Posted 01 July 2012 - 04:29 PM
Hi. Earlier this week, someone emailed me asking me why I talk about collagenase increasing tenderness below 60 °C after about six hours but that Heston in “In Search for Perfection” ages his steak at 50 °C and McGee says that fiber weakening enzymes “denature, become inactive, coagulate” at about 55 °C. I thought some of you might have the same question, so I've pasted my answer below:
There are a lot of different enzymes in meat. We're mainly interested in proteolytic enzymes that split proteins or peptides (which are chains of amino acids) and these enzymes are called proteases. Enzymes are named by adding an -ase onto what they act on; so any enzyme that breaks up collagen is called a collagenase. The enzymes that are important in aging or conditioning can be divided into calpains and lysosomal enzymes (including cathepsins):
• Calpains need calcium ions to be activated and act on muscle fibers (but not myosin or actin, which make up 65–70% of the myofibrillar proteins).
• Lysosomal enzymes act on muscle fibers (both myosin and actin) and (some) collagen.
See Lawrie's Meat Science for more details.
When meat is aged, typically at 1–3 °C for 1–4 weeks, it's mainly changes to the muscle fibers that increase tenderness. This is surprising because the break down of connective tissue (collagen and elastin) would seem to be the most likely cause of increased tenderness. Nonetheless, a famous experiment (Sharp, 1957) showed that almost no collagen is broken down during aging — even when he aged sterile meat for one year at 37 °C!
Since there is a great number of enzymes, there isn't a single temperature that they stop working at. Recall (from page 17 of my IJGFS article) that the sarcoplasmic proteins (which are mostly enzymes and myoglobin) start to denature around 40 °C and finishes around 60 °C. So, in other words, some enzymes stop working around 40 °C and most have stopped working around 60 °C.
The rapid aging that Heston — well, actually, Chris Young who was working for Heston at the time and told me that he based that recipe on what he'd been reading in Lawrie's Meat Science — is interested in is the break down of muscle fibers that occur in normal aging. Indeed, in another experiment that compared aging at 2 °C with 38, 43, and 49 °C found that the rate of aging was about 7 times faster at 38 °C and about 18 times faster at 49 °C than at 2 °C. This is what Myhrvold et al. (2011) is after when they suggest aging meat for even 4 hours at 45 °C can significantly increase tenderness.
While my recipes at above 55 °C and below 60 °C get some mild rapid aging while it heats up, I don't believe that this significantly increases the tenderness. What I'm after was first reported in Laakkonen et al. (1970), where they found that collagenase was active below about 60 °C and could significantly increase tenderness if held there for about six hours. Now, it seems that this collagenase only acts on some of the collagen and most of the collagen that's broken down at these temperatures (especially on the long, 1–3 day cooks) is a continuous nonenzymic breakdown. In another interesting experiment by Sharp (1964), he again held beef at 37 °C for 97 days but only after first heating to 70 °C for 15 minutes or 100 °C for 45 minutes; after heating to 70 °C, soluble hydroxyproline was 2% and raised to 23% after aging and after heating to 100 °C it raised from 12% to 55% — but certainly the enzymes are no longer active after heating so it'd seem that these changes are nonenzymic changes in the connective tissue. I don't know what exactly this implies, but it's certainly interesting.