Potato Protease Inhibitor

[Luann]

I've been thinking about this as I get back into having a couple of potatoes daily. Potatoes contain a protease inhibitor that has been studied for its therapeutic effects in rat models. However, a different protease inhibitor, leupeptin, is used in research to cause lipofuscin accumulation (rat study here) and is linked to brain cell degeneration.

I haven't found any research that shows potato protease inhibitors to be dangerous like leupeptin, but they both inhibit serine. Only leupeptin inhibits cysteine. I'm wondering just how different these two PIs are?

 

[Luann]

@Fractality @haidut @aquilaroja @Drareg @Mito @Mufasa @Douglas Ek
Any thoughts?

 

[Amazoniac]

Liuborg,

- Stability of Proteinase Inhibitors in Potato Tubers During Cooking
- Inactivation of trypsin inhibitors in sweet potato and taro tubers during processing

I don't know about specific effects of the remaining inhibitors. Are they in significant amounts after cooking?

 

[Luann]

Amazoniac, your piece of this puzzle fits perfectly into my peace of mind.
I researched which PIs in potato act on serine, similar to leupeptin, and it turned out to be Inhibitors I and II from the study you linked. From wiki: "Serine proteases fall into two broad categories based on their structure: chymotrypsin-like (trypsin-like) or subtilisin-like." From your link: "in cooked tubers, Inhibitor II was completely inactivated during all cooking treatments and Inhibitor I was partially inactivated, depending upon the cooking method."

If I knew how helpful your post would be I would not have forgotten to tag you.
Happily,
L

 

[Amazoniac]

Guru, the following is from the first link and it has nothing to do with the fact that you're about to listen to 'Seven days in sunny June' because I just reminded you of it.

"Concentrations of Inhibitor I, Inhibitor II, and CPI were determined in tissues from six locations within raw Russet Burbank potato tubers (Fig. 1). Cooking treatments for whole potato tubers without peeling were as follows: Boiling-potato tubers were heated in boiling water for 1, 3, 5, 10, 20, and 30 min; Oven baking potato tubers were baked in the 375°F preheated oven for 1, 3,10, 20, 40, 60 and 8(1 min; and Microwave baking-the potato tubers, in sets of two, were baked in a microwave oven (Litton Industry Model SOO, 2450 MHz) for 1, 3, 5, and 7 min."​

Now a random phrase just to change the subject with great formatting.

"Apical end cortical tissues exhibited a significantly higher concentration of all inhibitors than the other five locations and the levels of the inhibitor decreased in other tissues proceeding from the apical end of the raw tubers toward the stem end."

"Ryan et al. (1968) previously reported that Inhibitor I in the cortex tissue of freshly harvested potatoes were uniformly distributed throughout the tubers but it decreased significantly, primarily in stem end cortex, during storage for 1 yr. The tubers studied here were stored for 10-12 months and the Inhibitor I concentration was low in the stem end as expected (Ryan et al., 1968)."

"The center pith exhibited the lowest concentration of all inhibitors among the six selected locations (Table 1). Inhibitor II exhibited the highest concentration among the three proteinase inhibitors in raw potato tubers."​

Inhibitors Ein and Zwei we don't need to care about because they were destroyed with boiling:

"Inhibitor I was completely denatured by boiling and microwave baking. When cooked by microwave baking, Inhibitor I was almost completely denatured within 5 min. Boiling the tubers for 30 min was as totally efficient in denaturing this inhibitor as microwave baking. On the other hand, even after 80 min of oven baking a high percentage of Inhibitor I remained in the cortical tissues of the tubers."

"All three cooking methods completely denatured Inhibitor II (Fig. 3). During the first 5-10 min of boiling or oven baking the immunologically reactive Inhibitor II increased dramatically indicating that some Inhibitor II protein must be solubilized during heating. This suggests that some of the inhibitor is bound in uncooked tubers and is unavailable to the immunological assay; perhaps loosely bound to membranes or ionically bound to the cell walls. It is of interest that no such increase was observed during microwave cooking. However, denaturation may have been so rapid that release of the inhibitor may not have been detectable."

Each line represents a different part of the potato, all should converge to 0.
Boiling is effective much faster than baking.​

"On the other hand chymotrypsin inhibitory activity was entirely abolished by microwave baking but was less destroyed by oven baking or boiling. These results confirm that microwave baking very efficiently destroys proteinase inhibitor activity toward both trypsin and chymotrypsin, but that significant chymotrypsin inhibitor activity survived boiling and oven baking."

"The carboxypeptidase inhibitor is the most heat stable of the four inhibitors assayed. As shown in Figure 5 this inhibitor was stable to all three cooking methods. Boiling for 30 min or oven baking for an hour had little effect on the inhibitor. Microwave baking for 7 min denatured only about 25% of the polypeptide."

However! There are differences in content of each depending on the variety:

- Variability in the concentration of three heat-stable proteinase inhibitor proteins in potato tubers

But regarding tuber toxins in general, there are some things to does to increase the chances of destroying them:

- Cut prior to cooking (instead of processing them whole as it was the case above): If you're going to purée them, this is an option since the layered sheets will form an umbrella for..
- Steam
- Cook them for longer (instead of 30 min)

- Discard the part where they found most of the toxins (as long as it's a witness of history): check Table Ein, apical end cortex

This only applies if what they found is common to all potatoes:

Stem/proximal/stolon/attachment end is the umbilical cord to the original plant, it forms a scar when it disconnects. Underground plants usually grow in a conical shape for forcing against the funeling soil (which is inverted when they're hanging above the ground), therefore I suspect you can spot some sort of heart shape in these tubers, and the narrower part is this apical/distal/rose end where they found the concentration of toxins.

Modifications of Stem: Underground, Sub-aerial and Aerial Modification

Keep in mind that this is what I understood from what I read, so it might not be right.​

- Peel before cooking
- Peel it again to remove toxins that might concentrate near the peel: check again Table Ein, center pith. As expected, the closer you are to the extremities, the more toxins you'll find.​

Out of curiosity:
- Potato protease inhibitors inhibit food intake and increase circulating cholecystokinin levels by a trypsin-dependent mechanism (raw and purified)

@Drareg

*2016 - †2017

Sadly,
Broscientist

 

[Peater Piper]

I would expect pressure cooking to be even more effective than boiling when it comes to destroying potato toxins.

 

[Luann]

WOWW!!! I'm astonished by Amazoniac's thorough report on potato enzymes!! I hope your research and post was as interesting for you to write as it was for me to peruse. Now I know just how to cook my spuds.....and, I feel at peace knowing that there are handsome guys around to answer my questions!
Too bad about drareg.
Reading and re-reading your work with a big smile,
L

 

[Amazoniac]

WOWW!!! I'm astonished by Amazoniac's thorough report on potato enzymes!! I hope your research and post was as interesting for you to write as it was for me to peruse. Now I know just how to cook my spuds.....and, I feel at peace knowing that there are handsome guys around to answer my questions!
Too bad about drareg.
Reading and re-reading your work with a big smile,
L

Oh, I don't want to make it awkward or anything but I'm.. I'm married. We can still be friends that won't listen to 'Spending my time' thinking about the other. So as a (married) friend, I think this might interest you:

- The three-dimensional distribution of minerals in potato tubers

One more reason to discard the outer layers:

"[..]for all minerals other than iron, the tuber flesh contained more reserves of minerals than the surface layers. For example, tuber peel contained 17, 34 and 55% of the total tuber zinc, calcium and iron, respectively."

Spoiler: Distribution of minerals

"Tubers about 12 cm long were first washed, rinsed in deionized water and briefly air-dried. Five concentric skin (periderm and associated cortex) samples were carefully removed with a vegetable peeler to a depth of 0·3–0·4 mm from the apical to the stolon end of the tuber. The peeled potato tubers were then cut longitudinally into top (A), middle (B) and bottom (C) slices of equal thickness, the orientation of the tuber in the soil having previously been determined. Each of the three slices was further cut into five strips longitudinally and each of the five strips was then cut into five pieces (1–5) of equal length (Fig. 1)."

 

[Amazoniac]

I wonder how much of that sulfur occurs as cysteine..

"The bioavailability of minerals from plant tissues depends on their chemical form and the presence of promoter substances and anti-nutrients (White and Broadley, 2009). Potato tubers have high concentrations of promoter substances such as ascorbate, b-carotene, protein cysteine and various organic and amino acids that enhance the absorption of essential micronutrients (White et al., 2009)."​

- Why You Should Eat Glutathione (Chris Masterjohn)

"Potatoes deviate from the typical starch in that they’re a vegetable, and they’re a pretty decent source of glutathione."​

 

[Luann]

The link made me laugh! Well if I'm ever looking for love on the RPF I will count you out amazoniac, haha.
You have been so helpful to me on this thread :)

 

[Amazoniac]

The link made me laugh! Well if I'm ever looking for love on the RPF I will count you out amazoniac, haha.
You have been so helpful to me on this thread :)

I still have friends that are available on the market but it's going to depend on your goals. Telling me that you want to become a gold-digger, I recommend milk_lover; wanting to have your space and looking for someone distant, Diokine or Such; enjoying your dates like you enjoy your milk: 1% fat or less, Yon or paymanz; interested in a significant other from a significant country, Suikerbuik, Vinero or half of our forum members; someone to spoil you, waking you up in bed with a glass of freshly-squeezed DMSO and a bowl of apricot seeds, that's burtlan; and so on depending on your preferences. It's time to incline towards you with hands behind back to show a helpful disposition and ask you: do you have anything specific in mind?


But now to the irrelevant part, it's unbeliuboble how much conflicting information we find when searching for ascorbic acid changes due to potato cooking. However..

- Degrading it might be positive because it can prevent an interaction with iron.
- When you compared based on same weight, the skin of potatoes have much more iron (as pointed out above) and slightly more vit C than the flesh: one more reason to discard the outer layers prior to make a cook and consume only the interior.It must be good to reduce it to minimize in turn an interaction with iron.
- In terms of actual oxidation from air contact, boiling must be similar to steaming as long as the pan is covered as well. Oxygen can destroy exposed vit C, but it's always preferable to cook with moist than dry heat. So I guess it's out of question.
- The common losses noted with moist heat are usually due to leaching and not so much thermic degradation. Sometimes dry heat can also incur some loss when water is outcurred from the potato.
- Storage temperature and length is often neglected but plays an important rôle in affecting the content. There's a sharp drop and later it starts to stabilize and even between potatoes regardless of their initial amount.
- Peeling facilitates the loss of vit C to the cooking liquid, but since nutrients are also leached, it doesn't make sense to throw it away. You can lose 50% of the original potassium by discarding the liquor. Therefore it's indifferent if it's inside or outside the potato, it will eventually be mixed during digestion.
- The priority is to get the potato done and remove it from the heat when it has reached its tastiest state. Afterwards, any leached vit C will be degraded as the remaning liquid is concentrated into a sauce.
- There's a lot of degradation when potatoes are not consumed immediately. So reheated potatoes should have much lower contents.
- I'm not sure if heat can destroy some of the encased vit C before the cells burst and it has a chance of interacting with other stuff. If this is the case, it would favor steaming.

I'm considering adopting this kind of structure more often. Everyone must be fed up of poems.

- It seems to me that the most reliable way to degrade it without harsh cooking methods must be by boiling thin peeled potato slices in an open pan with extra liquid, removing them when they're done, mashing them and placing them aside while all the liquid is being reduced to a sauce. A great deal of vitamin C should be leached, extra water will require longer cooking but the potatoes were already removed, and with the waiting there will be further degradation. A lot of bubbles might form, and it's something that should be eased if apfels are cooked along. None of this is necessary considering that there's already substantial loss from storage, peeling, usual cooking and degradation until reheating.

I don't think the vit C/iron content is something to be dismissed when you realize that there are reported values of up to 50 mg of vit C/100 g of raw potato and below for example more than 1.5 mg of iron for every 100 g. Wagner started to question this some time ago.

Cron-o-meter values (1 kg):

Boiled with skin
Potassium - 5600 mg
Vit C - 120 mg
Iron - 16.5 mg

Boiled without skin
Potassium - 3300 mg
Vit C - 75 mg
Iron - 3.1 mg​

Assuming that these values are from potatoes of the same harvard, the skin content and its help in retention might explain the difference.

Spoiler

- In Glassy Bioaccessibility and Bioavailability of Iron from Potatoes with Varying Vitamin C, Carotenoid, and Phenolic Concentrations

- Effects of Cooking Methods on Nutritional Content in Potato Tubers (!)

"The measured effects of different cooking methods on potato nutrient quality depend on the type of cooking, temperature, cooking medium (such as oil or water), cooking duration, surface area exposed to water and oxygen, volume of solvent used for extraction, pH, and type of method used to quantify phytochemicals and their activity."

"Vitamin C is heat sensitive, and therefore any cooking methods lead to a substantial loss of Vitamin C content. Vitamin C is hydrophilic, and therefore cooking in water results in a severe loss of content. Love and Pavek (2008) have observed a disproportionate loss of AA among cultivars cooked with the same method: one cultivar exhibited a 10% to 20% loss, whereas another cultivar showed up to a 70% to 80% loss. This observation is a result of the dependence of vitamin C loss on heating intensity and time. Burg and Fraile (1995) and Navarre et al. (2010) have reported modest or no losses of AA unless potatoes are overcooked. Grudzińska et al. (2016) have found that higher losses of AA occur in microwaved tubers than steamed tubers (0.270 mg/g less AA in conventionally grown, microwaved tubers than steamed tubers; 0.140 mg/g less AA in organically grown, microwaved tubers than steamed tubers). Loss of AA has been suggested to be due to leaching rather than thermal degradation. Gołaszewska and Zalewski (2001) have found that dry cooked potatoes retain higher levels of AA than do wet cooked potatoes, probably because of AA leaching during cooking in water. Low AA levels, observed after microwaving or other dry cooking methods, are associated with loss of water content in potatoes (Burg and Fraile 1995). In a study involving multiple cooking methods, Tian et al. (2016a, b) have observed the greatest loss of AA after air-frying, frying in oil, and stir-frying (90.42, 83.35, and 55.5%, respectively), followed by baking, boiling, and steaming or microwaving. Similar observations have been made by Murniece et al. (2011), who have reported the greatest AA loss from roasting, then shallow-frying, and deep fat frying."

"The other major vitamins in potato, niacin and thiamin, are also affected by cooking as a result of heat sensitivity, irrespective of cooking method."​

- Effects of cooking methods on potassium and vitamin C retention in Irish Potato varieties

- The Effects of Boiling and Leaching on the Content of Potassium and Other Minerals in Potatoes

"The amount of K remaining in the potato samples was strongly dependent on the preparation method. Leaching [soaking over the nights] of cubed samples had little effect on K content, and the average amount of K remaining was 96% and 100% of control values in trials 1 and 2 (Figure 3), respectively. Shredded samples retained less of their K after leaching, but K amounts after overnight leaching were still 83% to 98% of control values (Figure 3).

Boiling samples immediately after cubing or shredding resulted in a much greater loss of K. Boiling cubed potato pieces reduced the amount of K remaining in the samples to 50% of control values (Figure 3). Boiling shredded samples resulted in an even larger loss of K. Total K content in the shredded and boiled samples was only 25% to 31% of that in control samples (Figure 3)."

"[..]boiling thinly sliced potatoes resulted in greater K loss than boiling diced potatoes (Tsaltas 1969)."

According to "dry weight is 20% of fresh weight" of potatoes.

Spoiler: Other minerals

- Vitamin C Destruction During the Cooking of a Potato Dish

"The vitamin C is a nutrient particularly sensitive to the process conditions. Temperature, as well as pH, water content (6,7), presence of oxidizing substances (8), oxygen (9), and the presence of metallic (10) and biological catalysts (11,12) influence its destruction rate. For some processes, like blanching in water, the vitamin C losses take place by diffusion of the vitamin C inside the potato, followed by leaching into the blanching fluid (14–16). In this case, authors generally neglect the existence of the enzymatic destruction of the vitamin C."​

- The effect of cooking and storage on the ascorbic acid content of potatoes (!)

"Potatoes are consumed throughout a period beginning before their maturity and extending from 5 to 10 months after the tubers are harvested. During this period they undergo metabolic changes, the character and extent of which depend upon the duration and conditions of storage as well as upon the variety of potato. These changes are accompanied by changes in the ascorbic acid content."

"Although the ascorbic acid content of raw potatoes is reported to vary between the wide limits of 53 mg. per 100 gm. (10)³ and 1.5 mg. per 100 gm. (13), the results of most investigations indicate that newly harvested potatoes are highest in vitamin C value and that this quantity diminishes very rapidly during the first part of storage and later more gradually (16, 18, 22, 26). Kröner and Steinhoff (8) noted this decrease followed by a rise during the latter part of storage, and Pett (18) reported that sprouting caused an increase followed by a rapid decrease."

"Olliver (16) reported a loss of 50 percent of the ascorbic acid from new potatoes after 15 days of storage and Pett (18) a loss of over 50 percent after 20 to 30 days. In the present study there was also a marked decrease in the ascorbic acid content of stored potatoes, very rapid during the first few weeks after harvesting, and more gradual thereafter."

"The immature Chippewa potatoes lost 22 percent of their ascorbic acid after 1 week's storage at 25.5° C. and 36 percent in 3 weeks (table 3).

"New" Green Mountain potatoes bought on the market decreased in ascorbic acid content from 20.0 to 17.3 mg. per 100 gm., a loss of 14 percent in about 10 days at 25.5° C. These potatoes had probably been harvested for at least a week before the first analysis for ascorbic acid was made.

The Maine-grown Green Mountain potatoes lost about 30 percent of their ascorbic acid in the first month of storage at 15.5°C. (fig. 1)."​

"It is interesting to note that in these potatoes the rate of loss of ascorbic acid at 15.5° and 25.5° C. was apparently independent of the variety of the potato, but dependent, to some extent at least, upon the ascorbic acid content of the tuber. The variation in ascorbic acid content from tuber to tuber was much smaller in potatoes that had been stored for a short period than in immature and new potatoes. This approach to uniformity during storage could only have been brought about if the loss of the vitamin were greater during the same period from tubers of high ascorbic acid content than from those of lower ascorbic acid content."


"Fixsen (4) has summarized the data that show the effects of cooking upon the ascorbic acid content of potatoes. No general agreement exists. The same method of preparation reported by some investigators as causing a loss in ascorbic acid value, has been reported by others as apparently causing a gain; and one investigator reports that the same method of cooking results in a loss of ascorbic acid in one variety and a gain in another. Richardson, Davis, and Mayfield (20) reported a gain of 28 to 75 percent in ascorbic acid as a result of baking potatoes, whereas Lyons and Fellers (12) using the same method of cooking noted a loss of 50 percent. The gain in ascorbic acid found after cooking has been ascribed variously to the presence in the raw tuber of a protein-ascorbic acid ester capable of hydrolysis on cooking (9, 13, 16, 19, 23), to the destruction of the ascorbic acid oxidase by heat (3, 14), and to the increased permeability of cooked tissue to the extracting medium, which results in greater ease of extraction of the vitamin from cooked than from raw potatoes (16, 25)."

"Preliminary experiments indicated that the distribution of ascorbic acid was not uniform throughout the tuber."

"It has been repeatedly stated (7, 13, 17) and apparently is usually assumed that the distribution of ascorbic acid in the raw potato is uniform or nearly so, although Rudra (21) found a higher concentration in the skin than in the flesh of new potatoes. Some of the potatoes used in the present study were examined after a period of storage, and it was found that, although there was a tendency for the concentration to be higher in the cortical [outer] layer, the difference between it and the medullary [inner] layer was slight."

"When wedge-shaped samples were taken from various parts of the potato it was noticed that the concentration of ascorbic acid was always higher in the bud end of the tuber than in the stem end."

"In the analysis of potatoes for their ascorbic acid content, the problem of representative sampling has often been overlooked or ignored. In the case of raw potatoes, grating or slicing and mixing, preliminary to sampling increases the possibility of formation of dehydroascorbic acid and of irreversibly oxidized ascorbic acid. However, unless the distribution of ascorbic acid is uniform throughout the tuber, there can be no assurance that a small single sample from a tuber is representative of the whole."

"The apparent inconsistencies of the gains reported for ascorbic acid in cooked potatoes suggest the operation of an accidental factor. If, for example, boiling pared potatoes causes an an apparent increase in ascorbic acid content, boiling unpared potatoes should have the same effect. However, if the type of distribution of ascorbic acid shown by Irish Cobbler potatoes at 15.5° C. were present but unrecognized, it is easy to see that the accidental selection of a disproportionately large number of samples from the bud end of the tubers would result in a relatively high ascorbic acid content as estimated from this group of samples. If, in addition, these samples represented cooked potatoes, the high value obtained would be thought to be a result of cooking rather than of sampling."


"In order that the potatoes cooked by different methods should attain the same stage of "doneness," cooking was discontinued when the internal temperature measured at the center of the tuber had reached 96° C."

Steaming

"Three unpared Irish Cobbler potatoes weighing between 153 and 180 gm. were used for each cooking. They were cooked in a covered steamer over 4 quarts of rapidly boiling water. This method required from 33 to 39 minutes except in one case when the time extended to almost 51 minutes. There was usually a gain in weight on the order of 1 gm. per potato. No ascorbic acid was recovered in the water. About 10 percent of the ascorbic acid originally present in the raw potatoes was destroyed."​

Boiling

"The potatoes were cooked by immersion in boiling, distilled water."

"Potatoes were boiled pared and unpared. Three potatoes weighing between 140 and 180 gm. were used in each case. The unpared potatoes (Irish Cobblers) required from 33 to 40 minutes cooking time and gained about 1 gm. apiece in weight. They lost about 10 percent of their original content of ascorbic acid, none of which was found in the cooking liquid."

"The pared potatoes required from 31 to 37 minutes for cooking."

"The Irish Cobbler potatoes gained from 2 to 6 gm. in weight and there was very little sloughing. The potatoes contained 76 percent of their original ascorbic acid and an additional 5 percent was recovered in the cooking liquid so that the destruction of the vitamin was 19 percent."

"The "new" Green Mountain potatoes were more mealy than the Irish Cobblers, which had been stored, and there was a great deal more sloughing during cooking. Nevertheless these potatoes retained 80 percent of their ascorbic acid value. The cooking liquid contained another 12 percent. The average destruction of ascorbic acid in these potatoes was 8 percent, but because of the magnitude of the standard error, this difference cannot be regarded as significant. If the ascorbic acid present in the cooking Kquid were disregarded the loss would be 24 percent in the Irish Cobbler and 20 percent in the Green Mountain potatoes."​

"Steaming and boiling unpared potatoes were the most conserving of vitamin C. Baking and pressure cooking caused slightly larger losses of ascorbic acid, while boiling pared potatoes was least conserving of the vitamin. However, the maximum loss of ascorbic acid due to a cooking method never exceeded 25 percent."


"The results of the present study indicate that the vitamin C value of potatoes is more dependent upon the time that elapses after harvesting before the potatoes are consumed and the temperature at which they are stored during that time than upon the method of preparation for consumption. The method of cooking most commonly employed, the boiling of pared potatoes, causes the greatest loss in ascorbic acid content, particularly since the cooking liquor is usually discarded. Losses due to other methods of cooking are somewhat smaller and fairly comparable. However, the decrease in the ascorbic acid content of potatoes during the first few weeks after harvesting exceeds even that which would be produced by the most destructive of the cooking methods."

"Another essential factor in the determination of the actual effect of a cooking method upon the ascorbic acid content of potatoes is the elimination of the effect of storage. This is particularly important when the potatoes are studied soon after being harvested since the rate of decrease in their ascorbic acid content is extremely rapid at this time. Unless determinations of the ascorbic acid content of both raw and cooked potatoes parallel each other in point of time, the result obtained for the effect of a cooking method may be very erroneous. In addition, the variation in ascorbic acid content from tuber to tuber is much greater soon after harvesting than it is after a period of storage. Consequently the sampling of an adequate number of tubers is imperative."​

- The effect of some processing methods on the vitamin C content of sweet and Irish potato

- Bioactive Compounds in Potato Tubers: Effects of Farming System, Cooking Method, and Flesh Color (dry matter)

- Changes in vitamin C content of Bintje potatoes during storage and usual culinary preparations (en français)

- Impact of potato processing on nutrients, phytochemicals, and human health

- Effect of Culinary Processes on the Content of Nitrates and Nitrites in Potato