Comparative study of physical and sensory properties of pre-treated potato slices during vacuum and

atmospheric frying

Ponente: José David Torres González

Docente: Ramiro Torres Gallo

Reología y Textura de los Alimentos

Maestria en Ciencias Agroalimentarias

Universidad de Córdoba - Monteria, 2015

E. Troncoso*; F. Pedreschi**; R.N. Zuñiga***.

*Chemical Engineering Deparment, Universidad de Santiago de Chile (USACH), P.O. Box 10233, Santiago, Chile,

**Department of Food Science and Technology, Universidad de Santiago de Chile (USACH), P.O. Box 10233, Santiago, Chile

***Chemical & Bioprocess Engineering Department, Pontificia Universidad de Chile, P.O. Box 306, 6904411, Santiago, Chile

LWT - Food Science and Technology 42 (2009) 187–195

RESUMEN

The objective of this research was to study the effect of different processing conditions on physical andsensory properties of potato chips.

Desirée and Panda varieties (diameter: 30 mm; thickness: 3 mm) were pre-treated in the following ways:

(i) control or unblanched slices without pre-drying.

(ii) blanched slices in hot water at 85 °C for 3.5 min and air-dried at 60 °C until a final moisture content of∼0.6 kg water/kg dry solid.

(iii) control slices soaked in a 3.5 kg/m3 sodium metabisulphite solution at 20 °C for 3 min and pH adjustedto 3.

Pre-treated slices were fried at 120 and 140 °C under vacuum conditions (5.37 kPa, absolute pressure) andunder atmospheric pressure until they reached a final moisture content of ∼1.8 kg water/100 kg (wet basis).

An experimental design (3 × 23) was used to analyze the effect of pre-treatment, potato variety, type offrying and frying temperature over the following responses: oil content, instrumental color and texture andsensory evaluation.

Vacuum frying increased significantly (p<0.05) oil content and decreased instrumental color and textural parameters.

Sensory attributes, flavor quality and overall quality, were significantly improved using vacuum frying.

The higher frying temperature (140 °C) increased ΔE, maximum breaking force, hardness and crispness and decreased L* and b* values. On the other hand, panda potato variety improved the color of the product.

A great improvement on color parameters was obtained using sulphited potato slices instead of the other pre-treatments.

Although, the better flavor was obtained for control potato chips, no significant differences were found for overall quality between control and sulphited potato chips.

Significant correlations (p < 0.01) between sensory and instrumental responses were found.

RESUMEN

IntroductionDeep-fat frying is one of the oldest and most popular food preparation techniques atboth domestic and industrial venues. Deep-fat frying can be defined as a thermalfood processing method in which food immersed in oil at a temperature of 150–190°C, which is well above the boiling temperature of water.

The heat and simultaneous mass transfer of oil and air promote a number of chemicalchanges, such as:

Water loss Denaturation protein

Oil uptake Reducing sugars

Crust formation Color via Maillard reactions

Starch gelatinization Hydrolysis or oxidation

Aromatization Oil polymerization

Fig. 1: Deep-Fat Frying Procesing (Aguilera, 1997)

Fig. 2: Scanning Electron Microscope of the crust

during deep fat frying potato (Singh, 1995).

Fig. 4: Phases during deep fat frying (Moreira, 1999)

Fig. 3: Mass and Heat Transfer during deep fat frying (Farkas, 1996)

1) Calentamiento inicial; 2) Ebullición superficial; 3) Velocidad

decreciente; 4) Punto final de burbujas (Baik y Mittal, (2003).

Pre-treatments for Deep-Fat FryingPre

treatm

ents

befo

red

eep

fat

fryi

ng

Blanching (Escaldado)

Drying (Secado)

Biofilms (Recubrimientos)

Osmotic dehydration (Deshidr. Osmótica)

Frozen (Congelación)

Combined methods (Combinados)

Material and methods

-Potatoes (varieties Desirée and

Panda) vegetable oil.

-Stored in dark room at 8 °C and 95% relative humidity.

-Slices (thickness of 3 mm) were cut from the pith of

the parenchymatous region of potato tubers using an

electric slicing machine (Berkel, model EAS65, UK).

-A circular cutting mold was used

to provide chips with a diameter of

30 mm.

-Sugar contents of Desirée and Panda varieties were

21.79 ± 0.58 and and 22.54±0,42 respectively.

Sample PreparationPotato slices were rinsed immediately after cutting for 1 min in distilled water

The following pre-treatments were used:

• (i) Control or unblanched slices without pre-drying.

• (ii) Blanched slices in hot water at 85 °C for 3.5 min were placed on a

wire tray, arranged in one layer of 28 samples, in a laboratory

convection dryer, with an average tray load of 1.75 kg/m2. Then the

samples were air-dried at 60 °C at an air velocity of 1.8 ± 0.1 m/s.

Weight loss was monitored periodically until the slices reached a final

moisture content of ∼0.6 kg water/kg dry solid.

• (iii) Potato slices were soaked in a 3.5 kg/m3 sodium meta-bisulphite

solution at 20 °C for 3 min and pH adjusted to 3.

The concentration of the metabisulphite

solution and the time of immersion was

chosen to obtain fried potato slices with a

sulphite level below 5 × 10−5 kg/kg,

according to the permissible limit the

General Standard for Food Additives

(GSFA) developed by the Codex

Committee on Food Additives and

Contaminants (CODEX STAN 192-1995,

1995).

After immersion, the samples were rinsed

and blotted with tissue paper before

frying.

Systems Deep-Fat FryingV = 0.012 m3 y stainless steel (type 316) vessel electrically heated and a temperature controller system (PID) ± 3 °C. Thefryer was filled with 0.0035 m3 of oil, which was pre-heated for 1 h prior to frying and discarded after 6 h of use.

Fig. 5: Schematic of the frying system

-The vessel was connected to a vacuum

pump. Level maxim of 5.37 kPa and at this

pressure the boiling point of water is 34 °C.

-Once the oil temperature reached the

target value (120 °C or 140 °C), the pre-

treated slices were placed inside the fryer

basket (Fig. 5).

-The vessel was depressurized. When the

absolute pressure in the vessel achieved

5.4 kPa, the basket was lowered and

immersed in the oil.

Vacumm

-Temperature (120 °C or 140 °C), the basket was immersed in the oil.

-Finalized the frying time, the basket was raised and the samples were removed

from the fryer, blotted with tissue paper and allowed to cool to room temperature

before analyses.

Atmospheric

Experimental design and statistical analysis• A statistical multifactorial experimental design was used to analyze simultaneously the effect of four

factors that affect the frying process through the following responses:

Fact

ors

Pre-treatment

c: control

bd: blanched

and dried

s: sulphited

Potato variety

d: Desirée

p: Panda

Type of frying

a: atmospheric

v: vacuum

Frying

temperature

120°C

140°C

Resp

onse

s

Oil content

Instrumental color

Instrumental texture

Sensory evaluation

Experimental Design And Statistical Analysis

The randomization of the experiments as well as

the statistical analysis of the experimental data

was done with the software Statgraphics Plus

(version 5.1).

The statistical significance of the effects of the

factors, and their interactions, was analyzed

simultaneously by means of Pareto charts and

ANOVA tests (p<0.05).

Table 1. Standard matrix of the multifactorial experimental design 3×23 runs.

Design responses

Sensory evaluation

Instrumental texture

Instrumental color

by computer vision

Oil contentWas determined by rapid method of total lipid extraction and purification (Bligh & Dyer, 1959). was expressed

as kg oil/kg dry solid.

A computer vision system (CVS), previously implemented, was used to measure representatively and

accurately the color of the potato chips, using 15 samples for each analysis. Diferencia de color total (QRT)

fried potato slices was defined as:

Measurements at temperature (20 °C) by a puncture test with a speed test of 10 mm/s using a punch with a

diameter of 2 mm (probe model P/2) performed in a Texture Analyzer TA.XT2i (Stable Micro System, Surrey,

UK). The peak or maximum force, defined as the force at which the punch penetrates the outer layer of the

surface of the fried potato slices, was obtained using the software Texture Expert (version 1.16). Each analysis

was conducted using 10 samples.

Thirteen assessors (eight male and five female), 20 and 30 years. This panel was trained during 12 sessions (2 h

each one) for quantitative descriptive analysis (QDA) of potato chips. The response variables were obtained by

means of a QDA on a 10 cm non-structured linear scale (Meilgaard, Civille, & Carr, 1991), and a quality rating

test (QRT) (Muñoz, Civille, & Carr, 1992) on a 7-point numerical scale (1 = very bad, 7 = very good).

Other analyses

Moisture content

Moisture content of potato chips was measured by drying the samples in a vacuum convection oven (SHEL

LAB, model 1410-2E, USA) at 30 kPa (vacuum pressure) and 70 °C per 24 h until reaching constant weight

(AOAC, 1984).

Sulphite content

The determination of sulphite content was performed according to the optimized Monier-Williams method

(AOAC, 1995). The total concentration of sulphite was determined by a titrimetric method in which sulphite

is converted into sulphur dioxide and further oxidized to sulphuric acid using hydrogen peroxide and

subsequently titrating against alkali. The analysis of all samples was performed in triplicate

Results And DiscussionTable 2. Response values of the multifactorial experimental design (3×23)

Table 3. Effects of the multifactorial experimental design (3×23)

Table 4. Percent change in the response by switching from low to high level of the factors

Fig. 2. Effect of pre-treatment on the statistically significant quality parameters of fried potato slices. Bars with different

letters indicate significant differences (p<0.05).

Fig. 3. Image gallery of pre-treated potato chips (Desiree and Panda varieties) fried at 120 C under vacuum and

atmospheric frying.

Table 5. Percent change in the response by switching from low to high level of the pretreatment

ConclusionsVacuum frying increased significantly oil content of potato chips and had a significant effect on theinstrumental and sensory parameters of color. Potato slices fried under vacuum had L* values significantlyhigher than the values corresponding to the slices fried under atmospheric conditions. The a*, b* and ΔEvalues were significantly higher for potato slices fried at atmospheric pressure than those for fried at thevacuum conditions.

Sensory evaluation confirmed these results, the color of the potato slices fried at atmospheric conditionswas evaluated as “darker” and “worst” than the potato slices fried at vacuum conditions. Instrumental andsensory textural parameters of chips, maximum breaking force, hardness and crispness, were significantlydecreased by using vacuum frying.

Nevertheless, texture quality was significantly “better” for vacuum fried chips and flavor quality and overallquality were improved when vacuum frying was used instead of atmospheric frying. Frying temperatureaffected significantly instrumental and sensory parameters of color and texture. The high level of fryingtemperature (140 °C) decreased L* and b* and increased ΔE, maximum breaking force hardness andcrispness. Panda potato variety improved the color of the product and the pre-treatment affectedsignificantly the instrumental parameters of color, flavor and overall quality of potato chips.

In general, sulphited potato slices improved significantly color parameters in comparison with control andblanched and dried pre-treatments. The better flavor was obtained for control potato chips, but nosignificant differences were found for overall quality between control and sulphited potato chips. Significantcorrelations (p<0.01) between sensory and instrumental responses were found indicating that both methodsare suitable for the measurement of color and texture parameters.

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