any of the oils mentioned would be detrimental at cold temperatures. Based on the text there is therefore no reason to suspect that any of the three oils mentioned would be bad for use in a cold salad dressing.
Q18.3 — A: Butter is healthier when heated up than when cold.
This assertion does not really make much sense. In light of the text, all we can reasonably conclude from the text is that butter is not likely to be more dan-gerous when hot than when cold, but it is far from saying that it would ac-tually be healthier when heated up. Looking at the other options:
• B: Hot butter is healthier than hot sunflower oil. We know from the text that hot butter does not produce many aldehydes whereas sun-flower oil does. So the assertion would be a logical conclusion, at least as far as heart disease is concerned.
• C: Cold butter is healthy to eat in small quantities. The last para-graph actually relates to saturated fats in general, and not just those which are heated up. We know from paragraph 3 that butter contains saturated fats and we also know from the last paragraph that eating saturated fats can cut hunger and stop other foods such as carbohy-drates from being eaten. So it is reasonable to conclude that small quantities of butter can be healthy to eat.
• D: Eating butter makes you less likely to eat other potentially un-healthy foods. This is dealt with in the last paragraph.
Q18.4 — B: The public was misled about the dangers of saturated fats.
It is explained in the last paragraph that saturated fats were unfairly vilified at the expense of the message on other issues such as overconsumption of carbohydrates. Looking at the other options:
• A: Potatoes fried in goose fat cannot cause heart disease. The text suggests that goose fat poses a minimal danger: however, it does link carbohydrate consumption (potatoes are carbohydrates, we are told) to heart disease and therefore we can conclude that this sentence is actually incorrect.
• C: Pasta with extra-virgin olive oil is healthier than pasta alone. This could only be concluded if the text explained how the addition of olive oil could produce a health benefit. All we know about olive oil is that it is stable when heated, not that it neutralises the harmful impact of carbohydrates on the risk of diabetes or heart disease.
• D: At room temperature, olive oil goes rancid more quickly than sunflower oil. We know that olive oil is more stable when going through the oxidation process (which also causes oil to go rancid when left at room temperature), whereas sunflower oil tends to oxidise more substantially. Based on the text, there is no reason to suspect that at room temperature the process would be the total opposite (i.e. olive oil going rancid faster than sunflower oil).
Q18.5 — B: Saturated < Monounsaturated < Polyunsaturated.
We know from paragraph 3 that oils rich in monounsaturated and saturated fatty acids are stable and produce far fewer aldehydes than polyunsaturated fats. Therefore both will feature at the bottom of the scale and polyunsaturated fats will be at the top (as they oxidise the most). We are also told that saturated fats hardly undergo any oxidation at all, which implies that the aldehydes produced by fats such as butter or olive oil come mostly from the monounsaturates. Hence saturated fats are at the bottom of the scale when it comes to oxidation, followed by monounsatu-rated fats.
Drafted by Juno Wong(UCAT Prep)