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)
scale (0.1 g precision is needed)
some general kitchenware
disposable plastic pipette (7 ml) or plastic syringe (10-20 ml)
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.
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)
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.
McGee, H.: McGee on Food and Cooking. London: Hodder and Stoughton 2004.
Lersch, M.: Hydrocolloid recipe collection
Isn't it fun? I made my own first "caviar" last year after acquiring a Texturas kit. We do some spherification and inverse spherification at work, but it's not the same as making your own pure flavoured drops at home.ReplyDelete
Yes, and great fun for the kids as well (for those of us having them in the age). I guess this might be a nice activity for science museums as wellReplyDelete
I did those pH calculations some while ago. They were based on the amount of citrate required to neutralize an acidic solution at a given pH.ReplyDelete
Now that I think about it (and contemplating on your observation): if the solution (in this case the strawberry mix) is buffered the calculation will be wrong. I'm no expert in strawberry chemistry, but it might very well be that strawberries are a buffered system, i.e. that you need more citrate than my simple calculations suggest. Do you agree?
I'll have to make a note of this and include it in the next update of "Texture".
It must be said that my pH measurements were quite inaccurate, used pH 0-14 strips. In the literature, McGee doesn't say much about this and Belitz et al. focuses mostly on flavour compounds (being carboxylic acids, ketones and esters). Maybe I should do a titration one of these days to see how it behaves? Strawberries contain per 100 g: Ca (20 mg), Na (1 mg), K (197 mg), Mg (14 mg) and P (29 mg). This indicates that there might be salts in there being able to act as buffers (...possibly?). What are the acid-base properties of polyphenolics such as anthocyanins and flavanoids which are said to be present in strawberries (and are the amounts large enough to make a difference)?ReplyDelete
I found one article that might give some answers, but I haven't got access: Wrolstad et al., J. Food. Science 2006, 35, 448
If you need some information about pH of fruits and vegetables I recommend having a look at this link http://en.molecularcuisine.org/forum/showthread.php?tid=51
Regarding the Citrate buffer I gave up long time ago trying to calculate the precise amount needed...just putting a little bit step by step until recovering the alginate solubility.
Thanks. This is very useful information, not only for spherification purposes. Great job :)ReplyDelete
To avoid "lumping" of the alginate, you could first mix the alginate and the sugar, and then add the liquid (strawberries)ReplyDelete