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I read many recipes of pretzels and they required to dip the raw dough in a bath of lye. As anyone should know for their own safety, lye is caustic and shouldn't be ingested.
What is the process involved that make them edible?
Edit: I am aware of the action of the lye. I wonder how the non-edible lye on the dough is transformed into something that is safe to eat.
Basically, the lye reacts with the CO₂ and moisture present during baking to form a non-toxic carbonate. This makes it safe to eat.
The reaction:
CO₂ (g) + H₂O (l) ⇄ H₂CO₃ (aq)
H₂CO₃ (aq) + 2 NaOH (aq) → Na₂CO₃ (aq) + 2 H₂O (l)
[EDIT]
Spurred by the comments, I have searched further.
tl;dr There is much going on wrt the lye dip. As far as safety goes, the lye is consumed in many reactions, including the above.
The effects of alkali dipping on starch, protein, and color changes in hard pretzel products have never been researched. Experiments were conducted to mimic reactions occurring on the pretzel dough surface. Dough was dipped in water or 1% sodium hydroxide solution at different temperatures between 50°C and 80°C. Protein and starch profile after dipping were analyzed. Color development on pretzel surface following the extraction of pigments from flour was investigated. Whole dough and pretzel samples were also made at pilot plant and the properties were analyzed. Only starch granules on the dough surface were gelatinized following dipping. Amylose-lipid complex dissociated at a lower temperature with alkali treatment but were not dissociated, even at high-temperature dipping in water. Treating the dough at 80°C in alkali solution resulted in the hydrolysis of proteins into smaller peptides that could be not precipitated by trichloroacetic acid (TCA). Dough surface color was different following pigment extraction from flour but not significantly different following baking. The results suggest that the color that developed on pretzel surface was not due to pigments present in the flour but was contributed by the reaction within or between the starch and protein hydrolysis derivatives during baking.
and what I think is the pertinent quote from the blog:
The protein results (2 in the list above [reproduced following]) indicate that the lye dip provides the smaller proteins needed for Maillard reactions, whereas the water dip does not. This seemed like perhaps the most important point to me.
- The dip resulted in the hydrolysis of protein into smaller peptides. This happened a little bit in 25°C water or lye dip, more in 80°C water, and a lot more in 80°C lye dip. Also, the smaller peptides in the hot lye dip had the smallest molecular weights; most of them “walked off” the electrophoresis gel, leaving no bands. The authors explain that the alkaline conditions of the lye dip result in like charges along the proteins, which repel and cause the proteins to unfold; this makes them more susceptible to hydrolysis.
Both the blog and the paper are worth reading.
My conclusion: the lye is consumed by the various reactions and therefore poses no safety concerns.
The purpose of dipping in lye (or other basic solution, like baking soda...or even baked, baking soda) is that it promotes coloring, as the solution reacts with the surface of the dough. It also promotes the Maillard reactions when the dough cooks. The result is even browning and that typical alkali flavor. If choosing lye, food grade is important, as commercial grades may include other, heavy metal, impurities. Lye is extremely caustic. So it must be used carefully! In pretzel and bagel making the solution is generally quite dilute...maybe around 3% lye in water. In both pretzel and bagel making, the product is typically given a brief bath in boiling water, after a dip in the lye solution. The boil and/or subsequent baking neutralized the alkali rendering it safe to eat.
The reason it is safe is three-fold.
First, the concentration is only 1% NaOH and the pretzels are only dipped for 10 seconds (see Snack Food Technology pages 180-182) which limits the amount of hydroxide per pretzel.
Second, the dough itself, for example protein of the dough, has acidic groups, such as amino acid side chains of lysine and tyrosine, which neutralize the hydroxide.
Finally, as explained in Effect of Alkali Dipping on Dough and Final Product Quality Journal of Food Science vol. 71, pages C209-C215, protein in the dough is partially hydrolyzed under the alkaline conditions. This exposes more terminal amino acid groups which also participate in neutralization.
The Snack Food Technology book cited above also explains:
If the caustic concentration becomes too high, there is not a complete conversion to sodium bicarbonate in the baking and drying cycles and the pretzels will be hot to the taste due to the residual sodium hydroxide
Lye will readily react with either amino-acids (producing respective sodium salts) or with fats (producing soaps), both reactants being readily present in dough. You don't need CO2 to neutralize it.
Ingesting small amounts of those end-products is indeed safe, and normally only a small quantity of lye is used in the process.
The references noted above have mostly looked at the specific chemical changes to the constituents of the dough and the species in solution. A few point to the Maillard reactions as a contributor to what is going on.
It is worth noting that the Maillard reactions are quite complex and involve a lot of intermediate products. However, in many cases the rate limiting factor is the pH of the constituents. It is possible to accelerate the reactions by raising the pH, and more Maillard products are produced if you let the process run for an extended period of time or raise the temperature which further increases the reaction rate. Some people don't believe that you can get the reaction to go at all at temperatures below 300°F, yet adding a little baking soda to a batch of onion soup and pressure cooking it (265°F) for 40 minutes will yield the same browning that much longer cooking of the onions produces in the classic technique.
So increasing the pH by using lye (pH 13) vs sodium carbonate (pH 10) vs sodium bicarbonate (pH 8) will facilitate a dramatic speed up the Maillard reaction rate, and subjecting the pretzel to high temperature in the oven will carry it out. What happens to the NaOH to detoxify it is most likely a combination of neutralization, dilution, and chemical conversion through interaction with other available species. I would not recommend eating dough dipped in lye without baking the dough first.
I am intrigued by the notion that the high pH breaks down the proteins into shorter amino acid sequences which facilitates the Maillard reactions but does not affect the rate constants.
