Journal of Food Engineering 47 (2001) 97±107 www.elsevier.com/locate/jfoodeng

Characterization of product quality attributes of tortilla chips during the frying process Marie Louise Kawas a, Rosana G. Moreira b,*

a Research and Development, Frito-Lay, Inc., Plano, TX-75024, USA b Department of Agricultural Engineering, Texas A & M University, 310 Scoates Hall, College Station, TX 77843-2117, USA Received 3 March 2000; accepted 19 June 2000

Abstract Characterization of product quality attributes (PQA) of tortilla chips during frying will provide critical information that can be used to develop fundamental models to describe the structural changes of a fried product during frying. Tortilla chips were prepared from nixtamalized dry-masa ¯our and fried in fresh vegetable oil for 60 s. The results indicated that most diameter shrinkages of tortilla chips happened during the ®rst 5 s of frying. The chip's thickness increased as a result of crust formation and some bubbles developed at the surface due to gas expansion. The chips become more porous (pore size increased in number and size). The pore size distribution became more uniform (normal) as frying time increased. The chips became crunchier as moisture decreased during frying. The combination of all these quality attributes is responsible for producting the best ®nal product as oil content is greatly a€ected by the mechanism of structure formation thus resulting in the desired product texture. Fundamental properties such as isotherms and glass transition temperatures were also evaluated. The Crapiste and Rotstein model provided the best correlation at the entire range of moisture content and temperatures. The glass transition termperatures were ®tted using the Gordon and Taylor equation. The glass transition curve for the fried chips with total oil content is higher than the one for the chips with partial oil content. A model was developed using the extreme value distribution to predict the pore size distribution of tortilla chips during frying. Ó 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Pores; Texture; Shrinkage; Expansion; Properties

1. Introduction the dough. Many manufacturers use dry-masa ¯our, as opposed to fresh masa, as it does not require much labor In the United States, the two most popular masa- and equipment. However, fresh masa is less expensive based snack products are tortilla chips and corn chips. and has a richer alkaline ¯avor (Serna-Saldivar, Gomez, Corn chips are fried directly from masa and contain & Rooney, 1990). more oil than tortilla chips. Tortilla chips are baked and Tortilla chips vary in oil content from 21% to 34% then fried, making them absorb less oil and have a (w.b.) depending on corn variety, cooking processes, ®rmer texture and a stronger alkaline ¯avor than corn grinding conditions, baking time, cooling time after chips. baking, and others (Lee, 1991). Nixtamalization is the process of cooking and steep- Moreira, Palau, and Sun (1995) observed that ing corn in alkaline solution, and then washing it to moisture loss and oil absorption rates were faster produce nixtamal. The nixtamal is stone-ground to form during the ®rst 15 s of frying, and became constant as a soft, moist dough called masa. Masa is then the raw frying continued; moisture loss rate increased as tem- material used to make tortilla chips. Dry-masa ¯our is a perature increased. The e€ect of temperature on oil product made by drying and grinding the fresh masa absorption was not signi®cant during the ®rst 15 s of into ¯our. Dry-masa ¯our has a longer shelf life (around frying, although, the ®nal oil content was higher for 1 yr) than fresh masa and requires only water to produce tortilla chips fried at 190C than at 150C for the some frying time. Moreira, Sun, and Chen (1997) measured the oil * Corresponding author. Tel.: +1-979-847-8794; fax: +1-979-845- 3932. content on the surface and at the core of tortilla chips to E-mail address: [email protected] (R.G. Moreira). determine the oil distribution during frying and cooling.

0260-8774/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 0 0 ) 0 0 104-7 98 M.L. Kawas, R.G. Moreira / Journal of Food Engineering 47 (2001) 97±107 During frying, only 20% was absorbed by the chips, and 2.2.2. Oil content 80% remained on the chip's surface. During cooling, 2.2.2.1. Total oil content. The total oil content of tortilla about 64% of the total oil content was absorbed by the chips was determined by using the Soxtec System HT chips, and 36% remained on the chip's surface. (Pertorp, Silver Spring, Maryland) extraction with pe- Oil content in fried foods has been related to initial troleum ether (AACC, 1986). The test was performed in moisture content (Gamble, Rice, & Selman, 1987; triplicate. Moreira, Palau, Sweat, & Sun, 1995), pre-frying treat- ment (Gamble & Rice, 1987), structural changes during 2.2.2.2. Internal oil content. The oil content on the sur- backing (Lee, 1991; McDonough, Gomez, Lee, Wan- face and at the core of tortilla chips was measured using iska, & Rooney, 1993; Rock-Dubley, 1993), and cooling the approach described by Moreira et al. (1997). The test time (Sun & Moreira, 1994). was performed in triplicate. Understanding the oil distribution in tortilla chips is imperative before trying to access good quality control. 2.3. Degree of shrinkage/expansion and puness Changes in structure and texture of tortilla chips during frying can greatly a€ect the mechanism of oil absorption The diameter, thickness, and pu€ height were mea- during cooling (Moreira, Castell-Perez, & Barrufet, sured using a steel caliper (MG Tool Company, NY). 1999). Therefore, it is important to fully characterize About 20 readings were made for ®ve samples of each tortilla chips during frying so that an accurate mathe- treatment. matical model can be developed to predict quality The degree of diameter shrinkage Si† was calculated changes. The objectives of this study were to: by · Characterize product quality attributes (PQA) such  do d t† as, shrinkage/expansion, puness, texture, pore size dis- Si ˆ 100 1† tribution, and porosity of tortilla chips during frying. do

· Measure fundamental properties such as isotherms The degree of thickness expansion and puness Ei† was and glass transition. calculated by · Develop a predictive model of pore size distribution  d t†do during frying. Ei ˆ 100 2† do

where do is the original dimension of baked sample (mm) and d t† is the dimension of sample with frying time (mm). 2. Materials and methods

2.1. Sample preparation 2.4. Solid density

Tortilla chips were prepared from nixtamalized dry To obtain the solid volume of tortilla chips, the pre- masa ¯our (NDMF) for tortilla chips (tortilla chip 1Y, weighed samples were ground using a co€ee grinder Valley Grain Products, Muleshoe, TX). The procedure (Braun, Model KSM2) and placed in a compressed he- is carefully detailed in Moreira et al. (1997). The tortillas lium gas multi-pycnometer (Quantachrome & Trade, 3 were fried in fresh vegetable oil for 60 s. Samples were NY). Solid density, qs kg=m †, was determined by di- collected at 5, 10, 20, 30, 40, 50 and 60 s frying time viding the weight of the sample by its solid volume. The interval for testing. The dry-masa ¯our used to make test was performed in triplicate. tortilla chips had a combination of 69% coarse particles (amount of particles that did not pass a US #70 sieve), 2.5. Bulk density 21% intermediate (amount of particles that passed the US #70 but not the US #100), and 10% ®ne (amount of The bulk volume was measured using the liquid dis- particles that passed a US #100 sieve). placement technique with toluene (Wang & Brennan, 1995; Lozano, Rotstein, & Urbicain, 1983). Bulk den- sity, q kg=m3†, was then determined by dividing the 2.2. Sample analysis b weight of the chip by its bulk volume. The test was performed in triplicate. 2.2.1. Moisture content Tortilla chip samples were ground in a co€ee grinder (Braun, Model KSM2) after frying. The moisture con- 2.6. Porosity tent of tortilla chips was determined by weight loss after drying 5 g samples in a forced air oven at 103±105C Porosity, /, was calculated as q (AACC, 1986) for 24 h. The test was performed in / ˆ 1 b 3† triplicate. qs M.L. Kawas, R.G. Moreira / Journal of Food Engineering 47 (2001) 97±107 99 2.7. Pore size distribution England) at an air¯ow rate of 1.2 m/s. The samples were dried at 10 min intervals up to 240 min when approxi- Three tortilla chip samples were analyzed for every mately 2% (w.b.) moisture content was reached. Water treament. Each tortilla chip sample was broken into nine activity was measured out using the Rotronic Hygro- pieces (10 mm L  7 mm W) for which three photomi- skop DT (Model DT-2, Rotronic Instrument , NY) crographs were taken in di€erent regions to have a good coupled to a Rotronic Measurement Station (Model representation of the treatement. The small pieces were WA-40TH, Rotronic Instrument, NY) for the mea- mounted on aluminum stubs with conductive adhesive surement of equilibrium relative humidity following the and viewed with no further sample preparation in an method described in Crapiste and Rotstein (1982). Electroscan Model E-3 ESEM (Electroscan, Wilming- Moisture content was determined by the forced air oven ton, MA) with an accelerating voltage of 15 kV. The method (AACC, 1986) previously explained in Section area and perimeter of the pores were analyzed by an 2.2.1. Isotherms were obtained at 25C, 48.8C, and image analysis software called Scion Image (National 68.8C. Temperature was kept constant using a water Institutes of Health, Bethesda, MD). Pore size distri- bath (Model K4R, Brinkman Instruments, Germany). bution histograms were developed to compare the dif- The system was calibrated with di€erent salt solutions. ferent treatments. To analyze the e€ect of total oil content on the iso- therms, control tortilla chips were prepared as described 2.8. Texture in Section 2.1. The e€ect of partial oil content (internal oil) was analyzed by frying control tortillas for 60 s and The texture of the tortilla chips was evaluated with a dipping them immediately in petroleum ether for 1 s to texture analyzerTM (TA-XT2, Texture Technology, NY) remove the surface oil. Water activity was measured using a 6.325 mm diameter ball probe and an 18 mm three times per sample and isotherms were evaluated as diameter hollow cylindrical base. The probe traveled at described above. a velocity of 0.1 mm/s until it cracked the sample; the Di€erent isotherm (desorption equilibrium) models lowest possible speed that the probe could travel was were evaluated to ®t the experimental data and to select chosen to get a very jagged curve and to be able to the best curve ®t. The models tested are shown in measure crunchiness. About 20 ¯at chips from each Table 1. Non-linear regression was performed using the treatment were tested. Levenberg±Marquardt iteration method with the graphics software package called PlotIt (version 3.1, 2.8.1. Crunchines 1994, Scienti®c Programming Enterprises). The crunchiness of tortilla chips fried at di€erent times was measured from the jaggedness or ``noise'' of the sample. The maximum distance traveled by the 2.9.2. Glass transition temperature (Tg) probe for each sample was recorded, and the data for Tg was determined as a function of moisture content each sample was saved for further analysis. The force and oil content. column of each ®ltered data ®le was copied into a text ®le and then run with the program to obtain the fractal 2.9.2.1. Sample preparation. To analyze the e€ect of Kolmogorov dimension of the mechanical signature. moisture content on glass transition temperature, torti- The Kolmogorov dimension is called the ``box count- lla samples with di€erent moisture contents were pre- ing'' dimension as a grid of squares is placed over the pared as described in Section 2.9.1.1. One-g of each boundary or the line pro®le and the number of squares sample was placed into stainless steel aluminum DSC through which any part of the line passes is counted. pans (Model 319-1536, Perkin Elmer). The samples were This process is repeated with di€erent grids having dif- scanned from 100C to 140C at a heating and cooling ferent size squares, and the number of squares that rate of 10C= min, a cooling rate of 40C/min, and a passed the pro®le was plotted vs the length of the side of DSC operating range of 5 mcal/s. The test was per- the square on a log±log scale (Russ, 1994). formed 20 times. The experimental data was modeled using the Gordon± Taylor equation (Roos, Karel, & 2.9. Fundamental properties models Kokini, 1996) w T ‡ kw T 2.9.1. Isotherms T ˆ 1 g1 2 g2 4† g w ‡ kw Desorption isotherms were determined as a function 1 2 of moisture content, temperature, and oil content. where Tg is the glass transition temperature (K), w the weight fraction, k is the empirical constant, and sub- 2.9.1.1. Sample preparation. Raw tortillas initially con- scripts 1 and 2 represent water and solids, respectively. taining 54% (w.b.) of moisture content were dried at Applications of the Gordon±Taylor equation to pre- 45 Æ 1C in the UOP8 Tray Drier (Arm®eld , Hamphire, dict water plasticization requires determination of the 100 M.L. Kawas, R.G. Moreira / Journal of Food Engineering 47 (2001) 97±107

Table 1 Characteristic parameters and coecients of determination of the models selected to predict the desorption isotherm data of masa samples dried for 240 min in a tray-dryer and tortilla chips with total and partial oil contentsa

2 Model Equation k1 k2 k3 k4 R   Crapiste and Rotstein 1 1 2427.26 364.14 0.32 12.38 0.97 k3 ln U†ˆk1 M exp k4M† (1986) T k2

Crapiste and Rotstein   8 480 0.32 21.21 0.98 1 1 k3 (total oil chips) ln U†ˆk1 M exp k4M† T k2 Crapiste and Rotstein   9 500 0.33 20.11 0.98 1 1 k3 (partial oil chips) ln U†ˆk1 M exp k4M† T k2  Pfost, Maurer, Chung, 4.8 7 17.91 ± 0.80 K1 7  10 and Milliken (1976) ln U†ˆk1 exp k3M† R T ‡ k2† Thompson (1972) 0.03 68 0.95 ± 0.94 k3 ln 1 U†ˆk1 T ‡ k2†M

Chund and Pfost k 8 105 000 17.91 ± ± 0.86 (1967) ln U ˆ 1 exp k M† RT 2 Hederson (1952) 0.00045 0.94 ± ± 0.87 k2 1 U ˆ exp‰k1T 100M† Š Smith (1947) 0.001928 exp(250.100/RT) ± ± 0.94 M ˆ k1 k2In 1 U†  Harkins and Jura k ln U ˆ k 2 exp()18.000/RT) 0.0033 ± ± 0.90 (1944) 1 M 2 a M ˆ moisture content (decimal d.b.); U ˆ water activity; T ˆ temperature (K); R ˆ gas constant (8.312 kJ/kg K); k1; k2; k3; k4 ˆ parameters:

empirical constant, k. The glass transition temperature 3. Discussion for amorphous water is often 135C based on the data of various studies (Roos, 1992). 3.1. E€ect of frying time on the product quality attributes To analyze the e€ect of total and partial oil contents (PQA) of tortilla chips on the glass transition temperature, tortilla chips were fried for 60 s at 190C. The e€ect of partial oil content The tortilla chips prepared in this study had an initial (internal oil) was analyzed by frying control tortillas for and ®nal moisture contents of about 42:0 Æ 0:2% (w.b.) 60 s and dipping them immediately in petroleum ether and 2:0 Æ 0:1% (w.b.), respectively. The total ®nal oil for 1 s to remove the surface oil. The tortilla chips were content, after 60 s of frying was 23:6 Æ 0:1% (w.b.). ground using a co€ee grinder to obtain powder samples. These chips absorbed about 9:5 Æ 0:2% (w.b.) oil during One-g samples were placed into stainless steel alumini- frying reaching about 17:6 Æ 02:% (w.b.) during cooling. um DSC pans (Model 319-1536, Perkin Elmer) and About 34% of the total oil content (i.e., 9:5 Æ 0:2% w.b.) scanned as described above. The test was performed 20 was absorbed by the chips during frying, and about 69% times. The experimental data was modeled using the of the total oil (i.e., 17:6 Æ 0:2% w.b.) was the internal Gordon±Taylor equation. oil content after cooling. The rest (8.9% w.b) was oil that remained at the chip's surface. 2.10. Data analysis Fig. 1 shows the e€ect of frying time on the di- mensional changes of tortilla chips. The diameter of The data was interpreted by Duncan's multiple tortilla chips decreased once the chips were immersed comparison test using SPSS software (Base 8.0, SPSS in oil. After 5 s of frying, the diameter continued to 1998). Statistical signi®cance was expressed at the decrease but at a slow rate. The chips' thickness did P < 0:05 level. Experimental pore size distribution dur- not change for the ®rst 30 s and then increased by ing frying was determined by the test of contrasts across only 10% and remained the same until the end of group means. The experiments were performed at least frying. The chips' puness increased to 72% during in triplicate. Desorption isotherm models were corre- the ®rst 20 s of frying, and continued to increase until lated using non-linear regression with the Levenberg± it reached 100% after 60 s of frying. As water evap- Marquardt iteration method. orated at a faster rate during the ®rst 10 s of frying, M.L. Kawas, R.G. Moreira / Journal of Food Engineering 47 (2001) 97±107 101

Table 2 Bulk density, solid density, and porosity of control tortilla chips during frying (60 s at 190C in fresh soybean oil)a

Frying Bulk density Solid density Porosity time (s) kg=m3† kg=m3†

0 880 Æ 70 1300 Æ 20 0:32 Æ 0:01 5 870 Æ 70 1270 Æ 30 0:30 Æ 0:02 10 790 Æ 90 1280 Æ 20 0:37 Æ 0:02 20 650 Æ 50 1290 Æ 20 0:49 Æ 0:02 30 540 Æ 70 1310 Æ 20 0:58 Æ 0:02 40 520 Æ 90 1280 Æ 10 0:59 Æ 0:01 50 540 Æ 70 1300 Æ 10 0:58 Æ 0:03 60 580 Æ 60 1300 Æ 20 0:55 Æ 0:03

a Tests were performed in triplicate. Fig. 1. Degree of shrinkage/expansion (diameter/thickness) and pu- ness of the conrtol tortilla chips during frying for 60 s in fresh soybean oil at 190C. The e€ect of frying time on the densities and porosity of tortilla chips is presented in Table 2. The solid density the material shrunk the most. As the crust was did not change signi®cantly during frying. The bulk formed, the chips' thickness increased and bubbles at density decreased from 880 to 580 kg/m3 after 60 s of the surface due to the barrier created by the tight frying. The bulk density decreased as the volume of the surface which resulted in vapor expansion inside the chips increased due to an increase in puness during chips' pores. frying. Porosity increased from 0.32 to 0.55 during fry-

Fig. 2. ESEM pictures of tortilla chip during frying. 102 M.L. Kawas, R.G. Moreira / Journal of Food Engineering 47 (2001) 97±107 ing. The bulk density decreased as the solid density re- shape. After 40 s of frying, the oval- shaped pores at mained the same and the chips became more porous as the core of the chips became larger as the cell walls they lost water during frying. collapsed; the crust becomes larger with many small Fig. 2 shows the ESEM micrographs of a tortilla pores. After 50 s of frying, the pores continued to in- chip during frying. The baked tortillas showed many crease in number, and then at 60 s of frying, the entire small pores randomly distributed and very few large structure was ®lled with pores of all sizes with little pores around a partially gelatinized starch matrix. starch matrix around them. After 5 s of frying, the tortilla chips' microstructure The e€ect of frying time on the chips' pore size dis- shows that the starch matrix became smoother con- tribution is shown in Fig. 3. The baked tortillas had taining a very small amount of randomly distributed about 60% pores with 38 lm in diameter. About 22% pores. After 10 s of frying, there were more pores with had a diameter of 63 lm, and the rest were in the range irregular shapes and the crust began to form at the of 88 and 138 lm in diameter. Between 5 and 20 s of chips' edges. After 20 s of frying, a large amount of frying, the range of pore size distribution was the same, small round and a few elongated pores are seen, where but the amount of pores of a certain size di€ered. After the crust is increasing in thickness showing small round 5 s of frying, the fraction of pores was about, 0.20, 0.30, pores. After 30 s of frying, the small and large pores 0.20, 0.12 with 38, 63, 88, 113 lm in diameter, respec- increased in size and started to change to an oval tively; the rest were in the range of 138±213 lm. After

Fig. 3. Pore size distribution in a tortilla chip during frying. M.L. Kawas, R.G. Moreira / Journal of Food Engineering 47 (2001) 97±107 103

Fig. 4. E€ect of frying time on Kolmogorov dimension of tortilla chips Fig. 5. Predicated equilibrium moisture content data for masa at dif- fried for 60 s in fresh soybean oil at 190C. ferent temperatures using the Crapiste and Rotstein model.

50 and 60 s frying, the pore size distribution looked like a normal distribution with a large amount of pores around 88 and 113 lm. The chips that were fried for 60 s showed a greater amount of large pores than the chips fried for 50 s. The e€ect of frying time on the crunchiness of tor- tilla chips is shown in Fig. 4. The level of jaggedness was related to the crunchiness of the tortilla chips during frying (Tesch, Normand, & Peleg, 1996; Harris & Peleg, 1996). The baked tortillas had a Kolmogorov dimension of 1.06, which increased continuously with frying time up to a value of 1.25 at the end of frying. The chips became crunchier as the moisture decreased, the porosity increased, and the pore size distribution increased. Fig. 6. Predicated equilibrium moisture content data for tortilla chips In conclusion, it can be said that the changes in the with total oil at di€erent temperatures using the Crapiste and Rotstein product structure, porosity, degree of diameter shrink- model. age, thickness expansion, puness, and texture are all related to moisture loss and oil absorption during the frying process (including cooling).

3.2. Fundamental properties of tortilla chips

3.2.1. Isotherms Water activity U† for tortilla chips was measured and isotherms at di€erent temperatures were developed. The characteristic parameters for the models used to ®t the experimental data and corresponding coecients of determination at the three temperatures are shown in Table 1. In general, the Crapiste & Rotstein model showed the best correlation (0.97) over the entire range of equilib- rium moisture contents and temperatures, followed by the Thompson (0.94) and Smith (0.94) models. Fig. 5 Fig. 7. Predicated equilibrium moisture content data for tortilla chips shows the e€ect of temperature on the desorption iso- with partial oil at di€erent temperatures using the Crapiste and Rot- therm of masa. stein model. 104 M.L. Kawas, R.G. Moreira / Journal of Food Engineering 47 (2001) 97±107 Figs. 6 and 7 show the e€ect on the desorption isotherm for tortilla chips with total and partial oil contents (internal oil content) as predicted by the Crapiste & Rotstein model, respectively. The equilib- rium moisture content decreased as temperature in- creased for the same water activity value. At very high water activities, moisture content at the three temper- atures changed very little. This behavior may be at- tributed to the e€ect of temperature on the solubility of starch. In addition, as the moisture content de- creases with increasing temperature, the enthalpy of vaporization is also expected to increase. For the same water activity value, the equilibrium moisture content decreaed as the amount of oil increased in the tortilla chips. Fig. 8. Glass transition temperature form masa.

3.2.2. Glass transition temperature, Tg The glass transition temperature Tg† of raw tortilla and tortilla chips with total and partial oil at di€erent moisture contents was measured to help understand their physicochemical behavior, i.e., the textural changes. The glass transition is a physical change that is promoted by the addition of heat and/or the uptake of water. The characteristic temperature, Tg, is an impor- tant parameter that can determine processability, product properties, quality, stability, and safety of food systems. In raw tortillas and tortilla chips, the starch is in an amorphous metastable state which is very sensitive to changes in temperature and moisture content. When the raw tortilla or the tortilla chips are exposed to at- mospheric temperature and humidity, structural changes occur. The most important change is noticed at the Tg, which involves transition from the solid ``glassy'' Fig. 9. Glass transition temperature for tortilla chips with total and to a liquid-like ``rubbery'' state. partial oil fried for 60 s in fresh soybean oil at 190C.

Figs. 8 and 9 illustrate the Tg curves for raw tortilla and tortilla chips, respectively, obtained by applying the Gordon±Taylor equation (Eq. 4). The k values The Tg of foods is related to water content, so tem- obtained experimentally were equal to 5.2. The drastic perature and mositure limits for product stability can be decrease of the Tg is a result of water plasticizing the approximated from state diagrams. For mositure con-  amorphous structure. The Tg of corn starch is 387 C, tents lower than 20% (w.b.), the tortilla chips with total and that of water is reported to be 135C (Roos, and partial oil contents must be stored at lower tem-

1992), which are the upper and lower ends of the Tg peratures than the masa to remain in the glassy state. In curve in Fig. 8. If raw tortilla and chips with total and partially crystalline polymers, water plasticization oc- partial oil (Figs. 8 and 9) are stored at a temperature curs only in the amorphous regions. The dehumidi®ca- below the Tg, then the products will be in a rigid glassy tion of starch proceeds more readily from initially state with a high viscosity where there is no molecular mobile amorphous regions leding to non-uniform mobility and di€usion occuring making the products moisture distribution (van den Berg & Bruin, 1978). stable for extended time periods. If the ambient tem- When comparing fried chips with masa, water plastici- perature is above the Tg, then the glassy products will zation occurred more easily for the fried chips resulting soften or become rubbery, and decrease in viscosity in a lower Tg. This is because the chips dried faster and increase in molecular mobility. This may result in during frying as they had previously formed amorphous physical and physicochemical deteriorations of the raw regions during baking; these amorphous regions were tortilla and chips. For example, the rate of deteriora- plasticized by water resulting in a lower Tg. For moisture tive reactions will be accelerated if raw tortillas with a contents greater than 20% (w.b.), the masa must be moisture content of 22% (w.b.) is stored at a temper- stored in lower temperatures than the tortilla chips with ature higher than 25C. total or partial oil content. M.L. Kawas, R.G. Moreira / Journal of Food Engineering 47 (2001) 97±107 105 It is interesting to note that the glass transition ferent frying times was normalized by dividing into the curve for the fried chips with total oil content is higher largest pore diameter (263 lm). This data was used to ®t than the one for the partial oil content chips (Fig. 9). the di€erent distributions using @RISK software (Pali- This is presumably because water plasticizes hydro- sade Corporation, New Field, NY). Among the distri- philic food components (Karel, 1985) and, even though butions used were the extreme value, logistic, lognorm, both treatments of chips had the same amount of moisture content, water was able to reach the starch of the chips with partial oil content more easily resulting in a lower Tg. In addition, previous studies have shown that the addition of lipids in starch can inhibit the starch gelatinization process (Eliasson, 1992). The in- hibition of starch gelatinization due to the addition of lipids may be related to the glass transition of food components, which would explain why glass transition occurs more readily for the chips with partial oil con- tent.

3.3. Pore size distribution

A prediction model for pore size distribution of tor- Fig. 10. Pore size distribution parameters a and b of tortilla chips fried tilla chips as a function of frying time was generated. for 60 s in fresh soybean oil at 190C using the extreme value distri- The pore diameter distribution of tortilla chips at dif- bution.

Fig. 11. Predicted pore size distribution using extreme value distribution for tortilla chips during frying. 106 M.L. Kawas, R.G. Moreira / Journal of Food Engineering 47 (2001) 97±107 lognorm II, Pearson V, Pearson VI, Weibull, and chips' structure in a normal distribution. Tortilla chips Rayleigh. The extreme value distribution was selected become harder up to 30 s of frying, and then crunchy as it best ®ts the experimental data at the entire range until the end of frying. The chips' crunchiness increases of frying time . The distribution characteristic param- during frying. eters a and b were gathered to be able to construct a Masa and tortilla chips with total and partial oil predictive model of the pore size distributions of tor- contents showed a desorption isotherm where moisture tilla chips. The parameter a describes the mode of content increases slowly at low water activities and sud- distribution, and b determines the variance (r) of dis- denly rises at high water activities. The model that best tribution as follows: ®ts the experimental data of masa on the entire range of equilibrium moisture content and temperatures was the b2p2 r : : 5† Crapiste & Rotstein with a coecient of determination 6 0.97. The Crapiste & Rotstein model also ®t the experi- The changes of parameters a and b with frying time mental data of the tortilla chips with total and partial oil are shown in Fig. 10. The following polynomial equa- contents well. Furthermore, equilibrium moisture con- tions were used to predict a and b as a function of frying tent decreased as temperature increased for the same time: water activity. For the same water activity value, the a t†ˆ 4  107t4 ‡ 5  105t3 0:002t2 ‡ 0:0262t equilibrium moisture content of tortilla chips with partial 2 oil was higher than that of the chips with total oil content. ‡ 0:1632; R ˆ 0:98; For moisture contents lower than 20% (w.b.), the tortilla chips with total and partial oil contents must be b t†ˆ 3  107t4 ‡ 4  105t3 0:0013t2 ‡ 0:0164t stored at lower temperatures than the masa to remain in ‡ 0:0837; R2 ˆ 0:99: 7† the glassy state. For moisture contents greater than 20% The distribution curve of the extreme value distribution (w.b.), the raw tortilla must be stored at lower temper- was represented as follows: atures than the tortilla chips with total or partial oil    content. x a F x†ˆexp exp : 8† The extreme value distribution can be used to predict b t† the pore diameter distribution of tortilla chips during The derivative of Eq. (8) was calculated to obtain the the entire range of frying time. density of the distribution   1 x a t† f x†ˆ exp References b t† b t†    x a t†  exp exp ; 9† AACC, (1986). Approved methods of the American Association of b t† Cereal Chemists. MN: American Association of Cereal Chemists. Chung, D. S., & Pfost, H. B. (1967). Adsorption and desorption of where F x† is the distribution, f x† the density, x the water vapor by cereal grains and their products. Transactions of the pore size, lm; a t† the mode as a function of time, t, American Society of Agricultural Engineers, 10, 549. b t† is the variance determinator as a function of time, Crapiste, G. H., & Rotstein, E. (1982). Prediction of sorptional t. Fig. 11 shows the predicted pore size distribution of equilibrium data for starch-containing foodstu€s. Journal of Food Science, 47, 1501±1507. tortilla chips for 10 and 60 s of frying. The prediction Crapiste, G. H., & Rotstein, E. (1986). 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