P OrOSity   and  P Ore  d evelOPment

The microstructure of fried foods is a significant factor that influences moisture loss and fat uptake during frying (Bouchon et al., 2001). Pores and their distributions are important components of the microstructure of fried products. The porosity of a product is normally a result of interactions between the product and its surrounding medium at elevated frying temperatures. Some food materials are naturally porous, especially such products as pastry, donut, and akara, which are whipped to incorpo-rate air into them before frying (Huse et al., 1998). Preexisting pores are normally quick and open channels for mass transfer during frying and could significantly affect the amount of oil entrained in the final product. For example, akara, which is a traditional deep-fat fried food in West Africa and is produced from cowpea (Vigna unguiculata) paste through a process of whipping air into it to form a foam-like paste, is among the products that are of interest to many investigators. The for-mation of pores by whipping prior to frying had a significant effect on the amount of oil absorbed by the product (Prinyawiwatkul et al., 1994; Huse et al., 1998; Singh et al., 2004; Huse et al., 2006). No data are currently available and no study has been reported on the porosity of this product, however.

During frying, pores are developed due to moisture evaporation from within a food material (Moreira et al., 1999; Kassama and Ngadi, 2005). The formed voids are filled with oil through different mechanisms. In certain cases, quick formation of an impervious crust leads to the formation of large pores within fried foods (Shyu et al., 2005). A crust is formed when the surface moisture content is reduced, thereby raising the surface temperature close to that of the frying oil. This elevated tempera-ture induces chemical changes such as starch gelatinization, protein denaturation, and gelation of polysaccharides such as hydrocolloids, which result in the forma-tion of crust in fried foods. Another major factor influencing pore development dur-ing frydur-ing is the frydur-ing conditions, which include frydur-ing duration and temperature.

Kassama and Ngadi (2005) reported an increase in the porosity of fried chicken meat with time. However, the change in the porosity leveled out after about 400 s of frying (Figure 2.2); this phenomenon was attributed to fat intrusion, which saturated

pore paths at a longer frying time. The effect of temperature was also clearly demon-strated in this experiment, where the increase in the frying temperature led to a sig-nificant increase in the porosity. Adedeji and Ngadi (2008) later reported the effect of formulation on pore development in the batter coating of chicken nuggets. The addition of hydrocolloid at various levels of rice flour, which was used as a substitute for wheat flour, resulted in a significant change in the porosity of the fried coating.

Pore characteristics have been evaluated by various physical methods, such as pycnometry (Kassama and Ngadi, 2004) and mercury intrusion porosimetry (Farkas and Singh, 1991; Karathanos et al., 1996; Ngadi et al., 2001), and imaging methods (Miri et al., 2006; Adedeji and Ngadi, 2008). The latter technique (imaging) is novel and includes a number of different approaches, namely, microscopy, computer vision (CV), and x-ray micro-computed tomography (micro-CT). The principle of poro-simetry is based on the non-wetting characteristics of mercury, which is used as the medium for pore properties’ determination. This technique involves the introduction of mercury into a porous food under pressure in a stepwise manner, and then the determination of the pore characteristics based on the relation between the amount of mercury allowed into the sample of a specific diameter and the pressure required.

This relationship is defined by the Washburn equation (Dullien, 1992; Giesche, 2006). For this expression to be valid, certain assumptions are made, e.g., that the pore shape is cylindrical. Pycnometry is a technique that utilizes density measure-ment to calculate the porosity of a material. An extensive documeasure-mentation on how to use this technique is given by Webb (2001).

2.2.3   S^hrinkage

Fried foods shrink when the moisture is lost and the food cells collapse as a conse-quence of heating and evaporation during frying. It is a phenomenon described as

Time (s)

0 200 400 600 800 1000

Porosity

–0.1 0.0 0.1 0.2 0.3

170°C 180°C 190°C

fIgure 2.2  Effect of frying conditions on pore development in fried chicken meat. (From Kassama, L.S. and Ngadi, M.O., Dry. Technol., 23, 907, 2005.)

a decrease in the product dimension when heat-induced evaporation/drying occurs (Krokida et al., 2000). The occurrence of shrinkage affects food microstructural properties, such as porosity, pore-size distribution, and textural characteristics.

Shrinkage can be expressed in terms of length or volume (Taiwo and Baik, 2007):

S D D

S and S_v are shrinkage expressed, respectively, in terms of dimension (thickness/

diameter) and volume D_o is the initial dimension V_o is the initial volume

D_(t) and V_(t) are the dimension and volume after frying at time t

Shrinkage starts as a surface occurrence, since drying during frying initiates at the surface, and then progresses into the sample with the frying time. When the dry-ing rate is high, the crust forms quickly, hence reducdry-ing the rate of moisture, which causes the shape of the product to form early with minimal shrinkage. Kawas and Moreira (2001), in their work on tortilla chips, reported that shrinkage was closely related to mass transfer changes during the frying operation. These investigators also showed that most of the diameter shrinkage experienced by tortilla chips occurred during the first 5 s of frying, which was attributed to quick crust formation.

Kassama and Ngadi (2003) reported an exponential decrease in the volumetric shrinkage of deep-fat fried deboned chicken breast meat. The rapid increase in shrink-age at an early stshrink-age of frying and the correlation with moisture loss were also observed in this study. Taiwo and Baik (2007) studied the effects of different pretreatments, such as blanching, freezing, air drying, and osmotic dehydration, on the shrinkage of fried sweet potato. They also reported quick crust formation, leading to early shrink-age setting in the samples. These investigators also established a relationship between shrinkage and moisture loss. However, they showed that fried sweet potato samples experienced some increase in volume after the initial decrease, and this was attributed partially to quick crust formation that led to pressure buildup within the sample, caus-ing puffcaus-ing. The effects of pretreatments were shown to be significant on the shrinkage of the samples. Shrinkage, apart from being an important quality definer, is also a very useful tool in transport phenomena modeling of the frying process.

2.2.4   texture

Texture is a sensory attribute that influences the acceptability of fried foods and is associated with the mechanical, geometrical, and acoustic configuration of the

foods (Szczesniak, 1987). Textural properties of fried foods are developed as a con-sequence of many physical, chemical, and structural changes taking place during frying, which include heat and mass transfer and chemical reactions (Pedreschi and Zúñiga, 2009; Troncoso et al., 2009). Starch gelatinization, protein denaturation, and pore formation are among the major factors that determine the textural properties of fried foods (Suderman, 1983; Loewe, 1993; Ngadi et al., 2007).

The degree of textural development in a fried food is closely related to the fry-ing temperature and time as well as the type of fryfry-ing oil. Food fried in oil with a high level of hydrogenation would result in a crisper texture, since hydrogenated oils are known to give higher heat transfer rates. Ngadi et al. (2007) reported that 100%

hydrogenated oil gave chicken nuggets with a higher maximum load, when tested on a universal testing machine, than samples from less hydrogenated oils. A similar result was reported by Kita et al. (2005). When vegetables and meat are fried in oil, a crust develops and certain microstructural changes are induced by the heat and mass transfer processes. These microstructural changes also define the textural nature of the food (Aguilera and Stanley, 1999; Aguilera, 2005).

Food texture is measured by sensory analysis or by an instrumental method.

Using a human inspector for a textural evaluation is subject to some errors because of variations in perception, even when trained panelists are used and a well-defined standard is referenced. However, Katz and Labuza (1981) compared sensory results and cohesiveness values from force–deformation curves for potato chips, popcorn, puffed corn curls as well as saltines, and obtained a good agreement between the two sets of data. A similar comparison was made by van Loon et al. (2007) for the crispness of French fries; comparable results were also noted.

An instrumental method is predicated on such parameters as force, power spec-trum, and fractal dimensions. The technique is easy to use, quick, and gives room for flexibility of control. The force–deformation curve or puncture/compression tests are most commonly applied, especially in potato and battered meat products.

The maximum force required to penetrate a sample at a certain displacement rate is interpreted as the textural quality of hardness, crispness, or crunchiness. Ateba and Mittal (1994) presented texture analysis results of fried meatballs, showing the effect of frying time. The break force peak was shown to increase with the frying time.

Potato chips always show a two-stage hardness, with initial softening characteristics, indicating cooking, and the later-stage hardness due to crust formation as a result of starch gelatinization. These characteristics of potato chips were elucidated by an instrumental method of force–displacement test according to Pedreschi and Moyano (2005). For French fries, the formation of hard crust provides a covering for the inner soft core, which presents the unique texture characteristic synonymous with these types of fried potato products (Agblor and Scanlon, 2000). Kumar et al. (2006) studied the hardness of a traditional Indian fried sweet, gulabjamun, using a com-pression test and demonstrated the effects of frying time and temperature. Higher temperature and longer frying time produced gulabjamun with a higher maximum break force. The modern approach in texture measurement involves the use of an imaging technique and an analytical/statistical method to obtain texture data nonin-vasively (Qiao et al., 2007a).

2.2.5   cOlOr

Color, because of its superficiality, is a major acceptability index of most foods by consumers. It is the first sensory attribute assessed, even before a food touches the mouth. Consumers are apt to relate color to other quality attributes of food, such as flavor, safety, and nutrition, because color indeed correlates well with these physical, chemical, and sensory qualities of food (Pedreschi et al., 2006). Color development during deep-fat frying is based on two main reactions. The first reaction is an inter-action between the carbonyl group of sugars (reducing sugars) and the nucleophilic amino group of the amino acids. These two groups form a variety of molecules/

compounds responsible for a range of flavors and colors, usually requiring thermal energy (heat). The second type of reaction is caramelization, which occurs as a result of pyrolysis of some sugars when intense heat is applied. Both processes produce similar results, although their paths of production differ. These reactions are respon-sible for the golden brown coloration in some fried foods. The extent of color devel-opment in fried foods is determined by the moisture content, water activity, frying oil quality, food composition, and heat intensity. The measurement of color in fried foods is performed in three ways: visual assessment, instrumentation evaluation, and use of CV. The latter is a new approach with added advantage over the conventional methods (Sun, 2000; Du and Sun, 2004; Pedreschi et al., 2006).

2.3   effeCt of oIl QualIty on physICoCheMICal  propertIes of frIed foods

The quality of oil is usually a function of its origin, composition, and frying condi-tions such as frying time and temperature (Garcia et al., 2004). Vegetable oils are generally known to contain no cholesterol. However, certain vegetable oils, such as coconut and palm oil, contain much saturated fat, which is relatively stable under storage but of great concern in terms of its trans fat content. On the other hand, other oils such as rapeseed, soybean, and sunflower oils contain more of unsaturated fatty acids compositions of triglycerides, which are more healthful but unstable during frying and storage (Ngadi et al., 2007; Chemicalland21, 2008). The hydrogenation process is frequently used to reduce the unsaturated-fatty-acids content of frying oil. This process not only changes the chemical constituent of frying oil but also its physical properties such that the lipid produced is more viscous than its unsaturated counterpart (Fernández et al., 2005).

The oil used for frying is often produced from a blend of oils of different attributes to balance the qualities and to impart the desired value to the final product (Ngadi et al., 2007). Obviously, oil quality defines its frying characteristics such as thermal (heat transfer coefficient), nutritional, sensory, and mass transfer properties (viscos-ity and surface tension). Oil qual(viscos-ity changes as it is used repeatedly, due to chemical changes such as hydrolysis, polymerization, and oxidation, leading to the formation of free fatty acids (FFA), volatiles, acylglycerols, and acrylamides (Lalas, 2009).

Hydrolysis occurs when moisture released into the frying oil reacts with triglycerides to form FFA. Oxidation results from oil exposure to the atmosphere during frying at elevated temperatures. Multiple double-bond unsaturated fatty acids, such as linolenic

acid, are more susceptible to oxidation than single-bond fatty acids (e.g., oleic acids) (Lalas, 2009). The rate of oxidation becomes higher at higher temperatures. It is indeed imperative to determine the “fry time” (point at which oil would no longer give accept-able product quality) of oil in order to prevent undesiraccept-able physicochemical changes of food.

Qualities of frying oil and of fried foods are very closely related (Blumenthal, 1991; Dunford, 2005), which is why adequate attention should be placed on the selection and condition of oil used in the frying operation. Indicators of oil quality such as foaming, smoke emission, color change, pH change, presence of sediments, and detection of free radicals through rapid means are important. A number of rapid means of determining these quality indicators have been developed (O’Brien, 1998).

Kazemi et  al. (2005) used the HSI technique to predict the quality of frying oil in terms of its acid value, total polar compounds, and viscosity. The investigators obtained comparable results with those obtained from conventional methods. Such a technique is discussed further in the chapter.

Correlations have been established between oil and fried food properties. Ngadi et al. (2007) studied the effect of oil hydrogenation on color change, texture, and oil content of chicken nuggets fried at various time intervals. Chicken nuggets fried in oil with a high level of hydrogenation were substantially different from samples fried in non-hydrogenated oil. Figures 2.3 and 2.4 show the effect of the level of oil hydrogenation on the moisture and oil contents during frying of chicken nuggets, respectively. There was also a significant increase in the maximum load (Figure 2.5) for penetration into the sample, which depicts crispness, as the level of hydrogena-tion in the frying oil increased. The color (chroma-value color saturahydrogena-tion) of the fried

2.3 2.1 1.9 1.7 1.5 1.3 1.1 0.9 0.7

0.50 30 60 90 120 150 180 210 240 270 300 Frying time (s)

Moisture content (g/g, db)

fIgure  2.3  Average moisture content of chicken nuggets fried in oils with different degrees of hydrogenation (•: 0%; ▪: 60%; ▴: 100%). The percentage refers to the w/w ratio of hydrogenated oil. Error bars show standard deviations. (From Ngadi, M.O. et al., LWT Food Sci. Technol., 40, 1784, 2007.)

0.24

0.22

0.20

0.18

0.16

0.14

0.12

0.100 30 60 90 120 150 180 210 240 270 300 Frying time (s)

Oil content (g/g, db)

fIgure 2.4  Average oil content of chicken nuggets fried in oils with different degrees of hydrogenation (•: 0%; ▪: 60%; ▴: 100%). The percentage refers to the w/w ratio of hydro-genated oil. Error bars show standard deviations. (From Ngadi, M.O. et al., LWT Food Sci.

Technol., 40, 1784, 2007.)

24 22 20 18 16 14 12 10 8 6 4 2

0 0 30 60 90 120 150 180 210 240 270 300 Frying time (s)

Maximum load (N)

fIgure 2.5  Average maximum load of chicken nuggets fried in oils with different degrees of hydrogenation (•: 0%; ▪: 60%; ▴: 100%). The percentage refers to the w/w ratio of hydro-genated oil. Error bars show standard deviations. (From Ngadi, M.O. et al., LWT Food Sci.

Technol., 40, 1784, 2007.)

chicken nuggets was equally affected by the degree of unsaturation of the frying oil, as shown in Figure 2.6. Moisture and oil contents decreased with the frying time as the degree of hydrogenation in the frying oil increased. Kita et al. (2005) concluded that French fries fried in hydrogenated oil were harder than those fried in less hydro-genated liquid rapeseed. The results further establish the significant influence of oil type and quality on the physicochemical properties of fried foods.

2.4   effeCts of pretreatMents on physICoCheMICal