Mango kernel fat based chocolate fat with heat resistant triacylglycerols: production via blending using mango kernel fat mid-fraction and palm mid-fractions produced in different fractionation paths

Abstract

Cocoa butter alternatives (CBAs) with heat resistance triacylglycerols (HRTs, including POP, POSt and StOSt) are increasingly popular in warm regions for producing hard chocolate. Three dominant palm mid fractions (PMFs), i.e., PMF-I from palm olein, PMF-II from the palm stearin and PMF-III from PMF-I, were found to be characterized by significant differences in physicochemical characterization and were selected as POP-rich fats. Mango kernel fat mid-fraction (MMF), a potential improver to increase the thermostability of chocolate, was enriched with POSt and StOSt and blended with PMFs in weight ratios of 10 : 90, 20 : 80 and 30 : 70 to prepare heat resistant CBAs. Non-fractionated mango kernel fat (MKF) was also mixed with PMFs in appropriate weight ratios as already reported to be contrasts. Chemical compositions and physical properties were analyzed to evaluate the qualities of the blends. The results revealed that non-fractionated MKF and partial fractionated fats PMF-I and PMF-II, contained higher levels of StOO, POO, OOO, PLP, PPP or diacylglycerols compared with those of cocoa butter (CB). Such ingredients would greatly change the thermal properties and soften the blends. While multi-stage fractionated PMF-III and MMF showed relatively high content of HRTs (83.3%), the blend with ratio 10 : 90 (PMF-III : MMF) was demonstrated to be best compared to the others because it resembled CB. It was recommended as the heat resistant CBA due to its improved thermal quality compared with CB.

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Introduction

Chocolate is a healthy and romantic food that is in favor around the world, and its consumption has increased sustainably, especially in emerging markets.^1,2 However, there is a problem with it: in the summertime and in most warm tropical countries, chocolate melts.³ Soft fat constituents, i.e. cocoa butter (CB), might be responsible for this problem.^1,3 Furthermore, the short supply of CB has also resulted in increasing demands for other hard fat alternatives. Heat resistance triacylglycerols (HRTs) that are applied to chocolate usually refer to high-melting symmetrical monounsaturated glycerides. Of which, 1,3-dipalmitoyl-2-oleoyl-glycerol (POP), 1-palmitoyl-2oleoyl-3-stearoyl-glycerol (POSt) and 1,3-distearoyl-2-oleoyl-glycerol (StOSt) are the most important ingredients for HRTs.¹ Their melting points are 37.3, 36.9 and 44.0 °C, respectively, in their most stable forms, which make them not easily melt in tropical and subtropical countries.⁴

The fats with increased amounts of HRTs are POP-rich fats (palm oil) and POSt/StOSt-rich fats (mango kernel fat (MKF), sal fat, shea butter, kokum kernel fat, and illipe butter).¹ Many studies were carried out to prepare cocoa butter alternatives (CBAs) and other structural fats through interesterification or fractionation using abovementioned POP-rich fats and POSt/SOSt-rich fats.^1,5–7 But interesterification is prohibited by the European Union (Directive 2000/36/EC), and is thought to exert negative efforts on human health when involved in the acyl migration that stearic is added on the sn-2 position of a triacylglycerol molecule.^8,9 Increasing studies, thus, have been focused on tailored fat preparation by mixture of non-interesterified fats such as palm oil, shea butter, sal butter, illipe fats and their fractions.^1,10–12

Palm oil fractions, especially palm mid-fractions (PMFs), are the widely used POP-rich fats.¹¹ Various PMFs with different triglyceride compositions and partial triglyceride levels have been industrially produced by two- or three-stage fractionation from palm oil.¹³ They may greatly affect the quality of the blends. But it is a pity that there is little information about how different PMFs is classified according to the characteristics and how it interact with other fats or oils in the blends. Moreover, it is also interesting to investigate the maximum amount of PMF that can be added to StOSt-rich fats in CBA formulas.¹⁴

Compared with PMF, the mentioned tropical POSt/StOSt-rich fats, i.e., shea butter, sal butter and illipe fat, are more difficult to obtain.¹ They generally grow in West Africa and Southeast Asia, and are not always available and can be of poor quality.¹ Moreover, such tropical fats as sal butter might lead to poor compatibility (especially the eutectic) because they contain some long chain fatty acids that are not found in CB.^1,11 Therefore, it is increasingly important to produce suitable POSt/StOSt-rich fats with stable qualities and without having unusual fatty acids. Mango is one of the most well-known commercial tropical fruits and is widely grown throughout the world with more than 1000 varieties available.¹⁵ MKF has attracted considerable interest due to its triacylglycerol compositions consisting of 0.3–8.9% POP, 5.7–17.3% POSt and 38.1–66.3% StOSt, which make it suitable for the chocolate products.^16–18 Recently, researchers have prepared CBAs by blending of non- or partial-fractioned MKFs and other fats.^16–18 However, such alternatives usually consist of high levels of diacylglycerols (2.5–5.8%) that are differ from that in CB (1.1–2.8%).^19,20 Also, they contain certain amounts of low-melting triacylglycerols, e.g., 1-stearoyl-2,3-dioleoyl-glycerol (StOO, 10.8–23.3%), 1-palmitoyl-2,3-dioleoyl-glycerol (POO, 1.8–10.8%) and 1,2,3-trioleoyl-glycerol (OOO, 2.5–8.6%). Such constituents might make the thermal properties, crystallographic forms or microstructures of the mixed fats quite different from those of CB.^16,19,21,22 Recent studies show that MKF stearins with 13–16% of POSt and 53–59% of StOSt are the most commonly fractionated products and valuable ingredients in CBA formulation.^18,23 But in general, high-quality fats are produced by multi-stage fractionation based on the experience of palm oil fractionation.¹¹ Thus, various MKF fractions characterized by different ratios of POSt/StOSt have been produced by multi-stage process.^24,25 Similar with palm oil fractions, the mid-fraction of MKF usually comprises of unique triacylglycerols, but its characteristics and applications have not been reported.

Since the great output of PMFs (reaching 1.5 million tons in 2014) and mango fruits (China is the second best mango producer), and the huge chocolate market potential in China, we examined the feasibility of mango kernel fat mid-fraction (MMF) mixed with PMF to produce CBAs with high contents of HRTs.^26,27 In addition, blends of non-fractionated MKF and PMF were also prepared as contrasts. It is the first study to report the binary CBAs prepared by blending of different PMFs produced in different fractionation paths with MMF.

Experimental

Materials

The most common PMFs in current China, mainly including PMF-I, PMF-II and PMF-III, were obtained from Wilmar Group (Shanghai, China). They were produced by acetone fractionation from different palm fractions as shown in Fig. 1. Three different batches of every PMF were sampled, and all the samples were kept frozen at −18 °C prior to analysis. MMF was provided by Wuxi Xinyao Biological Engineering Technology Co., Ltd. (Wuxi, China). MKF was extracted and purified as Sonwai et al. described.¹⁶ Three commercial CBs were donated by Wilmar Group (Shanghai, China), China Oil & Foodstuffs Corporation (Beijing, China) and Jinsihou Group (Shanghai, China), respectively.

Fig. 1 Different palm mid-fractions produced from palm oil fractionation.

Triacylglycerol standards, including POP, POSt and StOSt, were purchased from Larodan Fine Chemicals AB (Malmö, Sweden). 1,2-Diolein, 1,3-diolein and 2-olein acylglycerols were obtained from Sigma-Aldrich Chemical Co. Ltd. (Shanghai, China). Other reagents were of analytical or HPLC grade and were provided by Sinopharm Chemical Regent (Shanghai, China).

Blending of cocoa butter alternatives

The binary CBA blends were prepared by blending selected PMF, which were characteristic of significantly different characterizations, with MMF in weight ratios of 10 : 90, 20 : 80 and 30 : 70. MKF was also mixed with PMF to serve as contrasts. The best blends (PMF/MKF, 25/75 to 30/70) were produced according to Jahurul et al. reported.¹⁷

Determination of characterizations

Fat composition. Contents of triacylglycerol, diacylglycerol and monoacylglycerol were detected by a normal-phase high-performance liquid chromatographic system (Waters 1525, Waters, USA) equipped with an evaporative light scattering detector (ELSD of Allecth 3300, Alltech, USA) whose drift tube temperature reached 55 °C at a gas flow rate of 1.8 L min⁻¹. The fat composition was separated by a silica column (5 μm, 4.6 × 250 mm, Sepax, USA) with the oven temperature of 30 °C and were eluted with a binary gradient of hexane/isopropanol (A: 98/2, v/v) and hexane/isopropanol/acetic acid (B: 1/1/0.01, v/v/v) at a flow rate of 1.0 mL min⁻¹. The gradient elution program was performed as follows: solvent A was decreased from 100% to 80% over 10 min and decreased further to 70% from 10 to 14 min, and then was increased to 100% within 1 min and held lastly for 5 min. Samples were diluted by the solvent A to 0.5 mg mL⁻¹ with the injection volume of 5 μL. Triacylglycerol (the mixture of POP, POSt and StOSt), 1,2-diolein, 1,3-diolein and 2-olein acylglycerols were used as external standards to identify the peaks of triacylglycerol, diacylglycerol and monoacylglycerol, and to calculate their contents. In addition, free fatty acid level was obtained by titration according to the AOCS Official Method Ca 5a-40.²⁸

Triacylglycerol composition. Triacylglycerol composition was determined based on the AOCS Official Method Ce 5c-93.²⁸ A high-performance liquid chromatograph (Agilent 1200, Agilent, USA) equipped with a refractive index detector and two LiChroCART 18e columns (5 μm, 4.6 × 250 mm each, Merck, Germany) were used for the analysis. Samples were eluted with an isocratic solvent mixture of acetone/acetonitrile (75 : 25, v/v) at a low rate of 1.0 mL min⁻¹, and the concentration was 30 mg mL⁻¹ with 20 μL of the injection volume. Triacylglycerols were identified by the equivalent carbon numbers, and by comparing the retention time of triacylglycerol standards, i.e., POP, POSt and StOSt. Composition was reported in terms of the relative proportion.

Melting and crystallization characteristics. Melting and crystallization properties were obtained with a differential scanning calorimetry instrument (DSC Q200, TA, USA) according to the AOCS Official Method Cj 1-94.²⁸ In brief, samples (8.0 ± 0.5 mg) were rapidly heated to 80 °C and were held for 3 min to eliminate their thermal memory. The crystallization profiles were generated by cooling the samples from 80 to −40 °C at 10 °C min⁻¹ and holding for 5 min, then the melting profiles were obtained by heating the samples to 80 °C at 5 °C min⁻¹.

Solid fat content (SFC). SFC was measured using a pulse nuclear magnetic resonance (AM4000, Oxford, UK) according to the AOCS Official Method Cd 16b-93.²⁸ Melted samples (3.0 ± 0.1 g) were placed into the tubes and were held at 80 °C for 60 min. Subsequently, SFC was determined at 0, 5, 10, 15, 20, 25, 30, 35 and 40 °C and was maintained for 30 min at each setting temperature prior to measurement.

Slip melting points (SMP) and iodine values (IV). SMP was determined according to the AOCS Official Method Cc 1-25, and IV was analyzed according to the AOCS Official Method Cd 1-25.²⁸

Statistical analysis

All mentioned analyses were carried out in triplicates. Analysis of variance (ANOVA) and a LSD test were performed using the SPSS program version 19.0. A p < 5% was considered significant difference.

Results and discussion

Characterizations of palm mid-fractions, mango kernel fat mid-fraction and mango kernel fat

As shown in Fig. 1, PMF-I, PMF-II and PMF-III were acetone fractionated from palm olein, palm stearin and PMF-I, respectively. PMF with different characterizations will be selected as POP-rich fats in present study. The physical properties, i.e., IVs, SMPs and fat compositions of them, are shown in Table 1. All PMFs with low free fatty acids (<0.18%) and similar monoacylglycerol levels (0.3–0.4%) were found to have different contents of triacylglycerols and diacylglycerols. The triacylglycerol in PMF-III (98.4%) was significantly higher than that in PMF-I and PMF-II (94.2–94.6%), while the diacylglycerol showed the opposite result (1.3% in the former, and 4.0–4.3% in the later). Acetone is considered as an ideal solvent to selectively crystallize symmetrical triacylglycerols, which results in polar compositions tend to be enriched in the olein during the fractionation.²⁹ Kang et al.³⁰ also demonstrate that the acetone fractionated stearin contained only 1.5% of diacylglycerol, while the content in olein was higher reaching 5.9%.

Table 1 Characterisations of palm mid-fractions, mango kernel fat mid-fraction and mango kernel fat^a

	PMF-I	PMF-II	PMF-III	MMF	MKF
a PMF, palm mid-fraction; MMF, mango kernel fat mid-fraction; MKF, mango kernel fat; IV, iodine value; SMP, slip melting point; P, palmitic; St, stearic; O, oleic; L, linoleic; A, arachidic; U, unsaturated fatty acid; S, saturated fatty acid.b Mean ± SD (n = 3); values followed by different letters in the same row differed beyond a 5% significance.c tr, trace, <0.05%.
IV (g/100 g)	48.5 ± 0.3^a^b	45.0 ± 0.5^b	31.3 ± 0.8^c	32.9 ± 1.2^c	45.5 ± 0.8^b
SMP (°C)	26.1 ± 0.1^a	33.9 ± 0.9^b	33.1 ± 0.2^c	29.8 ± 0.3^a	30.1 ± 0.8^bc
Fat composition (%)
Triacylglycerol	94.6 ± 1.0^a	94.2 ± 1.9^a	98.4 ± 0.5^b	98.4 ± 0.3^b	96.7 ± 0.5^b
Diacylglycerol	4.3 ± 0.3^a	4.0 ± 0.3^a	1.3 ± 0.1^b	1.0 ± 0.1^b	2.1 ± 0.1^c
Monoacylglycerol	0.3 ± 0.1^a	0.3 ± 0.1^a	0.4 ± 0.0^a	0.4 ± 0.1^a	0.4 ± 0.1^a
Free fatty acid	0.18 ± 0.03^a	0.08 ± 0.01^b	0.04 ± 0.01^b	0.24 ± 0.03^a	0.16 ± 0.08^a
Triacylglycerol composition (%)
PLL	1.7 ± 0.3	1.5 ± 0.1	tr^c	tr	tr
OOL	1.6 ± 0.4	1.3 ± 0.3	tr	tr	tr
PLO	7.0 ± 0.5	5.5 ± 0.5	0.8 ± 0.2	1.2 ± 0.2	0.7 ± 0.2
PLP	8.3 ± 0.3	8.6 ± 0.4	6.3 ± 0.6	1.1 ± 0.1	tr
MOP	1.8 ± 0.2	1.7 ± 0.2	1.7 ± 0.3	tr	tr
OOO	3.0 ± 0.9	2.4 ± 0.4	0.6 ± 0.3	2.4 ± 0.7	6.4 ± 0.6
POO	16.9 ± 0.5	14.2 ± 1.2	2.9 ± 0.1	0.5 ± 0.4	4.1 ± 0.6
POP	40.4 ± 3.7	44.9 ± 2.1	65.0 ± 3.9	8.4 ± 0.7	3.5 ± 0.2
PPP	1.6 ± 0.1	5.1 ± 0.9	2.8 ± 0.5	tr	tr
StOO	2.1 ± 0.4	1.8 ± 0.3	0.5 ± 0.2	1.6 ± 0.2	21.9 ± 0.2
POSt	8.4 ± 0.3	8.2 ± 0.3	13.0 ± 0.2	38.3 ± 0.5	14.6 ± 0.6
StOSt	1.2 ± 0.3	1.0 ± 0.1	1.4 ± 0.2	38.9 ± 0.3	43.9 ± 0.8
StOA	tr	tr	tr	1.3 ± 0.3	1.6 ± 1.3
Total-UUU	4.6 ± 1.3^a	3.7 ± 0.6^a	0.6 ± 0.3^b	2.4 ± 0.7^d	6.1 ± 0.2^c
Total-SUU	27.7 ± 1.7^a	22.8 ± 1.9^b	4.2 ± 0.3^c	3.3 ± 0.4^c	26.6 ± 0.9^a
Total-SUS	60.1 ± 4.2^a	64.4 ± 1.8^a	87.5 ± 3.6^b	87.9 ± 1.2^b	63.3 ± 2.1^a
Total-SSS	1.6 ± 0.1^a	5.1 ± 0.9^b	2.8 ± 0.5^c	tr^d	tr^d

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IV that reflects the unsaturation of lipids were reported with the values of 45.0–48.5 g/100 g for PMF-I and PMF-II, and of 31.3 g/100 g for PMF-III. Triacylglycerols containing two or three molecules of unsaturated fatty acids, i.e. UUU (OOL and OOO) and SUU (PLL, PLO, POO and StOO), showed similar results (Table 1). On the contrary, higher values of SUS triacylglycerols were observed in PMF-III, while lower in PMF-I and PMF-II. In particular, highest HRT (mainly including POP and POSt) content was found in PMF-III with the value of 78.0%, while in other two PMFs only 48.8–53.1%. POP and POSt are easy to be selectively enriched in stearin by acetone fractionation has been demonstrated recently by several studies.^30,31 Therefore, a lower IV (less unsaturated) usually leads harder and better fat for CBA.

SMP represents temperatures at which columns of fats in open capillary tubes become fluid to run up the tubes when they subjected to controlled heating. PMF-II and PMF-III had higher SMPs with the values of 33.1–33.9 °C, followed by PMF-I with 26.1 °C. Compared with the lower one, high contents of high-melting triacylglycerols, mainly SUS (87.5% in PMF-III) and SSS (5.1% in PMF-II) in present study, were considered to be responsible for the significant difference.

PMFs obtained from different fractionation stages of palm oils were characterized by significant differences according to abovementioned properties. Therefore, all the three PMFs were studied as POP-rich fats in the CBA ingredients.

MMF and MKF were also found to be significant difference in terms of triacylglycerol compositions, IVs and SMPs (Table 1). The characterisation of the former was more similar to that of CB compared with the later. In particular, 6.4% OOO, 21.9% StOO, 14.6% POSt and 43.9% StOSt were the major triacylglycerols of MKF, while 8.4% POP, 38.3% POSt and 38.9% StOSt of MMF. POSt was enriched and StOSt was separated to some extent by the multi-stage fractionation, which made MMF become an ideal HRT-containing fat. The melting point difference between POSt and StOSt in the same polymorph form reaches 5–8 °C, making it feasible to separate the two triacylglycerols by fractionation especially in the solvent.^32,33 Similar research was also carried out to produce CB fraction with POP and POSt increasing from 54.7% to 60.0% and StOSt decreasing from 28.8% to 7.8% by acetone fractionation.³⁴

Triacylglycerol composition and iodine value of the blends

Triacylglycerols are the predominant form in lipids, the results of the binary blends are shown in Table 2. Similar to CB, the HRTs (POP, POSt and StOSt, total 73.5–83.3%) were demonstrated in the PMF and MMF blends, and were greatly influenced by the addition of PMF in MMF which lead to gradual increases in POP and decreases in POSt and StOSt, and eventually the three triacylglycerols tended to meet the demand of CB. For example, in PMF-I/MMF 10/90 the POP, POSt and StOSt values were of 12.4%, 35.5% and 34.9% respectively, and in PMF-I/MMF 30/70 they were of 17.2%, 29.1% and 27.2% respectively. The values were closer to that of CB (POP: 12.5–18.8%; POSt: 26.3–45.3%; StOSt: 19.2–37.2%). Similar results were also found in other blends except the PMF-III/MMF 30/70 whose POP level was higher reaching as high as 25.4%. POP is responsible for β′ form different from the β form of StOSt and may cause eutectic effects with the percentage of near 30% in the ternary phase of POP, POSt and StOSt.^35,36 For the blends of PMF and MKF, although POP and StOSt levels were similar with those of CB, the POSt (13.3–15.5%) was quite different from that of CB (26.3–45.3%). Blends of PMF and other non-fractionated MKF with the appropriate ratios were also shown <15% POSt content.^16,17

Table 2 Triacylglycerol composition and iodine value of the blends and cocoa butter^a

Samples	Triacylglycerol composition (%)									IV (g/100 g)
	PLO	PLP	POO	OOO	POP	PPP	StOO	POSt	StOSt
a PMF, palm mid-fraction; MKF, mango kernel fat; MMF, mango kernel fat mid-fraction; CB, cocoa butter; IV, iodine value; P, palmitic; St, stearic; O, oleic; L, linoleic.b CB, cocoa butters were obtained from Wilmar Group, China Oil & Foodstuffs Corporation and Jinsihou Group, respectively.c Sonwai, 2014;^14 Shahidi, 2005;^17 Gunstone, 2011 (ref. 20).d tr, trace, <0.05%.e —, not measured.
PMF-I/MMF 10/90	1.8 ± 0.0	1.7 ± 0.2	0.4 ± 0.3	2.1 ± 0.0	12.4 ± 0.9	0.1 ± 0.1	1.9 ± 0.1	35.5 ± 0.9	34.9 ± 0.9	35.0 ± 1.3
PMF-I/MMF 20/80	2.4 ± 0.3	2.2 ± 0.1	3.5 ± 0.3	2.4 ± 0.0	14.5 ± 0.5	0.4 ± 0.1	1.9 ± 0.1	32.2 ± 0.5	31.3 ± 0.3	35.8 ± 2.1
PMF-I/MMF 30/70	3.4 ± 0.4	3.1 ± 0.1	5.3 ± 0.8	2.7 ± 0.2	17.2 ± 0.1	0.5 ± 0.1	1.9 ± 0.1	29.1 ± 0.0	27.2 ± 0.8	36.2 ± 2.1
PMF-II/MMF 10/90	1.4 ± 0.1	1.7 ± 0.0	1.6 ± 0.1	2.1 ± 0.1	12.7 ± 0.5	0.4 ± 0.2	1.8 ± 0.0	34.6 ± 0.4	33.9 ± 1.5	34.3 ± 1.3
PMF-II/MMF 20/80	1.9 ± 0.1	2.4 ± 0.1	3.0 ± 0.1	2.0 ± 0.2	16.3 ± 0.5	1.0 ± 0.2	1.8 ± 0.1	31.6 ± 0.4	30.0 ± 1.1	34.9 ± 1.0
PMF-II/MMF 30/70	2.5 ± 0.5	2.9 ± 0.1	4.2 ± 0.3	2.2 ± 0.2	20.0 ± 1.0	1.9 ± 0.1	1.9 ± 0.1	29.1 ± 0.1	27.0 ± 0.7	35.5 ± 0.2
PMF-III/MMF 10/90	1.0 ± 0.1	1.6 ± 0.2	0.6 ± 0.1	1.9 ± 0.1	13.6 ± 0.5	0.3 ± 0.1	1.8 ± 0.1	35.6 ± 0.4	34.1 ± 1.7	33.2 ± 0.6
PMF-III/MMF 20/80	1.0 ± 0.0	1.9 ± 0.2	0.7 ± 0.1	1.8 ± 0.1	19.2 ± 0.4	0.5 ± 0.1	1.7 ± 0.1	33.0 ± 0.0	31.0 ± 1.4	32.6 ± 1.4
PMF-III/MMF 30/70	1.0 ± 0.0	2.7 ± 0.2	1.0 ± 0.2	1.6 ± 0.0	25.4 ± 0.5	0.8 ± 0.1	1.5 ± 0.0	30.8 ± 0.5	27.0 ± 0.9	32.0 ± 0.1
PMF-I/MKF 30/70	2.5 ± 0.3	2.7 ± 0.2	7.6 ± 0.7	5.3 ± 0.5	14.5 ± 0.7	0.6 ± 0.2	15.6 ± 0.8	13.4 ± 0.9	30.5 ± 0.9	47.5 ± 0.6
PMF-II/MKF 30/70	2.0 ± 0.0	2.6 ± 0.1	7.3 ± 0.4	5.0 ± 0.1	15.8 ± 1.2	1.4 ± 0.6	15.3 ± 0.7	13.3 ± 0.9	30.9 ± 0.2	46.0 ± 0.5
PMF-III/MKF 25/75	0.9 ± 0.1	1.8 ± 0.1	3.5 ± 0.5	4.5 ± 0.7	18.1 ± 0.2	0.5 ± 0.4	15.9 ± 0.9	15.5 ± 1.8	32.4 ± 1.2	40.9 ± 1.2
CB^b	1.0 ± 0.0	1.6 ± 0.2	3.3 ± 0.1	0.4 ± 0.0	18.8 ± 0.0	tr^d	3.0 ± 0.6	43.4 ± 0.6	25.6 ± 0.5	31.3 ± 0.6
CB^c	0.3–2.6	1.8–8.4	1.3–10.0	0.6–2.2	12.5–18.0	—^e	3.9–11.3	26.3–45.3	19.2–37.2	34.2–40.7

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Table 2 also shows IVs of the blends, the values of the PMF and MMF blends increased with the PMF-I or PMF-II increase in their respective blends, whereas opposite trend was observed in the PMF-III blends. The significantly higher SUU and UUU contents in PMF-I and PMF-II and lower values in PMF-III could be responsible for the results. It is worth mentioning that the difference in IVs ranged from 32.0–36.2 g/100 g among mentioned blends were small and were in agreement with that of CB (31.3–40.7 g/100 g). However, the IVs of PMF and MKF blends, i.e., 40.9–47.5 g/100 g, went beyond that of CB due to the higher StOO content in MKF. Only the PMF-III/MKF 25/75 with the value of 40.9 was closest to CB.

Thermal behaviors of the blends

SMP was involved in evaluating the applied ranges of the blends. As shown in Table 3, addition of 10–30% of PMF into MMF caused a slight decrease in the SMPs for the blends, from 26.1–29.9 to 25.1–28.1 °C, as a result of increasing in the contents of the low-melting point triacylglycerols mainly including PLO and POO in PMF (Table 2). Highest SMP was shown by the blends consisting of 10% PMF and 90% MMF, which indicated a better heat resistance compared with CB (25.3–26.1 °C). SMPs were found in PMF and MKF blends with the values of 25.8–27.7 °C were lower than those of PMF and MMF blends, but they were also close to that of CB. However, SMP has its own limitation and can only be used as a complementary parameter for identifying the specific use in general.³⁷ Melting and crystallization properties, therefore, were further discussed in this section.

Table 3 Slip melting point, melting and crystallization characteristics of the blends and cocoa butter^a

Samples	SMP (°C)	Melting property			Crystallization property
		Onset temp. (°C)	Offset temp. (°C)	Enthalpy (W g^−1)	Onset temp. (°C)	Offset temp. (°C)	Enthalpy (W g^−1)
a PMF, palm mid-fraction; CB; cocoa butter; SMP, slip melting point; MKF, mango kernel fat; MMF, mango kernel fat mid-fraction.b CB, cocoa butters were obtained from Wilmar Group, China Oil & Foodstuffs Corporation and Jinsihou Group, respectively.
PMF-I/MMF 10/90	26.1 ± 0.3	15.4 ± 0.5	31.0 ± 2.6	78.3 ± 3.4	18.1 ± 1.0	−20.5 ± 1.6	78.5 ± 1.3
PMF-I/MMF 20/80	25.9 ± 0.5	16.4 ± 0.8	27.8 ± 1.6	72.3 ± 1.3	17.1 ± 1.0	−20.9 ± 2.4	71.7 ± 2.1
PMF-I/MMF 30/70	25.1 ± 0.9	15.3 ± 1.0	27.1 ± 1.0	70.0 ± 1.8	16.3 ± 1.0	−23.0 ± 3.2	70.6 ± 2.1
PMF-II/MMF 10/90	29.3 ± 0.5	15.7 ± 0.5	29.2 ± 1.8	76.5 ± 2.5	18.2 ± 1.1	−18.5 ± 2.8	76.6 ± 1.7
PMF-II/MMF 20/80	28.7 ± 0.9	16.1 ± 1.1	28.0 ± 1.9	75.1 ± 1.3	17.0 ± 1.5	−21.3 ± 2.5	72.9 ± 2.0
PMF-II/MMF 30/70	28.1 ± 1.1	15.9 ± 0.2	28.2 ± 1.9	71.7 ± 2.8	16.6 ± 0.7	−21.6 ± 3.2	69.3 ± 4.2
PMF-III/MMF 10/90	29.9 ± 0.3	16.3 ± 1.0	29.7 ± 1.5	77.5 ± 1.3	18.2 ± 0.9	−16.4 ± 4.1	78.1 ± 2.2
PMF-III/MMF 20/80	28.9 ± 1.0	15.8 ± 0.7	28.2 ± 2.4	75.9 ± 2.8	17.2 ± 1.1	−17.9 ± 2.0	73.3 ± 1.7
PMF-III/MMF 30/70	27.7 ± 0.8	15.2 ± 0.9	27.8 ± 0.7	78.0 ± 1.5	15.9 ± 1.6	−19.1 ± 4.7	78.2 ± 1.9
PMF-I/MKF 30/70	25.8 ± 1.2	11.6 ± 1.6	28.3 ± 1.5	40.8 ± 2.2	14.7 ± 1.4	−20.2 ± 2.2	57.9 ± 2.1
PMF-II/MKF 30/70	27.7 ± 0.8	12.7 ± 1.4	26.2 ± 1.2	32.4 ± 3.1	15.5 ± 1.2	−18.8 ± 2.0	58.7 ± 2.1
PMF-III/MKF 25/75	27.1 ± 0.7	11.5 ± 2.0	28.7 ± 1.0	60.6 ± 2.5	15.9 ± 1.5	19.2 ± 1.6	65.3 ± 3.3
CB^b	25.3–26.1	13.0–14.9	26.8–28.7	58.7–75.2	15.7–18.7	−19.2–14.5	63.3–72.9

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Fig. 2(A) shows the melting profiles of the blends and CB. The profiles represent an indication of the amount of crystallized fats and occurrence of polymorphic transitions.³⁸ A single melting peak was started at 15.4–16.3 °C and ended at 29.2–31.0 °C in the blends with 10 : 90 of PMF and MMF, whereas 15.2–16.4 °C and 27.1–28.2 °C were observed in other blends with higher PMF ratios. The offset temperatures of the PMF and MMF blends containing 10% PMF were higher than those of the blends with 20% and 30% PMF. The trend was consistent with the reported SMPs, for the influence of the low-melting point triacylglycerols. There were also gradual decreases in the melting enthalpies of the blends in general, as the PMF increased. This also indicated the better heat-resistant blends obtained by adding moderate amounts of PMF. However, the melting profiles of the PMF and MKF blends with broader melting ranges were different from those of PMF and MMF blends. In particular, the PMF-I/MKF 30/70 and PMF-II/MKF 30/70 began to show two peaks and have lower melting enthalpies, indicating that the fats had high levels of undesirable triacylglycerols (mainly including POO, OOO and StOO) with melting temperatures which is different from CB. Only the PMF-III/MKF 25/75 showed similar melting profile and melting enthalpy (60.6 W g⁻¹) compared with that of CB.

Fig. 2 Melting (A) and crystallization (B) profiles of the blends and cocoa butter. PMF, palm mid-fraction; MMF, mango kernel fat mid-fraction; MKF, mango kernel fat; CB, cocoa butter.

Similar results were found in the crystallization properties of the blends as shown in Table 3. However, the crystallization profile (Fig. 2(B)) of every PMF and MMF blend showed a greater peak with a slight shoulder on its right, and the shoulder was increased with the addition of PMF. High-melting components increase, i.e., tri-saturated triglycerides (such as PPP) and diacylglycerols, might be responsible for the increasing shoulder (Table 2).³⁹ For the blends with 10% PMF and 90% MMF addition, the shoulders which were similar with those of CB were actually quite small, as it is shown in Fig. 2(B). Other high-melting triglycerides, especially StOSt with polymorphic phases (the main triglyceride of CB), might be responsible for the shoulders in such samples. The effect of solid transformation occurred in the β form, i.e., from stable β2 form to more stable β1 form, might be enhanced by a higher StOSt level.^1,35 Therefore, the blends consisting of 10% PMF and 90% MMF were more suitable to serve as an ideal heat resistant CBA sources according to their SMP, melting and crystallization properties. Conversely, the blends of PMF and MKF showed single crystallization peaks that were similar with CB, but they had lower crystallization temperatures and enthalpies, indicating they were more suitable to be soft chocolate materials. The results corresponded to the abovementioned melting behaviors.

Solid fat content of the blends

SFC is related to textural (hardness, softness and melting behavior) and sensorial properties of lipids.^39,40 The SFC is suggested to be high value (>60%) at <20 °C, while be 0% at body temperature for most of CB.²⁰ As shown in Fig. 3, all the PMF and MMF blends with high values of 66.6–86.7% at 20 °C and no solid at 37 °C met this demand. The SFC profiles of blends containing PMF-I and PMF-II showed gradual decreases as temperature increased from 20–35 °C, while the PMF-III blends showed steep decreases which were closer to that of CB. Complex triacylglycerols presented in PMF-I and PMF-II blends (mainly including POP, POSt, PLO, POO, PLP and PPP) and relative simple in PMF-III (POP, POSt and PLP) might be responsible for this difference (Table 1). Moreover, owing to the higher SFC than CB, the PMF-III and MMF blends, especially the PMF-III/MMF 10/90 (in consideration of above characteristic analyses), could be used to increase the hardness of chocolate and therefore might be advantageous for use in warm climates. For the PMF and MKF blends, the SFC profiles were lower compared with that of CB and were more gradual decreases during the detected temperatures, which were mostly suitable for soft CBA or icing fat.⁴¹ The results were consistent with the thermal behavior analyses.

Fig. 3 Solid fat content of the blends and cocoa butter PMF, palm mid-fraction; MMF, mango kernel fat mid-fraction; MKF, mango kernel fat; CB, cocoa butter.

Conclusions

Significantly different triacylglycerol composition and diacylglycerol level were found in studied PMF, MMF and MKF, both of them were able to greatly affect the SMP, SFC, melting and crystallization properties of the binary blends. Fat without fractionation or with partial fractionation, including MKF, PMF-I and PMF-II, contained high levels of undesired ingredients such as StOO, POO, OOO, PLP, PPP or diacylglycerol. The lower melting ingredients (i.e., StOO, POO, OOO and PLP) would soften the blends. While the higher ones (i.e., PPP and diacylglycerols) changed the melting properties, which make them quite different from that of CB. Therefore, the blends of PMF and MKF showed lower melting properties and were more suitable for soft CBA to lower the viscosity at temper in the processing of chocolate. Multi-stage fractionated palm oil and MKF products, i.e., PMF-III and MMF in present study, were enriched in HRTs and contained less undesired ingredients. The blend of PMF-III and MMF in weight ratio of 10 : 90 showed improved SMP, SFC, melting and crystallization properties compared with CB, indicating it has a better heat resistance.

Acknowledgements

The research was financially supported by the Natural Science Foundation of Jiangsu Province (Grants No: BK 20150137). Program of Science and Technology Department of Jiangsu Province (Grants No: BY2016022-33) and Graduate Research and Innovation Projects in Jiangsu Province (Grants No: KYLX16_0825).

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