air frying_ a new process for healthier french fried potatoes

7/29/2019 Air Frying_ a New Process for Healthier French Fried Potatoes

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AIR FRYING: A NEW PROCESS FOR HEALTHIER FRENCH FRIED POTATOES

Heredia, A.*; Mas, S.; Castell, M.; Andrs, A.

Institute of Food Engineering for Development, Universidad Politcnica de Valencia

Camino de Vera s/n, P.O. Box 46022 Valencia (Spain)

*mail contact: [email protected]

ABSTRACT

The aim of this work was to compare traditional deep-oil frying with air-hot frying, novel

technique which allows obtaining low-fat fried food, in terms of mass transfer phenomena

and volume changes associated. With this aim, fresh and pretreated, under blanching or

freezing, French fries were submitted to hot-air or deep-oil frying at 180 C. Obtained results

showed that oil uptake was rather lower under hot-air frying confirming this technique as a

healthier one. The analysis of the ratio R (mass loss/ water loss) along the process and the

correlation between water loss and volumen changes allowed to describe the different steps

of both frying processes in terms of mass transfer phenomena.

Keywords: French fries; Hot-air Frying; Deep-oil frying; Mass transfer phenomena

1. INTRODUCTION

Deep-oil frying, based on the immersion of the product in oil at a temperature of 150200 C,

is the most currently method to fry in order to obtain crunchy and tasty foodstuffs. The high

temperature causes an evaporation of the water, which moves away from the food and, oil is

absorbed by food at the same time, replacing some of lost water (Moreira, Palau, & Sin,

1995). Nevertheless, the reduction of the lipid content in fried foods is required mainly owing

to its relation with obesity and coronary diseases. With this aim, either alternative frying

technology such as vacuum, pressure or microwave, a pre-frying step or the use of edible

coatings have been studied. Hot-air frying is a novel method consisting on frying food with asmall amount of oil dispersed in a stream of hot air as external fluid. The product is

constantly moved in the air-frying chamber to favour the mass and heat transfer between the

product and the external fluid.

The purpose of the present work was to analyze the hot-air frying process in terms of mass

transfer phenomena and volume changes associated and to compare the results with those

obtained with deep frying, paying special attention to oil uptake fluxes. Concretely, fresh and

pretreated, under blanching or freezing, french fries were submitted to hot-air or deep-oilfrying at 180 C.


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2. MATERIALS AND METHODS

Raw material, Methodology and Equipments

Fresh potatoes (Solanum tuberosumL., Mona Lisa variety) were washed, sorted, peeled and

cut by means of a manual cutter into strips of 0.009 m x 0.009 m x 0.03 m. Frying process,

either deep-oil or hot-air frying, were carried out in (i) control or unpretreated strips, (ii)

strips blanched in hot water at 90 C for 1 min, and (iii) commercial frozen pre-fried potato

strips with an initial fat content of 3 % with similar dimensions. Experiments were carried out

at a fixed frying temperature of 180 C in commercial equipments for deep oil-frying (Solac)

and hot air-frying (Tefal). Three samples were removed from the frying equipment at 3 min

interval for hot air-frying experiments (total processing time: 30 min) and 2 min interval

(total processing time: 16 min) for deep-oil frying for analytical determinations.

Analytical Determinations

Water content was analysed by vacuum drying at 60 C until constant weight was achieved

(20.103 AOAC, 1980). The oil content was determined by the method of Bligh and Dyer

(1959). Volume was analyzed by means of picnometer using distilled water as a reference

liquid.

All analytical determinations were carried out by triplicate for each time of frying process.

Mathematical procedure

Net fluxes of total mass (Mot), and components (Mit), water (Mwt)and fat (Mfatt),were

calculated according to Heredia & Andrs (2008).

Volume variation (Vot) experimented by the samples at each time was obtained as follows

(Eq. (I)):

o

ooto

tV

VVV

0

0 )( (I)

WhereVot andVo0 arevolume sample at timetand 0, respectively.

3. RESULTS AND DISCUSSION

Figure 1 shows fat gain flux along deep-oil and hot-air frying. In general, it could be

observed that most of fat gain took place during 2 first min of the process, and especially for

untreated French fries. In fact, vegetal tissue contraction at the beginning of the process

favored oil penetration in the superficial layer. Nevertheless, this phenomenon was not

registered in blanched French fries since this contraction occurs during the previous

blanching step given as a result a reduction of oil absorption during frying. Frozen French

fries absorbed the least amount of oil during conventional frying; whereas a negative oil flux


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was registered under hot-air frying in spite of they presented the high oil content among the

samples fried in hot-air (data not shown). Finally, it might be pointed out that fat gain was

approximately ten times higher during deep-oil frying than hot-air frying, confirming this last

technique as healthier one. Regarding the mass and water, the R ratio value (R=Mot

/Mwt) can be used to understand the process. Figure 2 shows R ratio value at different

processing time. As can be observed, two steps were mainly identified: Step I characterized

by the intake (R1) of fat at the beginning of the process; Step II where R is

nearly equal to 1 indicating that total mass loss is exclusively due to water loss. It can be said

that the analysis of R ratio (mass loss/ water loss) along the process on one hand, and the

correlation between water loss and volumen changes (figure 3) on the other hand, allowed

describing the different steps of both frying processes in terms of mass transfer phenomena.Finally, figure 3 shows the different steps previously identified taken into account the

evolution of total volume in relation to water loss along frying: Step I or Volume Loss Step in

which volume loss is due to the tissue contraction by the thermal shock but mainly to water

loss; Step II or Volume Recovery Steponly observed in unpretreated and blanched samples

submitted to deep-oil frying, in which global heat transfer coefficient is high enough to

provoke water evaporation inside the potato tissue. Vapour expansion induces volume

recovery as a consequence of porosity increase; Step III or Volume constant step in which it

is observed a loss of water without change of volume due to the vitreous crust formation.

4. CONCLUSIONS

Obtained results showed that oil uptake was rather lower under hot-air frying confirming this

technique as a healthier one. Regarding to the effect of the sample pretreatment, freezed

samples reduced its initial oil content during air fying process and they showed the lowest oil

uptake under deep frying.The analysis of the ratio R (mass loss/ water loss) along the process

and the correlation between water loss and volumen changes allowed to describe the differentsteps of both frying processes in terms of mass transfer phenomena.

5. ACKNOWLEDGEMENT

Authors would like to thank to the Universidad Politcnica de Valencia (PAID-06-09-2876)

for the financial support given to this investigation.

6. REFERENCES

AOAC. (1980). Association of Official Analytical Chemist. Official methods of Analysis.

Washington DC.


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Bligh, E. G., & Dyer, W. (1959). A rapid method of total lipid extraction and purification.

Canadian Journal of Biochemistry and Physiology, 37, 911917.

Heredia, A. & Andrs, A. (2008). Mathematical Equations to Predict Mass Fluxes and

Compositional Changes During Osmotic Dehydration of Cherry Tomato Halves. Drying

Technology: An International Journal, 26(7),873-883.

Moreira, R. G., Palau, J., & Sin, X. (1995). Simultaneous heat and mass transfer during the

deep fat frying of tortilla chips.J ournal of Food Process Engineering, 18,307320.

0,00

0,01

0,02

0,03

0,04

0,05

0 2 4 6 8 10 12 14

Mfatt

Time (min)

Unpretreated Frozen Blanched

.. -0,02

-0,015

-0,01

-0,005

0

0,005

0,01

0,015

0,02

0 10 20 30

Mfatt

Time (min)


Figure 1. Net oil gain flux (Mfatt) in French fries at different times of deep-oil frying (closed symbols) and hot-air frying

(empty symbols).

0,0

0,2

0,4

0,6

0,8

1,0

1,2

0 2 4 6 8 10 12 14

R=MOt/

MWt

Time (min)


..

0

0,5

1

1,5

2

2,5

0 3 6 9 12 15 18 21 24 27 30

R=Mo

t/Mwt

Time (min)


Figure 2. Evolution of R ratioin French fries during deep-oil frying (closed symbols) and hot-air frying (empty symbols).

-0,8

-0,7

-0,6

-0,5

-0,4

-0,3

-0,2

-0,1

0,0

-0,8 -0,6 -0,4 -0,2 0,0

Vo

t

Mwt


III

IIII

I

..-0,8

-0,7

-0,6

-0,5

-0,4

-0,3

-0,2

-0,1

0,0

-0,8 -0,6 -0,4 -0,2 0,0

Vo

t

Mwt


I

III

Figure 3. Correlation between total volumen variation and water loss during deep-oil frying (closed symbols) and hot-air

frying (empty symbols).

air frying_ a new process for healthier french fried potatoes

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