Intrigued by Arthur's findings, thirty-year-old French chemist Louis Camille Maillard did substantial work on the reaction products created when amino acids and sugars were heated together. He first published his findings in 1912 and these reactions bear his name today. They describe and explain the characteristic flavors of chocolate, roasted coffee, bread crusts, maple syrup, soy sauce, cooked meat, and malt.
Maillard reactions can produce many of the same flavors and aromas as caramelization reactions, but at much lower temperatures. The mechanism for the reaction starts with the carbonyl groups in simple sugars and the free amino groups in amino acids. In addition to the common bread crust, caramel, cocoa, and coffee flavors, less pleasant aromas and flavors such as burnt, onion, solvent, rancid, sweaty, and cabbage can be created.
The reaction occurs in three steps. In the first step, an amino acid and a sugar combine with the loss of a water molecule to form an unstable compound. During the second step, this unstable compound undergoes Amadori rearrangement to form a ketosamine combination of a ketone and an amine). During the final step, the ketosamine undergoes further transformation, via one of three different pathways to produce one of three different products.
The first pathway further dehydrates the ketosamine, resulting in compounds like those formed in true caramelization reactions. The loss of three water molecules and additional reaction with amino acids characterize the second pathway, which results in the creation of large, colored polymeric compounds called melanoidins. In the third pathway, an intermediate product such as diacetyl is formed, which then undergoes Strecker degradation, conversion of an amino acid to an aldehyde, to form highly flavor active heterocyclic compounds, including pyrones like maltol and isomaltol, as well as furans and furfurals. Stronger flavored nitrogen heterocyclic compounds, including nitrosamines, tend to be more abundant in malt that has been kilned or roasted above 350 F.
The conditions necessary for Maillard reactions can be found both in the malt kiln and brewhouse. In fact, the majority of color in pale North American lager beer is generated in the brew kettle via these reactions. Roughly 10,000 distinct compounds, like maltol, the characteristic aroma of Munich malt, originate during brewing. The complexity of these formation pathways is such that the individual products cannot be precisely controlled, but maltsters can steer the flavors in a general direction. Although caramel flavors can be created in the kettle, they occur mainly due to Maillard reactions.
The task of classifying hundreds of varieties of malts into neatly ordered style categories is as daunting as trying to classify the thousands of beers made from them. It is possible to broadly separate malts based on process (kilned, caramelized, roasted), enzymatic activity, or even the color of the wort that they produce, but even similar products from different producers can have overlapping qualities and unexpected differences. When coupled with batch variation and fluctuations in raw materials, categorization can be a herculean labor.
When writing or revising the grist for a recipe, the brewer should always taste the malt. There is no substitute for the direct sensory experience gained from chewing malt. Tasting a blend of the individual grist ingredients mixed together in rough proportion gives a far closer approximation of what the finished beer will taste like than any amount of written description. Tweaking the recipe before brewing begins requires little effort or commitment, and can be vital to the success of the beer.
The listings that follow are roughly grouped by type, traditionally processed malts, caramel malts, drum roasted malts, malt made from other grains, and malts made with special processes and arranged from light to dark within these categories.
Pilsner malt is a base malt designed for very pale, all-malt beers. Traditional Pilsner malt production includes the use of low protein two row malts, lower modification during germination, and low temperature, high airflow kilning.
This malt should be very pale in color with moderate enzymatic potential. According to Brynildson, Pilsner malt has a distinctive flavor: a little green, with the smell and taste of fresh wort, which are particularly evident in European Pilsner style beers, like Bitburger or Warsteiner. Dimethyl sulfide (DMS) which has a cooked corn or cabbage flavor is present in all malt. The precursors S-methylmethionine (SMM) and dimethyl sulfoxide (DMSO) are created during malting, but removed at higher kiln temperatures. Because Pilsner malt is kilned at a low temperature it retains this flavor potential, which is considered acceptable at low levels in some beer styles, like German Pilsner.