Effects of elevated storage temperature on bread quality

Question

It's pretty well-known and scientifically established that rather cool temperatures are bad for bread: putting your bread in the refrigerator will tend to dry it out and accelerate chemical reactions in the starches that cause staling (as discussed, for example, in some answers to this question). Freezing, on the other hand, stops some of those reactions and is a common method for maintaining bread quality for longer storage. Regardless of quality issues, both types of cooling will help to prevent molding, so there are trade-offs.

My question is: what happens when we store bread at warmer temperatures, say in the 90-130F range (about 35-55C)? How will quality be affected? Will the shelf-life be altered? Are the effects only good, only bad, or mixed? I'm particularly interested in short-term holding (less than a day) at elevated temperatures.

Also, are there any food safety issues raised by a practice like this (i.e., worse than bread storage at room temperature)? Bread is not a particularly good growth medium for bacteria, but I imagine that Bacillus cereus or something might be at least a potential concern.

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[Background, for those who are curious: The reason I ask this question is because I sometimes need to store bread temporarily in a hot car. The only bakery near me that I trust to make decent bread sometimes has "day-old" bread on sale for half price. Their normal prices are, to my mind, excessively high; so if I can't buy their loaves for half-price, I usually just make bread myself. Occasionally, I'll pick up some loaves on the way to work, but it's inconvenient to take them back home immediately. And while I have sometimes carried them into work with me, it would be easier to just leave them in my car. I also can't pick up the bread on the way home, because the half-price bread (when they have it) generally sells out by mid-morning. Although I can sometimes park under a tree or something, the reality is that the summer sun will often raise the temperature of my car interior to above 100F. Thus, the bread will be subjected to quite a few hours of elevated temperatures. I've done this once or twice without a major change in quality, but I'm wondering if there are benefits to justify the inconvenience of keeping it near room temperature.]

Answer 1

Summary:

If the loaf is kept at an elevated temperature in a plastic bag for a period of 6-12 hours I believe you will see little to no difference compared to storing at room temperature.

Stored at an elevated temperature in a paper bag the loaf will start to dry out to a noticeable extent.

Note that the answer below does not address possible food safety issues.

Long-winded details:

My previous answer provided some information up to 36°C (97°F). Since the question asks about the temperature range 35-55°C I did some amateurish experimentation of my own.

Using my fan assisted oven at the lowest setting, I halved a store bought sourdough loaf and placed one half in the oven overnight and kept the other half at room temperature (25°C).

I should note that my oven temperature fluctuates between ~37°C and ~47°C (99-117°F) measured using using a Thermapen at various intervals. I'll also note that I wrapped the loaf-half that went into the oven in a tea towel to protect it from the oven fan.

In the morning I taste tested the two loaf-halves. The room temperature loaf had started to stale slightly but the half from the oven had also started to dry out. It was noticeably more difficult to cut through the loaf-half from the oven and I saw about 8mm of visibly dried bread extending inwards from the outer surfaces.

This test was quite obviously flawed in that you would neither use a tea towel to wrap your bread whilst in the car, nor would you first cut the loaf in half.

The next experiment I did used 5 smaller loaves of the same variety, from the same store. I placed two in a plastic bag and two more in the paper bag in which the loaves were purchased. The fifth loaf I kept at room temperature in order to compare later.

Using the same oven setting I kept the bagged loaves in the oven for 6 hours. After six hours I removed two loaves, one from each bag, and taste tested.

Comparing a small slice of the loaf from the plastic bag to a slice from the room temperature loaf I sensed no obvious difference. The loaf from the paper bag was noticeably drier. I marked the loaves from the oven and saved them for comparison again later.

Keeping the remaining two loaves in the oven for a further 6 hours, I did another taste test this morning.

The loaf that had been kept at room temperature had now slightly but noticeably started to stale. Comparing this to the 12 hour loaf from the plastic bag I noticed hardly any difference. I really couldn't say whether one was less stale than the other. Comparing the 12 hour loaf from the paper bag, once again drying was pronounced.

I also made a second comparison using the loaves that had been taken out at 6 hours. Again I sensed no obvious difference between the loaf from the plastic bag and the room temperature loaf. The 6 hour loaf from the paper bag was no less dry than it had been 6 hours before.

Update:

I followed up on @Athanasius question from the comments and did another test with the oven fan switched off. This time I had to fight with the oven thermostat to stay within the temperature range but managed to stay just under 130°F. I tested three small loaves of the same variety and from the same store as the previous tests. Again, I kept two in the oven in paper and plastic bags, and one at room temperature (also in a paper bag). As well as taste testing I also weighed the loaves before and after the test. Here are the figures for weight loss after 6 hours:

• Elevated temp, plastic bag: no measurable weight loss
• Elevated temp, paper bag: ~7% weight loss
• Room temp, paper bag: ~4% weight loss

While I don't have any objective means for comparing dryness from the previous experiment (I didn't weight the loaves in the previous test) it does seem like the oven fan led to increased drying. As a subjective measure I offer the fact that following yesterdays testing I discarded both the 6 hour and 12 hour loaves from the paper bag, but this morning I found the loaf from the paper bag good enough for breakfast despite the drying.

The words in the summary are, however, still correct: stored at an elevated temperature in a paper bag a loaf will start to dry out to a noticeable extent.

Answer 2

This article indicates that the elevated temperature would retard staling,

It has been shown that changes in the starch contributes about 93, 50 and 20 percent of the total crumb firmness at 20°C, 30°C and 36°C, respectively, during five days of storage. The results imply that changes in the starch in the crumb are about one-half and one-fourth as fast at 30°C and 36°C., respectively, than at 20°C. The results suggest that at elevated temperatures, some factor (changes in protein or moisture redistribution or both) in addition to the starch plays an important role in the firming process undergone by bread.

If anyone can offer some insight into how "crumb firmness" is measured that might help with understanding the statements above.

I've asked the author for sources and I'll update this answer if he is forthcoming with that information.

Update:

The author of the article, Noël Haegens, was good enough to get back to me. Here is his reply in its entirety:

The information is based on my personal experience. It is a known fact that the speed of staling depends on the temperature (Avrami constant - retrogradation kinetics). The maximum speed is around 0ºC and becomes less and less as the temperatures rises (or drops below -7ºC).

When I was working for a large industrial bakery we made tests keeping the bread at 35ºC (we even put air-conditioning in a truck) and measured the softness of the bread after 1, 4 and 7 days. The results were compared with bread stored at 0ºC. And indeed the bread stored at a higher temperature was softer.

The only thing which we “forgot” to take into account is that the bread stored at higher temperature got moulded quicker than the bread stored at 0ºC