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Food and Beverage Chemistry/Biochemistry
Isothermal crystallization kinetics of palm oil as influenced by addition of a commercial phytosterol ester mixture Eva Daels, Bart Goderis, Valerie Matton, and Imogen Foubert J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b05049 • Publication Date (Web): 22 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018
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Journal of Agricultural and Food Chemistry
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Isothermal crystallization kinetics of palm oil as influenced by addition of a commercial
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phytosterol ester mixture
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Eva Daelsa, Bart Goderisb, Valerie Mattona and Imogen Foubert*a
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a
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Belgium and Leuven Food Science and Nutrition Research Centre (LFoRCe), Kasteelpark Arenberg 20
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box 2463, 3001 Leuven, Belgium.
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b
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*Contact details: +32 (0) 56 24 61 73,
[email protected] 9
Keywords: phytosterol esters, functional food, crystallization, X-ray diffraction
Research Unit Food and Lipids, KU Leuven Kulak, Etienne Sabbelaan 53 box 7659, 8500 Kortrijk,
KU Leuven Polymer Chemistry and Materials, Celestijnenlaan 200f box 2404, 3001 Leuven, Belgium.
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1. Abstract
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In literature there is a good agreement on the health-promoting effects of phytosterols. However
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addition of phytosterol esters (PEs) to lipid (containing food products) may influence its crystallization
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behavior. This study investigated the crystallization kinetics of palm oil (PO) after addition of PEs in
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high concentrations (≥ 10%). The isothermal crystallization of the PE-PO blends was analyzed at a
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temperature of 20°C and at a supercooling of 18.7°C using differential scanning calorimetry and time-
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resolved synchrotron X-ray diffraction. At increasing PE concentrations PO crystallization at an
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isothermal temperature of 20°C started later, was slower and less crystals were formed. Furthermore a
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delay in polymorphic transition from α to β’ was observed. When the blends were isothermally
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crystallized at a supercooling of 18.7°C only two of these effects remained: the delay in polymorphic
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transition and the decrease in crystalline content.
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2. Introduction
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Palm oil (PO) is derived from the mesocarp of the fruit of the oil palm tree (Elaeis guineensis). It has
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surpassed soybean oil to be the most consumed vegetable oil in the world because of its unique 1 ACS Paragon Plus Environment
Journal of Agricultural and Food Chemistry
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physical properties1. It is very stable in the β’ crystal form which is the most preferred polymorphic
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form in food products like margarines and shortenings because β’ crystals provide a smooth and glossy
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texture1,2. Because PO is semi-solid at room temperature, it forms the ideal hard stock to be used in
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trans free food products3. PO contains many different types of TAG, diacylglycerols, free fatty acids
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and other minor components and it exhibits different polymorphic forms. The polymorphism of a lipid
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refers to the packing arrangement of the molecules within the crystals. The α polymorphic form has the
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lowest thermodynamic stability but it has the ability to transform into the more stable β′ or β forms4.
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Polymorphism of PO during isothermal crystallization has been extensively studied by several
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authors3,5–8 who reported that the isothermal crystallization mechanism of PO depends on the degree of
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supercooling. At a high degree of supercooling, PO displays a two-step crystallization with the
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formation of α crystals in the first step, followed by a transformation into β’ crystals and additional
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formation of β’ crystals directly from the melt. At a lower degree of supercooling PO crystallizes
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directly into the β’ polymorph without the prior formation of an α polymorph. The reported value of the
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cut-off temperature between both mechanisms varies between 22 and 25°C probably because of a
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difference in experimental conditions on the one hand and a difference in PO composition on the other
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hand3.
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Phytosterols have recently gained much scientific and commercial interest due to the introduction of
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cholesterol-lowering foods enriched in plant sterols. Phytosterols contain over 250 different compounds
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including plant sterols and their hydrogenation products, plant stanols9. Since the chemical structure of
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phytosterols is very similar to that of cholesterol, an optimal intake of plant sterols and stanols
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decreases the intestinal cholesterol absorption and thereby reduces serum LDL cholesterol
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concentrations by 6-12%10. Phytosterols are already present in the normal diet for example in vegetable
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oils, cereal grains and nuts but the levels are too low to provide significant cholesterol-lowering
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properties11. The most efficient dietary tool to lower serum cholesterol levels are functional foods
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enriched with plant sterols. A meta-analysis on 124 studies showed that with an intake of phytosterols
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of 1.5-3 g/day a dose-dependent reduction in the blood levels of LDL-cholesterol of 7-12.5% can be 2 ACS Paragon Plus Environment
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Journal of Agricultural and Food Chemistry
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obtained10. Among the currently available sterol-based foods, spreads are by far the most common food
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type, followed by dairy foods and drinks12. The concentration of phytosterol esters (PEs) that is applied
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in the fat phase in these products is in the range of 10 to 50% depending on the product type. Evidently,
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with only 10% PE addition the chance of reaching the advised daily phytosterol intake remains rather
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low13. Food products with rather high PE loading therefore seem very relevant.
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In literature there is a good agreement on the health-promoting effects of phytosterols, however
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addition of PEs to lipid (containing food products), in particular at high amounts, may influence its
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crystallization behavior and may thus lead to problems occurring during the production process or with
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the macroscopic properties of the end product. Fundamental research on this topic is very scarce. In a
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previous study we examined the effects of the addition of a commercial PE mixture on the non-
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isothermal crystallization behavior of PO and constructed morphology maps that show the different
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polymorphic forms as a function of temperature and PE concentration during cooling and heating14. It
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was shown that the PEs crystallized separately from PO and formed two different ordered structures,
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namely the higher-melting truly crystalline structure named PEx which was composed of stearic and
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palmitic acid esterified PEs and the lower-melting liquid crystalline structure PEy which mainly
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contained phytosteryl oleate.
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These results enabled research on the impact of adding PEs to PO under industrially more relevant
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crystallization conditions. More specifically we investigated the influence of PE addition on the
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‘equilibrium’ crystallization state of PO in order to evaluate whether PE addition gives desired
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properties to the end product15. It was found that PE addition has a major influence on each level of the
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crystalline structural hierarchy (primary crystallization, polymorphism, network formation and textural
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properties). Next to the effects related to the final ‘equilibrium’ state, it would be interesting to verify
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whether PE addition also influences the crystallization kinetics of PO. Diluting PO with PE is expected
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to delay or slow down the PO crystallization. The extent of this delay and the impact on the rate of
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polymorphic transitions is however, less predictable. To the best of our knowledge the effect of adding
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PEs on the isothermal PO crystallization kinetics has not been reported in literature before. More 3 ACS Paragon Plus Environment
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generally, the effect of addition of many other types of lipids like mono- and diglycerides on the
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crystallization kinetics of different edible oils and lipids has been studied, but mostly
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concentrations (< 10%) since these minor components can already influence the kinetics at a
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concentration