A toast to resistant starch

Freezing bread is a great way of preventing food waste but might there be other reasons for doing it? The internet is awash* with claims of health benefits resulting from freezing and / or toasting bread. Upon what are these claims based and have they been proven as fact?

*Well, an algorithm-driven social media echo chamber in our corner of the interweb, at least…

In short

Research (albeit limited) suggests that freezing starchy carbs (pasta, rice, potatoes, bread, industrial dough products etc.) might increase its level of resistant starch, which might be enhanced further by then toasting / reheating.

Changing starch to this less digestible form might have a range of health benefits. Before anyone gets over-excited and starts promoting toast as a cure for diabetes, though, things need unpacking.

Resistance is useful

The main part of a wheat berry / grain is the endosperm, the largest proportion of which is starch. This complex carbohydrate is a polysaccharide, comprising long chains of glucose molecules. Starch acts as the main energy store in the seed, ready for the first stages of a wheat plant’s growth.

Some forms of starch are digested completely in the small intestine, breaking down into simpler sugars and then released as glucose into the bloodstream. Forms that aren’t digested in the small intestine are referred to as resistant starch (RS), which is a form of dietary fibre that passes through to the large intestine. As RS does not release glucose, it’s less likely to contribute to sugar spikes, which is particularly useful for people with diabetes or who otherwise need to control blood sugar levels.

When RS reaches the large intestine, it acts as a prebiotic that helps to ‘feed’ the ‘good’ bacteria there. A growing body of evidence suggests that a healthy gut microbiome might be beneficial to overall health of our bodies and minds, but that’s a nebula too large to explore in this relatively short article about starch. The by-products of RS fermentation include short-chain fatty acids (SCFAs), which also seem to be beneficial for health: some studies have associated SCFAs with lower rates of colorectal cancers, lowering inflammation and supporting the immune system.

Starch retrogradation

RS has been classified into five categories. RS1 and RS2 occur naturally in some foods, while the form we’re considering here (RS3) is created by starch retrogradation.

Starch in wheat flour is semi-crystalline in structure and fairly stiff in state. When flour is mixed with water and heated to sufficient temperature, starch bonds with water, swelling and eventually bursting out of the granules in which it’s held. The matrix of starch and water forms a flexible gel, which (along with gluten) becomes a major building block of bread’s soft structure. It’s what makes wheat-based bread gummy if you slice it while it’s still hot, and why flour and liquid thickens sauces when heated.

When bread starts to cool, starch retrogradation begins. This change, from a gel-like state to a less flexible, crystalline one, is (on combination with evaporation) a key reason for bread becoming drier and stiffer over time. Starch retrogradation – and, therefore, staling - happens more quickly at around 3-4°C, which is an argument against storing bread (or industrial dough product) in a fridge.

A toast to the resistance

Now to the health claims… 

In 2007, a paper titled The impact of freezing and toasting on the glycaemic response of white bread was published. Researchers fed seven women and three men homemade white bread and white factory loaves, which had been prepared and stored in four different ways. Compared to eating the bread and the Chorleywood Process loaf in their original state, eating them in the other three states (toasted; frozen then defrosted; toasted after freezing and defrosting) resulted in a lower two-hour incremental area under the glucose response curve (glycaemic response).

A similarly small study involving 12 subjects, published in 2008, found that the GI of industrial dough product rolls was lower when they were frozen and defrosted before consumption. It was lower still for rolls manufactured by part-baking, freezing and then rebaking – the ‘bake off’ process that lies behind much of the ‘freshly baked in store today’ type marketing used by UK supermarkets.  

Glycaemic response to frozen stored wheat rolls enriched with inulin and oat fibre (2012) found that freezing bread contributed to a 34% reduction in GI but only when done in combination with adding dietary fibre to the dough: adjusting either of these variables alone had no significant impact on GI.

Effect of freezing and heating white bread on the glycemic response of healthy individuals (2023) involved 32 adults eating fresh bread, fresh bread that had been toasted, and bread that had been frozen then defrosted and toasted. The authors reported that eating toasted bread resulted in a 25% reduction in glycaemic response compared to fresh bread, while freezing and toasting combined lowered the response by 40%. The authors reported that increasing the period of freezing (between three and seven days) had no significant effect on the amount by which blood sugars levels decreased. 

The results above resonate with a 2015 in vitro (lab, rather than live) study that looked at the effect on digestion of boiling and deep freezing a wholemeal wheat flour and water slurry. The more times the cook/freeze cycle was repeated, the higher the level of RS in the samples and the more 
SCFAs they generated.

One swallow does not a summer make

A caveat that we always underline when talking about scientific studies is: good quality research is carried out under very specific, controlled conditions. This means that the result of a study can’t necessarily be extrapolated to real life situations, especially ones involving humans, as each of us is unique. As can be seen above, studies on this issue so far have had relatively small sample sizes, so have low statistical significance.

Something else to consider before stating something is ‘healthier’ or otherwise ‘better’ is whether you want to base that assertion on a single factor. Someone once triumphantly tweeted (it was a while ago) that one of these studies proved that supermarket loaf tanning salon products were ‘better’ than freshly-made Real Bread. We asked if the claimant had factored in the extra energy consumption of baking twice; the faster staling and, therefore, increased likelihood of food waste; socio-economic factors; or anything about the composition (ingredients and addtives) of a particular frozen and rebaked product. The tweeter didn’t tweet a reply.

Three more thoughts for now: 

• Resistant starch can be problematic for some people; for example due to a bowel condition.
• Like other forms of high-temperature cooking (roasting frying etc.) of starchy foods, toasting bread can result in the formation of acrylamide, which official advice cautions against. 
• None of the studies reported freezing or reheating resulting in any change in calorific value of bread.

Which pretty much takes us back to the start of this article. Please don’t take any of the above as health advice, or pass on any of it as such to other people.

See also

• What’s so good about Real Bread?
• Wholemeal bread
• Sourdough bread mythbuster
• Real Bread vs. ultra-processed food
• Focus on phytate and FODMAPs