according to the quiche recipe by Dorie Greenspan (2009), “as soon as the crust comes out of the oven, lightly beat an egg white with a fork and brush the white over the inside of the crust. . . . [I]t will provide a kind of waterproof lining between the crust and the quiche filling. Quiches are so much nicer when you can pair their soft, creamy custard with the slight crunch of a crust.”
The fact is that baked dough products, like quiche and pizza, deteriorate rapidly after they are removed from the oven. Absorption of water into the crust is the main problem, which contributes to the deterioration of the crispy texture and the presence of an unpleasantly soggy crust. Let us look at how crispness is achieved, perceived, and analyzed in foods with a crispy crust and a soft, moist interior.
From a scientific point of view, texture is related to the structural, mechanical, and surface properties of food. Texture is detected by seeing, hearing, touching, and kinesthetics (the ability of the body to sense its position, location, orientation, and movement—and those of its parts). Only human beings can perceive and describe food texture, and we detect it as one integrated sensation, which nevertheless comes from the various senses—all stimulated by the eating process. The perception of food is gleaned from the information collected by the senses through the process of eating. This includes observing the food product outside or inside its container and handling the food item before consumption (cutting roast beef or pouring soup), as well as actually eating it. Texture plays an important role in determining likes and dislikes, and its importance is accentuated when expectations are violated, for example, when one tastes meat that is not tender, lumpy puree, or ice cream that is crystallized rather than creamy.
Crispness and crunchiness are not so much characteristics of the food item as they are aspects of its structure and the physical state of its components. Whether these two English terms refer to different concepts is still open to question (for a discussion of the sensory experience of food and crispy-crunchy-crackly foods in particular, see chapter 2). In other languages (for example, Spanish and French), similar questions arise. What crispness and crunchiness actually mean is not clear because the perception of crispness and crunchiness is the sum of the information received by several senses (such as sight, hearing, and touch). Crispy is, for instance, defined by Laurence Fillion and David Kilcast (2002) as “a light and thin texture producing a sharp clean break with a high-pitched sound when a force is applied, mainly during the first bite with the front teeth.” Crunchy is described as “a hard and dense texture that fractures without prior deformation.” Lisa M. Duizer, Osvaldo H. Campanella, and Geoff R. G. Barnes (1998) define crispness as “a combination of the noise produced and the breakdown of the product as it is bitten entirely through with the back molars.” Confusion abounds! Zata Vickers (1984) initially suggested that the pitch of the sound was the distinguishing factor between crispness and crunchiness; however, the same author later said that “most foods were about as crispy as crunchy.” Therefore, for our purposes, we will speak exclusively in terms of crispness, while not forgetting that crispness and crunchiness are complex concepts with interpretations that depend on the food item, the background of the eater, and their food culture.
Foods that are crispy because they have a crust tend to be fresh, recently baked products (like French bread). Crispness can take the form of an added element, such as nuts in salads or croutons in soups. These added “touches” create pleasing, creative, or surprising textur-al combinations. Recent developments in the art of cooking impart new textures to traditional preparations or combine various textures in the same dish made with the same raw materials. For example, we have seen these items used in combination with various other foods: orange gelatin on sliced fresh oranges, caramelized orange peel in orange sherbet, and candied orange peel in orange ice cream.
In a study performed in our lab, a total of two hundred consumers were interviewed about crispy food (Varela et al. 2007). Foods these consumers most frequently described as crispy were so-called dry-crisp products, such as toast, cookies (biscuits), potato chips (crisps), and breakfast cereals; these foods are firm and brittle. The second most mentioned group of crispy foods was what could be called wet-crisp products, which include raw or hardly cooked vegetables and raw fruits, such as apples. In wet-crisp foods, the crispy characteristic comes from the water pressure inside the plant cells, which are what make up the vegetable or fruit. A third group was foods with a crust, which, as the name already implies, are foods with a dry, hardened exterior and a soft, high-water-content core. Familiar examples are french fries (chips), French-style bread, and battered-and-breaded deep-fried foods. Also included in this group were foods that combine a crispy layer and a high-water-content topping. The best-known examples are pizzas, pies, quiches, and tarts. For reasons we will discuss shortly, retaining the crispy character of the dry part is very difficult.
All products with a crust present common characteristics and therefore challenges because of their mixed structure, normally having a high-water-content, soft, and deformable interior, surrounded by, or attached to, a dry, firm, and brittle crust. The key issue in these kinds of products is how to maintain the crispy character after preparation; in general, the loss of crispness is due to the diffusion of water from the high-water-content part to the low-water-content, crispy part.
Water content in any given sample indicates how “humid” the food is. However, it is more accurate to speak about water activity (aw). The water activity is a simple thermodynamic measure of the dryness of food. Water activity is related to how free, available, or bound the water is. In addition to the water content, water activity values depend on the concentration and type of dissolved substances in the food—mainly sugars and salt, the existence and distribution of pores, and the relation of water to the “thirsty” sites of the food matrix. Proteins, carbohydrates, and starches possess many sites to which water molecules can strongly bind. Critical water activity values are those in which the products become sensorially unacceptable because they completely lose their crispness.
Water diffusion from moist (high aw to dry (low aw layers causes moistening of the components of the crispy parts. At a critical water activity value, further movement of water causes a change from a glassy state (in which the materials behave as hard and brittle) to a rubbery state (in which the materials behave as leathery, soft, or sticky). As the food is transitioning between the glassy and rubbery states, a loss of crispness takes place.
Tempura or battered-and-breaded fried foods—like fish, seafood, poultry, cheese, and vegetables—are good examples of foods with crisp external crusts. They are favored and appreciated by consumers. Coated fried foods are dipped in a flour-based batter before they are fried. Because battering and frying have been traditional methods for preparing foods, empiricism has dominated their application for centuries. Although considerable geographical variations occasionally exist because of the raw materials available, versions of batter-coated or breaded foods are found in the traditional or regional cuisines of practically every part of the world.
In one type of coating, known as tempura, the liquid batter comes into direct contact with the hot oil, which makes it coagulate around the piece of food, forming a crispy crust. All the final quality characteristics of the tempura-type coated food largely depend on a good formulation of the ingredients that constitute the raw batter. The list of ingredients used has become much longer than just wheat flour and water: different starches, gums, milk, seasonings, and many other items are added. The behavior of each ingredient is very different, determining the final performance of the product.
This batter is a single outer covering for the substrate. Unlike adhesion batters (as we will see), this type of batter normally contains leavening agents (beer, for example) that contribute to the formation of small bubbles (carbon dioxide). Therefore, the batter expands when fried, developing a number of larger gas cells and, consequently, a spongy structure. Celebrated chef Heston Blumenthal of the Fat Duck in Berkshire, England, created a novel way to get significantly crisper fried fish. This was done by manipulating the type of starch used—by nucleating the batter in a syphon with nitrous oxide and by incorporating vodka (alcohol takes far less energy to evaporate than does water). All this rendered an irregular and rather unique crust while ensuring that the fish did not overcook (for a nonsyphoned, but nonetheless crisp, battered fish recipe, see chapter 2).
An emulsifier, such as the lecithin present in egg yolk, can keep the growing bubbles from collapsing or bursting. During frying, the batter foam dries out and takes on a completely solid and spongelike structure. The use of a leavening agent reduces the density (less weight for the same volume) and increases the volume of the coating so that it is lighter on the tongue.
The leavening agent produces gas. This helps aerate the structure; however, only when small bubbles are already present in the batter will the released gas be captured by them—normally, no new bubbles are formed. Instead, those bubbles that are already present grow. This is why the initial beating stage is very important in this type of batter. The aeration caused by the leavening agent contributes to crispness and facilitates water loss (in the form of steam) during frying. Steam also helps to expand the coating.
The structural characteristics of tempura-like batter must be such that they lend to the fried product a uniform external layer with good adhesion to the substrate—and good coverage. Tempura-type batters form a crispy, continuous, aerated, and uniform layer over the food substrate that constitutes the batter’s final aspect. They protect the natural juices of foods, thereby ensuring a final product that is tender and juicy on the inside and crispy on the outside.
Another type of batter is one that acts as the glue to an external layer of bread crumbs, creating a battered-and-breaded final product. The choice of the batter ingredients is not as delicate a matter as in tempura-type products. Essentially, the batter needs to act as a good adhesive—and, ideally, it should not be distinguishable during consumption. The crispy characteristics of the end product in this case mainly depend on the bread-crumb coating: the bread or grains from which it is made, the shape and size of the crumbs, and the crumbs’ regularity, degree of toasting, and so on. After a few minutes of frying, coated foods have a pleasant golden-colored exterior with a crunchy texture, whereas the interior usually remains tender and juicy. These are the characteristics that make these products so appetizing.
Frying is the most common method for cooking or reheating tempura or battered-and-breaded foods. Apart from a variety of chemical reactions occurring, several changes take place in the frying process, such as gelatinization of starch, denaturation of protein, and reduction of moisture (the product is dehydrated until it provides a crispy texture).
One problem associated with the consumption of battered-and-breaded deep-fried foods is the great amount of oil absorbed during the frying process. Recently, there has been a trend to reduce the fat content in fried foods by changing the formulations or developing new cooking methods to avoid one of the frying steps. A good solution for this is the use of prefried frozen or refrigerated battered or battered-and-breaded food pieces. For instance, only 30 seconds of prefrying is required in the case of battered shrimp. Such products may then be finished by alternative cooking methods; this constitutes a useful option for caterers or hospitality kitchens, for example, with the added advantage of leading to a lower fat content than the equivalent fully deep-fried products.
The oven is increasingly used as an alternative cooking or reheating method for prefried battered and breaded foods. Baking is a good way to avoid the excessive absorption of fat that occurs in deep frying. New methods in microwave heating are another option. The preparation of battered-and-breaded food items in standard microwave cooking notoriously leads to serious texture problems: sogginess, lack of crispness, and lack of browning. This is because the microwave radiation heats up the moist interior of the food item. It thus drives off the water (in the form of vapor), forcing it to move from the interior and outward to the crust, where the water condenses because the crust is still relatively cold.
However, new technologies offer a way to specifically heat the surfaces of food items in microwave cooking. One of the more recent methods that can be applied to cook or reheat prefried products in the microwave uses susceptor materials. The name is derived from susceptance, a property of certain materials that gives them the ability to convert electromagnetic energy (from the microwave) into heat. In fact, the popcorn susceptor bag, in which the microwave temperature ultimately reached is high enough to cause corn kernels to pop, is a well-known example. Susceptor materials are metallized plastic films laminated with paperboard; the heat generated in the susceptor material during microwaving is transferred to the product, creating areas hot enough to evaporate water and render the food surface crisp. Susceptors can work reasonably well as long as the surface of the food item is in contact with or very close to the susceptor material (figure 37).
Figure 37 Nugget surrounded by the susceptor casing. The external surface is made of cardboard; the nternal surface is constructed of a specially designed material that acts by heating up the sample surface through radiant heating.
In products with a crust, what we perceive as texture in the outer layer during eating, be it soggy or crispy, is certainly a result of the characteristics of the crust. The perception of crispness is partly related to auditory sensations, as all crispy foods are noisy when eaten. Scientifically, the study of the crispy or crunchy textures can be performed through recording the sounds emitted by the food piece while it is compressed or cut (for a detailed explanation of how this is done, see chapter 2).
The same prefried chicken nuggets were cooked by four procedures:
• Deep-frying (360°F [180°C]) in a domestic fryer (3 quarts [3 L] oil; 3 minutes)
• Electric oven (440°F [225°C]) with convection (11 minutes)
• Microwave oven (700 watts; 1 minute, 15 seconds)
• Microwave oven (700 watts; 1 minute, 15 seconds), with susceptor material (see figure 37)
Crispness is strongly linked to auditory sensations. Sounds contain important information related to the fracture properties of crispy foods. Not surprisingly, the instrumental methods developed by scientists to evaluate crispy foods in the laboratory have focused on measuring the sound they emit during fracture.
In this example, a microphone was used to record the sounds emitted while the sample was cut with a plastic blade.
The results can be evaluated by observing the curves, which relate the force required to cut the products with the sound they emit during cutting.
• Deep-fried samples. When the samples are very crispy, as indeed they were in this case, the curves obtained are highly jagged, with lots of peaks (figure 38a). This means that small cracks or fractures occur in the nugget crust as the blade penetrates the samples (imitating the teeth-biting action) and sudden drops of force take place producing audible noises.
Figure 38 Force and noise emitted during the cutting of a nugget. Each graph contains two curves: the black one represents the force, and the gray one represents the noise. The force is measured in newtons (N) and the noise level is expressed as sound pressure level (SPL) in decibels (dB): (a) deep-fried sample; (b) oven sample; (c) microwave sample; (d) microwave plus susceptor sample.
• Oven-cooked samples. These samples show a curve with fewer force peaks. The force values are higher because a harder, drier product is obtained. This is caused by the greater water loss during the long exposure to the high oven temperature (figure 38b).
• Microwaved samples. The force plot obtained from microwave cooking is dramatically different from the others (figure 38c): it does not present peaks in force, meaning that no fracture events have happened. These samples obviously were not at all crispy but gummy and tough. The toughness is reflected in the force values, which are higher than in the deep-fried samples, indicating tougher pieces of food.
• Susceptor-microwaved samples. A noticeable improvement in crispness is seen when the susceptor material surrounds the product: some ups and downs appear in the force, together with some sound events (figure 38d). There was an enhancement of the crispy character, but these nuggets were still not as good as the deep-fried samples.
In deep-frying or in oven cooking, the water evaporation occurs mainly at the surface because heat comes from outside (from the hot oil or from the hot oven walls and air). Nuggets (or any other food item) heated in a microwave oven undergo internal water evaporation because the microwave energy heats all the water at the same time; this inner hot water tends to escape to the surface, which is surrounded by cold air (in a microwave oven, the air does not heat), causing a moistening of the crust.
When a nugget is microwaved in a susceptor material, the heat that the material generates (with the microwaves’ help) allows the water to evaporate and the nugget surface to dry. The susceptor material therefore acts as a small oven. The use of susceptors constitutes the first attempt at preventing sogginess in microwaved food. There are already a number of industrial food products for home cooking that make use of this development, focusing on ready-to-heat crusted or composite foods for the microwave (sandwiches, pastries, and pizzas, for example).
The use of susceptors is just one example of the possibilities that food science and technology offer in the vast panorama of improving crispness in battered-and-breaded food. Much research into microwaving and other cooking methods is yet to be done, with many questions still to be answered, such as how effective could ovens that combine infrared heating and microwaves be?
4 chicken thighs
3½ ounces (100 g) plus 10 g all-purpose (wheat) flour
Salt, baking powder, and black pepper
2 egg whites
4 ounces (125mL) cold sparkling water*
Preheat the oil to 390°F (200°C). Pat the chicken thighs dry with a paper towel. Put the 10 g of extra flour on a plate and use it to coat each thigh, patting off the excess. Mix together the remainder of the flour, a pinch of salt, a pinch of baking powder, and some pepper. Lightly whisk the egg whites until bubbly but not stiff. Pour the sparkling water into the flour mix, whisking gently and briefly. Gently stir in the whisked egg whites just to mix. Retain as many bubbles as possible so that the batter stays light.
Dip two thighs in the batter to coat, let the excess drip off, and then immerse them in the hot oil (390°F [200°C]) using a slotted spatula. Fry for 5 to 6 minutes, making sure the oil temperature remains constant. When the batter is set, turn the pieces of chicken over and cook until they are an even golden brown. Lift out with the spatula and drain on a paper towel. Check that the oil has returned to 390°F (200°C), and then repeat with the remaining chicken thighs.
* The bubbles from the sparkling water provide many tiny bubbles that act as nuclei. These grow in size when the carbon dioxide generated from the baking powder is released. Together, the bubbles and carbon dioxide contribute to the structure of the batter during frying, producing a spongelike, crispy texture.
Other ways to enhance the crispness of coated foods involve the use of ingredients such as coarser bread crumbs, mixes of different crumb sizes, dried Japanese bread crumbs, whole grains, multigrains, seeds, or nuts incorporated into the batter. Also, starches lend crispness to batter coatings, particularly those coming from new vegetal sources, like sago, new rice cultivars, and the like. But the main question is, Are cooks ready to follow each of these crispy trails?
The authors are grateful to the Ministerio de Ciencia e Innovación of the Spanish government for its financial support (AGL 2009-12785-C02-01) and to INDAGA network (AGL2009-05765-E).
Duzier, L. M., O. H. Campanella, and G. R. G. Barnes. 1998. “Sensory, Instrumental, and Acoustic Characteristics of Extruded Snack Food Products.” Journal of Texture Studies 29:397–411.
Fillion, L., and D. Kilcast. 2002. “Consumer Perception of Crispness and Crunchiness in Fruits and Vegetables.” Food Quality and Preference 13:23–29.
Greenspan, D. 2009. “Quiche: And Now for the Crust.” Available at http://www.doriegreenspan.com/print/2009/02/quiche-and-now-for-the-crust.html.
Varela, P., A. Salvador, A. Gámbaro, and S. Fiszman. 2007. “Texture Concepts for Consumers: A Better Understanding of Crispy-Crunchy Sensory Perception.” European Food Research and Technology 226:1081–1090.
Vickers, Z. 1984. “Crispness and Crunchiness: A Difference in Pitch?” Journal of Texture Studies 15:157–163.
