Subscriber access provided by UNIV OF DURHAM
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
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 39
Journal of Agricultural and Food Chemistry
1
Isothermal crystallization kinetics of palm oil as influenced by addition of a commercial
2
phytosterol ester mixture
3
Eva Daelsa, Bart Goderisb, Valerie Mattona and Imogen Foubert*a
4
a
5
Belgium and Leuven Food Science and Nutrition Research Centre (LFoRCe), Kasteelpark Arenberg 20
6
box 2463, 3001 Leuven, Belgium.
7
b
8
*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.
10
1. Abstract
11
In literature there is a good agreement on the health-promoting effects of phytosterols. However
12
addition of phytosterol esters (PEs) to lipid (containing food products) may influence its crystallization
13
behavior. This study investigated the crystallization kinetics of palm oil (PO) after addition of PEs in
14
high concentrations (≥ 10%). The isothermal crystallization of the PE-PO blends was analyzed at a
15
temperature of 20°C and at a supercooling of 18.7°C using differential scanning calorimetry and time-
16
resolved synchrotron X-ray diffraction. At increasing PE concentrations PO crystallization at an
17
isothermal temperature of 20°C started later, was slower and less crystals were formed. Furthermore a
18
delay in polymorphic transition from α to β’ was observed. When the blends were isothermally
19
crystallized at a supercooling of 18.7°C only two of these effects remained: the delay in polymorphic
20
transition and the decrease in crystalline content.
21
2. Introduction
22
Palm oil (PO) is derived from the mesocarp of the fruit of the oil palm tree (Elaeis guineensis). It has
23
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
Page 2 of 39
24
physical properties1. It is very stable in the β’ crystal form which is the most preferred polymorphic
25
form in food products like margarines and shortenings because β’ crystals provide a smooth and glossy
26
texture1,2. Because PO is semi-solid at room temperature, it forms the ideal hard stock to be used in
27
trans free food products3. PO contains many different types of TAG, diacylglycerols, free fatty acids
28
and other minor components and it exhibits different polymorphic forms. The polymorphism of a lipid
29
refers to the packing arrangement of the molecules within the crystals. The α polymorphic form has the
30
lowest thermodynamic stability but it has the ability to transform into the more stable β′ or β forms4.
31
Polymorphism of PO during isothermal crystallization has been extensively studied by several
32
authors3,5–8 who reported that the isothermal crystallization mechanism of PO depends on the degree of
33
supercooling. At a high degree of supercooling, PO displays a two-step crystallization with the
34
formation of α crystals in the first step, followed by a transformation into β’ crystals and additional
35
formation of β’ crystals directly from the melt. At a lower degree of supercooling PO crystallizes
36
directly into the β’ polymorph without the prior formation of an α polymorph. The reported value of the
37
cut-off temperature between both mechanisms varies between 22 and 25°C probably because of a
38
difference in experimental conditions on the one hand and a difference in PO composition on the other
39
hand3.
40
Phytosterols have recently gained much scientific and commercial interest due to the introduction of
41
cholesterol-lowering foods enriched in plant sterols. Phytosterols contain over 250 different compounds
42
including plant sterols and their hydrogenation products, plant stanols9. Since the chemical structure of
43
phytosterols is very similar to that of cholesterol, an optimal intake of plant sterols and stanols
44
decreases the intestinal cholesterol absorption and thereby reduces serum LDL cholesterol
45
concentrations by 6-12%10. Phytosterols are already present in the normal diet for example in vegetable
46
oils, cereal grains and nuts but the levels are too low to provide significant cholesterol-lowering
47
properties11. The most efficient dietary tool to lower serum cholesterol levels are functional foods
48
enriched with plant sterols. A meta-analysis on 124 studies showed that with an intake of phytosterols
49
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
Page 3 of 39
Journal of Agricultural and Food Chemistry
50
obtained10. Among the currently available sterol-based foods, spreads are by far the most common food
51
type, followed by dairy foods and drinks12. The concentration of phytosterol esters (PEs) that is applied
52
in the fat phase in these products is in the range of 10 to 50% depending on the product type. Evidently,
53
with only 10% PE addition the chance of reaching the advised daily phytosterol intake remains rather
54
low13. Food products with rather high PE loading therefore seem very relevant.
55
In literature there is a good agreement on the health-promoting effects of phytosterols, however
56
addition of PEs to lipid (containing food products), in particular at high amounts, may influence its
57
crystallization behavior and may thus lead to problems occurring during the production process or with
58
the macroscopic properties of the end product. Fundamental research on this topic is very scarce. In a
59
previous study we examined the effects of the addition of a commercial PE mixture on the non-
60
isothermal crystallization behavior of PO and constructed morphology maps that show the different
61
polymorphic forms as a function of temperature and PE concentration during cooling and heating14. It
62
was shown that the PEs crystallized separately from PO and formed two different ordered structures,
63
namely the higher-melting truly crystalline structure named PEx which was composed of stearic and
64
palmitic acid esterified PEs and the lower-melting liquid crystalline structure PEy which mainly
65
contained phytosteryl oleate.
66
These results enabled research on the impact of adding PEs to PO under industrially more relevant
67
crystallization conditions. More specifically we investigated the influence of PE addition on the
68
‘equilibrium’ crystallization state of PO in order to evaluate whether PE addition gives desired
69
properties to the end product15. It was found that PE addition has a major influence on each level of the
70
crystalline structural hierarchy (primary crystallization, polymorphism, network formation and textural
71
properties). Next to the effects related to the final ‘equilibrium’ state, it would be interesting to verify
72
whether PE addition also influences the crystallization kinetics of PO. Diluting PO with PE is expected
73
to delay or slow down the PO crystallization. The extent of this delay and the impact on the rate of
74
polymorphic transitions is however, less predictable. To the best of our knowledge the effect of adding
75
PEs on the isothermal PO crystallization kinetics has not been reported in literature before. More 3 ACS Paragon Plus Environment
Journal of Agricultural and Food Chemistry
Page 4 of 39
76
generally, the effect of addition of many other types of lipids like mono- and diglycerides on the
77
crystallization kinetics of different edible oils and lipids has been studied, but mostly
78
concentrations (< 10%) since these minor components can already influence the kinetics at a
79
concentration
