By student contributors Randy Hurd, Ben Roden and Tadd Truscott
We have all taken that terrifying risk, with our chubby, cookie-laden fingers wedged into a glass of ice-cold milk, asking if we can wait just a little longer. We imagine that perfectly soaked, soft and flavorful cookie, while simultaneously overcome with the fear of what may result if we wait too long. Many a soul has wondered: “Is there a better indicator for cookie dunking time? Must I depend on gut instinct alone and merely hope I can avoid watching that sweet piece of sugary goodness sink to a mushy, milky grave?” In response to this inquiry, we present an analysis of the rate at which milk absorbs into cookies and the time to structural failure for dunked cookies.
Cookies (or biscuits, as they are called in the United Kingdom) first appeared in Rome between 200-300 B.C. These Roman “bis coctum” or “twice baked” treats were made with little water content to promote a long shelf life. However, this approach meant the biscuit was quite hard and difficult to chew, so the Romans dipped the treats in wine to soften them . In 19th century England, with the emergence of afternoon tea, biscuit dunking grew into common behavior, and the custom was quickly spread around the globe. In more modern times dunking culture has expanded to donuts and cake with dunking mediums including milk, coffee and tea.
Len Fisher has received an Ig Nobel Prize for his scientific investigation of the physical mechanisms governing cookie dunking. Fisher noted that cookie dunking consists of a liquid penetrating a porous medium due to capillary action, which is already described by the Washburn Equation: Where the expression defines the penetration length L after a specific amount of time t, where γ is the surface tension, D the average pore diameter and η is liquid viscosity .
Understanding that most dunkers want the taste benefits of a moist cookie without the undesired loss of structural integrity, Fisher suggests a dunking method where the cookie is dunked horizontally rather than vertically. This method allows the underside of the cookie to become moist while the upper side is able to maintain necessary rigidity . However, in spite of the logic of this method, many cookie dunkers still prefer a completely soft cookie.
In 1999, Fisher went a step further, seeking to find the ideal dunking liquid. His work showed that milk drinks, rather than tea or coffee provided a more flavorful and satisfying dunking experience. By sampling air from the nostrils of volunteers, Fisher showed that a milk drink, and the tiny particles of fat suspended therein, keep the flavor-laden food particles on the tongue longer, allowing the particulates to be pulled into the nose. Coffee and tea, on the other hand, are much more likely to wash the food particles into the throat without allowing as much time for tasting .
However, while suggesting an improved method for dunking, Fisher’s work does not indicate when failure is expected in a traditional cookie dunking experience. The common human is still faced with the agonizing task of hoping their cookie won’t tumble to the bottom of their glass before they decide to bring it to their mouth. For this reason, we present an analysis of milk percolation rates and breakoff times for some favorite American cookies. We hope this data will help you avoid cookie dunking tragedy and increase your cookie eating experiences for years to come.
Methods and Results
Four types of commercially available cookies were investigated: Oreos, Chips Ahoy, Nutter Butter and Graham Crackers. Cookies were dipped halfway in two percent milk for various amounts of time and weighed both before and after to measure the mass of absorbed milk, with dunk times varying from 0.5 to seven seconds. Additionally, cookies were dunked for a standard dunk time of six seconds and then clamped horizontally until the cookie broke and fell, as shown in Figure 1. Time to failure was recorded for each cookie.
Figure 2 shows that the cookies tested absorb more than 80 percent of their liquid weight relatively quickly (less than two seconds) and all have absorbed 99 percent of their liquid weight capacity within five seconds. More porous cookies, like graham crackers and Chips Ahoy cookies, absorb liquid significantly faster than hard-pressed cookies. This data indicates that for the tested cookies, keeping your cookie in the glass any longer than five seconds does not lead to any additional milk entering the cookies, and 95 percent of the absorbed liquid can be achieved via a two-second immersion.
The results from the cookie-breaking experiment shown in Figure 1 are summarized in Table 1. These experiments show that softer, more porous cookies grow soft and break faster than hard-pressed cookies. However, any dunk under five seconds should be safe from a premature breakoff that would result in a soft cookie at the bottom of a glass.
Finally, though this final observation may seem painfully obvious, it was observed that moist cookies continue to grow softer over time. In other words, it is not necessary to suspend a cookie in a liquid until the desired level of softness is reached, rather a cookie may be dunked for a relatively short period of time, followed by an out-of-glass waiting period until the ideal consistency is achieved.
Several commercially available cookies were dunked in milk for various amounts of time, with the weights measured before and after. The collected data suggest that these cookies absorb 80 percent of their potential liquid within two seconds and 99 percent within five seconds. This indicates that a cookie suspended in milk for five seconds may represent a point of diminishing returns in the realm of cookie dunking. Furthermore, we have identified eight seconds as a lower bound at which breakoff can occur. Finally, we observe that dunked cookies will continue to soften with time, even when not suspended in milk. Thus, short dunking duration between two to five seconds, followed by a brief waiting period seems to be the ideal approach for a cookie dunker to reach a desired degree of softness without the worry of losing the cookie to the bottom of the glass.
1. Kruszelnicki, K.S. “Biscuit Dunking Physics” Australian Broadcasting Company. Web. Read 25 Aug. 2015
2. Frost, V. “Crunch time: Why Britain loves a good biscuit” The Guardian. Web. Read 25 Aug. 2015
3. Fisher, L. “Physics takes the biscuit” Nature. Vol. 397. 11 Feb. 1999
4. “Cold milk takes the biscuit” BBC News. Web. Read 26 Aug. 2015
Randy Hurd is a grad student in mechanical engineering. He didn’t think this writing project was quite what the Statesman was looking for, but decided to send it along anyway. He hopes that is will enrich readers lives. firstname.lastname@example.org