Umami Science Part III - Umami Synergy

Have you ever thought about why we dip sushi in soy sauce?  We know it makes the fish taste a lot more delicious, but why?  Today we'll learn that when we combine the glutamate in soy sauce with the nucleotides in fish, we amplify the umami taste sensation well beyond what either ingredient alone produces.  This fascinating phenomenon is called umami synergy.

Umami synergy involves the relationship between umami-causing glutamate (MSG) and two additional molecules, the nucleotides inosinate (IMP) and guanylate (GMP).  IMP and GMP do not cause umami on their own.  But when present alongside glutamate, they are capable of amplifying the umami taste fifteen-fold.  Not only is the umami taste magnified, it is more sustained and longer lasting, too.  This phenomenon, only recently unraveled at the molecular level, plays a role in worldwide cuisine, driving us to combine ingredients rich in glutamate with those rich in IMP and GMP.  

Umami Synergy. Please stop by the soy sauce on the way into my mouth. Thx. REUTERS/Issei Kato

Umami Synergy. Please stop by the soy sauce on the way into my mouth. Thx. REUTERS/Issei Kato

Maximizing Umami Taste: Umami Synergy

Umami synergy is a sort of culinary gestalt: the whole is far greater than the sum of the parts. Glutamate produces umami.  Nucleotides (GMP and IMP) by themselves produce no umami. Together, they produce an umami sensation that dwarfs the sensation caused by just glutamate.  

This phenomenon has a significant influence on the foods we eat.  One source explains that "major world cuisines have traditionally relied on umami synergy for deliciousness by combining protein foods with IMP and vegetables with glutamate."  We've learned how umami synergy drives the flavor of Japanese dashi broths, by combining the glutamate of kombu with the IMP of bonito flakes or the GMP of dried shiitakes.  But Western cuisine is full of examples, too.  Tomato sauce (glutamate) combined with meat (inosinate) in pasta Bolognese; cheeses (glutamate) and beef (inosinate) in French onion soup or cheeseburgers; and cheese plus anchovies (inosinate) in Caesar salad.  The point is that humans implicitly understood umami synergy, using it to their culinary advantage, well before "umami" was discovered in the early 1900s.  

Now let's talk about the umami compounds and why they're in our foods.  Then we'll learn how the ways we prepare our food increase the concentration these molecules, maximizing our umami taste experience.   

Umami Synergy.  This is a new way to think about beef stew. Umami Information Center.

Umami Synergy.  This is a new way to think about beef stew. Umami Information Center.

The Umami Compounds

The fact that our foods are often rich in glutamate, inosinate, and guanylate isn't surprising. Present in every living thing, these molecules are absolutely central to biology on our planet.

As we learned in Part II, Professor Kikunae Ikeda discovered umami in 1908 when he extracted purified glutamate from kombu.  Just a few years later, one of Ikeda's students, Shintaro Kodama, succeeded in identifying IMP as an umami compound when conducting studies on katsuobushi (dried bonito flakes).  It wasn't until 1960 that another Japanese scientist, Akira Kuninaka, uncovered GMP's umami-enhancing properties when he isolated it from shiitake mushroom broth. Kuninaka also discovered the umami synergy phenomenon.

Glutamate, as we've learned, is an amino acid.  Amino acids are the building blocks of proteins. Proteins, in turn, are large amino acid polymers, characterized by complex three-dimensional structures and sophisticated biological functions.     

Amino Acids and Proteins. Structures of the 20 naturally occurring amino acids, called as such because they possess an "amine" (NH2) group and a carboxylic acid (COOH) group. Glutamate at lower right (left). Proteins are chains of amino acids that fold themselves into highly complex three-dimensional structures (right).  

Amino Acids and Proteins. Structures of the 20 naturally occurring amino acids, called as such because they possess an "amine" (NH2) group and a carboxylic acid (COOH) group. Glutamate at lower right (left). Proteins are chains of amino acids that fold themselves into highly complex three-dimensional structures (right).  

All of the amazing complexity and diversity of proteins derives from just 20 amino acid building blocks in mammals.  Of those 20, the human body can synthesize only 10.  We have to get the other 10--the "essential" amino acids--from our food.  And unlike fats and starches, our bodies don't store excess amino acids; we need to consume them on a daily basis to keep all our internal trains moving on time.  Amino acids have a range of important biological functions beyond just populating proteins.  For example, our entire umami discussion here is premised on free glutamate's role in triggering a complex cellular signaling pathway en route to the taste of umami.  

Recipe for "chicken stock with umami synergy" at http://umami2u.blogspot.com/2012/12/chicken-stock-with-umami-synergy.html.

Recipe for "chicken stock with umami synergy" at http://umami2u.blogspot.com/2012/12/chicken-stock-with-umami-synergy.html.

IMP and GMP, in contrast, are nucleotides.  Nucleotides are the building blocks of the nucleotide polymers, DNA and RNA.  Three things make a nucleotide a nucleotide is (1) a 5-carbon sugar, connected to (2) a phosphate group, and (3) a nitrogen-containing ringed structure called either a purine or a pyrimidine.  Inosinate is the common precursor to two of the five nucleotides used in DNA and RNA, adenylate and guanylate (the other umami nucleotide).  Free nucleotides--like ATP, the fundamental currency of energy in living things--are just as important in biology as their polymeric counterparts.  IMP is high in meats and fish because muscle cells in animals need a lot of ATP to function.  One source explains that "[o]lder animals with very well exercised muscles tend to have more umami, as do fish that are heavy swimmers, such as mackerel, salmon, and tuna."  In fact, it's well-known that older stewing hens make better chicken stock than young birds do (see here, here, and here).

Nucleotides. Phosphate connected to sugar connected to base. Nucleotides are responsible for umami synergy. http://www.uic.edu/classes/bios/bios100/lectures/dna.htm

Nucleotides. Phosphate connected to sugar connected to base. Nucleotides are responsible for umami synergy. http://www.uic.edu/classes/bios/bios100/lectures/dna.htm

Maximizing Umami Compounds: The Power of Cooking and Fermentation

Just as important for our food discussion is the fact that these biomolecules--proteins, DNA, RNA, ATP--are broken down after an organism dies, increasing the amounts of glutamate and the umami nucleotides in the food source that the organism becomes.  

The key thing to note is that food preparation has a significant role in maximizing umami.  As one author explains, "[p]rocesses such as cooking, boiling, steaming, simmering, roasting, braising, broiling, smoking, drying, maturing, marinating, salting, ageing and fermenting all contribute to the degrading of the cells and macromolecules of which the foodstuff is made."  Enzyme-mediated breakdown through processes like fermentation is particularly effective in bringing out umami.  

One great example is katsuobushi (dried bonito flakes):  drying the bonito fish can increase the inosinate concentration 30-fold as cellular ATP is broken down!  The same principle applies at sushi restaurants, where the tastiest tuna has likely been aged for a few days before service to a customer, maximizing its umami.   

ATP Breakdown = Umami.  Well, for a while.  ATP content is high in animal muscle, because it is needed to power contraction.  ATP stops being produced after an animal is slaughtered, and immediately begins to degrade.  Inosinate (IMP) is produced within 24 hours, but degrades further in subsequent days.  Increasing amounts of inosine and hypoxanthine indicate decreasing freshness.  http://www.novocib.com/Freshness_Assay_Kits.html.

ATP Breakdown = Umami.  Well, for a while.  ATP content is high in animal muscle, because it is needed to power contraction.  ATP stops being produced after an animal is slaughtered, and immediately begins to degrade.  Inosinate (IMP) is produced within 24 hours, but degrades further in subsequent days.  Increasing amounts of inosine and hypoxanthine indicate decreasing freshness.  http://www.novocib.com/Freshness_Assay_Kits.html.

Next time, we're going into some serious science here at Ramen Chemistry.  We're finally ready to understand how umami happens at the molecular level.  Stay tuned.