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ABSTRACT

OMEARA, MEGHAN. Determination of the Interfacial Tension between Oil/Steam and Oil/Air at Elevated Temperatures. (Under the direction of Dr. Brian Farkas.) Immersion frying is a widely used cooking technique that involves heating foods in oil above

100°C. During frying, the heating profile of the product causes rapid moisture loss at its

surface resulting in crust formation without burning. The rate of heat transfer, moisture loss and oil uptake are all affected, in part, by interfacial tension between the oil, steam and food surface. Understanding the relationship between frying oil temperature and the oil-steam interfacial tension may yield insight into the mechanism of boiling heat transfer and thus moisture loss. The pendant drop method was used to determine oil-air and oil-steam interfacial tension for five cooking oils (Canola, corn, olive, peanut, and soybean) at temperatures up to 200°C. Initial oil and air values started at room temperature and oil and steam values started at 110°C. The pendant drop method relates density difference, force of gravity, drop shape, and drop radius of curvature to interfacial tension. To determine interfacial tension, the density of the oil must to be known at each temperature. Density was determined from room temperature to the smoke point of each oil using the Archimedean method. This method relates fluid density to the buoyancy and volume of a submersed object of known physical properties. All oils demonstrated a nearly linear decrease in density with increasing temperature (R2>0.99) from a high of 915.7 kg/m3 (soybean at 22°C) to a low of

801.5 kg/m3 (peanut at 200°C). Density trends for all oils were similar but density values

were statistically different. Coefficient of variations of triplicate measurements at each temperature were less than 0.13%, indicating that the method demonstrated high precision for measuring the density of food oils at high temperatures. All interfacial tension values decreased linearly as temperature increased (R2>0.99) from a high of 32.14 mN/m (Canola oil at 23.5°C) to a low of 20.04 mN/m (fresh peanut and fresh corn oil at 200°C). Interfacial tension trends were similar for all oils but values were statistically different between oils at a given temperature (p<0.0001). No significant difference was found between the oil-steam and oil-air interfacial tension values e.g. 25.21 mN/m, 25.29 mN/m, respectively, for Canola at 120°C. The coefficient of variation for quadruple measurements at each temperature ranged from 0.06 to 0.85%, indicating a precise method. The results indicate that oil-air interfacial tension may be used as an estimate for oil-steam interfacial tension. Additionally, the method and data generated through this research may be used in analysis of processes involving food oils at high temperatures including frying and atomization of biodiesel fuels.

© Copyright 2011 by

All Rights Reserved

Determination of the Interfacial Tension between Oil-Steam and Oil-Air at Elevated

Temperatures

by

Meghan OMeara

A thesis submitted to the Graduate Faculty of

North Carolina State University

in partial fulfillment of the requirements for the degree of

Master of Science

Food Science

Raleigh, North Carolina

2012

APPROVED BY:

_______________________________ ______________________________

Dr. Jan Genzer Dr. K.P. Sandeep

________________________________

Dr. Brian Farkas

Committee Chair

ii

DEDICATION

I dedicate this page to my mom and dad. I would also like to dedicate this to my wonderful fiancé, who helped me throughout this time. iii

BIOGRAPHY

she studied Chemical Engineering. She is finishing her Masters of Food Science from North Carolina State University with a minor in Chemical Engineering. She will go on to work for Nestlé USA at the manufacturing plant in Mt. Sterling, KY. She will be working as a food technologist on Hot Pockets doing scale-up and process modifications. iv

TABLE OF CONTENTS

List of Tables .......................................................................................................................... v

List of Figures ......................................................................................................................... vi

CHAPTER 1 ........................................................................................................................... 1

INTRODUCTION ............................................................................................................ 1

GOALS AND OBJECTIVES ........................................................................................... 9

REFERENCES ................................................................................................................. 10

CHAPTER 2 ........................................................................................................................... 12

ABSTRACT ...................................................................................................................... 12

INTRODUCTION ............................................................................................................ 12

MATERIAL AND METHODS ........................................................................................ 15

Materials ..................................................................................................................... 15

Archimedean Method.................................................................................................. 16

RESULTS AND DISCUSSION ....................................................................................... 16

CONCLUSION ................................................................................................................. 26

REFERENCES ................................................................................................................. 27

CHAPTER 3 ........................................................................................................................... 29

ABSTRACT ...................................................................................................................... 29

INTRODUCTION ............................................................................................................ 30

MATERIAL AND METHODS ........................................................................................ 34

Materials ..................................................................................................................... 34

Pendant Drop .............................................................................................................. 34

Interfacial Tension Instrumentation ............................................................................ 35

Steam Delivery............................................................................................................ 36

Interfacial Tension Determination .............................................................................. 37

RESULTS AND DISCUSSION ....................................................................................... 38

Accuracy of Method ................................................................................................... 38

Oil-Air Interfacial Tension ......................................................................................... 48

Oil-Steam Interfacial Tension ..................................................................................... 40

Predictive Equations ................................................................................................... 44

Soybean Oil Brands .................................................................................................... 46

Effect on rate of Heat Transfer ................................................................................... 47

CONCLUSION ................................................................................................................. 48

REFERENCES ................................................................................................................. 49

v

LIST OF TABLES

Table 2.1 Density values for five common cooking oils from room

temperature to 200°C ...................................................................................... 17

Table 2.2 Y-intercept, slope and correlation coefficient ................................................. 18

Table 2.3 Determined surface tension effect and corrected densities ............................. 19

Table 2.4 R2 values from experimental densities compared to different literature

densities........................................................................................................... 20

Table 2.5 Comparison of current study olive oil density to Jamieson (1927) densities

at room temperature ........................................................................................ 22

Table 2.6 Current study, predicted density using Modified Rackett equation (Halvorsen et al. (1993), Percent absolute error (%AE) for each oil at

various temperatures ....................................................................................... 23

Table 2.7 Variables for equation 5 from Coupland and McClements (1997)................. 24 Table 2.8 Comparison of predicted density from Coupland and McClements (1997)

and current study predicted data ..................................................................... 24

Table 3.1 Current study interfacial tension of decane compared to literature data for

decane ............................................................................................................. 38

Table 3.2 Oil-Air interfacial tension for five common cooking oils for various

temperatures .................................................................................................... 39

Table 3.3 Y-intercept, slope and correlation coefficient ................................................. 40

Table 3.4 Oil-Steam interfacial tension for five common cooking oils for various

temperatures .................................................................................................... 41

Table 3.5 Variables for equation 5 from current study ................................................... 46

Table 3.6 Interfacial tension for four brands of soybean oil for various temperatures ... 47

Table 3.7 Results from equation 6 at room temperature and at 200°C ........................... 48

vi

LIST OF FIGURES

Figure 1.1 Schematic of Du Nouy ring method ............................................................... 5

Figure 1.2 Schematic of Wilhelmy Plate method............................................................. 5

Figure 1.3 Schematic of Pendant Drop ............................................................................ 6

Figure 1.4 Schematic of sessile drop ................................................................................ 7

Figure 1.5 Schematic of maximum bubble method ......................................................... 8

Figure 2.1 Density data for various oils from room temperature to 200°C (‹) soybean oil, (...) canola oil, (Ÿ) olive oil, (Y) peanut oil, (‘) corn oil...................... 18 Figure 2.2 Comparison of Peanut oil published densities. Mange and Wakeham (1944) (‹), current study (...), Subrahmanyam et al. (1994) (S) ................. 20 Figure 2.3 Comparisons of soybean oil published densities. Nouredinni et al. (1992) (‹), Current study (...), Mange and Skau (1945) (ǻ), Ackman and Eaton (1977) (Ÿ), Johnstone et al. (1940) (Y), Rice and Hamm

(1988) (|) ....................................................................................................... 21

Figure 2.4 published densities to experimental densities. Nouredinni et al. (1992) (‹), Current study (...), Ackman and Eaton

(1977) (Ÿ) .................................................................................................... 21

Figure 2.5

Noureddinni et al. (1992) (‹), Current study (...) ......................................... 22

Figure 3.1 Dimensions of drop ......................................................................................... 35

Figure 3.2 Environmental cell used to control environment during pendant

drop method .................................................................................................... 35

Figure 3.3 Advance goniometer model 300 and CCD digital camera for drop

image analysis ................................................................................................. 36

Figure 3.4 Schematic of instrumentation with steam flow ............................................... 37

Figure 3.5

air as the interface. Soybean oil (‹), Canola oil (†), olive oil (ʆ),

peanut oil (Y), corn oil (‘) ........................................................................... 40

vii

Figure 3.6 Interfacial tens

the interface. Soybean oil (‹), Canola oil (†), olive oil (ʆ), peanut

oil (Y), corn oil (‘) ....................................................................................... 41

Figure 3.7 Interfacial tension for corn oil from 100°C to 200°C with steam or air as the interface. Corn oil and air (‹), corn oil and steam (†) ........................... 42 Figure 3.8 Interfacial tension for peanut oil from 100°C to 200°C with steam or air as the interface. Peanut oil and air (‹), peanut oil and steam (†) ..................... 42 Figure 3.9 Interfacial tension for olive oil from 100°C to 200°C with steam or air as the interface. Olive oil and air (‹), olive oil and steam (†) ......................... 43 Figure 3.10 Interfacial tension for soybean oil from 100°C to 200°C with steam or air as the interface. Soybean oil and air (‹), soybean oil and steam (†) ............... 43 Figure 3.11 Interfacial tension for Canola oil from 100°C to 200°C with steam or air as the interface. Canola oil and air (‹), Canola oil and steam (†) .................... 44 experimental interfacial tension for Canola oil ............................................... 45 Figure 3.13 Interfacial tension for different brands of soybean oil from room temperature to 200°C with air as the interface. Carlini (‹), Crisco (†),

LouAna (ʆ), Wesson (‘).............................................................................. 47

1

CHAPTER 1

1.1 INTRODUCTION

In the last 25 years, there has been a significant increase in the number of obese adults in the United States [1.1]. In 1985, the majority of states had less than 10% obesity, but in 2009, every state had above 20% obesity except Colorado. Obesity has been connected to coronary heart disease by the American Heart Association [1.2]. Oil consumption has also been liked to health concerns such as coronary heart disease [1.3]. In the United States, fried food is a multibillion dollar industry that relies heavily on deep-fat frying [1.4]. Frying is the process of cooking foods in edible oils at temperatures above the boiling point of water [1.5]. After frying, a 1.5 mm thick potato slice may contain up to 35% oil [1.3, 1.6]. One way to decrease the oil content in fried foods would be to reduce the length of frying. During frying, a product goes through four stages [1.7]. In the second stage, the moisture content at the surface of the product is rapidly lost and crust formation occurs. The rate at which the moisture escapes is related to the interfacial tension between the oil and the steam. In order to decrease oil content, the relationship between frying temperatures and the interfacial tension between oil and steam needs to be understood. Interfacial tension (Ȗ) is the force acting at the interface of two liquids or between a liquid and a solid. Molecules aligning at the interface of each liquid and the interaction between the molecules are what affect interfacial tension. The interface can also be between a gas and a liquid however, this tension is termed surface tension (ı). The tension at the surface of a liquid drop in a gas is what gives the liquid its shape. The interfacial between two liquids (oil-water 24.4 mN/m) is less than the surface tension of the liquid (oil 32.6 2 mN/m) [1.8]. Interfacial tension is seen throughout the food industry and in nature manifesting in emulsion formation (mayonnaise) walk on water [1.9] to water beading up on a waxy leaf [1.10]. The ability for water skeeters to walk on water, oil and water to form mayonnaise, and a food to fry all have one thing in common. That is that interfacial tension plays a role in the ability for each to occur. Water skeeters are known as the insect that can walk on water. The ability for this to occur is because of two reasons. First, water skeeters have hydrophobic hairs that line the legs and allow for a high interfacial tension between the water skeeters and the water [1.9]. Second, water skeeters have long, slender legs that distribute their weight over a large surface area. The combination of these two attributes allows water skeeters to remain on the surface. Oil and water are immiscible liquids. However, they can form a stable mixture when blended with a surfactant. An example is the use of soap while cleaning hands [1.10]. When is because the interfacial tension between the oil and water is high. To remove the oil or grease, soap needs to be used. The soap is a surfactant and decreases the interfacial tension between the water and oil allowing for the oil to be removed from the hands. Mayonnaise is one of many foods that use an emulsion to make a final product. An emulsion is a stable mixture of two immiscible liquids [1.10]. One way to assist the formation of an emulsion is to reduce the liquid-liquid interfacial tension to a point where the immiscible liquids will form a stable mixture. An emulsifier, or a surfactant, reduces interfacial tension and can be added to the oil or water fraction. The emulsifier also plays a 3 role in the stability of the emulsion, in which it reduces the ability of the oil or water drops to coalescence. One way to reduce coalescence of an emulsion is by forming smaller drops of the dispersed phase within the continuous phase [1.10]. To aid in formation of smaller drops an emulsifier needs to be added to reduce the interfacial tension between the two liquids. Interfacial tension plays a role in heat and mass transfer during the frying process. During frying, moisture escapes the product as crust on the product is formed [1.7]. This moisture loss is dependent, in part, on the rate of heat transfer [1.5]. The rate of heat transfer during boiling may be related to interfacial tension by [1.11]: [1.1] Where ȝL is the liquid viscosity (Pa s), hfg is the latent heat of vaporization (J/kg), g is gravitational force (m/s2), ȡL ȡv) is the density difference between the liquid and vapor (kg/m3ıpL is the heat capacity for the liquid (J/K), (Ts Tsat) is the difference between the surface and saturated-liquid temperatures (K), csf is a coefficient that varies with surface-liquid combination, and PrL is the liquid Prandlt number. After frying, as the product cools, oil uptake occurs and is in part related to the tension between the food and the oil through capillary flow [1.12]: where h is the length of oil uptake (m), Ȗ is interfacial tension between product and oil

(mN/m), cos(ș is the contact angle between the oil and the product, ȡ is the density of the oil

(kg/m3), g is gravitational acceleration (m/s2) and r is the radius of the pore section [1.12]. 4 Interfacial tension is affected by temperature and composition. For many pure liquids, an increase in temperature causes a linear decrease in interfacial tension [1.10]. The equation [1.10]: where M is molar mass and ȡ is density. Also affecting interfacial tension is the presence of surfactants. Surfactants are ultimately used to decrease the interfacial tension between two liquids, a liquid and a solid, or a liquid and gas. Surfactants are amphiphiles that contain a hydrophobic tail and a hydrophilic head [1.10]. The balance between the two results in the hydrophilic-lipophilic balance (HLB). The higher the value the more hydrophilic the surfactant is and the lower the value, the more lipophilic the surfactant is. An HLB between 9 and 18 is desired for oil in water emulsions and a value between 3.5 and 6 is desired for water in oil emulsions [1.10]. For example, glycerol monostearate has an HLB of 3.8 which makes it a good surfactant for water in oil emulsion [1.10]. There are several common methods to measure interfacial tension. They may be divided into steady state and dynamic methods [1.10]. Steady state methods measure the interfacial tension when the system is at equilibrium and the interfacial tension is no longer changing. The Du Nouy ring, Wilhelmy plate, pendant drop and sessile drop can all measure steady state interfacial tension. The Du Nouy ring is a detachment method in which the required force to pull a ring through the interface is measured (Figure 1.1) [1.10]: 5

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