COMPARISON OF THE STABILITY OF BUTTERY AROMA IN GELS PREPARED WITH MAIZE OR POTATO STARCH

Grażyna Bortnowska
Department of Food Technology, West Pomeranian University of Technology, Szczecin, Poland

ABSTRACT

Stability of the buttery aroma in gels prepared with instant or hot gelated potato and waxy maize starches was investigated with application of the chromatographic and sensory analyses. It was shown that persistence index of buttery aroma increased with higher concentration of studied starches and mostly in gels made with instant potato starch. The stability of buttery aroma was affected by viscosity (r ≥ 0.96) and consistency (r ≥ 0.93) and also ability of gels to hold water (r ≥ 0.72). The highest viscosity at shear rate of 3 (s^-1) and consistency index, irrespective of thickener concentration, exhibited gels prepared with instant potato starch. The buttery aroma intensity of aromatized gels decreased with storage time particularly markedly in samples containing 1–3% starch.

Key words: aroma stability, maize starch, potato starch, static headspace analysis, persistence index, water holding capacity.

INTRODUCTION

Starch is the basic thickener and source of carbohydrate in food and one of the most abundant substances in nature [7]. Nutritional and sensory characteristics of starch-based dairy desserts favour their consumption by several groups of consumers, such as children or elderly people [26]. Furthermore, dietary starch products belong to a specific category of prebiotics [17]. The sources of starch include maize, wheat, and potato, which can contain between 14% and 27% amylose (a linear chain of glucose linked by a-1,4-glucosidic bonds) and 73–86% amylopectin (a branched chain of glucose linked by α-1,4-glucosidic bonds with α-1,6-glucosidic bonds at the points of branching) [1]. Depending on the origin starches exhibit different physicochemical and functional properties [20]. Moreover, applied as the thickeners may also affect aroma compounds release due to diffusion decrease in the media and molecular interactions between flavour compounds and matrix constituents [10,13,15,23]. Native starches are frequently limited in their food applications, due to cohesive texture, heat and shear sensitivity, lack of clarity, opacity and low viscosity. Therefore, there is requirement to produce starches, which can provide consistent results specific to the needs of the products [24]. Physicochemical properties of starch can be improved by physical, biological or chemical modification [8]. Górecka et al. [12] reported that application of physically modified starches allowed to produce products exhibiting expected by consumer properties. Many systematic studies have been carried out on aroma-carbohydrate interactions, but there are not reports concerning correlation between stability of hydrophilic aroma compounds and storage time of modified starch-based products.

The aim of this work was to study the influence of type and concentration of selected modified waxy maize or potato starches on buttery aroma stability with respect to the storage time of gels.

MATERIALS AND METHODS

Materials
The following starches were used: hot gelated waxy maize starch and potato starch as well as instant waxy maize starch and potato starch, received from National Starch Food Innovation (Hamburg). Buttery flavouring soluble in water, manufactured by Chr. Hansen Ireland Limited Rohan Industrial Estate, Little Island, Co. Cork Ireland, received from Chr. Hansen Poland Sp. z o.o. (Cząstków Mazowiecki).

Gel preparation
The gels were prepared with four different starches: hot gelated waxy maize starch or potato starch and instant waxy maize starch or potato starch at concentrations of: 1.0, 2.0, 3.0, 4.0, 5.0 and 6.0 wt%. All starches were mixed with distilled water for 2 min using a hand blender. Then, the mixtures containing hot gelated waxy maize starch or potato starch were placed in a 95°C water bath for 20 min. Constant stirring was maintained throughout to ensure complete dissolution of solutes. All gels were refrigerated at 4±0.5°C for 12 h prior to analysis [2]. For the static headspace and sensory analyses, a precise amount of buttery flavouring was added to give a final concentration of 0.1 wt%. Then, the gels were stirred for 1 min at 1200 rpm to ensure equal distribution of the volatile compounds and the jars were immediately sealed.

Methods
Static headspace analysis
For the analysis, 5±0.01 g of the gel was transferred into a 22.3 ml headspace vial and afterwards the aromatized gels, containing different starches, in opened vials were stored in a refrigerator at 4.0±0.5°C for: 0.5, 1.0, 2.0, 4.0, 6.0, 10.0, 14.0, 18.0, 22.0, 34.0 and 48.0 h. Before headspace analysis the vials with gels were capped and placed into an automatic sampler for headspace analysis (TurboMatrix 16 – Perkin Elmer International C.V. Switzerland) combined with gas chromatograph and interfaced with mass spectrometer (GC-MS, electron ionization – EI) where the samples were incubated at 70°C for 15 min. The GC was equipped with a PE-SMS capillary column (30 m length, 0.25 mm i.d. and 0.25 µm film thickness) and helium carrier gas was used at a constant flow rate of 0.7 ml·min^-1. The temperature program was as follows: initial temperature 50°C, program rate 10°C·min^-1 and final temperature 150°C. The aroma compounds peaks were identified by comparison them with Wiley's mass spectrum library and also GC retention indices were compared against standard.

Persistence measurements
The intensity of diacetyl released from the gels (I) was monitored as a function of storage time (t) and data were fitted to a 1st-order kinetic equation according to Giroux et al. [7]

I = I₀ · EXP(-ct)

where: I₀ is the maximum intensity and c is the decay rate constant for flavour release.

A persistence index (PI) was calculated from the decay rate constant according to the equation:

PI = 1/c

The intensity (concentration) of diacetyl released to the headspace was calculated according to method described by Kolb and Ettre [16].

Sensory analysis
The samples were evaluated towards odour intensity of buttery aroma as described in PN-ISO 4121 [21] by an internal panel consisting of eight assessors, selected according to PN-ISO 8586-1 [22]. Samples were presented at 22·1°C in coded 50 ml brown sealed flasks and evaluated after: 0.0, 6.0, 12.0, 24.0 and 48.0 h of storage time in a refrigerator at 4.0±0.5°C in opened flasks. A 9-points category scale adapted from the technique of Guyot et al. [14], was used as a reference scale for the odour intensity evaluation. The low intensity of buttery aroma was scored as 1 and the highest one as 9.

Water holding capacity of gels
The water holding capacity (WHC) was measured according to procedures described by Weel et al. [27] and Braga et al. [5]. A 8 g portion of gel was put into a 10 ml centrifuge tube. The gel was centrifuged at 380 × g for 10 min in a MPW 350 centrifuge (Med.-Instruments, Warsaw, Poland). After centrifugation the serum was removed and weighed. The WHC values were calculated from the equation:

WHC (%) = 100 [1-(Water_loss /  Water_gel)]

where: Water_gel (g) is the amount of water in the gel before centrifugation and Water_loss (g) is the difference between amount of water in the gel before and after centrifugation.

Rheological measurements
Rheological measurements were carried out in a controlled shear stress rheometer (Rheotest 2 – 50 Hz – type RV2, equipped with S/S1 cylinder) with increasing shear rate from 3 to 1312 s^-1 at 22±0.5°C. Experimental data were fitted to Ostwald-de Waele model [18, 26]:

  σ =  K · γⁿ

where: σ is the shear stress (Pa), γ is the shear rate (s^-1), K is the consistency index (Pa · sⁿ) and n is the flow index.

Statistical analysis
Each experiment was conducted three times and the data were statistically treated using Statistica^® 6.0 program PL. Data of persistence index and water holding capacity were subjected to two-way analysis of variance (ANOVA). Significant differences between means were identified by the LSD procedure (p ≤ 0.05). The extent of correlation between persistence index of buttery aroma and viscosity at shear rate of 3 (s^-1), water holding capacity (WHC), consistency index (K) as well as between odour intensity of buttery aroma and storage time was determined by Pearson's correlation coefficients.

RESULTS AND DISCUSSION

The quantity of diacetyl, the main component of buttery aroma, released from gels to the headspace considerable depended on starch type and concentration. It was observed that in majority of the studied samples with increasing starch concentration also increased persistence index of diacetyl. For example, in gels containing 1% instant or hot gelated waxy maize starch and instant or hot gelated potato starch, the persistence indexes were registered respectively: 32.4 or 53.4 and 64.1 or 46.8 (Fig. 1). Whereas, increase of starch concentration to 6% caused nearly threefold increase of diacetyl persistence index, calculated as average of all studied samples and the highest values were observed in gels prepared with instant waxy maize starch. The highest and in majority statistically significant differences (p ≤ 0.05) of diacetyl persistence indexes were observed in samples containing more than 4% starch. However, in samples made with instant potato starch changes of its concentration also in range from 1 to 4% caused very clear differences in release of investigated aroma compound. Furthermore, it was noticed that using 3% instant potato starch, it was possible to increase persistence index of buttery aroma to the level as in gels prepared with 5% hot gelated waxy maize starch or potato starch (Fig. 1). It was also observed that the highest diacetyl persistence indexes, in majority of the samples irrespective of starch concentration, demonstrated gels containing instant potato starch and the lowest values of this parameter were registered in  gels prepared with instant waxy maize starch.

The hot gelated waxy maize starch increased more diacetyl persistence index than the instant waxy maize starch whereas, the samples containing potato starches showed inverse values concerning measured parameter, it means that demonstrated higher persistence in gels containing instant starch than hot gelated starch. The sensory analysis showed that intensity of buttery aroma was affected markedly more by storage time of aromatized gels than by concentration of starches used to prepare gels. It was observed, that with increasing storage time of studied samples, decreased intensity of buttery aroma released from gels  prepared with instant or hot gelated waxy maize starch and instant or hot gelated potato starch respectively by: 45.6 or 32.5% and 25.4 or 39.1% (Fig. 2).

The studies of Pearson's correlation coefficients of buttery aroma against storage time of gels revealed that these parameters were highly correlated and in majority statistically significant, mostly in samples containing 1–3% starch (Table 1). It was also found that intensity of buttery aroma mostly decreased, similarly as it was registered by instrumental analysis, in samples prepared with instant waxy maize starch however, the lowest changes during storage were noticed in samples prepared with instant potato starch (Figs. 1, 2). Moreover, sensory analysis demonstrated that in samples containing hot gelated starches the influence of starch concentration on intensity of buttery aroma was clearly lower than in samples containing instant starches (Fig. 2). Increase of starch concentration in gels changed water holding capacity (WHC) in range from 12.5 to 100% and the highest values were indicated in samples made with hot gelated potato starch irrespective of its concentration (Fig. 3). It was detected that increase of starch concentration from 1 to 6% increased WHC by: 62.8, 82.2, 50.6 and 54.1% in samples prepared with instant maize starch, hot gelated maize starch, instant potato starch and hot gelated potato starch, respectively.

Results from the ANOVA indicated that both starch type and starch concentration caused a significant effect on WHC (p ≤ 0.001). Additionally, ANOVA showed that also interaction between starch type and starch concentration affected WHC in all studied samples (p ≤ 0.001) (Table 2). The thickening properties of hydrocolloids are widely described in the literature [2,6] however, there are a little studies where water holding capacity parameter was calculated. Braga et al. [5] reported that increase of sodium caseinate concentration from 2 to 6% increased water holding capacity of studied gels by about 40% depending on glucono-d-lactone/caseinate ratio.

Also, Weel et al. [27] showed that increase of whey protein concentration from 4.0 to 11.0% resulted in higher water holding capacity by about 15%. Comparing these results it is possible to suppose that studied maize and potato starches exhibited much better properties to hold water however, considerable differences in WHC values between hot gelated potato starch and other investigated starches may suggest dependence between origin and type of modification in molecular structure of applied starches [7,19,20]. Changes of WHC parameter were statistically significantly correlated (p ≤ 0.05 or p ≤ 0.01) with persistence index (PI) in all samples with the exception of those prepared with hot gelated potato starch (Table 3).

These results suggest that diacetyl might be retained by starch solutions through steric phenomena what also was reported by Secouard et al. [25] with respect to the limonene in xanthan solutions and by Weel et al. [27] in whey protein gels containing diacetyl. In contrast, relatively low correlation between diacetyl persistence index and WHC in gels prepared with hot gelated potato starch may suggest that also other factors affected stability of buttery aroma. All samples exhibited non-Newtonian flow behaviour with increasing shear rate (Fig. 4).

The highest apparent viscosity demonstrated gels prepared with instant potato starch and the lowest made with hot gelated potato starch and waxy maize starch. The studies also showed increase of apparent viscosity with higher concentration of starches in gels. Furthermore, it was registered a very high correlation (r ≥ 0.96) between viscosity values and persistence indexes (Table 3). The results depicted in Table 4 record that regardless of origin and technological preparation of the starches, increase of their concentration caused that the samples exhibited also higher consistency index (K). The highest differences of this parameter (15.3 Pa·sⁿ) between samples containing 1 and 6% starch were indicated when the gels were prepared with instant potato starch and the lowest (2.9 Pa·sⁿ) between gels containing hot gelated potato starch. Moreover, it was shown that consistency indexes were highly (r > 0.93) and statistically significantly (p ≤ 0.01 or p ≤ 0.001) correlated with appropriate persistence indexes of buttery aroma (Table 3). The values of  flow index (n) showed that studied gels were shear-thinning although not clear dependence in all samples was observed (Fig. 4, Table 3). This type of behaviour is partly in accordance with observations of other authors on samples containing starch. Tárrega and Costell [26] reported that increase of starch concentration increased consistency index and decreased flow index in all investigated samples. Similar results also showed González-Tomás et al. [11] with respect to the custard desserts containing different amount of starch. The presented studies showed small differences of typical relations between consistency and flow indexes what might be caused by specific technological treatment of the starches and relatively large in some cases values of LSD (Table 4).

The high correlation between persistence and rheological parameters (viscosity and consistency) may be explained as the result of the presence of an entangled polymer network in thickened systems which inhibit transport of small molecules, such as flavour volatiles from within the gel system to the surface [1] because, diffusion of flavour molecules is reduced with increase of solution viscosity as predicted by the Stokes-Einstein and Wilke-Chang equations [15,23]. Similar results were also demonstrated with respect to the polysaccharide matrices by Secouard et al. [25] and Bortnowska [3] what confirms that release of hydrophilic aroma compound in gels containing starches might be on diffusion dependent. Besides, nonspecific interactions between starch and hydrophilic volatile of buttery aroma also should be taken into consideration since starch is a polar stationary phase capable of forming hydrogen bonds with aroma compounds. This assumption is in good agreement with results reported by Boutboul et al. [4], who observed that retention on different starch matrices increased with the polarity of the flavour molecules and concluded that aroma-starch interactions mainly resulted from an adsorption phenomenon involving hydrogen bonds and not from inclusion complexes.

CONCLUSIONS

1. Increase of starch concentration caused improvement of buttery aroma stability in gels, mostly in those prepared with instant potato starch.

2. Intensity of buttery aroma decreased with storage time of gels, particularly markedly in samples containing 1–3% starch.

3. Persistence index (PI) of buttery aroma depended more on viscosity and consistency of gels than on water holding capacity (WHC) values irrespective of applied starches.

4. Taking into account results obtained from investigations it is possible to suppose that studied starches can be utilized to manufacture a wide assortment of food products exhibiting differentiated consistency and relatively high stability of hydrophilic odorants.

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Accepted for print: 30.07.2009

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