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Low oil French fries produced by combined pre-frying and pulsed-spouted microwave vacuum drying method
food and bioproducts processing 9 9 ( 2 0 1 6 ) 109–115
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Food and Bioproducts Processing journal homepage: www.elsevier.com/locate/fbp
Low oil French fries produced by combined pre-frying and pulsed-spouted microwave vacuum drying method Xiaojian Quan a , Min Zhang a,b,∗ , Zhongxiang Fang c , Huihua Liu d , Qiaosheng Shen e , Zhongxue Gao f a
State Key Laboratory of Food Science and Technology, Jiangnan University, 214122 Wuxi, Jiangsu, China Jiangnan University(Yangzhou) Food Biotechnology Institute, Yangzhou 225002, China c Faculty of Veterinary and Agricultural Sciences, The University of Melbourne, Parkville, Victoria 3010, Australia d School of Health Sciences, Federation University Australia, VIC 3353, Australia e Snack R&D Center, General Mills(China) Co., Shanghai 200336, China f Wuxi Delin Boat Equipment Co., Wuxi 214191, Jiangsu, China b
a r t i c l e
i n f o
a b s t r a c t
Article history:
French fries were prepared by a combined method of pre-frying and pulsed-spouted
Received 20 November 2014
microwave vacuum drying (PSMVD). The impact of pre-frying and PSMVD on the quality
Received in revised form 12 April
(oil content, color, texture, microstructure and shrinkage in volume) of French fries was
2016
studied and the sample was compared with that of only vacuum fried ones. The results
Accepted 25 April 2016
indicated that the French fries prepared by the combined method had lower oil content
Available online 30 April 2016
(25%, db) than that of vacuum-fried sample (40%). Other property analysis also showed that the combined technique processed samples exhibited comparable food texture and color
Keywords:
to that of vacuum fried samples, and had a porous microstructure with relatively smaller
French fries
pore size. It was concluded that the combined method of pre-frying and PSMVD could be an
Vacuum frying
alternative method to produce high quality French fries with low oil content.
Pulsed-spouted microwave- vacuum
© 2016 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
drying Oil content Texture Microstructure
1.
Introduction
French fries are worldwide popular food thanks to their pleasant taste and typical flavor (Van Loon et al., 2005). French fires generally produced by immersion in edible oil or fat at a temperature above the boiling point of water. It is a rapid process of simultaneous heat and mass transfers, which can be used as a drying operation, resulting in flows in opposite directions of water vapor (bubbles) and oil at the surface of the piece. However, this frying process always leads to a high oil content of about 40% in the French fries (Gupta et al., 2010), which is
not considered a good property as too much fat intake would have negative impact on human health (Krokida et al., 2001a). Customers demand for low-fat even fat-free products with a long time storage has been the driving force of the food industry to produce lower oil content fried potatoes that still retain desirable texture and flavor (Song et al., 2007a). Vacuum frying is defined as a frying process which is carried out under pressures well below atmospheric levels (Andrés-Bello et al., 2011). Due to the lowered pressure, the boiling points of both the oil and the moisture in the foods are also lowered. This offers some advantages: (1) reduced oil
∗ Corresponding author at: School of Food Science and Technology, Jiangnan University, 214122 Wuxi, Jiangsu, China. Tel.: +86 13606179162; fax: +86 0510 85807976. E-mail address: [email protected] (M. Zhang). http://dx.doi.org/10.1016/j.fbp.2016.04.008 0960-3085/© 2016 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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content in the fried product, (2) preservation of natural color and flavor of the product due to the low temperature and oxygen content during the process, and (3) reduced adverse effects on oil quality (Song et al., 2007b). In addition, the frying process significantly affects the microstructure and porosity of the dried products (Dehnath et al., 2003). During the vacuum frying, water evaporates very rapidly from the surface of the food and the locations from which moisture is driven out become dry and creates porous microstructures, which benefits to the crisp texture of the product. Lulai and Orr (1979) reported that the most clearly defined factor influencing oil uptake during fried chips production is the initial solids content of the tubers. Furthermore, Moreira et al. (2009) found that the rate of oil uptake, but not the final oil content, was greatly affected by the frying conditions. More specifically, it had been demonstrated that using several pretreatment (like osmotic dehydration, coating and pre-drying) before frying could effectively reduce the oil content. For instance, Bunger et al. (2003) reported that soaking with 3% NaCl solution for 50 min significantly reduced oil uptake from 0.13 to 0.10 g oil/g dry matter. But, it also was reported that the decrease in oil absorption using osmotic dehydration pretreatment was attributed to the increase in solids content occurring during the osmotic dehydration process rather than a reduction in the amount of oil taken up (Moreno and Bouchon, 2008). So it has been debated recently that the importance of selecting an adequate basis to carry out comparisons properly. Moreover, even if the oil content had been significantly reduced in such products with pretreatment, they still had not met the requirement of low-fat content fried potatoes. Moreover, the excessive osmotic dehydration and pre-drying will lead to destruction of the taste and texture of French fries. To further improve the French fries’ favorable color while reduce the oil content, combined technology such as by introducing microwave finishing after pre-frying was reported (Porter et al., 1973). The combination of microwave drying significantly increases the heating rates and reduces the drying time (Salazar-González et al., 2012; Zhang et al., 2006), which in turn reduces the frying time and oil content while improves the product quality. However, the major drawback associated with microwave heating is the non-uniform temperature distribution, resulting in hot and cold spots in the heated product, which not only affects the quality but also raises the issue of food safety (Vadivambal and Jayas, 2010). Recently, a new equipment of pulsed-spouted microwave vacuum dryer (PSMVD), invented in our laboratory, was used to solve this problem (Zhang and Wang, 2012). The added “pulsed spout” feature in the microwave vacuum dryer provided a more uniform temperature distribution in the dryer when compared with those conventional rotating turntable microwave vacuum dryer, and consequently the product quality was significantly improved (Wang et al., 2013). The objective of this work was to produce lower oil French fries by combining pre-frying with PSMVD. The effect of this new method on the product quality was compared with the vacuum frying process.
2.
Materials and methods
Potatoes were purchased from a local market in Wuxi, China, and stored in a refrigerator at 5 ◦ C until used. Palm oil (Jia-li Co. Ltd., Lianyungang, China) was also purchased from the local market.
2.1.
Pretreatment of French fries
Fresh potatoes were washed, peeled and cut into 10 mm × 10 mm × 40 mm slices with a manual potato cutter. The potato slices were blanched in a water bath at 90 ± 1 ◦ C for 5 min followed by cooling under running tap water for 1 min. Excess water was removed by draining on tissue papers. Finally, the potato slices were frozen in a refrigerator at −18 ◦ C prior to frying.
2.2.
Vacuum frying (VF)
A lab-scale vacuum fryer equipped with a centrifuge (Nan Feng Company, Wuxi, China) was used to prepare the vacuum-fried samples. The fryer was filled with 5 L of palm oil as the frying medium. The frying temperature and vacuum pressure was set at 90 ± 1 ◦ C and 16 kPa, respectively. Six batches of 100 g potato slices each, after the pretreatment, were fried for 5, 10, 15, 20, 25 and 30 min. After frying, the potato slices were centrifuged at 300 rpm for 8 min under vacuum to remove the frying oil. The centrifuged potato slices were transferred to a plastic bag and then put into the equipment of PSMVD in 3 min to prevent oxidation. All experiments were conducted in triplicate.
2.3. Pulse-spouted microwave vacuum drying (PSMVD) A newly developed experimental apparatus, which was explained in details in a recent patent (Zhang and Wang, 2012), was used for pulse-spouted microwave vacuum drying (Fig. 1). Different from the conventional rotating turntable microwave vacuum dryer, the PSMVD apparatus includes a pulsed-spouted system with a set of adjustable air flow and distributive unit as well as a set of air handing units at the bottom of drying chamber (Wang et al., 2013). A schematic diagram of the equipment was present in Fig. 1. During the pulse-spouted microwave vacuum drying, the vacuum-fried samples were spouted at a specific time interval so as to ensure the air can flow periodically into the duct drying chamber via controlling the electromagnetic valve (2 s on/60 s off). The pressure was fluctuated within the drying chamber from 11 to 18 kPa. From our preliminary experiments, potato slices par-fried 15 min (moisture content of about 18.5% and oil content of about 31.94%) (dry base, db), was selected for PSMVD to a final moisture content of about 3%. The selected par-fried samples were subjected to pulse-spouted microwave vacuum drying with different microwave power levels of 10, 15, 20 W/g sample (Table 1). At the end of each process, dried samples were packed in separate plastic bags and kept in a desiccator for further quality analysis.
2.4.
Determination of moisture content
The moisture content of the samples was measured by using the oven method (Fan et al., 2006). Approximately 3 g of ground French fries were placed in the oven and dried at 102 ± 3 ◦ C until the mass of the sample did not change further.
2.5.
Determination of oil content
The oil content was determined by using the Soxhlet extraction method (Fan et al., 2006). Ground sample was oven-dried
food and bioproducts processing 9 9 ( 2 0 1 6 ) 109–115
111
Fig. 1 – Schematic diagram of pulse-spouted microwave vacuum drying system. 1, feeding ball valve; 2, plate valve with 3 mm diameter hole; 3, microwave heating cavity; 4, magnetron at 915 MHz; 5, circulating water unit; 6 and 21, drying chamber; 7, CPCA-140Z pressure sensor; 8, solid–gas separator; 9, condenser; 10, vacuum pump; 11, gas flow electromagnetic valve; 12, gas flow adjustable valve; 13, pressure gauge; 14, air handling unit; 15, control panel; 16, water loading pipe; 17, gas distributor; 18, spout pipe; 19, silicon rubber stopper; 20, sample; 22, fixed unit for drying chamber holder; 23, fiber-optic temperature sensor. and extracted with petroleum ether (b.p. 30–60 ◦ C) for 4 h. The oil content (db) in this paper was considered as defatted dried solids.
French fries were prepared. For each sample, ten repetitions were carried out.
2.8. 2.6.
Color measurements
The color parameters (L*, a*, b*) of the samples were measured using a CR-400 Chroma Meter (Konica Minolta Sensing Inc., Tokyo, Japan) which had been calibrated with a standard white board. Parameters L*, a* and b* indicate the intensity of lightness, redness and yellowness of the sample, respectively. Each sample was measured four times at different positions and the average values were reported.
2.7.
Determination of shrinkage
Shrinkage in volume (Sv ) was evaluated by Eq. (1) Sv =
V0 − Vf V0
× 100%
(1)
where V0 is the original volume of the sample (cm3 ) and Vf is the volume (cm3 ) of the sample finished. Both the volume of the sample can be calculated by V = S × L, where S is crosssectional area of the sample (cm2 ) and L is the sample’s length (cm). Ten samples were tested to determine shrinkage for each condition at the given equilibrium time.
Determination of texture 2.9.
The texture was determined using a texture analyzer (TAXT plus, Stable Micro Systems, Ltd., Surrey, UK) fitted with a three-point bending rig (HDP/3PD). The texture analyzer has an adjustable gap between the two lower supporting blades and the upper blade at an equidistant position above the center was moved to gently fracture the sample. Hardness was measured in terms of breaking strength, which is the force required to break the French fries. The pre-speed, test-speed, and post-speed were set as 1.0, 2.0, and 10.0 mm/s, respectively. The trigger force was 5 g. The peak force (in grams, g) which indicates the textural hardness was obtained from the force vs distance curves using the Texture Exponent (version 5.0.7.0) software associated with the texture analyzer. This textural hardness was measured on the same day when the
Scanning electron microscopy (SEM) analyses
Microstructure of the samples was observed using a scanning electron microscope (S-4800, Hitachi, Tokyo, Japan). The samples were cut along the cross section (10 mm × 10 mm × 3 mm) and defatted by using the Soxhlet extraction, then coated with gold under vacuum condition for the SEM observation, using an accelerating voltage of 5 kV at 40× magnification.
2.10.
Statistical analysis
Data were analyzed by using the Statistical Analysis System (SAS, version 8.0, SAS Institute Inc., Cary, North Carolina). Analyses of variance were performed by the ANOVA procedure.
Table 1 – Experimental design of different French fry preparation methods. Methods VF VF + PSMVD-10 VF + PSMVD-15 VF + PSMVD-20
Procedure Vacuum frying until a moisture content of about 3% Vacuum frying 15 min and pulse-spouted microwave vacuum drying at 10 W/g to a moisture content of about 3% Vacuum frying 15 min and pulse-spouted microwave vacuum drying at 15 W/g to a moisture content of about 3% Vacuum frying 15 min and pulse-spouted microwave vacuum drying at 20 W/g to a moisture content of about 3%
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Results and discussion
3.1.
Vacuum frying characteristics
450
20 18
400
16
Moisture content (%, db)
3.
3.2. Pulsed-spouted microwave vacuum drying characteristics The plot of moisture content vs PSMVD at different microwave power levels is present in Fig. 3, which shows that the moisture content was decreased with drying time at all microwave power levels. As the microwave power increased, the time for reaching the final moisture content (about 3%, db) was decreased, which indicated that the drying rate was increased with microwave power increasing. This is confirmed by Fig. 4 that the microwave power significantly (p < 0.05) affected the drying rate. This could be attributed to the acceleration of interior moisture migration under higher microwave power conditions. Such acceleration of the interior moisture
450 40 400 35 30 300 25
250 Vacuum degree: 16kPa 200
20
150
15
100
10
50
5
0
0
5
10
15
20
25
30
Oil content (%, db)
Moisture content (%, db)
350
0 35
Frying time (min)
Fig. 2 – Variation of moisture content and oil content with time in French fries during vacuum frying.
300 250 200
10 W/g 15 W/g 20 W/g
14 12 10 8 6 4 2
150
0 15
100
20
25
30
35
40
45
50
Drying time (min)
50 0
0
5
10
15
20
25
30
35
40
45
50
Time (min) Fig. 3 – Variation of moisture content with time in French fries during vacuum frying and pulsed spouted microwave vacuum drying combination (Microwave power levels 10, 15, 20 W/g).
1.2
1.0
Drying rate (g/min)
Moisture loss and oil uptake are two most important mass transfer processes during frying of food materials. The moisture and oil content of French fries during vacuum frying are presented in Fig. 2, which indicated that the moisture content decreases rapidly but the oil content increases with frying time. This was in agreement with other researchers (Garayo and Moreira, 2002; Krokida et al., 2001b). When the potato slices were placed into the hot oil under vacuum condition, water evaporates very rapidly through the pore which creates a porous microstructure. The moisture from the interior diffuses out as steam and creates a pressure gradient. As the loss of moisture content continues, the locations from which moisture is driven out losing their hydrophobicity and become dry (Bravo et al., 2009). The de-oiling devices in the system removes excessive oil on the sample surface before vacuum broken, and prevent the oil permeating into interior during vacuum breakage. Then the oil adheres to the dry portion of French fries and even enters the porous areas. Meanwhile, desired or undesired physicochemical changes can take place and affect the sensory attributes such as texture and color (Garayo and Moreira, 2002). Therefore, partially frying the potato slices in hot oil followed by a drying process, using PSMVD could be used to produce crisp French fries with desired oil content. From the preliminary experiments (data not shown here), partially frying the potato slices for 15 min was the best condition for the following PSMVD process.
Moisture content (%, db)
350
0.8
0.6
10 W/g 15 W/g 20 W/g
0.4
0.2 0
2
4
6
8
10
12
14
16
18
20
Moisture content (g/100g, db) Fig. 4 – Variation of drying rate with moisture content at different microwave power levels during pulsed spouted microwave vacuum drying (Microwave power levels 10, 15, 20 W/g). migration consequently increased the surface water evaporation. Fig. 4 also shows that the effect of microwave power on the drying rate was markedly higher at high moisture levels, except for 10 W/g. This is because the amount of microwave energy absorbed by the material is depended on its dielectric properties and the electric field strength (Salema et al., 2013). Materials with higher moisture content are able to absorb more microwave energy since the values of the dielectric constant and loss factors are higher at higher moisture content. Thus, the present samples with higher moisture showed a higher drying rate when other drying conditions were identical. However, under the lowest microwave power level of 10 W/g in this study, the highest drying rate was observed at the moisture content of around 12%. Both higher or lower than this moisture level led to a lower drying rate (Fig. 4). The mechanism needs further investigation. Fig. 5 presents the oil content of the partially vacuum fried potato slices during PSMVD, which shows the oil content was decreased regardless the microwave power levels. This could
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35
70
30
60
L* a* b*
10 W/g 15 W/g 20 W/g
15 10
Color
40
20 Oil content (%, db)
Oil content (%, db)
50
25
30 20 10 0
5
VF
VF+PSMVD-10
VF+PSMVD-15
VF+PSMVD-20
Drying time (min)
0
5
10
15
20
25
30
35
40
45
50
Time (min) Fig. 5 – Variation of oil content with time in French fries during vacuum frying and pulsed spouted microwave vacuum drying combination. be attributed to the facts that some surface oil was flow out with the moisture evaporation under the microwave condition, or some oil also vaporized from the surface of the product under the microwave condition (Agnieszka and Adam, 2008), or some stuck on the drying chamber. Fig. 5 also suggested that different microwave power levels only affected the rate of oil loss, but the total amount of oil loss was almost same. Moreira et al. (2009) also reported that the drying condition normally did not affect the total oil transfer. During the PSMVD of the vacuum fried potato slices, the oil decrease was mainly due to the co-evaporation of oil with the internal moisture. Because the total moisture loss for these samples were almost identical, the total oil loss for them were also same.
3.3. Comparison on the quality of French fries produced by different methods 3.3.1.
Oil content
With the increasing awareness of human health, many fried foods are not very welcome by consumers mainly because of their high oil content and potential hazardous substances generated during frying. Due to high thickness, it is difficult to reduce oil content in French fries by using traditional frying methods. Although the application of vacuum frying has successfully reduced the formation of acrylamide (Granda et al., 2004), their oil content are still as high as about 40% (Gupta et al., 2010). The oil content of French fries produced by the combination of vacuum frying and PSMVD was decreased to 25% (db), about 40% decrease of oil content when compared with the vacuum fried sample. The decrease of oil content could be attributed to its synergy effect during the combination process. Firstly, the short time vacuum-frying process reduced the oil uptake, as the moisture and oil content of 15 min vacuum-fried French fries was about 16% and 30%, respectively (Fig. 2). Secondly, during the subsequent PSMVD process, the oil content of the product was further decreased by several factors as discussed above. The final oil content of the French fries was around 25% (db).
3.3.2.
Color
Color is the first impression of a product that affects customers’ choice. For the snack fried foods, their golden yellow color was one of the key factors that attract people’s attention.
Fig. 6 – Comparison on the color of French fries produced by different methods. The lightness parameter L* is commonly used as a quality control parameter for fried foods (Mariscal and Bouchon, 2008). Higher L* value indicates a lighter color, which is favorable for fried foods. Fig. 6 presents the color values of French fries produced by different methods, which shows the lightness (L*) of vacuum-fried French fries was lower than that of the combination method processed sample. It was reported that the L* values were decreased with frying time increasing (Bunger et al., 2003; Shyu and Hwang, 2001), and the longer the frying time, the more the non-enzymatic browning reactions occur. Therefore the formation of vacuum-fried darker color (low L* values) could be associated with more non-enzymatic browning reactions caused by longer frying time. However, such reaction was not so obvious in the PSMVD process. Fig. 6 also shows that vacuum-fried French fries possessed the highest yellowness (b*) value, suggesting they had stronger golden yellow color. This could also be related to the frying process. The longer vacuum-frying time would lead to more oil penetrated into the sample and more chemical oxidation, and eventually led to stronger golden yellow color. For the PSMVD samples, the b* values decreased at the first stage and then increased with the microwave power level increasing (Fig. 6).
3.3.3.
Texture
Fig. 7 presents the hardness of French fries produced by different methods. The French fries produced by vacuum frying was 2042.79 g, which was similar to the hardness of French fries produced by combination of vacuum frying and PSMVD
2000
1500
Hardness (g)
0
1000
500
0 VF
VF+PSMVD-10
VF+PSMVD-15
VF+PSMVD-20
Fig. 7 – Comparison on the hardness of French fries produced by different methods.
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Fig. 8 – Scanning electron micrographs of (a) vacuum-fried French fries and (b) combination produced French fries. The magnification was set as 40×.
3.3.4.
Shrinkage in volume
Maadyrad et al. (2011) reported that volume shrinkage during early stages of frying was nearly equal to the volume of water loss, but this turned to smaller in the final stages of
30 25 20 15 10 5
Microstructure
Structural properties are also important for the characterization of the quality of fried product. Krokida and Maroulis (2001) reported that the processing method significantly affects the microstructure and porosity of dried products. Fig. 8 presents the scanning electron micrographs of the cross-section of French fries prepared from different methods, which shows that the sample produced by combination method (Fig. 8b) exhibited a morphology featured as regular and uniform pore distribution, whereas that of vacuum fried samples exhibited bigger but less regular holes (Fig. 8b). Jiang et al. (2013) also observed that the different processing method can affect the pore size of food materials. During vacuum frying, the heat flux from oil reaches the surface of the samples and is transferred via the solid matrix to the deeper parts of the material. This results in the dried area moved from the surface to the core with a slow and constant speed that leads to the microstructure with large size pores and the longer the frying time, the larger the pore size. The combined method in the present study reduced the frying time and also the pore size. The combined method also showed advantages of internal heating and high heating rates due to dielectric cores within the drying body under PSMVD (Araszkiewicz et al., 2007). The numerous vapor evaporated by moisture via the solid matrix created a mass of channels to let the vapor leave the matrix, which resulted in a porous structure with small pore size in the sample of combination of vacuum frying with PSMVD.
3.3.5.
35
Shrinkage in volume (%)
at the power level of 20 W/g, suggesting their texture were comparable. Meanwhile, it was also observed that the sample hardness of combination process was increased with the microwave power levels. When the microwave power was as low as 10 W/g, there might be not enough internal moisture evaporation and vapor pressure generated. Thus, the radial force was unable to expand the French fries, and the sample became stiff due to loss of water and shrinkage. Upon increasing the microwave power, the internal moisture was evaporated more rapidly, which created a high vapor pressure and porous structure that reduced the sample hardness.
0
VF
VF+PSMVD-10
VF+PSMVD-15
VF+PSMVD-20
Fig. 9 – Comparison on the shrinkage of French fries produced by different methods. drying. Therefore, the volume of shrinkage is depended on the water transfer within the product. As shown in Fig. 9, the volume shrinkage took place in all French fries due to water loss, but the shrinkage degree of vacuum-fried ones was smaller, which may be resulted from its longer frying time that led to the surface becoming more rigid, thus producing a resistance to volume change. The final volume shrinkage for combination processed sample was observed to decrease upon microwave power level increasing. This was mainly due to the interior force created under the microwave power. When the microwave power was 10 W/g, the generated interior force was too low to make enough internal moisture evaporation and vapor pressure. Thus, the radial force was not sufficient to induce the French fries to expand and therefore had a larger volume shrinkage due to moisture loss. With microwave power levels increasing, the internal moisture was evaporated more rapidly, and the high vapor pressure pushed the French fries to expand and led to less volume shrinkage.
4.
Conclusions
French fries were produced by a combined technique of vacuum-frying and PSMVD. Both moisture content and oil content were decreased during the PSMVD process, and the microwave power level did not affect the final oil content. Compared with the traditional vacuum-fried sample, French
food and bioproducts processing 9 9 ( 2 0 1 6 ) 109–115
fries prepared from the combination method showed advantages of low oil content with still comparable properties in color, texture and shrinkage. It was concluded that combination of vaccum frying with PSMVD could be an alternative method to produce French fries with low oil content without sacrificing its gold color and crunchy texture.
Acknowledgements We acknowledge the financial support by Key Laboratory of Space Nutrition and Food Engineering, China Astronaut Research and Training Center (Contract No. SNFE-KF-15-11), Jiangsu Province (China) “Collaborative Innovation Center for Food Safety and Quality Control” Industry Development Program, Jiangsu Province (China) Infrastructure Project (Contract No. BM2014051) which have enabled us to carry out this study.
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Krokida, M.K., Oreopoulou, V., Maroulis, Z.B., Marinos-Kouris, D., 2001a. Effect of pre-drying on quality of French fries. J. Food Eng. 49 (4), 347–354. Krokida, M.K., Oreopoulou, V., Maroulis, Z.B., Marinos-Kouris, D., 2001b. Effect of osmotic dehydration pretreatment on quality of French fries. J. Food Eng. 49 (4), 339–345. Lulai, E., Orr, P., 1979. Influence of potato specific gravity on yield and oil content of chips. Am. Potato J. 56 (8), 379–390. Maadyrad, A., Ghiassi Tarzi, G., Bassiri, A., Bamenimoghadam, M., 2011. Process optimization in vacuum frying of kiwi slices using response surface methodology. J. Food Biosci. Technol. 1, 33–40. Mariscal, M., Bouchon, P., 2008. Comparison between atmospheric and vacuum frying of apple slices. Food Chem. 107 (4), 1561–1569. Moreira, R.G., Da Silva, P.F., Gomes, C., 2009. The effect of a de-oiling mechanism on the production of high quality vacuum fried potato chips. J. Food Eng. 92 (3), 297–304. Moreno, M.C., Bouchon, P., 2008. A different perspective to study the effect of freeze, air, and osmotic drying on oil absorption during potato frying. J. Food Sci. 73 (3), E122–E128. Porter, V., Nelson, A., Steinberg, M., Wei, L., 1973. Microwave finish drying of potato chips. J. Food Sci. 38 (4), 583–585. Salazar-González, C., San Martín-González, M.F., López-Malo, A., Sosa-Morales, M.E., 2012. Recent studies related to microwave processing of fluid foods. Food Bioprocess Technol. 5 (1), 31–46. Salema, A.A., Yeow, Y.K., Ishaque, K., Ani, F.N., Afzal, M.T., Hassan, A., 2013. Dielectric properties and microwave heating of oil palm biomass and biochar. Ind. Crops Prod. 50, 366–374. Shyu, S.-L., Hwang, L.S., 2001. Effects of processing conditions on the quality of vacuum fried apple chips. Food Res. Int. 34 (2–3), 133–142. Song, X.-J., Zhang, M., Mujumdar, A.S., 2007a. Optimization of vacuum microwave predrying and vacuum frying conditions to produce fried potato chips. Drying Technol. 25 (12), 2027–2034. Song, X.J., Zhang, M., Mujumdar, A.S., 2007b. Effect of vacuum-microwave predrying on quality of vacuum-fried potato chips. Drying Technol. 25 (12), 2021–2026. Vadivambal, R., Jayas, D., 2010. Non-uniform temperature distribution during microwave heating of food materials—a review. Food Bioprocess Technol. 3 (2), 161–171. Van Loon, W.A.M., Linssen, J.P.H., Legger, A., Posthumus, M.A., Voragen, A.G.J., 2005. Identification and olfactometry of French fries flavour extracted at mouth conditions. Food Chem. 90 (3), 417–425. Wang, Y., Zhang, M., Mujumdar, A.S., Mothibe, K.J., Roknul Azam, S., 2013. Study of drying uniformity in pulsed spouted microwave-vacuum drying of stem lettuce slices with regard to product quality. Drying Technol. 31 (1), 91–101. Zhang, M., Tang, J., Mujumdar, A., Wang, S., 2006. Trends in microwave-related drying of fruits and vegetables. Trends Food Sci. Technol. 17 (10), 524–534. Zhang, M., Wang, Y., 2012. A device and its application for uniform microwave vacuum drying in a spouted mode. China Patent No. ZL 2010 10572843.0, April 18.
Contents lists available at ScienceDirect
Food and Bioproducts Processing journal homepage: www.elsevier.com/locate/fbp
Low oil French fries produced by combined pre-frying and pulsed-spouted microwave vacuum drying method Xiaojian Quan a , Min Zhang a,b,∗ , Zhongxiang Fang c , Huihua Liu d , Qiaosheng Shen e , Zhongxue Gao f a
State Key Laboratory of Food Science and Technology, Jiangnan University, 214122 Wuxi, Jiangsu, China Jiangnan University(Yangzhou) Food Biotechnology Institute, Yangzhou 225002, China c Faculty of Veterinary and Agricultural Sciences, The University of Melbourne, Parkville, Victoria 3010, Australia d School of Health Sciences, Federation University Australia, VIC 3353, Australia e Snack R&D Center, General Mills(China) Co., Shanghai 200336, China f Wuxi Delin Boat Equipment Co., Wuxi 214191, Jiangsu, China b
a r t i c l e
i n f o
a b s t r a c t
Article history:
French fries were prepared by a combined method of pre-frying and pulsed-spouted
Received 20 November 2014
microwave vacuum drying (PSMVD). The impact of pre-frying and PSMVD on the quality
Received in revised form 12 April
(oil content, color, texture, microstructure and shrinkage in volume) of French fries was
2016
studied and the sample was compared with that of only vacuum fried ones. The results
Accepted 25 April 2016
indicated that the French fries prepared by the combined method had lower oil content
Available online 30 April 2016
(25%, db) than that of vacuum-fried sample (40%). Other property analysis also showed that the combined technique processed samples exhibited comparable food texture and color
Keywords:
to that of vacuum fried samples, and had a porous microstructure with relatively smaller
French fries
pore size. It was concluded that the combined method of pre-frying and PSMVD could be an
Vacuum frying
alternative method to produce high quality French fries with low oil content.
Pulsed-spouted microwave- vacuum
© 2016 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
drying Oil content Texture Microstructure
1.
Introduction
French fries are worldwide popular food thanks to their pleasant taste and typical flavor (Van Loon et al., 2005). French fires generally produced by immersion in edible oil or fat at a temperature above the boiling point of water. It is a rapid process of simultaneous heat and mass transfers, which can be used as a drying operation, resulting in flows in opposite directions of water vapor (bubbles) and oil at the surface of the piece. However, this frying process always leads to a high oil content of about 40% in the French fries (Gupta et al., 2010), which is
not considered a good property as too much fat intake would have negative impact on human health (Krokida et al., 2001a). Customers demand for low-fat even fat-free products with a long time storage has been the driving force of the food industry to produce lower oil content fried potatoes that still retain desirable texture and flavor (Song et al., 2007a). Vacuum frying is defined as a frying process which is carried out under pressures well below atmospheric levels (Andrés-Bello et al., 2011). Due to the lowered pressure, the boiling points of both the oil and the moisture in the foods are also lowered. This offers some advantages: (1) reduced oil
∗ Corresponding author at: School of Food Science and Technology, Jiangnan University, 214122 Wuxi, Jiangsu, China. Tel.: +86 13606179162; fax: +86 0510 85807976. E-mail address: [email protected] (M. Zhang). http://dx.doi.org/10.1016/j.fbp.2016.04.008 0960-3085/© 2016 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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content in the fried product, (2) preservation of natural color and flavor of the product due to the low temperature and oxygen content during the process, and (3) reduced adverse effects on oil quality (Song et al., 2007b). In addition, the frying process significantly affects the microstructure and porosity of the dried products (Dehnath et al., 2003). During the vacuum frying, water evaporates very rapidly from the surface of the food and the locations from which moisture is driven out become dry and creates porous microstructures, which benefits to the crisp texture of the product. Lulai and Orr (1979) reported that the most clearly defined factor influencing oil uptake during fried chips production is the initial solids content of the tubers. Furthermore, Moreira et al. (2009) found that the rate of oil uptake, but not the final oil content, was greatly affected by the frying conditions. More specifically, it had been demonstrated that using several pretreatment (like osmotic dehydration, coating and pre-drying) before frying could effectively reduce the oil content. For instance, Bunger et al. (2003) reported that soaking with 3% NaCl solution for 50 min significantly reduced oil uptake from 0.13 to 0.10 g oil/g dry matter. But, it also was reported that the decrease in oil absorption using osmotic dehydration pretreatment was attributed to the increase in solids content occurring during the osmotic dehydration process rather than a reduction in the amount of oil taken up (Moreno and Bouchon, 2008). So it has been debated recently that the importance of selecting an adequate basis to carry out comparisons properly. Moreover, even if the oil content had been significantly reduced in such products with pretreatment, they still had not met the requirement of low-fat content fried potatoes. Moreover, the excessive osmotic dehydration and pre-drying will lead to destruction of the taste and texture of French fries. To further improve the French fries’ favorable color while reduce the oil content, combined technology such as by introducing microwave finishing after pre-frying was reported (Porter et al., 1973). The combination of microwave drying significantly increases the heating rates and reduces the drying time (Salazar-González et al., 2012; Zhang et al., 2006), which in turn reduces the frying time and oil content while improves the product quality. However, the major drawback associated with microwave heating is the non-uniform temperature distribution, resulting in hot and cold spots in the heated product, which not only affects the quality but also raises the issue of food safety (Vadivambal and Jayas, 2010). Recently, a new equipment of pulsed-spouted microwave vacuum dryer (PSMVD), invented in our laboratory, was used to solve this problem (Zhang and Wang, 2012). The added “pulsed spout” feature in the microwave vacuum dryer provided a more uniform temperature distribution in the dryer when compared with those conventional rotating turntable microwave vacuum dryer, and consequently the product quality was significantly improved (Wang et al., 2013). The objective of this work was to produce lower oil French fries by combining pre-frying with PSMVD. The effect of this new method on the product quality was compared with the vacuum frying process.
2.
Materials and methods
Potatoes were purchased from a local market in Wuxi, China, and stored in a refrigerator at 5 ◦ C until used. Palm oil (Jia-li Co. Ltd., Lianyungang, China) was also purchased from the local market.
2.1.
Pretreatment of French fries
Fresh potatoes were washed, peeled and cut into 10 mm × 10 mm × 40 mm slices with a manual potato cutter. The potato slices were blanched in a water bath at 90 ± 1 ◦ C for 5 min followed by cooling under running tap water for 1 min. Excess water was removed by draining on tissue papers. Finally, the potato slices were frozen in a refrigerator at −18 ◦ C prior to frying.
2.2.
Vacuum frying (VF)
A lab-scale vacuum fryer equipped with a centrifuge (Nan Feng Company, Wuxi, China) was used to prepare the vacuum-fried samples. The fryer was filled with 5 L of palm oil as the frying medium. The frying temperature and vacuum pressure was set at 90 ± 1 ◦ C and 16 kPa, respectively. Six batches of 100 g potato slices each, after the pretreatment, were fried for 5, 10, 15, 20, 25 and 30 min. After frying, the potato slices were centrifuged at 300 rpm for 8 min under vacuum to remove the frying oil. The centrifuged potato slices were transferred to a plastic bag and then put into the equipment of PSMVD in 3 min to prevent oxidation. All experiments were conducted in triplicate.
2.3. Pulse-spouted microwave vacuum drying (PSMVD) A newly developed experimental apparatus, which was explained in details in a recent patent (Zhang and Wang, 2012), was used for pulse-spouted microwave vacuum drying (Fig. 1). Different from the conventional rotating turntable microwave vacuum dryer, the PSMVD apparatus includes a pulsed-spouted system with a set of adjustable air flow and distributive unit as well as a set of air handing units at the bottom of drying chamber (Wang et al., 2013). A schematic diagram of the equipment was present in Fig. 1. During the pulse-spouted microwave vacuum drying, the vacuum-fried samples were spouted at a specific time interval so as to ensure the air can flow periodically into the duct drying chamber via controlling the electromagnetic valve (2 s on/60 s off). The pressure was fluctuated within the drying chamber from 11 to 18 kPa. From our preliminary experiments, potato slices par-fried 15 min (moisture content of about 18.5% and oil content of about 31.94%) (dry base, db), was selected for PSMVD to a final moisture content of about 3%. The selected par-fried samples were subjected to pulse-spouted microwave vacuum drying with different microwave power levels of 10, 15, 20 W/g sample (Table 1). At the end of each process, dried samples were packed in separate plastic bags and kept in a desiccator for further quality analysis.
2.4.
Determination of moisture content
The moisture content of the samples was measured by using the oven method (Fan et al., 2006). Approximately 3 g of ground French fries were placed in the oven and dried at 102 ± 3 ◦ C until the mass of the sample did not change further.
2.5.
Determination of oil content
The oil content was determined by using the Soxhlet extraction method (Fan et al., 2006). Ground sample was oven-dried
food and bioproducts processing 9 9 ( 2 0 1 6 ) 109–115
111
Fig. 1 – Schematic diagram of pulse-spouted microwave vacuum drying system. 1, feeding ball valve; 2, plate valve with 3 mm diameter hole; 3, microwave heating cavity; 4, magnetron at 915 MHz; 5, circulating water unit; 6 and 21, drying chamber; 7, CPCA-140Z pressure sensor; 8, solid–gas separator; 9, condenser; 10, vacuum pump; 11, gas flow electromagnetic valve; 12, gas flow adjustable valve; 13, pressure gauge; 14, air handling unit; 15, control panel; 16, water loading pipe; 17, gas distributor; 18, spout pipe; 19, silicon rubber stopper; 20, sample; 22, fixed unit for drying chamber holder; 23, fiber-optic temperature sensor. and extracted with petroleum ether (b.p. 30–60 ◦ C) for 4 h. The oil content (db) in this paper was considered as defatted dried solids.
French fries were prepared. For each sample, ten repetitions were carried out.
2.8. 2.6.
Color measurements
The color parameters (L*, a*, b*) of the samples were measured using a CR-400 Chroma Meter (Konica Minolta Sensing Inc., Tokyo, Japan) which had been calibrated with a standard white board. Parameters L*, a* and b* indicate the intensity of lightness, redness and yellowness of the sample, respectively. Each sample was measured four times at different positions and the average values were reported.
2.7.
Determination of shrinkage
Shrinkage in volume (Sv ) was evaluated by Eq. (1) Sv =
V0 − Vf V0
× 100%
(1)
where V0 is the original volume of the sample (cm3 ) and Vf is the volume (cm3 ) of the sample finished. Both the volume of the sample can be calculated by V = S × L, where S is crosssectional area of the sample (cm2 ) and L is the sample’s length (cm). Ten samples were tested to determine shrinkage for each condition at the given equilibrium time.
Determination of texture 2.9.
The texture was determined using a texture analyzer (TAXT plus, Stable Micro Systems, Ltd., Surrey, UK) fitted with a three-point bending rig (HDP/3PD). The texture analyzer has an adjustable gap between the two lower supporting blades and the upper blade at an equidistant position above the center was moved to gently fracture the sample. Hardness was measured in terms of breaking strength, which is the force required to break the French fries. The pre-speed, test-speed, and post-speed were set as 1.0, 2.0, and 10.0 mm/s, respectively. The trigger force was 5 g. The peak force (in grams, g) which indicates the textural hardness was obtained from the force vs distance curves using the Texture Exponent (version 5.0.7.0) software associated with the texture analyzer. This textural hardness was measured on the same day when the
Scanning electron microscopy (SEM) analyses
Microstructure of the samples was observed using a scanning electron microscope (S-4800, Hitachi, Tokyo, Japan). The samples were cut along the cross section (10 mm × 10 mm × 3 mm) and defatted by using the Soxhlet extraction, then coated with gold under vacuum condition for the SEM observation, using an accelerating voltage of 5 kV at 40× magnification.
2.10.
Statistical analysis
Data were analyzed by using the Statistical Analysis System (SAS, version 8.0, SAS Institute Inc., Cary, North Carolina). Analyses of variance were performed by the ANOVA procedure.
Table 1 – Experimental design of different French fry preparation methods. Methods VF VF + PSMVD-10 VF + PSMVD-15 VF + PSMVD-20
Procedure Vacuum frying until a moisture content of about 3% Vacuum frying 15 min and pulse-spouted microwave vacuum drying at 10 W/g to a moisture content of about 3% Vacuum frying 15 min and pulse-spouted microwave vacuum drying at 15 W/g to a moisture content of about 3% Vacuum frying 15 min and pulse-spouted microwave vacuum drying at 20 W/g to a moisture content of about 3%
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Results and discussion
3.1.
Vacuum frying characteristics
450
20 18
400
16
Moisture content (%, db)
3.
3.2. Pulsed-spouted microwave vacuum drying characteristics The plot of moisture content vs PSMVD at different microwave power levels is present in Fig. 3, which shows that the moisture content was decreased with drying time at all microwave power levels. As the microwave power increased, the time for reaching the final moisture content (about 3%, db) was decreased, which indicated that the drying rate was increased with microwave power increasing. This is confirmed by Fig. 4 that the microwave power significantly (p < 0.05) affected the drying rate. This could be attributed to the acceleration of interior moisture migration under higher microwave power conditions. Such acceleration of the interior moisture
450 40 400 35 30 300 25
250 Vacuum degree: 16kPa 200
20
150
15
100
10
50
5
0
0
5
10
15
20
25
30
Oil content (%, db)
Moisture content (%, db)
350
0 35
Frying time (min)
Fig. 2 – Variation of moisture content and oil content with time in French fries during vacuum frying.
300 250 200
10 W/g 15 W/g 20 W/g
14 12 10 8 6 4 2
150
0 15
100
20
25
30
35
40
45
50
Drying time (min)
50 0
0
5
10
15
20
25
30
35
40
45
50
Time (min) Fig. 3 – Variation of moisture content with time in French fries during vacuum frying and pulsed spouted microwave vacuum drying combination (Microwave power levels 10, 15, 20 W/g).
1.2
1.0
Drying rate (g/min)
Moisture loss and oil uptake are two most important mass transfer processes during frying of food materials. The moisture and oil content of French fries during vacuum frying are presented in Fig. 2, which indicated that the moisture content decreases rapidly but the oil content increases with frying time. This was in agreement with other researchers (Garayo and Moreira, 2002; Krokida et al., 2001b). When the potato slices were placed into the hot oil under vacuum condition, water evaporates very rapidly through the pore which creates a porous microstructure. The moisture from the interior diffuses out as steam and creates a pressure gradient. As the loss of moisture content continues, the locations from which moisture is driven out losing their hydrophobicity and become dry (Bravo et al., 2009). The de-oiling devices in the system removes excessive oil on the sample surface before vacuum broken, and prevent the oil permeating into interior during vacuum breakage. Then the oil adheres to the dry portion of French fries and even enters the porous areas. Meanwhile, desired or undesired physicochemical changes can take place and affect the sensory attributes such as texture and color (Garayo and Moreira, 2002). Therefore, partially frying the potato slices in hot oil followed by a drying process, using PSMVD could be used to produce crisp French fries with desired oil content. From the preliminary experiments (data not shown here), partially frying the potato slices for 15 min was the best condition for the following PSMVD process.
Moisture content (%, db)
350
0.8
0.6
10 W/g 15 W/g 20 W/g
0.4
0.2 0
2
4
6
8
10
12
14
16
18
20
Moisture content (g/100g, db) Fig. 4 – Variation of drying rate with moisture content at different microwave power levels during pulsed spouted microwave vacuum drying (Microwave power levels 10, 15, 20 W/g). migration consequently increased the surface water evaporation. Fig. 4 also shows that the effect of microwave power on the drying rate was markedly higher at high moisture levels, except for 10 W/g. This is because the amount of microwave energy absorbed by the material is depended on its dielectric properties and the electric field strength (Salema et al., 2013). Materials with higher moisture content are able to absorb more microwave energy since the values of the dielectric constant and loss factors are higher at higher moisture content. Thus, the present samples with higher moisture showed a higher drying rate when other drying conditions were identical. However, under the lowest microwave power level of 10 W/g in this study, the highest drying rate was observed at the moisture content of around 12%. Both higher or lower than this moisture level led to a lower drying rate (Fig. 4). The mechanism needs further investigation. Fig. 5 presents the oil content of the partially vacuum fried potato slices during PSMVD, which shows the oil content was decreased regardless the microwave power levels. This could
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35
70
30
60
L* a* b*
10 W/g 15 W/g 20 W/g
15 10
Color
40
20 Oil content (%, db)
Oil content (%, db)
50
25
30 20 10 0
5
VF
VF+PSMVD-10
VF+PSMVD-15
VF+PSMVD-20
Drying time (min)
0
5
10
15
20
25
30
35
40
45
50
Time (min) Fig. 5 – Variation of oil content with time in French fries during vacuum frying and pulsed spouted microwave vacuum drying combination. be attributed to the facts that some surface oil was flow out with the moisture evaporation under the microwave condition, or some oil also vaporized from the surface of the product under the microwave condition (Agnieszka and Adam, 2008), or some stuck on the drying chamber. Fig. 5 also suggested that different microwave power levels only affected the rate of oil loss, but the total amount of oil loss was almost same. Moreira et al. (2009) also reported that the drying condition normally did not affect the total oil transfer. During the PSMVD of the vacuum fried potato slices, the oil decrease was mainly due to the co-evaporation of oil with the internal moisture. Because the total moisture loss for these samples were almost identical, the total oil loss for them were also same.
3.3. Comparison on the quality of French fries produced by different methods 3.3.1.
Oil content
With the increasing awareness of human health, many fried foods are not very welcome by consumers mainly because of their high oil content and potential hazardous substances generated during frying. Due to high thickness, it is difficult to reduce oil content in French fries by using traditional frying methods. Although the application of vacuum frying has successfully reduced the formation of acrylamide (Granda et al., 2004), their oil content are still as high as about 40% (Gupta et al., 2010). The oil content of French fries produced by the combination of vacuum frying and PSMVD was decreased to 25% (db), about 40% decrease of oil content when compared with the vacuum fried sample. The decrease of oil content could be attributed to its synergy effect during the combination process. Firstly, the short time vacuum-frying process reduced the oil uptake, as the moisture and oil content of 15 min vacuum-fried French fries was about 16% and 30%, respectively (Fig. 2). Secondly, during the subsequent PSMVD process, the oil content of the product was further decreased by several factors as discussed above. The final oil content of the French fries was around 25% (db).
3.3.2.
Color
Color is the first impression of a product that affects customers’ choice. For the snack fried foods, their golden yellow color was one of the key factors that attract people’s attention.
Fig. 6 – Comparison on the color of French fries produced by different methods. The lightness parameter L* is commonly used as a quality control parameter for fried foods (Mariscal and Bouchon, 2008). Higher L* value indicates a lighter color, which is favorable for fried foods. Fig. 6 presents the color values of French fries produced by different methods, which shows the lightness (L*) of vacuum-fried French fries was lower than that of the combination method processed sample. It was reported that the L* values were decreased with frying time increasing (Bunger et al., 2003; Shyu and Hwang, 2001), and the longer the frying time, the more the non-enzymatic browning reactions occur. Therefore the formation of vacuum-fried darker color (low L* values) could be associated with more non-enzymatic browning reactions caused by longer frying time. However, such reaction was not so obvious in the PSMVD process. Fig. 6 also shows that vacuum-fried French fries possessed the highest yellowness (b*) value, suggesting they had stronger golden yellow color. This could also be related to the frying process. The longer vacuum-frying time would lead to more oil penetrated into the sample and more chemical oxidation, and eventually led to stronger golden yellow color. For the PSMVD samples, the b* values decreased at the first stage and then increased with the microwave power level increasing (Fig. 6).
3.3.3.
Texture
Fig. 7 presents the hardness of French fries produced by different methods. The French fries produced by vacuum frying was 2042.79 g, which was similar to the hardness of French fries produced by combination of vacuum frying and PSMVD
2000
1500
Hardness (g)
0
1000
500
0 VF
VF+PSMVD-10
VF+PSMVD-15
VF+PSMVD-20
Fig. 7 – Comparison on the hardness of French fries produced by different methods.
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Fig. 8 – Scanning electron micrographs of (a) vacuum-fried French fries and (b) combination produced French fries. The magnification was set as 40×.
3.3.4.
Shrinkage in volume
Maadyrad et al. (2011) reported that volume shrinkage during early stages of frying was nearly equal to the volume of water loss, but this turned to smaller in the final stages of
30 25 20 15 10 5
Microstructure
Structural properties are also important for the characterization of the quality of fried product. Krokida and Maroulis (2001) reported that the processing method significantly affects the microstructure and porosity of dried products. Fig. 8 presents the scanning electron micrographs of the cross-section of French fries prepared from different methods, which shows that the sample produced by combination method (Fig. 8b) exhibited a morphology featured as regular and uniform pore distribution, whereas that of vacuum fried samples exhibited bigger but less regular holes (Fig. 8b). Jiang et al. (2013) also observed that the different processing method can affect the pore size of food materials. During vacuum frying, the heat flux from oil reaches the surface of the samples and is transferred via the solid matrix to the deeper parts of the material. This results in the dried area moved from the surface to the core with a slow and constant speed that leads to the microstructure with large size pores and the longer the frying time, the larger the pore size. The combined method in the present study reduced the frying time and also the pore size. The combined method also showed advantages of internal heating and high heating rates due to dielectric cores within the drying body under PSMVD (Araszkiewicz et al., 2007). The numerous vapor evaporated by moisture via the solid matrix created a mass of channels to let the vapor leave the matrix, which resulted in a porous structure with small pore size in the sample of combination of vacuum frying with PSMVD.
3.3.5.
35
Shrinkage in volume (%)
at the power level of 20 W/g, suggesting their texture were comparable. Meanwhile, it was also observed that the sample hardness of combination process was increased with the microwave power levels. When the microwave power was as low as 10 W/g, there might be not enough internal moisture evaporation and vapor pressure generated. Thus, the radial force was unable to expand the French fries, and the sample became stiff due to loss of water and shrinkage. Upon increasing the microwave power, the internal moisture was evaporated more rapidly, which created a high vapor pressure and porous structure that reduced the sample hardness.
0
VF
VF+PSMVD-10
VF+PSMVD-15
VF+PSMVD-20
Fig. 9 – Comparison on the shrinkage of French fries produced by different methods. drying. Therefore, the volume of shrinkage is depended on the water transfer within the product. As shown in Fig. 9, the volume shrinkage took place in all French fries due to water loss, but the shrinkage degree of vacuum-fried ones was smaller, which may be resulted from its longer frying time that led to the surface becoming more rigid, thus producing a resistance to volume change. The final volume shrinkage for combination processed sample was observed to decrease upon microwave power level increasing. This was mainly due to the interior force created under the microwave power. When the microwave power was 10 W/g, the generated interior force was too low to make enough internal moisture evaporation and vapor pressure. Thus, the radial force was not sufficient to induce the French fries to expand and therefore had a larger volume shrinkage due to moisture loss. With microwave power levels increasing, the internal moisture was evaporated more rapidly, and the high vapor pressure pushed the French fries to expand and led to less volume shrinkage.
4.
Conclusions
French fries were produced by a combined technique of vacuum-frying and PSMVD. Both moisture content and oil content were decreased during the PSMVD process, and the microwave power level did not affect the final oil content. Compared with the traditional vacuum-fried sample, French
food and bioproducts processing 9 9 ( 2 0 1 6 ) 109–115
fries prepared from the combination method showed advantages of low oil content with still comparable properties in color, texture and shrinkage. It was concluded that combination of vaccum frying with PSMVD could be an alternative method to produce French fries with low oil content without sacrificing its gold color and crunchy texture.
Acknowledgements We acknowledge the financial support by Key Laboratory of Space Nutrition and Food Engineering, China Astronaut Research and Training Center (Contract No. SNFE-KF-15-11), Jiangsu Province (China) “Collaborative Innovation Center for Food Safety and Quality Control” Industry Development Program, Jiangsu Province (China) Infrastructure Project (Contract No. BM2014051) which have enabled us to carry out this study.
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