Facts about miracle fruit (miraculin revisited - part 2:2)

Short introduction in Norwegian: I anledning at jeg deltok i en episode om mirakelfrukt på Schrödingers katt på NRK (og YouTube) publiserer jeg to blogginnlegg om temaet. Det første innlegget handler om smakstesting av mirakelfrukt. Innlegget nedenfor er del 2 av 2 og er en samling fakta om mirakelfrukt med referanser til forskningslitteratur. Siden denne bloggen normalt er på engelsk fortsetter jeg herved på engelsk.

On the occasion of me attending an episode of the Norwegian popsci TV series "Schrödingers katt" (and YouTube) about miracle fruit I post two entries on miracle fruit and its key constituent miraculin. The first post describes a tasting of miracle fruit with a number of sour foods. The second post below is a collection of facts about miracle fruit based on research literature. Part 2:2 below is divided into the following main topics:

• Miracle fruit/berry - properties
• Traditional/historic use of miracle fruit
• Taste sensation when using miracle fruit/miraculin
• Cultivation and production
• Use in commercial products, potential applications
• Health risk and governmental/public approval
• Elimination of the sweet-inducing effect
• Mechanism of taste-modifying activity
• Some other sweet and taste-modifying substances
• References

Miraculin revisited - part 1:2

Introduction in Norwegian: I anledning denne ukas episode om mirakelfrukt på Schrödingers katt på NRK (og YouTube) publiserer jeg to blogginnlegg om temaet. Det første er en reprise av et tidligere innlegg, dog utvidet med noen flere smakstester. Det påfølgende er en samling fakta om mirakelfrukt med referanser til primærlitteraturen. Siden denne bloggen normalt er på engelsk fortsetter jeg herved på engelsk. Innlegg nr. 1 av 2 følger nedenfor.

On the occasion of me attending this week's episode of the Norwegian popsci TV series "Schrödingers katt" (and YouTube) about miracle fruit I'll post two entries on miracle fruit/miraculin. The first is a reposting on a previous entry, expanded with a few more tasting notes. The second post will be a collection of facts about miracle fruit including references to primary literature. Part 1:2 follows below.

The following entry was previously published 7 August 2010, slightly revised.
Note: the original blog entry has some interesting comments worth having a look at.

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For some time now, there has been somewhat of a hype about the miraculous berry that makes everything sour taste sweet. Some time ago, I ordered a packet of dried and powdered miracle fruit tablets and gave it a try. The following post gives some background and the results of a truly fascinating experience.

The miracle fruit is a a berry containing the glycoprotein miraculin with the unlikely effect that when your taste buds meet this substance, you taste sour foods as they were sweet. That is, your perception of sourness is altered. In certain parts of the world, the substance has been used for quite long, whereas in USA and Europe it has not yet been cleared for use as additive. The berry in itself is allowed, but unfortunately they don't keep for long and are apparently not suited for shipping fresh. However, a freeze dried version made into tablets does exist and this is the version I tried.

There is quite some amount of research on the effect and mechanism of miraculin on our tongue as a google scholar search for "miraculin" reveals. The first scientific report was in Nature as early as in 1968 (correction: first time published in 1965). There is also research indicating that other plants exhibit similar effects, such as curculin from the Curculigo latifolia plant. The miraculin protein structure shown here is taken from the Swiss protein structure homology-modeling service.*

Miraculin!

For some time now, there has been somewhat of a hype about the miraculous berry that makes everything sour taste sweet. Some time ago, I ordered a packet of dried and powdered miracle fruit tablets and gave it a try. The following post gives some background and the results of a truly fascinating experience.

The miracle fruit is a a berry containing the glycoprotein miraculin with the unlikely effect that when your taste buds meet this substance, you taste sour foods as they were sweet. That is, your perception of sourness is altered. In certain parts of the world, the substance has been used for quite long, whereas in USA and Europe it has not yet been cleared for use as additive. The berry in itself is allowed, but unfortunately they don't keep for long and are apparently not suited for shipping fresh. However, a freeze dried version made into tablets does exist and this is the version I tried.

There is quite some amount of research on the effect and mechanism of miraculin on our tongue as a google scholar search for "miraculin" reveals. The first scientific report was in Nature as early as in 1968. There is also research indicating that other plants exhibit similar effects, such as curculin from the Curculigo latifolia plant. The miraculin protein structure shown here is taken from the Swiss protein structure homology-modeling service.*

An ordinary google search gives various producers and web shops for buying the stuff. Adding to the fun are the conspiration theory-like suggestions (two refs.) of the sugar industry's ways of stopping miraculin approval in the USA since the product might reduce the population's consumption of sugar (which of course is beneficial for everyone except the sugar industry). There are also efforts being made on producing the miraculin glycoprotein using genetic engineering methods, and I guess the hope is that one might efficiently produce miraculin or a relative using common plants or organisms such as lettuce or E-coli bacteria (same as is done with production of other proteins/enzymes such as medicinal insulin or rennet for cheesemaking).

Blue cheese and plum soft ice cream (TGRWT #18)

Pastry chef Aidan Brooks has posted the invitation to join in on episode #18 of "They go really well together" (TGRWT). The idea is that foods with one or more flavour compounds in common will taste well in combination; the concept of flavour pairing. This month's ingredients are plum and blue cheese.

I've found that simple dishes such as milk shakes, smoothies and ice cream (this post) provide excellent matrixes for testing flavour pairings. Things are kept simple allowing for the pure flavours to come out, not being swamped by lot of (distracting) flavours from other ingredients. As such, milk and/or cream pose a good base. So this time, I went for a very simple straightforward recipe for soft serve ice cream. At the same time, it gave me the opportunity to test our new Bamix immersion blender (a common blender or smoothie machine will probably work well too).

My first spherification

Yes, I know. I'm six years late to be among the cool guys, but who cares? To me it's all about having fun and learning, and then being late is no issue.

As far as I know, spherification and it's related methods were introduced by El Bulli chef Ferran Adrià et al. some time after the turn of century. I has got a lot of attention, and a You Tube search on the term gives quite a few hits on demonstrations of DIY spherification.

The phenomenon is based on using hydrocolloids, that is compounds that can generate gels, mostly with water but other media are also know (oils and alcohol mixtures, that is). A good place to start is Martin Lersch's hydrocolloid recipe collection "Texture". A number of different gelling agents are being used, the most common household substances being gelatin and fruit pectin, the latter often used when making jams and jellies.

In this case I got my hands on some sodium alginate that I wanted to play with. When a mixture containing sodium alginate comes in contact with a calcium solution, the alginate starts to cross-link and a gel is formed. In this case, dripping a alginate-containing solution into calcium chloride generates small beads that are gelatinous on the surface and liquid in the centre. Alginate is somewhat sensitive to the pH, and sodium citrate might be used as a buffer to stabilise the pH at ca. 4-5 (all this information is found in the Textures recipe collection).

Sodium alginate is a polymeric carbohydrate-like compound which is soluble in water. When it reacts with calcium ions, cross-links are formed giving large three dimensional webs that become viscous/gel-like and holds water.


Strawberry spheres in sparkling drink (for lava lamp effect)
(Sparkling Chardonnay or non alcoholic cider are both fine)

Equipment
(immersion) blender
scale (0.1 g precision is needed)
some general kitchenware
disposable plastic pipette (7 ml) or plastic syringe (10-20 ml)
(pH strips)

Ingredients
frozen and thawed strawberries, 200 g
sugar, 25 g
sodium alginate, 1.9 g
sodium citrate¹, 2 g
calcium chloride², 2.5-4 g
water, 500 ml
Sparkling Chardonnay or non-alcoholic drink (i.e. apple cider)

Procedure (see You Tube for informative demonstrations)
For template, the recipe for Melon cantaloupe caviar taken from El Bulli's texturas recipes: The strawberries were blended and mixed with the sugar. pH measured to be ca. 3 (somewhat uncertain since the berries gave some colour to the strips). Sodium citrate was added gradually, stopping at a total of 2 g to get a pH of ca. 4-5.¹ Sodium alginate was added and blended (the alginate partially turned into lumps; should have added the alginate to a small portion, mixed this, and then added the rest. Lots of blending did the trick). The mixture was strained through a sieve. For easier dripping (see below), the mixture was diluted 1:1 with water (the initial strawberry mixture was rather viscous, resulting in oblong or drop-shaped "caviars"). This would of course affect gelation, hence the amounts here are deduced on a try-and-fail basis.

Calcium chloride was dissolved in the water. The strawberry mixture was dripped into the calcium chloride solution, the drops forming small strawberry beads, and left for 1/2 to 1 minute.³ The beads were strained, rinsed in water and added to the sparkling wine or cider.

(Tri)sodium citrate functions as a buffer due to its three carboxylic acid functional groups.


Verdict
The strawberry beads/spheres/caviars tasted good, no detectable flavour from the matrix. Simply strawberry. While mixing, the strawberries turned somewhat greyish. Not surprising, since the colour is an anthocyanin pigment (anthocyanin colours are pH dependent, often bright red in acidic environment and more on the green/blue side in basic conditions).

The reason for using Chardonnay was simply that I found Chardonnay to match well with strawberries at the food pairing database, and that this might be a fun aperitif (although I would maybe not spend money on an expensive wine and then put strawberry in it).

What might be taught

• chemical reactions might occur between chemical compounds
• experimental and cooking skills (weighing exact amounts, diluting etc.)
• dispersions: gels (hydrocolloids) and macromolecules
• pH, acidity and buffers (citrate)
• density (the beads float up together with the CO₂ bubbles, and sink when the bubbles burst)

The procedure is somewhat complicated, and I'm glad I didn't bring the kids the first time. When the table was set, and the solutions were ready, the kids loved dripping the solution making beads. Now I've got some experience, and next time I'll bring them along from the beginning.

Notes/comments
¹ It was difficult to assess the pH correctly, and the amounts of sodium citrate suggested in the textures recipe collection did not (seemingly) have the desired effect. Hence, citrate was added until the desired pH, adding up to 2 g.

² The CaCl₂ must be dry/dehydrated. In my case, it had absorbed moisture from the air and gone all wet (quite hygroscopic). It was in left in shallow bowls in the oven at 150-200 °C stirring occasionally. A couple of hours later, a white crystalline/powdery salt was left.

³ Using 2.5 g CaCl₂ per 500 ml water and leaving the beads 30 seconds in the bath resulted in rather soft beads. Leaving them for one minute gave beads that were solid almost throughout. I wanted firmer beads with a soft interior. Increasing the concentration to 4 g CaCl₂ per 500 ml water did the trick: firm shell, and liquid interior when the beads were left in the bath for somewhat less than a minute.

References
McGee, H.: McGee on Food and Cooking. London: Hodder and Stoughton 2004.
Lersch, M.: Hydrocolloid recipe collection

Deciphering an old preserves recipe

Teaching food preservation methods combined with science often results in the classical why-questions; why does the recipe tell us to do things this way, and why is that so important (...and is it really that important?)

Autumn means ripe fruit, berries and vegetables, and many of us look forward to harvesting for the coming year. Only a few areas of the world have the benefit of continuous supply of fruit and vegetables (especially we up here in the north), and this has lead to loads of ways to preserve food. Nowadays, however, many of these methods are used for culinary purposes rather than survival. Recently, Martin wrote about cherry jam (I have given a few on his post as well). Also, Hervé This has written about culinary proverbs and old wives tales, "culinary precisions" as he terms them, some being sound advice from a scientific viewpoint, some being directly misleading, and others probably not making that a big difference.

So, I thought this might be a good time to take a recipe for sugar-preserved pears and have a closer look at it, step by step. It is in fact a quite fun exercise.

Traditional sugar-preserved pears «the old fashioned way» (generic recipe)

1. Heat jar in oven or water by slowly increasing the temperature to above 100 °C and keeping it at that temperature for a certain amount of time (i.e. 15 minutes). Cool.
   
2. Peel pears. Cut in half and remove seeds and core
   
3. Make syrup from water and sugar by boiling the syrup until the sugar is dissolved. Leave to cool
   
4. Fill a jar with the pears. Fill up with syrup, cold or lukewarm, but not hot
   
5. Put the lid loosely on, do not tighten
   
6. Place jar in a pot with cold water almost up to the rim of the jar. Heat slowly and keep at boiling point for a certain amount of time (i.e. 8 minutes)
   
7. Remove jar from pot and
   Either: leave to cool for a certain amount of time (i.e. 15 minutes). Then tighten the lid
   Or: Tighten the lid while boiling hot
   
8. Turn jar upside down and
   Either: store upside down
   Or: leave upside down for a certain amount of time (i.e. 15 minutes)

Taking a closer look
Firstly, the main reason for all these operations is to keep the pears edible for a long time, more specific until the next time ripe pears are at hand (a year, usually...). The two main ways that fresh fruit is spoilt are chemical (enzymes and reaction with oxygen from the air) and biological (microorganisms: bacteria, moulds and yeasts). Even though the methods were developed before the discovery of microorganisms and enzymes, the results of these were evidently clear. The source of enzymes is the fruit itself, whereas microorganisms are ubiquitous: the fruit itself, hands, tools, jar and in the air. So a perfectly sterile fruit would over some time be contaminated by just sitting in the air.
Two important facts: microorganisms thrive and multiply at temperatures between 10 and 40 °C, and die at high temperatures. The trick is thus to avoid the 10-40 °C window. Also, microorganisms need water to thrive and multiply.

So, how much of this procedure makes sense from a scientific point of view? I've marked the steps being most «fishy» with a red asterisk:

1. Heat jar and lid in oven or water by slowly increasing the temperature to above 100 °C and keeping it at that temperature for a certain amount of time (i.e. 15 minutes)
   Makes sense, but cooling leaves the jar and lid ready for infection. However, they're dry and not prone to being infested
   
   
2. Peel pears. Cut in half and remove seeds and core
   No effect other than possibly infecting the pears from hands, tools and surrounding air
   
   
3. Make syrup from water and sugar by boiling the syrup until the sugar is dissolved. Leave to cool
   Sugar has a preserving function due to its dehydrating effect on microorganisms (a later post will deal with this). Sugar won't kill microorganisms, but inhibit growth. If the syrup is thoroughly boiled, microorganisms in the water and sugar are killed. Cooling the syrup is not recommended if it can be avoided (10-40 °C window)
   
   
4. Fill a cold jar with the pears. Fill up with cold or lukewarm, but not hot, syrup
   Warning: we are in the 10-40 °C window. From a microbiological view, the best would be to add hot/boiling syrup. However, this might damage the fruit, such as turning the surface mushy (in the case of plums, the skin would most likely break).
   
   
5. Put the lid loosely on, do not tighten
   Makes sense. During heating, the contents expand and trapped and dissolved air is expelled (gas solubility is lower at higher temperature). The air above the fruit expands when heated and needs to go somewhere
   
   
6. * Place jar in a pot with cold water almost up to the rim of the jar. Heat slowly and keep at boiling point for a certain amount of time (i.e. 8 minutes)
   We are in the 10-40 °C window for quite some time during slow heating. The historic reason is most likely that old glass types had tensions/stress that would result in the jar cracking from shock heating or cooling. Modern glass production methods solve this by i.e. annealing. My experiences with ordinary jam jars is that they manage shock heating and cooling quite well. If the fruit survives, rapid heating is preferable.
   
   
7. * Remove jar from pot and either leave to cool for a certain amount of time (i.e. 15 minutes). Then tighten the lid, or tighten the lid while boiling hot Leaving the jar open at this point is certainly not a good idea. Cooling results in the air above the fruit contracting, sucking in air from the surroundings (see note). Even though microorganisms might die when entering, thermally stable spores might enter which might develop at a later point. Tightening the lid right away is by far preferable. If you use modern jars with aluminium lids that pop down due to reduced pressure inside the jar, this should happen when the jar cools (if you buy a jar of jam and the lid doesn't pop when you open it, return it and get another). If the lid pops up before or during storage, consume immediately («oh, what a disappointment» ;)).
   
   
8. Turn jar upside down and either store upside down or leave upside down for a certain amount of time (i.e. 15 minutes)
   Makes sense. Getting the lid sterile is always a difficult task. Turning the jar lets the hot contents come in contact with the lid, sterilising it getting rid of a majority of the microorganisms present. One might speculate whether keeping the lid seals moist might also be a reason (why wine is stored lying), but a biologist friend of mine meant that the atmosphere above the fruit would result in a moist enough atmosphere for that purpose, and that the sterilising effect of killing microorganisms is the point here. If that is true, it should not make a difference which way the jars are stored.

Remember that operations conducted in a kitchen are far from sterile procedures. For this reason, many such recipes rightfully ask for two and sometimes three actions with apparently the same purpose.

Finally, while hard cheeses with unwanted mould often are safe to eat if the mould is removed by cutting away a layer of the cheese, stored preserves such as jams, sugar preserved fruit and syrups should be discarded if the seals are broken or visible mould is seen. In the cheese, the microorganisms cannot travel due to the dry and solid structure, whereas diffusion is jams etc. occurs rather easily and the whole jar might very well be contaminated even though the visible mould is removed.

What might be learned/taught
Loads of microbiology. However, since I'm primarily a chemist, I won't venture too far into this. To me, taken that microorganisms are everywhere and that they die when they are heated, the most important things to teach would be (again, from the top of my head):

• temperature-volume relationships of gases and gas-liquid relationships (popping lids, see note below)
• critical thinking, don't always believe what you read
  
• working systematically, always questioning why should I do this? (see Five cardinal rules in cooking)

In fact, this experiment/recipe might be among the few cases where one can test and predict something rather complicated based on a very limited amount of knowledge, but are often stated as among the most important treats in enquiry based science teaching (quite a paradox, really). Such situations are scarce, and I'm thrilled every time I stumble upon one.

Erik


Note: Heat expanding air is easily illustrated by putting a blown-up balloon into the freezer; it contracts. Take it out, and it expands back to (almost, at least) original size. However, when the jar contents are boiling, the headspace will in fact be filled by mostly steam from the preserves. When this water condenses (in the closed jar), the volume of the steam (now liquid water) is reduced by a factor of ca. 1300(!), resulting in a considerably lowered pressure.

Addendum (6. August)
some statements about sterilisation are modified as common canning does not result in sterile product (hence the need for preservatives such as sugar, salt, acid/vinegar etc.). This way of canning/preserving more resembles high pasteurization which is common in some milk products.