Friday, May 29, 2015

The Science and Engineering of S'mores

We realize this post is a few days too late for the first major camping weekend of the summer, but the truth is, we needed a tune-up on our s'mores theory and practice ourselves.  So now, while the extensive refresher training we completed over the last few days is still at the front of our minds, we wanted to put out a short treatise on the intricacies and nuances of marshmallow roasting and s'more eating.

The first step in creating a s'more is to initiate the thermal oxidation of some woody biomass.  To avoid marshmallow ignition, we want radiant heat from the coals rather than heat from the gas-phase oxidation occurring in flames. That requires full primary combustion of the wood, which takes a considerable amount of time.

The second step is to select a marshmallow roasting device from a nearby tree or shrub, taking care to avoid the species Toxicodendron vernix.  The geometric outlay of the device is to some degree a matter of personal preference, but we've found that a half inch diameter at the base tapering to approximately a quarter inch at the business end, and 2.5-3 feet long, to be optimal for most common campfire heat intensities and standard-size marshmallows.

Similarly, the optimal linearity of the device is a subjective matter.  Some prefer a higher degree of linearity to facilitate a uniform axial rotation during roasting. Others prefer some curviness to allow the roaster to reach preferred roasting locales within the campfire from any position around the fire ring, including those toward which the smoke is not traveling, and independent of other s'more engineers who may be occupying prime roasting real estate.

Nearly all experts agree, however, on the advantage of a barbed tip to prevent marshmallow disengagement from the device during the later stages of roasting, when the rigidity of the melty marshmallow core has deceased significantly.  Similarly, there is nearly universal agreement that removal of the bark from the tip of the device prevents inadvertent transfer of bark particles to the marshmallow.

In theory, pure radiative heat from glowing embers produces the most satisfactory roasting experience.  However, in practice, maintaining sufficient heat flow from a bed of embers over the course of tens of minutes that comprise a typical roasting session is challenging (as is having the patience to wait for the wood to finish burning down to coals).  Thus, the optimal sustained roasting environment often requires a combination of actively burning wood and pockets of glowing embers.

With the marshmallow applied to the roasting device, and roasting commenced over an appropriate heat source, the sugars in the outer shell of the marshmallow will begin to caramelize.  The primary chemical challenge during roasting is to uniformly caramelize the entire shell without charring or igniting any part of it.  The preferred technique among experts is a slow, rotisserie-style rotation at 4-6" from the embers, though few possess the patience to functionally sustain such an activity for the duration of the roasting operation. Another challenge is roasting the surface closest to the base end of the roasting device.  There may be some advantage to loading the marshmallow onto the device axially to minimize the surface area facing the base end (since the flat side of the marshmallow is smaller than the curved section), and then lightly smooshing the marshmallow to convert some of the base-facing surface into side-facing surface.

When the marshmallow is satisfactorily caramelized, it can be assembled into a s'more, with graham crackers and chocolate as co-ingredients.  In some circles, a preferred embodiment of the s'more is one which can be eaten cleanly (i.e., without loss of melted marshmallow to the eater's face or hand, or to the ground).  In such an embodiment, the chocolate and graham must be of the correct pliability, such that biting into the s'more results in neither excessive compression of the marshmallow nor shattering of the graham cracker components.  The desired pliability can be achieved by mild heating of the graham and chocolate for 4-5 minutes while roasting the marshmallow (resulting in a final temperature of 100 - 105 °F).  More intensive heating melts the chocolate and toasts the graham, effectively exacerbating the problem.

A proper grip on the s'more can also help keep the s'more intact while biting.  As this hand model is demonstrating, a firm grip in one quadrant of the graham, using the end section of the first two fingers and the thumb, is optimal for minimizing transfer of marshmallow to the hand while maximizing the exposed area on which to apply the bite.  Large bites, up to the size of the entire s'more, also minimize brittle fracture of the graham into the hand.

After roasting one- or two dozen marshmallows, the roasting device can be saved for future use, but a bit of maintenance and cleaning will help preserve its integrity and avoid attracting ants.  A first step in cleaning is commonly to remove as much marshmallow residue as possible by mouth.

Particularly recalcitrant residue can be removed by charring the tip, followed by wiping with an appropriate material, such as a t-shirt or nearby grass.  The health benefits of any residual char that may be transferred to future marshmallows are unclear, but related products are highly valued in some types of cuisine (an explanation of the scientific function of biochar in the digestive tract can be found near minute 19 in the video accompanying the previous link, although the entire episode provides a much more compelling narrative if one is amused by the non-sequitor nature of Japanese cartoons).  The roasting device can be stored somewhere relatively clean and out of the fire pit until needed again.

In conclusion, many factors must be considered to produce an optimally roasted marshmallow and a satisfactorily engineered s'more.  We hope this primer has helped elucidate some of those factors, but we welcome additions and suggestions to incorporate into the second edition of this text.

What is your preferred marshmallow-roasting protocol?


  1. My only contribution is this: spread the graham cracker with a layer of peanut butter before applying chocolate and marshmallow. You won't be sorry.

    1. Excellent! We are huge fans of PB, and will definitely give that a try.