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. 2020 Apr 28;57(12):4404–4413. doi: 10.1007/s13197-020-04477-4

Rambutan (Nephelium lappaceum) kernel olein as a non-hydrogenated fat component for developing model non-dairy liquid creamer: effect of emulsifier concentration, sterilization, and pH

Tanawan Wanthong 1, Utai Klinkesorn 1,✉
PMCID: PMC7550482  PMID: 33087954

Abstract

In this work, the effects of the emulsifier concentration, sterilization process, and pH on the properties and stability of the model liquid creamer were evaluated. Applying diacetyl tartaric acid ester of mono- and diglycerides or DATEM at a concentration of 0.3% (w/w) in the presence of 2% (w/w) sodium caseinate produced stable model liquid creamers (10% (w/w) rambutan kernel olein) with a small particle size (Z-average ≈ 200 nm) and a narrow size distribution range (PDI < 0.24). These creamers were stable regarding creaming and coalescence, having non-flocculated particles and a constant flow behavior index (n) after sterilization using autoclaving (121 °C, 1.1 bar for 15 min) and during storage for 150 days at 25 °C. The model liquid creamers were unstable at pH values near the isoelectric point of caseinate (pH 4–5). However, these were stable after mixing with hot coffee solutions based on no observed feathering or sedimentation. The whitening performance of the model liquid creamers compared well with commercial ones. Non-hydrogenated fat-based model non-dairy liquid creamer was successfully formulated using rambutan kernel olein as a fat component. The results obtained in this study are useful for the possible application of fractionated rambutan kernel fat in food products.

Keywords: Rambutan kernel olein, Non-dairy liquid creamer, Autoclave sterilization, Emulsion stability, Whitening performance

Introduction

Non-dairy creamers, also called coffee creamers, coffee whiteners or coffee lighteners, are formulated systems homologous in functionality and attributes to ordinary dairy products (Golde and Schmidt 2005; O’Brien 2009). These are widely used to whiten beverages such as coffee, cocoa and tea, soften the acidic taste and impart a desired flavor and texture (Golde and Schmidt 2005; Sher et al. 2013). Non-dairy creamers can be in powder, frozen and liquid forms. Dominant advantages of liquid creamers over other creamer forms include that these more closely simulate dairy creamers and provide a homogeneous beverage (Sher et al. 2013). Typically, non-dairy liquid creamers comprise vegetable fat, protein (sodium caseinate or soy protein isolate), carbohydrates (maltodextrin or corn syrup solid), emulsifiers or stabilizers, buffering salt and other food additives (sweetener, flavor and color) (O’Brien 2009; Sher et al. 2013).

The fat concentration in non-dairy liquid creamers can vary from 5 to 18% with melting temperatures of 35–42 °C (O’Brien 2009). The vegetable fats may comprise partially or fully hydrogenated oils with one or more of soybean oil, coconut oil, palm oil, palm kernel oil, corn oil, cottonseed oil, sunflower oil and blends thereof (Sher et al. 2013). However, using hydrogenated oils may have a negative impact on health, particularly partially hydrogenated oils (PHOs) as these oils are a source of trans-fat which has been reported as a factor that significantly increases the risk of cardiovascular disease (Dhaka et al. 2011). Moreover, the hydrogenation process of oils produces a number of spent metal catalyst wastes which could cause environment problems because of their hazardous nature (Hassan and Richter 2002). To overcome these concerns, non-hydrogenated vegetable fats contain natural saturated fatty acids could be an alternative ingredient for non-dairy creamer products.

To extract fat from plant materials, conventional solvent extraction and several emerging technologies, such as ultrasound, microwave, pulsed electric field, and high voltage electrical discharge either as a pretreatment method or assisted extraction are applied (Galanakis 2013). From the previous work, conventional solvent extraction was used to extract fat [approximately 33–37% (w/w)] from the rambutan (Nephelium lappaceum) seed kernel. The solvent-extracted fat is a white, soft solid at room temperature and contains oleic acid (C18:1; 36.8%) and arachidic acid (C20:0; 34.3%) as the main fatty acids (Sirisompong et al. 2011). The in vivo oral and skin irritation testing of rambutan fat has indicated that it is a non-toxic fat (Eiamwat et al. 2014). From our previous study (Mahisanunt 2016), the solvent fractionation of rambutan kernel fat successfully conducted and obtained the fractionated fats that had specific physical and chemical properties. Fractionation is a technique for changing the physical properties of fat by controlling the crystallization followed by physical separation of the low melting or liquid fraction (olein) from the high melting or solid crystalline fraction (stearin). Solvent (e.g. acetone, hexane, ethanol, and isopropanol) is often applied in fractionation to enhance the separation efficiency and purity of the fat fractions (Gibon 2006). The rambutan kernel olein fraction obtained from double acetone fractionation had a melting range of 39–43 °C and iodine values of 62.3. Its major fatty acids were oleic acid, arachidic acid, gondoic acid and stearic acid. This olein was soft semi-solid at room temperature which indicated its potential for use as an alternative fat to replace hydrogenated oils (Mahisanunt 2016).

Acceptable non-dairy liquid creamer should be stable during storage without creaming, gelation, sedimentation or color change. In addition, it should retain a uniform emulsion with constant viscosity over time. Therefore, we investigated the effect of the emulsifier concentration and sterilization process on the properties and stability of a model non-dairy liquid creamer prepared with rambutan kernel olein. Besides the stability, the creamers should also disperse quickly with no feathering or oiling off, provide a good whitening capacity, and have a delicate, superior flavor and taste when added to a beverage, such as coffee or tea (Golde and Schmidt 2005; Sher et al. 2013). Thus, the effects of pH and whitening performance of the model rambutan kernel olein-based liquid creamer were also evaluated by comparison with a commercial liquid creamer. From this study, model liquid creamer may suitably be used to whiten and soften the acidic taste of beverage products, especially coffee. Using rambutan kernel olein to replace hydrogenated fat may be a potential solution for reducing the processed trans-fat in foods.

Materials and methods

Rambutan seeds (Nephelium lappaceceum L.) were kindly donated by Malee Sampran Public Co., Ltd. (Nakhon Pathom, Thailand). Diacetyl tartaric acid ester of mono- and diglycerides (DATEM) made from edible fully hydrogenated rapeseed oil was a gift from Berli Jucker Public Co., Ltd. (Bangkok, Thailand). Maltodextrin [dextrose equivalent (DE) 19] and sodium caseinate [93.7% (w/w) protein] were purchased from Vicchi Enterprise Co., Ltd. (Bangkok, Thailand). Commercial liquid creamer (~ 11% (w/w) fat) and dark roast coffee powder were purchased from a local supermarket. Analytical grade di-potassium phosphate and sodium azide (NaN3) were products of Ajax Finechem (New South Wales, Australia). Acetone and petroleum ether were products of Merck (Darmstadt, Germany) and Avantor Performance Materials (Pennsylvania, USA), respectively.

Extraction of rambutan kernel fat

The rambutan kernel fat was extracted according to the method modified from Sirisompong et al. (2011). After removing the seed shell, the kernels were dried in a hot-air tray dryer at 65 °C for 6 h [final moisture content ~ 7% (w/w)]. These dried kernels were then ground into fine particles using a blender (MX-T210GN, National Co., Thailand). The ground-dried kernels were put into an extraction bag and transferred into Soxhlet extraction equipment (Glassco Laboratory Equipments Pvt. Ltd., India). The fat was extracted using petroleum ether for 8 h with a kernel-to-solvent ratio of 1:8. After that, the petroleum ether in miscella (a solution of fat in solvent) was evaporated using a vacuum rotary evaporator (R-300, Buchi Ltd., Switzerland) at 60 °C and the residual solvent was further removed by nitrogen flushing. The extracted fat was kept at − 18 °C before using for the preparation of olein.

Preparation of rambutan kernel olein

The rambutan kernel olein was prepared using acetone fractionation according to the method of Mahisanunt et al. (2017) with some modifications. The fat was melted to destroy the crystalline form at 80 °C and then mixed with warm acetone (~ 60 °C) at a ratio of fat-to-solvent of 1:6 (w/v). The fat-acetone mixture was kept at 25 °C for 24 h to enable high melting fat crystallization. After that, the fat crystals were separated using vacuum filtration through a Whatman No. 4 filter paper. The fat-acetone liquid fraction was kept at 20 °C for a further 10 h. Then, the liquid phase and solid fat crystals were separated. The liquid fraction obtained at this step was collected and the acetone was evaporated using a vacuum rotary evaporator. The remaining solvent was further removed from the residual fat (which was then called rambutan kernel olein (RKOle) in the present work) using nitrogen flushing. The RKOle fraction was kept at − 18 °C before being used to develop the model liquid coffee creamer.

Production of model liquid coffee creamer

The model liquid coffee creamers were produced by homogenizing 10% (w/w) RKOle with 90% (w/w) aqueous phase. The aqueous phases containing 2.5% (w/w) maltodextrin, 2.5% (w/w) sucrose, 2% (w/w) sodium caseinate, 0.15% (w/w) di-potassium phosphate and various concentrations of DATEM [0.1, 0.3 and 0.5% (w/w)] were prepared by hydrating all components in hot deionized water (~ 60 °C). Then, these aqueous phases were blended with RKOle at the same temperature using a handheld homogenizer (Ultra-Turra® xT25 basic, Ika®-Werke Gmbh&Co., Staufen im Breisgau, Germany) at 9500 rpm for 4 min. Subsequently, the coarse emulsions were passed through a high-pressure valve homogenizer (APV-2000, SPX Flow Technology Rosista GmbH, Holzwickede, Germany) three times at 300 and 30 bar for the first and second stages, respectively. The resulting hot fine emulsions were cooled to 25 °C in a cold-water bath (~ 20 °C) and sodium azide [0.02% (w/w)] was added before keeping at 25 °C in a temperature control incubator for 24 h. For each creamer, the droplet characteristics (zeta-potential, droplets size and distribution) and stability were determined.

Sterilization by autoclaving

This experiment used the model liquid coffee creamers stabilized by 0.3% (w/w) DATEM which had been prepared according to the procedure described above. The creamers were filled into 250 ml glass bottles (Duran, DWK Life Sciences GmbH, Wertheim, Germany) and sterilized using a steam autoclave (Model 25x-2, Wisconsin Aluminum Foundry Co., Inc., WI, USA) at 121 °C and 1.1 bar for 15 min. After cooling, the bottles were stored at 25 °C for 1 and 150 days, after which their droplets characteristics, color and rheology were analyzed.

Effect of pH

The initial pH of the sterilized model RKOle-based liquid coffee creamers was 6.4. A sample of 200 ml of each creamer was transferred to a 250 ml glass bottle and then the pH of the liquid creamer was adjusted to the desired pH between 4 and 7 by adding HCl or NaOH solution (1 M). All creamer samples were stored at 25 °C for 24 h prior to analysis of zeta-potential, droplets size and distribution, color, creaming index and microstructure.

Preparation of coffee mix

The mixes between coffee solution and liquid creamer with and without adding sugar were prepared to evaluate the whitening capacity of the model RKOle-based liquid creamer. A black coffee solution was prepared in hot deionized water (85 °C) at 1.1% (w/v) according to the manufacturer’s recommendation. Then, 15 ml of commercial or one of the RKOle-based liquid creamers was added to each 180 ml coffee solution in the absence and presence of 4 g sugar with stirring until homogeneous (~ 5 s). The visual appearance, color, pH and droplets characteristics were measured.

Measurement of zeta-potential, droplet size and distribution

The zeta-potential, droplet size and distribution of each liquid creamer were measured using a dynamic light scattering instrument (Zetasizer Nano-S90; Malvern Instruments, Malvern, UK) at 25 °C. Before measurement, the liquid creamers were diluted with deionized water (reflective index, dielectric constant and viscosity were 1.331, 78.5 and 0.8872, respectively) to a droplet concentration of approximately ~ 0.1% (w/w) for eliminating the multiple scattering effects. The electrophoretic mobility of the oil droplets was measured and then the zeta-potential was calculated using the Henry equation. For the droplet size and distribution, the instrument analyzed the intensity of scattered light from droplets at an angle of 90 using a 4 mW He–Ne laser at 633 nm. The droplet size was therefore reported in terms of the volume intensity mean diameter or the Z-average. While the distribution of droplets was expressed using the polydispersity index (PDI).

Creaming index determination

In the present work, the creaming index (CI) was used to indicate the physical stability of model liquid creamers. Immediately after preparation, 10 ml of each creamer emulsion was poured into a glass test tube (internal diameter ≈ 1.40 cm, height ≈ 16 cm). The total height of the creamer emulsion (HE) in each tube was measured and all samples were then stored at 25 °C in a temperature-controlled incubator (Sanyo Electric Co., Ltd, Osaka, Japan). After storage, the phase separation of creamers was observed and the height was measured for a bottom serum phase (Hs). The creaming index was calculated as:

CI%=HS/HE×100 1

Microstructure observation

The microstructure of the RKOle-based liquid creamers was observed using an optical microscope (Axiolab®, Carl Zeiss Ply Ltd., Jena, Germany). Briefly, a drop of creamer emulsion (~ 10 µl) was placed on a microscope slide and carefully covered with a cover slip. The microstructure of samples was observed at an objective magnification of 40× where an image of each slide was recorded using digital image processing software (Image-Pro Plus™, version 6).

Color measurement

The color of the liquid creamers and coffee mixes was measured using an instrumental colorimeter (UltraScan® PRO, HunterLab, Reston, VA, USA). The instrument was calibrated using a light tab and white color standard tile before measurement. Each creamer sample (~ 30 ml) was poured into the measurement bottle and the color was measured using daylight (D65) as the standardized light source. The color of the sample was expressed in terms of the L*, a* and b* values. From these color values, the whiteness of samples was calculated as:

WhitenessIndex=100-100-L∗2+a∗2+b∗212 2

Rheology measurement

The rheology of the RKOle-based liquid creamers was measured at 25 °C using a dynamic shear rheometer (Physica MCR 301, Anton Paar, Graz, Austria). The creamer (~ 19 ml) was contained in a single-gap measuring cylinder cell (CC27-SS, diameters of the inner and outer cylinders were 26.7 and 28.9 mm, respectively). Flow curves were determined as a function of shear rates of 2–150 s−1. The power law model (σ = K·γn) was used to fit the experimental flow curves, where σ is the shear stress, K is the consistency index (Pa.sn) and n is the flow behavior index.

pH measurement

The pH values of the liquid creamers and coffee mixes were measured using a JENCO pH meter (Jenco Electronics Co., Ltd., Shanghai, China). The pH meter was calibrated with standard buffer solutions of pH 4.0 and 7.0 before use.

Statistical analysis

A completely randomized design was used for the experiments. All experiments were conducted at least twice with triplicate measurements in each experiment. Data were statistically analyzed using analysis of variance (ANOVA). Differences in means were determined using Duncan’s multiple range test with the P < 0.05 level denoting statistical significance.

Results and discussion

Influence of DATEM concentration on droplet characteristics and stability of model RKOle-based liquid creamer

The model RKOle-based liquid creamers (2% (w/w) sodium caseinate, 2.5% (w/w) maltodextrin, 2.5% (w/w) sucrose, 0.15% (w/w) di-potassium phosphate and 10% (w/w) RKOle) were prepared using different DATEM concentrations [0.1, 0.3 and 0.5% (w/w)]. The droplet characteristics of the prepared liquid creamers are shown in Table 1. The particle size of samples decreased with increasing concentration of DATEM from 0.1 to 0.3% (w/w) (P ≤ 0.05). This decrease was probably due to more emulsifier being adsorbed on the surface of newly developed particles during homogenization leading to their protection from coalescence and the production of small particles. Increasing the DATEM concentration to 0.5% (w/w) had no further effect on the particle size (P > 0.05). This could be attributed to all the lipid particle surfaces in the dispersion being saturated by the emulsifiers (Tcholakova et al. 2004; Yerramilli and Ghosh 2017). The PDI is an indicator used to determine the particle size distribution of the dispersion system. This value ranges from 0.0 (perfectly uniform system) to 1.0 (highly polydisperse system), with values less than 0.3 being considered as optimum values and indicating a monomodal distribution in the system (Danaei et al. 2018). The PDI values of all liquid RKOle model creamers prepared in the present work were less than 0.24, indicating monodispersal with a narrow size distribution of samples. No significant difference was noted between the PDI values of the RKOle creamers produced at different DATEM concentrations.

Table 1.

Particle size (Z-average), polydispersity index (PDI), zeta-potential, and creaming stability of RKOle-based liquid creamer prepared with different DATEM concentrations

Characteristics DATEM concentration [% (w/w)]
0.1 0.3 0.5
Particle size (nm) 199.81 ± 2.91a 189.28 ± 1.94b 187.89 ± 3.01b
PDIns 0.238 ± 0.065 0.209 ± 0.039 0.187 ± 0.060
Zeta-potential (mV)ns − 53.49 ± 0.82 − 53.58 ± 0.91 − 53.65 ± 0.29
Creaming stability1 +,− +,+ +,+
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a,bDifferent letters in the same row represent significant difference (P ≤ 0.05)

nsWithin the same row represent no significant difference (P > 0.05)

1Creaming stability of RKOle-based liquid creamer after storage at 25 °C for 24 h and 28 days, respectively. +, stable; −, unstable

Similarly, there was no significant effect of DATEM concentrations on zeta-potential of RKOle creamers (Table 1). The zeta-potential provides information on the adsorption of ionic compounds on the droplet surface and can be used to predict the physical stability of an emulsion system. Normally, emulsions that have high absolute zeta-potentials (≥ 30 mV) tend to have good physical stability. The zeta-potential values of all the RKOle creamers were negative and ranged from − 53 to − 54 mV, indicating acceptable physical stability. This negative charge was caused by the adsorbed DATEM and sodium caseinate at the interface (Perugini et al. 2018; Yesiltas et al. 2018). DATEM is anionic emulsifier that produces a negatively charged zeta-potential which is attributed to the presence of negative groups in the molecules (Sahafi et al. 2018). Typically, for the caseinate stabilized emulsion, when the pH of the emulsion is higher than the isoelectric point or pI value (~ pH 4.6), the oil droplets surface became negatively charged. Therefore, all the model RKOle creamers at the preparing pH (~ pH 6.3–6.4) had negative values of surface charge. The high negative zeta potential values in the present work was consistent with the values reported in previous works which ranged from − 40 to − 70 mV for caseinate alone and for mixed caseinate/small molecule surfactants (Perugini et al. 2018; Yesiltas et al. 2018).

The physical stability of the RKOle liquid creamers was also observed visually during storage. All creamer samples appeared homogeneous with no phase separation after storage for 28 days. However, a small lipid layer on the surface was observed in the creamer prepared with 0.1% (w/w) DATEM (data not shown). This was consistent with the above results which showed that RKOle creamer stabilized with 0.1% (w/w) DATEM had largest particle size and highest PDI (Table 1). All results in this experiment showed that using a DATEM concentration at 0.3% (w/w) could produce a stable model liquid creamer having both a small particle size and a narrow range of size distribution. Therefore, this emulsifier concentration was used in subsequent experiments.

Influence of sterilization on droplet characteristics, color indices and rheological parameters of RKOle-based liquid creamers

In this section, the model liquid creamers were prepared using 10% (w/w) RKOle, 2% (w/w) sodium caseinate and 0.3% (w/w) DATEM. These were sterilized using autoclaving at 121 °C and 1.1 bar for 15 min. The unheated and sterilized liquid creamers were stored for 150 days at 25 °C. The particle size and distribution, zeta-potential, flow behavior index (n) and color parameters of the RKOle creamers were determined as shown in Table 2.

Table 2.

Influence of sterilization on particle size (Z-average), polydispersity index (PDI), zeta-potential, flow behavior index (n) and color parameters of model RKOle-based liquid creamer at 24 h and 150 days storage at 25 °C

Characteristics Fresh creamer Sterilized creamer
24 h 150 days 24 h 150 days
Particle size (nm) 222.43 ± 2.91a 215.00 ± 3.16b 223.25 ± 3.54a 217.32 ± 1.36b
PDI 0.186 ± 0.012b 0.218 ± 0.014a 0.178 ± 0.012b 0.220 ± 0.010a
Zeta potential (mV) − 57.73 ± 1.62a − 42.33 ± 3.80c − 56.51 ± 1.04a − 48.18 ± 1.01b
n 0.95 ± 0.01c 0.99 ± 0.02a 0.96 ± 0.01bc 0.97 ± 0.01b
L* 89.32 ± 0.41a 87.68 ± 0.08b 87.91 ± 0.17b 87.08 ± 0.14c
a* − 0.74 ± 0.04c − 0.02 ± 0.06b − 0.09 ± 0.14b 0.09 ± 0.10a
b* 4.18 ± 0.03d 7.35 ± 0.05b 5.03 ± 0.27c 8.03 ± 0.30a
Whiteness index 88.51 ± 0.38a 85.65 ± 0.09c 86.90 ± 0.16b 84.78 ± 0.05d
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a–dDifferent letters in the same row represent significant difference (P ≤ 0.05)

The results showed that sterilization had no significant effect on the particle size, PDI and zeta-potential of the RKOle-based creamers. This indicated a synergistic effect between the protein and DATEM which resulted in the emulsion being heat-stable (Campbell and Trueck 1998). The positive combination effect of DATEM with sodium caseinate was also reported by Barfod and Sparso (2007). One possible mechanism is the co-absorption of DATEM and protein on the oil/water interface. Another mechanism may by the adsorption of the DATEM-proteins complex resulting in a thicker and stronger interfacial layer with enhanced emulsion stability at high temperature (Agboola et al. 1998).

After 150 days of storage, no growth in the particle size was observed; the particle size for both the unheated and sterilized RKOle model creamers was between 215 and 223 nm (Table 2). However, the PDI increased slightly after storage from approximately 0.18–0.22. Nevertheless, these values are within the range of monodispersion (PDI less than 0.30), indicating a narrow particle size distribution and satisfactory long term stability (Danaei et al. 2018). The magnitude of the zeta-potential of the RKOle-based creamers stored at 25 °C for 150 days decreased to − 42.3 and − 48.2 for the unheated and sterilized samples, respectively. The decrease in the zeta-potential during storage of emulsion systems was observed in previous works (Cheong and Nyam 2016; Hu et al. 2006) and might have been due to the rearrangement of the solid fat crystals in the creamer emulsions which can result in changes in the particle surface charge (Witayaudom and Klinkesorn 2017). There was a lesser reduction in the negative surface charge for the sterilized RKOle-based creamer compared with the unheated sample (Table 2). This was probably due to the moist heating during autoclave sterilization that led to the liberation and accumulation of free fatty acids in the creamer samples. Adsorption of this component at the particle surface during storage generated more negative zeta-potential (Tamilvanan et al. 2010). Typically, absolute values of the zeta potential observed in this work were still higher than the minimum recommendation of ± 30 mV, resulting in sufficient electrostatic repulsion between the droplets and consequently avoiding droplet aggregation and coalescence (Silva et al. 2012). This was consistent with the physical stability of the samples that showed no phase separation after storage for 150 days (data not shown). In addition, the flow behavior index (n) value obtained from the power law regression (R2 ≈ 0.995–0.997) confirmed the stability of the liquid creamer systems. All samples had a flow behavior index (n ≈ 0.95–0.99) close to unity (n ≈ 1) and so exhibited a Newtonian fluid behavior (Table 2). This indicated that the RKOle liquid creamers were stable regarding creaming and coalescence, with non-flocculated particles after sterilization and storage.

The effect of autoclaving sterilization and storage on the color attributes of the RKOle liquid creamers was determined. The L* value represents the lightness of model liquid creamers. A negative value of a* and a positive value of b* indicate greenness and yellowness, respectively. As the values of a* and b* were very small in the current study, the color of fresh prepared or unheated model creamer emulsions could be described as whitish with a slightly greenish-yellow shade. It was clear that all color indices of the sterilized creamers were significantly different from the unheated sample (P ≤ 0.05). The L* value decreased from 89.3 to 87.9 as sterilization progressed, which the whiteness of the sterilized creamers decreased slightly from 88.5 to 86.9. The lower negative a* value and the higher positive b* value of the sterilized creamers than those of the unheated sample indicated there was a bit more red and yellow in the sample (Table 2). The decrease in whiteness and the change to the greenish-yellow shade after sterilization may have been due to a non-enzymatic browning reaction between reducing sugar and proteins molecules in the liquid creamers, which was enhanced by heating. A similar observation on a reduction in the whiteness value of food emulsion systems after heat treatment has been reported (Bernat et al. 2015; Zaaboul et al. 2019). After storage, the whiteness index and L* of the RKOle-based creamers decreased, while the a* and b* values increased (Table 2). This was indicated by the creamer emulsions becoming darker, redder and yellower due to the formation of brown pigments resulting from non-enzymatic browning or the Maillard reaction which was also reported in previous works (Bernat et al. 2015; Cano-Ruiz and Richter 1998; Zaaboul et al. 2019). However, the color indices of the sterilized liquid creamer after storage for 150 days was still within the range of typical values for sterilized liquid emulsion food products which has been reported as about 64 to 84, − 0.5 to − 3.5 and 4.7 to 13.2 for the L*, a* and b* values, respectively (Chiewchan et al. 2006; Seo et al. 2018).

Effect of pH on stability of liquid RKOle model creamers

The effect of pH on the stability of the liquid RKOle model creamers was investigated in this experiment. This is a need to evaluate the utilization of the model creamer to add to coffee solutions which has acidic behavior, with the pH ranging from 4.8 to 5.7 (Gurol 1998). The visual appearance and optical microscopic images (Fig. 1) indicated that the sodium caseinate-DATEM stabilized model RKOle-based creamers were unstable at pH ≤ 5, showing phase separation into lower serum and upper cream phases (Fig. 1a). The creaming index of the RKOle creamers was approximately 46 and 34% at pH 4 and 5, respectively (Table 3).

Fig. 1.

Fig. 1

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Visual appearance (a) and optical light microscopic images of model rambutan kernel olein (RKOle)-based liquid creamer at various pHs (b pH 4.0, c pH 5.0, d pH 6.0–7.0)

Table 3.

Creaming index, particle size (Z-average), polydispersity index (PDI) and zeta-potential of model RKOle-based liquid creamers at various pHs

pH Creaming index (%) Particle size (nm) PDI Zeta-potential (mv)
4.0 45.81 ± 1.60a – – –
5.0 34.12 ± 1.75b – – –
6.0 0.00 ± 0.00c 235.20 ± 2.89b 0.182 ± 0.014a − 48.57 ± 1.42c
6.41 0.00 ± 0.00c 233.86 ± 2.75b 0.163 ± 0.018b − 53.73 ± 1.51b
7.0 0.00 ± 0.00c 239.67 ± 2.00a 0.149 ± 0.006b − 57.25 ± 1.38a
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1Initial model RKOle-based liquid creamer

a–cDifferent letters in the same column represent significant difference (P ≤ 0.05)

At pH 4, the model creamers were highly flocculated (Fig. 1b). When the pH of the creamers increased to pH 5, there was lesser droplet aggregation (Fig. 1c). At these two pH levels, the droplet characteristics including particle size, PDI and zeta-potential could not be measured due to the resolution limit of the instrument. These results were probably due to the pH levels of these creamers being close to the pI of the adsorbed sodium caseinate molecules (≈ pH 4.6) (Surh et al. 2006), which resulted in a relatively small charge on the surface of droplets, which in turn resulted in insufficient electrostatic repulsion to prevent aggregation; consequently droplet flocculation was promoted (Liu et al. 2012; Shepherd et al. 2000; Surh et al. 2006). At a pH level higher than 5, there was no evidence of phase separation (Fig. 1a), with individual small oil droplets in the range 233–240 nm (Fig. 1d, Table 3). The stability of the RKOle-based creamers at higher pH levels was confirmed by the PDI values which were in the range 0.15–0.18, indicating a narrow particle size distribution (Danaei et al. 2018).

The results clearly indicated that the zeta-potential of the RKOle-based creamers was significantly influenced by pH (P ≤ 0.05). The zeta-potential of freshly prepared model creamers (pH ~ 6.4) was approximately − 54 mV, which was caused by the adsorbed DATEM and sodium caseinate at the interface as described in the previous section. When the pH of the creamers decreased to 6, there was a slight reduction in the magnitude of the zeta-potential to approximately − 49 mV. In contrast, there was a small increase in the zeta-potential to approximately − 57 mV as pH increased from 6.4 to 7 (Table 3) because the decrease and increase in the net negative charge is governed by the ionization degree of the amino groups (–NH2) of the protein molecules (Johnston et al. 2015). Similarly, the effect of pH on the zeta-potential changing of other charge biopolymers, such as pectin, was also reported (Nagarajan et al. 2019).

Influence of liquid creamers and addition of sugar on whiteness and characteristics of coffee mixes

In this experiment, the whitening capacity of the model RKOle-based liquid creamers in hot coffee was determined and compared with a commercial liquid creamer. The black coffee solution prepared in the present work (~ 1.1%) was a clear dark brown solution. The instrumental color values were 5.45, 1.5, 1.2 and 5.43 for L*, a*, b* and whiteness index, respectively (Fig. 2).

Fig. 2.

Fig. 2

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Color parameters L* (a), a* (b), b* (c) and whiteness index (d) of coffee solution (CF), liquid creamers (LC) and their mixes in the absence (CM) and presence of adding sugar (CMS) for commercial liquid creamer (CLC) and model rambutan kernel olein (RKOle)-based liquid creamer (RLC)

Results showed that the commercial liquid creamer had an off-white color with lower L* and whiteness index values than the model RKOle liquid creamer (Fig. 2). When the creamers were mixed with the coffee solution, no oiling off or feathering or sedimentation was observed in any of the coffee mixes, suggesting that the liquid creamers were stable. The commercial liquid creamer-coffee mixes had a darker brown color with higher a* and b* values and lower whiteness index compared to the RKOle liquid creamer-coffee mixes (Fig. 2). These differences were probably due to the larger size and higher PDI of the oil droplets in the commercial liquid creamer (~ 781 nm and 0.61, respectively) compared with the model RKOle creamer (~ 241 nm and 0.21, respectively) as shown in Table 4. A decrease in the measured L* value and an increase in the b* value as the droplet size increased has been reported for emulsion systems (Chanamai and McClements 2001). The zeta-potential of the coffee solution changed from approximately − 16.3 mV to approximately − 30.6 and − 28.9 mv when mixed with the commercial and model creamers, respectively. The increasing zeta-potential was probably due to the ability of the creamers to neutralize the acidity of the coffee solutions by raising the pH (Table 4), which would lead to more negative droplets (Chung et al. 2017). Typically, consumers may add different amounts of sugar to sweeten their hot coffee. Therefore, the effect was also investigated of sugar addition on the appearance and characteristics of coffee mixes. Results showed that the addition of sugar had no effect on the properties of the coffee mixes (Fig. 2, Table 4) and that the model RKOle-based liquid creamers prepared in this study had similar whitening power to their commercial counterparts.

Table 4.

Particle size (Z-average), polydispersity index (PDI), zeta-potential and pH of coffee solution, liquid creamers and their mixes in the absence and presence of adding sugar

System composition Particle size (nm) PDI Zeta potential (mv) pH
CF – – − 16.34 ± 0.69e 4.89 ± 0.02f
CLC 780.84 ± 61.28c 0.607 ± 0.09a − 40.47 ± 0.75b 6.51 ± 0.00a
CCM 890.52 ± 23.40a 0.426 ± 0.19b − 30.56 ± 0.45c 5.70 ± 0.01d
CCMS 848.40 ± 28.92b 0.543 ± 0.26ab − 30.92 ± 0.26c 5.73 ± 0.04c
RLC 241.20 ± 8.69d 0.209 ± 0.02c − 53.96 ± 1.36a 6.31 ± 0.01b
CRM 238.06 ± 5.50d 0.213 ± 0.02c − 28.93 ± 1.64d 5.21 ± 0.01e
CRMS 240.63 ± 4.29d 0.232 ± 0.02c − 28.39 ± 2.17d 5.23 ± 0.01e
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CF coffee solution, CLC commercial liquid creamer, CCM coffee-commercial liquid creamer mix, CCMS coffee-commercial liquid creamer mix containing sugar, RLC RKOle-based liquid creamer, CRM coffee-RKOle-based liquid creamer mix, CRMS coffee-RKOle-based liquid creamer mix containing sugar

a–fDifferent letters in the same column represent significant difference (P ≤ 0.05)

Conclusion

This study suggested that rambutan kernel olein has potential to be used as the fat component for fabricating non-hydrogenated fat-based non-dairy liquid creamer. The proper concentration of DATEM as emulsifier to produce a stable model creamer with desirable characteristics was 0.3% (w/w) in the presence of 2% (w/w) sodium caseinate. Sterilization could be applied to prolong the storage stability of liquid creamer without any effect on emulsion stability. The model liquid creamer had a more whitish appearance than the commercial one. Coffee solutions mixed with the model liquid creamer did not have any feathering and were similar to that of the commercial creamer, tough they did have a higher whiteness level. The study results indicated the efficient whitening power of the model liquid creamer. However, more information regarding the taste and flavor of coffee mixes containing rambutan kernel olein-based liquid creamer is necessary and future work is required. Besides, the effect of antioxidant especially natural extracts, such as plant polyphenols on the oxidative stability of this model liquid creamer may also need to study.

Acknowledgements

The authors would like to acknowledge the Center of Advanced Studies for Agriculture and Food (CASAF), Kasetsart University for their financial supports under the Post Master Researcher Scholarship (PM070). We are also grateful to Malee Sampran Public Company Limited (Nakornpathom, Thailand) for providing the rambutan seed samples.

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.

Footnotes

Publisher's Note

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