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Home , Gastronomy, Molecular gastronomy

Leading Edge Commentary

Biophysics of Molecular

Michael P. Brenner1,* and Pia M. So¨ rensen1 1School of Engineering and Applied , Harvard University, 29 Oxford Street, Cambridge, MA 02138, USA *Correspondence: [email protected] http://dx.doi.org/10.1016/j.cell.2015.03.002

Chefs and scientists exploring biophysical processes have given rise to . In this Commentary, we describe how a scientific understanding of recipes and techniques facilitates the development of new textures and expands the flavor palette. The new dishes that result engage our senses in unexpected ways.

Molecular gastronomy’s beginnings can first to carry out precise temperature- sures the resistance of the material to be traced to the 1970s and an emerging controlled on an egg is unclear, deformation. Roughly speaking, the 3 curiosity about the scientific aspects of but a remarkable process was discov- elastic modulus E = Uinteraction=l , where   the we eat. As Nicolas Kurti, Profes- ered: between 60 C and 70 C, critical Uinteraction is the binding energy between sor of at Oxford, famously stated, biophysical transitions occur in chicken the crosslinks, and l is the typical dis- ‘‘It is a sad reflection on our civilization eggs; this is the range in which the tance between the crosslinks. The mate- that while we can and do measure the different proteins within the yolk and white rial thus becomes harder to deform temperature in the atmosphere of Venus, unfold. Strikingly, differences of less than when the binding energy or density of we do not know what goes on inside our 1C lead to significant textural and struc- the crosslinks increases. Another aspect souffle´ s’’ (McGee, 1984). Acting upon tural changes. Indeed, a well-trained of texture is plasticity, which occurs this sentiment and in collaboration with can predict the temperature of the when crosslinks easily remodel; a material the physical chemist Herve´ This, Kurti water bath within a half a degree based with high plasticity will not recover its orig- began to convene regular gatherings in on the texture of the egg, and eggs inal shape when deformed. Yet another Erice, , bringing together professional cooked even just a couple degrees apart concept is the yield stress, which is the chefs and scientists—among them, Pierre have very different culinary applications minimal stress that can be applied to the Gilles de Gennes, the Nobel prize winning (Figure 1). material for the crosslinks to break and physicist who popularized the study of At its core, the biophysical transitions the material to fracture. Soft condensed physics of soft materials, as well as Har- that happen when cooking an egg are matter has given a qualitative un- old McGee, who wrote the remarkable the same ones that biology regulates derstanding of how these macroscopic treatise ‘‘On and Cooking’’ (McGee, against: the unfolding and subsequent properties are related to the microstruc- 1984). aggregation of proteins. Textures arise ture of materials. But how they apply in This questioning of the biophysical ba- because denaturing proteins gradually detail to the intricate suite of textural tran- sis of foods coincided with a burgeoning expose previously buried fragments, sitions in an egg is not well understood; movement in the culinary world, spurred enabling crosslinks to form between we do not know how the microscopic largely by the chef Ferran Adria` , who different proteins so that the protein structure of the egg’s crosslinked interior aimed to use emerging knowledge about mixture becomes a gel. As the tempera- changes as a function of temperature the science behind recipes to rethink ture increases, more of the proteins are nor how the different parts of egg proteins and reengineer foods and create new exposed, and the crosslinks become contribute to the final texture. Importantly, textures and tastes. Part of his program stronger and more numerous. This is we also don’t know which physical prop- involved developing more precise meth- due to both the continued unfolding of in- erties correspond to the mouthfeel of a ods for controlling cooking protocols, dividual proteins and the ensuing unfold- food. such as immersion heaters, rotovaps, ing of more stable ones. Microscopically, These questions are not just aca- and centrifuges. When these are applied the unfolded proteins bond in complex demic: for example, if the biophysics of to foods, a panoply of rich transitions are patterns, and the resulting macroscopic the transitions in an egg were better un- uncovered, leading to new textures and texture arises from the nature of these derstood, it might be possible to rede- tastes. arrangements. Thus, manipulating the sign the egg to have different properties The simplest example of creating new crosslinked matrix by precisely adjusting as it is heated. One could imagine mixing textures is the common task of cooking the cooking temperature allows the con- different concentrations of the protein an egg. Chefs routinely cook eggs in trol of macroscopic texture. constituting egg whites to manipulate boiling water, but an immersion heater al- Several scientific concepts describe the textural transitions, or mixing egg lows cooking them at any desired temper- the macroscopic aspects of texture proteins taken from different organisms ature and observing sensitive changes in and how they relate to the underlying with distinct denaturation and aggrega- the texture that have been otherwise microscopic arrangement. One is quanti- tion properties. Moreover, tuning individ- impossible to note. Which chef was the fied by the elastic modulus, which mea- ual proteins to prescribed macroscopic

Cell 161, March 26, 2015 ª2015 Elsevier Inc. 5 from the food to the nasal passage. While many flavor exist as preformed compounds in the cooking ingredient and are released when the tissues of foods are damaged, as is the case with the pungent molecules in mustard seeds, others are formed during the cooking process through the application of heat, mechani- cal force, or other procedures. This is the case for many fruity aromas, which are created when tissue damage releases en- zymes that react with molecules in fruit cells to produce esters. Figure 1. Eggs Cooked at Different Temperatures in a Temperature-Controlled Water Bath There are three noteworthy methods for Even small differences in temperature result in significantly different textures. Image courtesy of Modernist producing small flavor molecules in mod- . ern . The first is to capture and concentrate them. Modernist chefs properties could have utility well beyond melting temperature and also a low yield use a variety of techniques to extract, cooking. stress. This material enabled Heston Blu- isolate, and concentrate flavor and aroma Another aspect of Adria` ’s culinary revo- menthal to create a famous dish, hot and molecules, many of which draw on lution involved introducing new ingredi- cold (Blumenthal, 2009), in which a methods and equipment from biology ents. One such set of ingredients are mol- glass of tea is divided into two halves: and laboratories. These range ecules called hydrocolloids, which can be the left half is served cold, whereas the from filtering to centrifugation to vacuum used to create new textures in the form of right half is served hot. Gellan strengthens infusions. For example, different types of , foams, and the gel sufficiently to separate the two uses centrifugation as a way to isolate gels, all hallmark components of the halves of the tea cup but breaks up imme- flavorful carotene butter from carrot juice modernist . A particular explosion diately upon pouring. The textures of the and laboratory sieves and agar filtering occurred with gelling agents, a subgroup two teas are identical, allowing the con- to concentrate purees and soups (Myhr- within this category, which mostly con- sumer to simultaneously hot and vold et al., 2011). One of the most popular sists of carbohydrates. Whereas a century cold tea on different sides of the tongue. pieces of equipment for isolating and ago, the major gelling agent in Western- To date, chefs have capitalized on the concentrating flavor is the rotovap. Long was derived from tremendous diversity of gelling agents a stalwart of chemistry laboratories but animal collagen, today’s chefs routinely produced by organisms in the natural introduced in the last decade into high- use gelling agents derived from a diverse world to create a wide range of foods in end cooking, rotovaps allow gentle, range of organisms that allow control of new parameter regimes. There is much low-heat separation of certain molecules texture and properties in new regimes. room for further , with increased based on their volatility. Thus, com- These ingredients include methylcellu- understanding of why the different gelling pounds that would easily boil off or lose, xanthan gum, agar, and gellan, agents work as they do. For example, one decompose at standard boiling tempera- each having distinct biophysical proper- could imagine engineering organisms to ture can be captured and served as part ties that differ from the standard gelling produce gelling agents with distinct prop- of a dish. For example, Joan Roca, of agents. For example, whereas gelatin erties. the restaurant Can Roca in Spain, uses a gels between 4C and 35C, methylcellu- Thus far we have discussed culinary rotovap to capture the flavor molecules lose gels between 50C and 90C. This manipulations of food texture; the second of eucalyptus leaves and citrus peel, property, allowing creation of a hot gel, major aspect of cooking affected by the both consisting of molecules that easily led to Adria` ’s invention of hot ice cream, culinary revolution is flavor. Flavor arises vaporize and disappear; the distillates which has the same texture and rheology from the combination of several sensory are then used as a base for sorbets and as ordinary ice cream but only when experiences—most notably, the thou- other cold desserts (Figure 2B). Roca served at high enough temperatures. sands of taste receptors on our tongues also uses the same technique to make a Similarly, spherification, one of Adria` ’s and the olfactory receptors in the upper gel from distilled forest soil, which he signature dishes and characteristic of mo- nasal passages. While the number of then serves with an oyster, creating a lecular gastronomy, also makes use of a tastes that can be experienced by hu- completely novel flavor combination that gelling agent: alginate, a polymer found mans are limited to the well-known would be impossible to achieve without in sea weed, which forms crosslinks with five—sweet, salty, sour, bitter, and the rotovap. calcium ions, resulting in a thin gel that en- —we can sense orders of magni- A second method for producing flavor capsulates a sphere of liquid flavor tudes more smells. Flavor compounds is creating small flavor molecules through (Figure 2A). Another example is gellan, tend to be small molecules, and in the chemical reactions. Aspects of this prac- derived from the bacterium Pseudo- case of olfaction they are also volatile, tice have clearly been a part of cooking monas elodea, which has both a high which allows them to be transported long before the advent of molecular

6 Cell 161, March 26, 2015 ª2015 Elsevier Inc. Figure 2. Examples of Dishes from (A) Ferran Adria` ’s spherified pea ravioli. Image courtesy of Ferran Adria` . (B) Joan Roca’s rotovapped citrus peel. Image courtesy of Blua Producers. (C) Close-ups of ’s pork bushi, a novel take on the Japanese fermented fish known as katsuobushi. Image courtesy of Momofuku. (D) Wylie Dufresne’s deconstructed dish, . Image courtesy of Takahiko Marumoto. gastronomy—simply heating or cutting from the plethora of flavor molecules re- Chang, for example, works with uncon- into foods can initiate chemical reac- sulting from these reactions. ventional combinations of foods and mi- tions—but recent developments in tech- A third method of producing new fla- crobes to create new recipes, such as niques and equipment have not only vors in foods is fermentation, which relies pomegranate seeds with lactic acid bacte- brought advances in this field, but have on microbes breaking down large mole- ria, commonly used in dairy fermentations, also suggested further potential for dis- cules in and animal products, or locally grown farro with Aspergillus covery. One example is the diversity of such as proteins, carbohydrates, and oryzae, a mold traditionally used in Asian flavors produced by caramellization and fats, into an array of much smaller mole- for miso and sake production Maillard reactions. Caramellization de- cules. Fermentation is an age-old method (Figure 2C). The design and characteriza- notes the decomposition of sugars at of producing intense, complex, and tion of such novel fermentation processes high heat, whereas Maillard reactions incredibly diverse flavors: coffee, soy are still in their infancy but hold much result from complex reactions initiated , cheese, chocolate, wine, and vine- potential as a field for further discovery. by amino acids reacting with sugars. gar—the contemplation of almost any list We hope that we’ve succeeded in Both reactions are highly temperature of fermented foods is a telling illustration bringing across the message that the bio- and pH sensitive and must be carefully of the powerful relationship between physical properties of molecules underly- controlled not to result in strong and bitter fermentation and flavor. In many cases, ing cooking provide much interesting flavors. However, if correctly manipu- flavor production is the primary purpose fodder for scientific inquiry. The culinary lated, these reactions contribute to the of the fermentation reaction, such as in revolution has, however, pushed this complex flavors of diverse foods ranging soy sauce and traditional fermented fish notion further, and the design of new from steak to maple syrup to champagne. . But in other cases, the flavor mol- dishes has itself become a highly experi- Modernist chefs found ways to enhance ecules are delicious byproducts of what mental enterprise. In the process of opti- these effects using pressure cookers, might be taken to be the primary goal of mizing the incorporation of new ingredi- which not only speed up flavor produc- the process. For instance, in wine produc- ents and equipment into the kitchen, tion, but also produce more intense fla- tion, where the preserving and intoxicat- chefs experiment in ways similar to vors. Though Maillard and caramellization ing properties of from glucose scientists. Failure of a given idea gives reactions create marvelous results even drive the fermentation, it is the myriad rise to new ones, eventually leading to with a relatively modest knowledge of enzymatic byproducts that create the creations that might not even be related the underlying chemistry, both could complex and nuanced flavors of wine. to the original idea. potentially be deconstructed and A relatively recent trend in haute cuisine described this process in the context of controlled even more carefully, with the is the experimentation with fermentation creating the recipe for hot and cold opportunity of isolating novel flavors reactions to create new foods. David tea as involving ‘‘endless trials’’ and

Cell 161, March 26, 2015 ª2015 Elsevier Inc. 7 ‘‘infinitesimal precision’’ (Blumenthal, play on the faculties of all senses, and minibar uses spherification to make 2009). This is an attitude to cooking that including notions of emotions and mem- eggs in which the white has been re- sticks to this day and is apparent in ories evoked by tastes and aromas, our placed by a rich solution of parmesan many aspects of molecular gastronomy. expectations of food, and the element of cream. For example, a common theme in surprise as a dish turns out to be some- In summary, the past two decades modernist kitchens is the frequent and thing different from what it appears to have seen an emergence of striking complete overturn of the menu preceded be. As an example, a common theme in complementarity between the methods, by intense periods of research and exper- the modernist kitchen is the decon- ideas, and of the culinary world imentation. Ferran Adria` initiated this structed dish, which separates the com- and basic science. The chef’s goal of trend at his famed restaurant elBulli, ponents of a familiar recipe into individual transforming the properties of cells and which closed for 6 months of the year pieces with novel and imaginative tex- proteins into another form is complemen- while his team withdrew to a workshop tures. Wylie Dufresne’s take on eggs tary to scientist’s efforts to understand in order to exclusively focus on the crea- benedict, which consists of deep-fried them. There is much left to be learned at tion of next year’s menu. Interestingly, served with concen- this intersection. the emphasis on culinary experimentation trated egg yolk shaped into columns, is not only expanded cooking into the realm an example of this approach (Figure 2D). REFERENCES of science, but it also allowed it to take on Another common theme is playing with Blumenthal, H. (2009). Cookbook characteristics from the . With the the diner’s expectations by presenting (Bloomsbury). realization that new ingredients and foods that look different from what they McGee, H. (1984). On Food and Cooking: The Sci- equipment do more than simply offer an are. Accordingly, Joan Roca famously ence and Lore of the Kitchen (Scribner). opportunity to tweak traditional recipes, serves an ice cream dish that is indistin- Myhrvold, N., Young, C., Bilet, M., and Smith, R.M. chefs began exploring the creation of guishable in shape from that of a cigar, (2011). Modernist Cuisine: The and Science of uniquely novel dishes. These creations and Jose´ Andre´ s of the restaurants Jaleo Cooking, First Edition (Cooking Lab).

8 Cell 161, March 26, 2015 ª2015 Elsevier Inc.