All of a sudden, everyone is talking about 'resistant starch.' This actually isn't anything new. Food science has known about resistant starches since the late twentieth century. One early paper that uses today's common terminology of 'resistant starches' and 'retrogradation' is a 1995 Institute of Food Research publication [1].

"There is emerging evidence that a fraction of starch in our diet escapes digestion and absorption in the small intestine (Englyst rt al., 1992). Most of the starch that we eat has been processed by a heat/moisture treatment which disrupts the native granular structure. If the material is cooled, the solubilised and partially solubilised starch polysaccharides reassociate or retrograde. Retrograded starch is partially resistant to hydrolysis by mammalian amylolytic enzymes in vitro."

The study of starches themselves goes back to the very earliest use of the microscope. Antonie Van Leeuwenhoek, the Dutch researcher who invented the field of microbiology, wrote in 1719:



What Van Leeuwenhoek was looking at were the grains of starch in the meal (probably wheat flour). So the study of starches goes back a very long way.


Resistant starch is being widely promoted now in various supplements and 'superfoods.' Unfortunately, as with turmeric, misinformation and misconception is widespread. For example, resistant starch is seen as one way to offset the poor content of Western diets, rich in rapidly digested starches that release large quantities of glucose into the bloodstream. Rather than urging a change in the diet itself, the food industry promotes the addition of resistant starches to processed foods, to reduce insulin spikes and decrease blood glucose levels without changing the preferred taste and mouth-feel of starchy foods. Needless to say, we do not present turmeric's starches in that light.

So why should we care about resistant starches in turmeric? There are two reasons: first, resistant starch is itself a desirable food component. When it breaks down during fermentation in the large intestine, the products of that fermentation feed beneficial bacteria in the large intestine. Butyrate, one of those products of fermentation, helps keep the immune system in balance by reducing excessive immune system responses. It has many other functions in the body as well [2]. In addition, the development of resistant starch during the preparation of turmeric products governs how mammalian bodies absorb the primary active compounds in turmeric. So it is an important subject.

To understand what resistant starches are, we need to go back to some basic food chemistry. Starch is the form in which plants store energy (sugar, in other words) for use at night, or at other times when photosynthesis is not available. Starch is stored in both aboveground and underground structures (in the seeds of grains, for example, and the tubers of potatoes-and the rhizomes of turmeric). Starches exist in two forms, amylopectin and amylose. Amylose is made up of long linear chains of sugar molecules, while amylopectin has a complex crystalline structure of bundles of branching molecules. Although more investigation is still needed, amylose is often visualized as occupying the spaces between the amylopectin bundles.


If you read about starches, you'll also see the mention of A-type, B-type and C-type starches (though many sources discuss only A and B). These terms have to do with how the bundles or clusters of molecules are arranged. B chains connect to other clusters or bundles, whereas A chains are at the ends of strings of clusters. The C-type of starch connects at only one point. Amylopectin in almost all turmeric species is composed of the B type.

There is one turmeric species, Curcuma karnatakensis, whose amylopectin is primarily of the A type [3]. However, it is cultivated in only one small region of India and so far, at least, has not been exported.

The percentage of both total starch and of amylose in turmeric has been reported at various levels, but one trial found an average amylose level in turmeric starch of about 48%. At that percentage, turmeric has more amylose than either potato starch or rice (both of which are frequently mentioned as benchmarks for starch content). It is less, however, than high-amylose corn, which can be as much as 70-80% of the total starch content, and much more than waxy maize or waxy rice, which have no amylose at all.

When foods that contain these starches are cooked in water, the starches absorb water, lose their crystalline structure and become an amorphous viscous gel: that is, a thick (viscous) amorphous (meaning without a discernible structure) solution. This is what happens to the starch in turmeric during the traditional first processing step, when turmeric rhizomes are cooked for roughly 45 minutes before being dehydrated. If you're a member of the Facebook Original Turmeric User Group, you will have seen the admins say repeatedly that turmeric's starches partially regain their crystalline structure after their initial post-harvest cooking period. In the current online discussions of resistant starches, you're likely to come across the term 'retrogradation.' Retrogradation is exactly that same process--acquiring a partial crystalline structure again after being gelatinized by cooking in boiling water.

This new crystalline structure is referred to as 'resistant' starch, meaning that it is not broken down by the pancreatic enzymes found in the small intestine (primarily amylase). Therefore, its sugar molecules are not absorbed through the lining of the small intestine and do not enter the bloodstream.

So what are the different types of resistant starches? For years, only four types were defined, but a fifth type has recently been added:

RS1 - starch that is inherently resistant to digestion because of its structure. For example, the starch in most grains is surrounded by dense cellular walls that protect it both from water during cooking and from enzyme activity in the small intestine after consumption. When those grains are milled into flour, however, some of that structural protection is lost. Whole wheat berries may contain as much as 14% resistant starch of the RS1 type, but wheat flour typically has only about 2%. That's because milling the wheat into flour breaks down the mechanical barriers and exposes the starches to digestive processes.

RS2 - this type of resistant starch is found in raw potatoes, unripe bananas and high-amylose maize (corn). This starch behaves just the opposite from the mechanisms described for cooked resistant starch. When it is cooked (or when bananas ripen), it reverts from a resistant starch to one that is digestible in the small intestine. That's why dieters are told that green bananas are lower in sugar than ripe bananas. The absolute amount of carbohydrates is no lower, but much of the starch is not digestible until the bananas ripen. So the amount that's absorbed into the body is less for green bananas than for ripe bananas.

RS3 - this is the kind of starch described above as 'retrograded' starch, shared by turmeric and many other rhizomes, corms, tubers and grasses. When the starch in these plants is cooked in water, it absorbs the water and becomes a gel. Technically speaking, the chains of amylose and amylopectin 'unpack' and allow water to penetrate their previously dense structure. This process also frees curcumin particles that were trapped within the starch matrix. When the starch cools, some of it retrogrades (i.e. goes back) to a crystalline structure that has minimal binding sites for digestive enzymes. Therefore it is much less digestible in the small intestine.

RS4 - This starch is made by changing the molecular structure of a natural starch. RS4 starches are typically made from the starches of potatoes, maize (corn) and tapioca. They may be used as additives to other starch-containing foods to increase the percentage of resistant starches, and are also being investigated as a base for food-safe packaging films.[4]

RS5- these are combinations of a starch and a fat. They may occur naturally, or may be manufactured to provide greater resistant starch levels than a product naturally contains.

One immediate question is whether all the starch in a food is converted to resistant starch during the first heating/cooling cycle. Experience with turmeric in the form of golden paste indicates that this is not the case. Research has indeed found that repeated cycles of heating and cooling increase levels of resistant starch [5].

In the case of golden paste, user experience has shown significantly greater benefit when traditionally processed turmeric powder was again subjected to heating in water. The pasting ability of this turmeric powder is immediately obvious, showing that some water solubility of the starch is still present even though traditionally processed turmeric would already have been cooked in water prior to being dehydrated and milled into powder. One reason for the increased benefit is that turmeric's primary active compounds (the curcuminoids) are embedded along with the starches within cell walls whose pectin and cellulose content strongly resists breaking down [6]. So not all the starches are subjected to solution in water during the initial post-harvest cooking. In addition, the presence of predominately B-type starches in turmeric means that not all the starch is accessible to water during each cooking cycle, as are, for example, A-type cereal grains that are almost completely gelatinized during their initial cooking. Research has suggested also that some amount of the previously retrograded starch may experience melting during subsequent heating, allowing repeated gelatinization of the starch. That would also allow additional curcuminoids to be available [7].

So what does all this mean in terms of the effective consumption of whole turmeric?

First, it confirms what we've been saying all along, that raw turmeric is not readily broken down in the digestive tract of humans and other monogastrics. With those active compounds sequestered within plant cell membranes, the digestive enzymes take so long to reach them that food portions will have mostly or completely transited the small intestine before they are available for absorption. The repeated chewing in ruminant digestive systems, along with the repeated exposure to enzyme action, means that cows, goats and other ruminants will have better access to the curcuminoids. But group experience has shown that even they do better with traditionally processed turmeric powder than with raw turmeric rhizomes or raw turmeric powder. The traditional processing of turmeric rhizomes, a roughly 45-minute period of simmering in water. breaks down cell membranes and gelatinizes the starches contained within them. Curcuminoid clusters are more readily available in that amorphous water-saturated form, to be acted on by lipids in the turmeric itself and by dietary fats consumed along with it.

What happens when this initial cooked form of turmeric is cooled and dehydrated? Some of the amylose and amylopectin in the turmeric is converted to an RS3 resistant starch, the 'retrogradation' described above and in other texts on resistant starches. This is what the turmeric group admins have referred to as 'partial recrystallization' in comments in the group. Because of its dense structure, resistant starch is poorly accessible to the pancreatic enzymes in the small intestine. So it passes on through, almost completely unaffected, to the large intestine, where it does break down in a fermentation sequence. The products of fermentation provide energy to the beneficial bacteria that colonize the large intestine, and are also beneficial in many ways to us and our pets [8].

In practical terms, dehydrated turmeric is stored throughout the remainder of the year, after the annual harvest, either as whole fingers that are milled into powder at the time of use (the traditional way of storing turmeric) or as prepared turmeric powder. In either case, the turmeric powder is cooked at the time of use in a water-soluble preparation (in the various cuisines of India) or-hopefully-as golden paste. Much of the remaining amylose in the turmeric now dissolves in the water, in the case of golden paste, or in the water-based vegetable or meat broth in the dietary recipe. This again forms an amorphous gel which, again, allows the curcuminoids trapped within it to become available when the prepared food or golden paste is consumed. And again, when the preparation cools down and is refrigerated, some of those starches will undergo retrogradation to become RS3 resistant starch. Reheating of foods that contain turmeric will probably result in at least one more cycle of available curcuminoids and formation of resistant starch. The reason that is not recommended for golden paste is that the black pepper is much more sensitive to heat than the curcuminoids, and will be damaged by repeated reheating. Just adding golden paste to a hot beverage won't destroy the black pepper, but simmering in a soup or in the hot dry environment of baking is likely to do so.

Citations:

1. Molecular order and structure in enzyme-resistant retrograded starch
https://www.sciencedirect.com/science/article/abs/pii/0144861796813877

2.Resistant starch and "the butyrate revolution"
https://www.sciencedirect.com/science/article/abs/pii/S0924224402001310?via%3Dihub

3. Physicochemical properties of starch obtained from Curcuma karnatakensis - A new botanical source for high amylose content
https://www.sciencedirect.com/science/article/pii/S2405844020300141

4. Physical Properties of Chemically Modified Starch(RS4)/PVA Blend Films-Part 1
https://link.springer.com/article/10.1007/s10924-006-0040-5

5. Studies on effect of multiple heating/cooling cycles on the resistant starch formation in cereals, legumes and tubers
https://pubmed.ncbi.nlm.nih.gov/19562607

6. Enzyme Assisted Turmeric Starch Hydrolysis in Hydrotropic and Supercritical Conditions
https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3701183

7. Retrogradation enthalpy does not always reflect the retrogradation behavior of gelatinized starch
https://pubmed.ncbi.nlm.nih.gov/26860788/

8. Resistant starch as a prebiotic and synbiotic: state of the art
https://www.cambridge.org/core/journals/proceedings-of-the-nutrition-society/article/resistant-starch-as-a-prebiotic-and-synbiotic-state-of-the-art/EFE1F432B77DEE660BB609DEF449D05A