Freezing is the best way to extend the shelf life of bread, but there’s an intriguing claim that freezing and then toasting bread can also lower its glycaemic index (GI), a measure of how quickly a food raises your blood sugar. Is this claim true, and where does it come from? Let’s look at the original study on the subject, which was published all the way back in 2007.
In the study, researchers wanted to investigate how the way in which white bread was prepared affected its impact on subjects’ blood sugar. They recruited 10 healthy people aged 22–59, and gave them homemade and commercial white bread that had been stored and prepared in four different ways. The bread was either eaten fresh, frozen and defrosted, toasted, or frozen and then toasted. Each participant received the differently treated breads in a random order, and researchers measured their blood sugar over a two hour period following ingestion.
The researchers found that in comparison to eating fresh bread, all treatment methods led to a significant reduction in the blood sugar response, which was measured using a metric called the incremental area under the glucose response curve (iAUC). While iAUC is not the same thing as the glycaemic index, it is used to calculate GI, with higher iAUC translating to higher GI. A lower GI is desirable, because it means your blood sugar increases slowly and remains lower over a longer period of time, rather than ‘spiking’ over a short period of time. The latter means that the pancreas has to release a lot of insulin at once to keep blood sugar under control, which promotes insulin resistance and type II diabetes.
Back to the bread: the researchers found that just toasting the homemade bread decreased its iAUC by about 25%. Freezing and then defrosting the bread was more effective, decreasing the iAUC by about 30%. When these two methods were combined, they decreased iAUC by about 40%. The results were similar for commercial bread, though interestingly, freezing followed by toasting was not significantly more effective than toasting alone.
So, it does seem as though freezing and toasting bread can reduce its glycaemic index, and by quite a lot. But how? It’s no news that the way in which food is cooked can influence its chemical structure and therefore its effects on the body when ingested. Extreme heat can break down starch granules, making the starch easier to break down into glucose and increasing the absorption rate. Alternatively, extreme heat can destroy some of the starch, which is presumably what is happening in the toast.
The freezing part is more interesting and is probably due to the formation of something called retrograded starch. Retrogradation occurs when cooked starch is cooled down, which causes starch molecules to form a crystalline structure that is resistant to degradation. This makes it harder for the gut to digest and absorb, delaying the entry of glucose into the blood. Retrogradation gives bread a less pleasant texture and is involved in staling, so bread manufacturers use additives to reduce it. This would explain why freezing appeared less effective in reducing the iAUC of the commercial bread.
You might be wondering whether the duration of freezing has an effect on the bread’s glycaemic index. Does freezing the bread for longer durations lower the GI further? Scientists actually answered this question in a recent study. They conducted a randomised controlled trial with 32 healthy participants, and tested white bread that had been frozen for 3, 5, or 7 days. They showed that fresh bread increased participants peak blood sugar more than defrosted bread, reinforcing the previous findings. However, they found that freezing the bread for longer periods did not increase the amount of retrograded starch, nor did it have a significant effect on participants’ blood sugar. So, it looks as though extended freezing time isn’t necessary, at least beyond three days.
It seems as though freezing and toasting starch-containing foods could make them healthier by lowering their glycaemic index. When it comes to healthy eating, we tend to focus on what we eat over how we eat it. There’s unexplored potential to improve our health by changing how we prepare food at home. With that being said, what we eat is still the most important factor, and wholemeal bread is usually going to be a healthier alternative to white bread (frozen, toasted or otherwise) due mainly to its fibre content and lower glycaemic index. This is a generalisation since both white bread and wholemeal bread vary in their nutritional value, and there’s some evidence that the blood sugar response to bread actually depends more on inter-person variability than it does on the nature of the bread. However, the scientific consensus is that long term consumption of wholemeal bread is preferable health-wise.