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Several sources have claimed that canned beans (after discarding the water in the can) are significantly lower in FODMAPs, particularly galacto-oligosaccharides, compared to cooked dried lentils. I'm interested in reducing galacto-oligosaccharides as a way of reducing flatulence and indigestion symptoms.

For instance, https://www.glnc.org.au/resources/hot-topics/how-to-enjoy-legumes-on-a-low-fodmap-diet/ seems to claim a two-fold reduction:

Canned legumes are much better tolerated than dried legumes, due to having a lower FODMAP content. Canned lentils are safe at 1/2 cup, whilst butter beans and chickpeas are low FODMAP at 1/4 cup. Keep in mind that if you choose dried over canned, it is likely the same portion won’t be tolerated if you are sensitive to GOS.

Dried red and green lentils: dried lentils require a little more prep and will take some time to cook prior to being ready to eat. Keep these to 1/4 cup serve of cooked lentils – they’re are a fabulous addition to curries and soups!

Lentils may be cooked without prior soaking, but presumably cooking dried chickpeas involves soaking them, which should reduce the galacto-oligosaccharides. Why would canning reduce this process further? Could soaking the dried legumes for a longer period of time and replacing the water a couple times reduce the FODMAPs to the same level as canned beans (maybe in the fridge to prevent bacterial growth)? Maybe I should soak the beans, cook them, and then soak them further? Or is there something else special about the canning process compared to using dried beans?

  • Hmmm. My first theory was that beans are canned at higher temperatures than you cook them at home ... at like 120C instead of 95C. However, it seems that GOS is highly resistant to breakdown due to heating, so that's not it. – FuzzyChef Dec 15 '19 at 2:08
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    @FuzzyChef : but it'd be pressure cooked in the can, which might have an effect – Joe Dec 15 '19 at 2:59
  • Joe: yeah, but I can't find any evidence that causes GOS to break down. – FuzzyChef Dec 15 '19 at 3:38
  • It does seem like it would take near caramelization temps to break down those sugars. I suspect a combination of pressure canning and then extended soak times before the can is used. – Sobachatina Dec 15 '19 at 15:37
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Why would canning reduce this process further [than soaking]?

Because canned beans are cooked. Cooking adds to further breakdown of many flatulence-causing elements. In fact, if the primary concern is flatulence and indigestion, several authoritative sources have recommended skipping soaking and just using longer cooking. I'll quote from my answer to a previous question here:

The main reason often cited for soaking is to prevent flatulence. However, if you throw out the soaking water, you also throw out lots of nutrients. Recent research suggests that long slow cooking is a better solution and probably gets rid of more of the flatulence-causing components than a soak followed by a quick cook. And you get to retain more nutrients. To quote Harold McGee from On Food and Cooking:

One kind of troublesome carbohydrate is the oligosaccharides [which are water soluble].... But the latest research suggests that the oligosaccharides are not the primary source of gas. The cell-wall cements generate just as much carbon dioxide and hydrogen as the oligosaccharides--and beans generally contain about twice as much of these carbohydrates as they do oligosaccharides.

Based on this research, McGee suggests:

[Soaking] does leach out most of the water-soluble oligosaccharides--but it also leaches out significant quantities of water-soluble vitamins, minerals, simple sugars, and seed-coat pigments: that is, nutrients, flavor, color, and antioxidants. That's a high price to pay. An alternative is simple prolonged cooking, which helps by eventually breaking down much of the oligosaccharides and cell-wall cements into digestible simple sugars.

Specifically regarding galacto-oligosaccharides, there are studies that have attempted to quantify the role of soaking and cooking. That linked study concluded:

During soaking, total α-GOS content decreased between 10% (lentil and faba bean) and 40% (chickpea). [...] Cooking further decreased α-GOS and increased total dietary fibre content.

The explanation for the cooking effect is given later in the article:

Cooking after soaking led to a further decrease in raffinose (−32%), stachyose (−25%) and verbascose (−35%) and to a significant increase in galactose content (+54%) in the whole dish. [...] This should be attributed to further enzymatic degradation, due to better conditions for the expression of α-galactosidase activity. Alpha-galactosidase from lentils are active in the temperature range 20–50 °C and up to 65 °C, and have optimal pH of 4.7, 5.5 or 6.1, depending on their isoforms. [...] During heating, the temperature increased progressively and conditions were met for higher α-galactosidase action.

Basically, elevated cooking temperatures increase the activity of the enzyme alpha-galactosidase, which is naturally found in beans and lentils (and is perhaps better known as the active ingredient in many anti-flatulence supplements/medications).

Hence, canned beans that are thoroughly cooked likely get both the breakdown of the cell-wall cements that McGee mentions as well as increased breakdown of GOS through elevated enzyme action.

I would note, however, the detail in the abstract to that study that each type of bean or lentil will likely respond somewhat differently, as each has its own particular makeup of problematic elements (and those elements will be affected in different ways by cooking). For example, this study on red kidney beans showed significant reduction in lectins, raffinose, and stachyose during soaking, and significant further reduction in lectins and raffinose during cooking. But cooking provided no significant reduction to stachyose.

Could soaking the dried legumes for a longer period of time and replacing the water a couple times reduce the FODMAPs to the same level as canned beans (maybe in the fridge to prevent bacterial growth)?

Well, as seen in the charts for example in the last linked study, increased soaking time shows diminishing returns. Replacing the water could help a bit, but refrigeration may also slow down or stop some enzyme activity that could help in breakdown. (The linked article notes that 77F was chosen for a soaking temperature because it is used by commercial canned bean producers.)

Maybe I should soak the beans, cook them, and then soak them further?

Again, while this is possible and likely to have some effect, there will be diminishing returns. And with each replacement of soaking water, you will like lose more beneficial nutrients from the legumes anyway (along with the things you are trying to get rid of).

Or is there something else special about the canning process compared to using dried beans?

Well, the only thing that might be special about the canning process is the high temperatures canned beans are subjected to for sterilization. In a cursory search, I didn't immediately come up research that quantifies what that might do, but my guess is that it could lead to further breakdown of some flatulence-causing elements. However, I'm not sure about the effect of high-temperature cooking on galacto-oligosaccharides in particular, as the first article linked above postulates that much of the breakdown is caused by enzyme activity that wouldn't happen at canning temperatures. So, in that case, it's just the long cooking process for canned beans in general that may have some benefits.

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  • Didn't know that legumes naturally had alpha-galactosidase. Would slow cooking at temperatures below 65° be advisable to prevent their breakdown, compared to cooking at higher temperatures? Of course, kidney beans should be cooked at high temperatures to break down toxins. – mic Dec 15 '19 at 19:20
  • @mic: Well, that's what that one study suggests. I don't really know much more about the details of temperature vs. enzyme activity than that study suggests. It also implies that the enzyme would be active at room temperature soaking temperatures, though looking at other references suggests maximum activity around 50C. (My anecdotal evidence is that years ago I stopped soaking and instead start dry beans in cold water with a very low flame and let them come up to temp very gradually; I found that method, along with long slow cooking, helped with flatulence more than other things I tried.) – Athanasius Dec 15 '19 at 21:18
  • Obviously if you're trying to maximize the reduction of these things, you'd probably also want to soak and discard the soaking water. But I personally found cooking at low temp for a long time has helped too. (I mostly did it because it helped the beans gradually inflate, sort of combining soaking with the beginning of cooking. But the study gives some reasons why maybe it's a good idea for other reasons...) – Athanasius Dec 15 '19 at 21:20
  • What temperature do you cook at and for how long? cooking.stackexchange.com/questions/12219/… says 140°F (60°C) is a safe minimum temperature for cooking, so it seems that there's a fine line between being hot enough to prevent harmful bacterial growth, and being cold enough for α-galactosidase to work. – mic Feb 14 at 23:25

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