IPA: Myth or Microbiology

Many craft beer drinkers have heard the tale. The India Pale Ale (IPA) was developed to make the transcontinental journey from Britain to its burgeoning colonies in the Far East. The month’s long journey across land and sea, often in sauna like environments, caused the traditional British ales to spoil before reaching their destination.  A new brew was needed to make the arduous journey. George Hodgson and his Bow Brewery are credited with the development of the IPA which was initially, strictly an export⁽¹⁾ to the British colonies. Other British beers were also being exported to India at that time, including porters, but the IPA or “Hodgson’s pale ale” had the lion’s share of sales⁽¹⁾. Whether you believe that the IPA was invented specifically to survive the long distance voyages to India or it was simply a new style that was marketed to British colonists is a debate for another posting. One thing is clear however. Hops, the ingredient that lends the characteristic aroma, flavor, and bite to the IPA has one more trick up its sleeve.

Hops (Humulus lupulus) have some amazing anti-microbial properties which are largely attributed to the hop’s acids. Alpha acids are represented by humulone and its congeners co-humulone, adhumulone, prehumulone, and pos-thumulone. The beta acids are lupulone and its congeners colupulone, adlupulone, prelupulone, and postlupulone (Fig 1)^{(2, 3)}. While these can be a mouthful to say, the alpha acids provide the majority of antimicrobial activity as they are the most soluble in wort during the brewing process and a major component in the taste and appearance of hops.

Hop acids are mainly active against Gram-positive bacteria, which includes strains of Staphylococcus aureus, Streptococcus, Clostridium tetani (tetanis), Clostridium botulinum (botualism), and Listeria monocytogenes (listerosis)⁽⁴⁾. There is some evidence that hop acids can be effective against bacteria (Streptococcus mutans) that cause dental caries (cavity causing); just a thought the next you find yourself without a toothbrush⁽⁴⁾. Hop acids are also effective against Mycobacterium which includes the family of bacterium responsible for tuberculosis (Mycobacterium tuberculosis) and leprosy (Mycobacterium leprae)⁽⁵⁾.

Fig 1. From Srinivasan et al. 2004

Hop compounds (lupulone, humulone, isohumulone and humulinic acid) induce leakage of the cell membranes of certain gram-positive bacteria. This breakdown of the cell membrane inhibits active transport of sugars and amino acids across the membrane. Thus, cellular respiration and protein synthesis are interrupted⁽³⁾. Additional studies have demonstrated that hop bitter acids disrupt the transmembrane pH gradient. Thisis an important component of proton motive force (PMF) which is required for  energy (ATP) production within the cell and without it metabolism is inhibited.(summarized in⁽⁶⁾).

While there are other barriers to microbial growth in wort and eventually beer, including boiling, ethanol, pH, and C0₂ levels, hop acids provide long term microbial suppression. They also help to stabilize the flavor of beer during storage, which is an important factor in long distance distribution⁽⁷⁾. All these factors help suggest why “Hodgson’s pale ale” became so popular in the British colonies, but today we owe the Hop for the tale tell aroma and flavor all us “Hop Heads” have fallen in love with. So next time you enjoy your favorite IPA you may be literally drinking to your health. Prost!

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1. Pryor, A. 2009. India Pale Ale: an Icon of Empire. University of Essex. Commodities of Empire Working     Paper No.13
2. Srinivasan et al. 2004. Contributions to the Antimicrobial Spectrum of Hop Constituents. Economic Botany. 58: S230-238.
3. Vriesekoop et al. 2012. Bacteria in Brewing: The Good, the bad, and the ugly. J. Inst. Brew., 118, 335– 345.
4. Bhattacharyae et al. 2003. Inhibition of Streptococcus and Other Oral Streptococci by Hop (Humulus Lupulus L.)
5. Chin et al. 1949. Antituberculosis activity and toxicity of lupulone for the mouse. Proceedings of the Society for Experimental Biology and Medicine 70: 158-162.
6. Suzuki, K. (2012) 125th anniversary review: microbiological instability of beer caused by spoilage bacteria. J. Inst. Brew., 117, 131–155.
7. Schönberger, C. and T. Kostelecky. 2011. The Role of Hops in Brewing. J. Inst. Brew., 117, 259–267.