Freezing, Defrosting, Toasting, and Your Glucose Response to White Bread | Additives Ruin Part of the 30-37% Benefit

Who would have thought that: More convenient, in the eyes of most people better tasting and still better for your blood glucose management: Take white bread from the freezer, defrost at room temperature overnight, and toast it for a 30% reduction in postprandial glycemia even if you're perfectly healthy!
In the SuppVersity Kitchen Science article-series, I address questions that may have passed your mind when you've been cooking, doing the groceries, or looking at the photos of your favorite Insta-Foodies. Today's article addresses the reality of "Making white bread great(er) again!"  Or, shall I write "making the glucose excursions from eating white bread smaller again" - having more than 30% less glucose trickling into the blood of absolutely healthy subjects who were recruited for a study at the Oxford Brookes University (Burton 2007).(three male and seven female) sounds "great", no?
Review older articles about bread & co. and your health at the SuppVersity

Best Bread Depends on Bugs

Fat Bread = Better Bread!?

Resistant Starch Health Bagels

Gluten in Your Head?

Gluten-Tolerance in a Small Pill

Is Gluten Intolerance Real?
To participate, subjects were required to be between 18 and 59 years of age, with a body mass index (BMI) <30 kg/m2. Interested subjects were asked to complete a health screening questionnaire to check against possible health problems, including, and most importantly, clinically abnormal glucose metabolism (fasting blood glucose >6.1 mmol/l) and any medical conditions or medications that might affect glucose regulation, gastric emptying, body weight, appetite or energy expenditure.

In other words: This study is also interesting because we're talking about a clientele that is for once not overweight/obese and sick 😎 and thus significantly more relevant for those of you who have hitherto managed to avoid insulin resistance or type II diabetes.

On different occasions, the subjects were fed either a homemade white bread (the study used a standard "Breadman Pro" by Russell Hobbs, regular flour, no additives) or a commercial white bread - both in all of the following four conditions:
  1. fresh - tested on the morning following baking or purchase
  2. frozen and defrosted - frozen bread (2-7 days) was defrosted overnight at room temperature 
  3. toasted - fresh bread was toasted on the morning following baking or purchase 
  4. toasted following freezing and defrosting - bread was toasted after defrosting 
In all cases, the toaster was set to medium heat, "ensuring consistent moderate toasting" (ibid.), and, I should add, avoiding acrylamide formation (e.g. moderate toast 10.9–213.7 µg/kg vs. dark toast 43.7–610.7 µg/kg; both modest in comparison to fries which have up 2130 µg/kg acrylamide | Jackson 2005). The study used a randomized repeated measures design and measured incremental blood glucose, peak glucose response, 2 h incremental area under the glucose response curve (IAUC).
Israeli scientists found no difference in glycemic response between 'healthy' sourdough and 'unhealthy' white bread when they analyzed their whole study population. On an individual level, however, the microbiome dictated whether the white or the sourdough bread spikes blood glucose; and this disposition cannot be modified in the short run.
Which bread is right for you? Yes, you've read that before: White bread isn't worse for everyone. Israeli scientists from the Weizmann Institute (Korem 2017) were the first to show not only that the glycemic response to the two types of bread varies greatly across people but that the variation they observed "accurately predicts glycemic-response" to sourdough vs. white bread. Furthermore, a more recent paper from the Mayo Clinic (Mendes-Soares 2019) confirms and highlights that "the microbiome, in addition to nutrient content [not CHO content]" should "inform tools to control the glycemic responses" in both, healthy and diabetic individuals.

Now, having the right microbiome is one thing researchers are looking at. The influence of additives is another one. In the study at hand, enzymes and co. in the commercial bread reduced the healthfulness of bread indirectly by inhibiting resistant starch formation.

Other additives are yet thought to improve the healthfulness and nutritious value of bread. The addition of pulses, for example, increases the protein content, improves the amino acid composition and could also have beneficial effects on the fiber/RS content (Boukid 2019). The Macrominerals P, Mg, and K (El Khoury 2019), or soluble fiber like arabinoxylan (Falchi 2016) improve glycemia and the addition of wheat germ-enriched bread (Moreira-Rosário 2019) will even yield changes in your microbiome that could then change how other types of bread affect your glucose and overall metabolism. It should be said, though, that not every additive that seems promising will also work: the addition of polyphenol extracts, for example, failed to produce improvements in glycemic response or satiety (Coe 2016).
In order to standardize the tests, subjects were asked to restrict their intake of alcohol and caffeine-containing drinks and to refrain from intense physical activity (e.g., long periods at the gym, excessive swimming, running, aerobics) on the day preceding the individual testing sessions. Moreover, ...
Figure 1: Nutritional composition (per 100 g) of test breads, Hovis Classic and homemade white bread made from Carr's white strong bread flour | Burton & Lightower (2009)
"[...t]o minimize the possible influence of the second meal effect, subjects were asked to refrain from eating an extra-large evening meal or have an unusually high or low food intake throughout the day preceding a test[; and w]here possible, subjects ate a similar meal type on the evening before testing; however subjects were asked to avoid consuming pulses for this meal to avoid the effects of colonic fermentation on postprandial glycaemic" (Burton 2009).
All foods were tested in subjects after a 12 h overnight fast, which is why we can assume that their overall insulin sensitivity was slightly better than what you'd see with fully charged liver glycogen later in the day (breakfast effect) and a different position of the hands of your internal clock.
Did you know that your habits determine your breakfast response?!
"As blood glucose responses vary within subjects from day to day, the reference food (glucose) was tested 3 times in each subject. 
Thus, subjects tested each test bread once and the reference food 3 times in random order on separate days, with at least a one-day gap between measurements to minimize carry-over effects.

All test slices of bread and the reference food were served with 200 ml water. A further 200 ml water was given during the subsequent 2 h" (Burton 2009).
Subjects were asked to eat the test breakfast within a 10–12 min period to reduce the influence of chewing on particle size and plain glucose was used as a comparator yielding the same 50g of non-fiber (='available') carbohydrate (see detailed macro-break-down in Fig.1).
With commercial bread, you get more than you want... additives like dough conditioners and improvers. These ingredients (often 'designer enzymes' and proteins) are widely used in commercial bread and are designed to optimize dough formation and quality, reduce staling rate and maintain water retention during baking (Goesaert 2005) - they are the reason you can buy shelf-stable bread for a few cents per slice at the supermarket.

As the authors of the study under review point out, these magic ingredients could, however, be the reason why freezing and defrosting works magic for homemade (additive-free) but not commercial bread: "[W]ater content and activity within the dough facilitates the retrogradation process. Moreover, amylopectin retrogradation, together with moisture transfer between bread components, may be reduced by the use of dough improvers (Baik et al., 2003)".
Figure 2: Changes in blood glucose for homemade (left) and commercial (right) bread and 2h-glucose AUC (measure of total glucose influx over time | bottom) - Blood glucose response curve for commercial white bread: Glucose (*); fresh (○); frozen, defrosted (•); fresh, toasted (□); frozen, defrosted, toasted (▪).
Details about the additives (e.g. enzymes) in the commercial bread (in the EU and the US many don't have to be labeled, anyway) are unfortunately not available. Hence, we cannot fact-check their individual effects. Personally, I'd say, though, that it's very likely that the changes in glycemia are due to additive interference w/ the structural changes in the starch molecules. How's that? Well, legally undisclosed additives in commercial white bread could also explain why the commercial bread reacts less favorably to all three treatments because enzymes or other additives inhibit the formation of resistant starch RS3 (Goesaert 2005 | see red box), of which you've learned in previous SuppVersity articles that it's mostly fermented to SCFA in the colon and will thus not increase your blood glucose.
More cooking science from the SuppVersity Archives: "Is Air Frying Healthier Than Deep Frying?" | read it!
But wait, does that even matter? Well, it does matter for taste. After all, most people prefer toasted over floppy sandwiches. As far as the differences in glucose peaks and the 2h-AUC are concerned, actual health benefits will accumulate over several years, only, and we cannot say exactly how relevant they're going to be. What we can say for sure, at least, is (a) that the carcinogenic Polycyclic aromatic hydrocarbon #PAH content won't explode when you toast your white bread properly, i.e. without direct contact to the heat source, like it would occur in a pan (this is what you should not do!), on coal, on an open fire, etc.
Addendum on storage effect: While there's no evidence from studies using wheat bread, breads based on other grains (here tef), show a significant decline in readily available starch when stored. So significant that after only 5 days the tef bread in Shumoy et al. went from a GI of 75 to 57 (-24%)!
Moreover, the study at hand shows that it's not just OK to 'conserve' your (ideally homemade) white bread by storing it in the freezer. No, it's more than OK! Maybe even "the better way"! The defrosted slices of additive-free bread may, after all, yield glucose 2h-AUC improvements of -39% if you also toast the bread! Plus, even if you like it floppy (don't toast), there's a smaller but still significant glycemic advantage (-31%) over fresh homemade bread. While the still significant values are smaller for commercial toast, even the additive-laden white bread benefits - especially from toasting, but less from freezing and defrosting. Speaking of toasting, the -28% reduction you get with commercial bread upon toasting the slices tells you that the short time it takes to toast your sandwich slices in the AM is well-spent for texture and health | While your bread's baking, defrosting or been toasted, comment on Facebook!
  • Baik, Moo-Yeol, L. Charles Dickinson, and Pavinee Chinachoti. "Solid-state 13C CP/MAS NMR studies on aging of starch in white bread." Journal of agricultural and food chemistry 51.5 (2003): 1242-1248.
  • Boukid, Fatma, et al. "Pulses for bread fortification: A necessity or a choice?." Trends in Food Science & Technology (2019).
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  • Coe, Shelly, and Lisa Ryan. "White bread enriched with polyphenol extracts shows no effect on glycemic response or satiety, yet may increase postprandial insulin economy in healthy participants." Nutrition Research 36.2 (2016): 193-200.
  • El Khoury, Rania, et al. "Macromineral enrichment of white bread reduces postprandial glycaemia without altering sensory properties: a crossover study." East Mediterr Health J (2019).
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  • Mendes-Soares, Helena, et al. "Assessment of a personalized approach to predicting postprandial glycemic responses to food among individuals without diabetes." JAMA network open 2.2 (2019): e188102-e188102.
  • Moreira-Rosário, André, et al. "Daily intake of wheat germ-enriched bread may promote a healthy gut bacterial microbiota: a randomised controlled trial." European journal of nutrition (2019): 1-11.
  • Rey-Salgueiro, Ledicia, et al. "Effects of toasting procedures on the levels of polycyclic aromatic hydrocarbons in toasted bread." Food Chemistry 108.2 (2008): 607-615.
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