Effects of Drying Temperature and Particle Size on Hydration Properties of Dietary Fiber Powder from Lime and Cabbage By-Products

Abstract

The use of by-products from fruit and vegetable processing plants as sources of dietary fiber and bioactive agents is currently of interest, both from the waste management and from the production of value-added products points of view. This work aimed at studying the use of lime residues and outer leaves of white cabbage to produce dietary fiber powder, which included the study of the product pretreatment and preparation, drying, and grinding dried residues into powder. Dietary fiber content during each processing step was determined to calculate the loss of functionality of the products through the process. Hydration properties, i.e., water holding capacity (WHC) and swelling capacity (SWC), of the two residues were also determined after hot air drying at 60–80°C. The effect of particle size of dietary fiber powder, which was obtained by grinding dried residues, in the range of 63–450 μm, on the hydration properties was also evaluated and discussed.

Keywords:

• Dietary fiber powder
• Drying
• Swelling capacity
• Water holding capacity
• Particle size
• Quality

INTRODUCTION

Fruits and vegetables have been scientifically proven to be good sources of dietary fiber. Many high dietary fiber formulated foods are currently being developed to serve the demand of health conscious consumers. To be acceptable, dietary fiber added to a food product should possess satisfactory characteristics of food ingredients such as the amount of total dietary fiber of above 50%, moisture content of lower than 9% (wet basis), low content of lipid, low caloric value, and neutral flavor and taste.[1] In addition, fibers should have a soluble dietary fiber to insoluble dietary fiber (SDF/IDF) ratio close to 1:2 to exert the physiological effects associated with both fractions of dietary fiber.[2,3]

Plant fibers from different sources exhibit very widely functional properties.[4,5] Hydration properties, viscosity and gel formation capability are common qualities of concern when considering plant fibers as food ingredients. These properties are in turn related to the microstructure of materials which changes during processing.[6] Many attempts therefore have been made to investigate the effects of various processing operations on the characteristics of different plant fibers.

Larrauri[1] studied the change of the water holding capacity (WHC) of orange fiber during washing and reported that washing dramatically increased the value of WHC of the fiber due to the removal of sugars within the fiber, which led to water activity reduction. Other investigators studied the effect of blanching on the quality of dietary fibers.[7,8] Blanching influences the physicochemical properties of dietary fiber, e.g., proportion of soluble and insoluble fiber, viscosity and molecular weight. During wet heat treatment, it has been shown that insoluble fiber might solubilize or even degrade into smaller fragments and, as a consequence, be lost to the processing water.[7,8]

Heating or drying also affects the structure and functional properties of fiber.[9–11] Femenia et al.[12] studied the hydration properties of cauliflower and reported that the WHC and swelling capacity (SWC) of cauliflower dried at 75°C were significantly lower than those dried at 40°C since drying temperature affected directly the structure and functional properties of the fiber. On the other hand, Moonsoor[13] reported that various drying methods, i.e., freeze drying, spray drying and vacuum drying, had very little or no effect on the solubility of soy hull pectin. Grinding dried fiber to fine powder, however, modified the fiber matrix structure, hence affecting the WHC and SWC of dietary fiber.[14] Cadden[15] studied the effect of particle size reduction on WHC of wheat bran and found that the reduction of particle size decreased WHC of hydrated wheat bran due to the collapse of its fiber matrix. Grover et al.[16] also reported that particle size affected the surface properties, including hydration capacity, of apple pomace powder.

The use of by-products from fruit and vegetable processing plants is currently of interest from the waste reduction viewpoint because these products contain various nutritional substances including dietary fiber and useful bioactive agents. For citrus juice production, by-products account for 50% of the original amount of whole fruit.[17] In addition to citrus fruits, cabbage residue is also of interest. For industrial preparation, the outer leaves of cabbage are generally removed and used as animal feed or fertilizer. By-products from cabbage are potential sources of commercial dietary fiber powder. White cabbage (Brassica oleracea var. capitata), for example, contains nearly 90% carbohydrates (dry weight), which one third is dietary fiber and two thirds are low molecular weight carbohydrates.[18]

This work aimed at studying the effects of hot air drying temperature and particle size on the hydration properties of high dietary fiber powder prepared from lime and white cabbage residues. The suitable processing conditions obtained can be used as guidelines for the production of high quality dietary powder in the future.

MATERIALS AND METHODS

Sample Preparation

Fresh limes (Citrus aurantifolia Swing.) were purchased from a local market. Lime residues were prepared by squeezing fresh limes and removing the seeds. The residues were boiled in hot water at 95°C for 5 min to produce fiber with high water holding capacity.[17] The samples were centrifuged for 5 min to remove excess water. The boiled lime residues were then soaked in 3% (w/v) sodium chloride solution for 30 min to reduce their bitterness; the samples were centrifuged again for another 5 min. Subsequently, the samples were chopped into small pieces using a chopper (Waring WCG75E, USA). The weight and dietary fiber content of the samples at each step of preparation were determined to calculate the yield losses. Outer leaves of cabbages were obtained from Thai Q.P. Co., LTD. (Ratchaburi, Thailand). The samples were washed gently to avoid or minimize loss of soluble fiber component, free sugar and ash.[2] After draining, it was cut into 0.5 × 5 cm² pieces.

Drying Experiments

After preparation, 1000 g of lime residues and 600 g of outer leaves of cabbage were dried under the same condition using a laboratory scale hot air oven (Termaks TS8136, Norway) at 60, 70, and 80°C; the ambient air humidity was controlled at around 50% via the use of an air conditioner within a room that the dryer was located. Each sample was placed on a stainless steel tray and efforts were made to ensure that each sample experienced the same drying conditions by placing the tray at the same location within the dryer in each experiment. 3–5 g of samples was taken at different time intervals to determine the sample moisture content. Drying was continued until the equilibrium moisture content was reached at each condition. The moisture content of the samples was determined using a hot air oven at 105°C.[19]

Determination of Dietary Fiber Contents

Total dietary fiber (TDF), soluble dietary fiber (SDF) and insoluble dietary fiber (IDF) contents were determined using enzymatic-gravimetric method.[19] In brief, dried samples were gelatinized with heat stable α-amylase (Sigma-Aldrich, Germany). After gelatinization the sample was digested with protease and amyloglucosidase (Sigma-Aldrich, Germany) to remove the present protein and starch. Subsequently, IDF was filtered and washed with warm distilled water. The filtrate and washed water were combined and added with 4 volumes of 95% ethanol at 60°C to precipitate SDF. The precipitates were weighed after drying at 105°C in a hot air oven (Memmert 800, Germany) until the constant weight was obtained. Protein and ash content of the SDF and IDF were determined by Kjeldahl procedure and by incinerating the samples at 525°C in an incinerator (Neyo 2–160, USA), respectively.[19] The SDF and IDF contents were calculated by:

(1)

and TDF content is the sum of the SDF and IDF contents.

Hydration Properties

The samples were placed on the largest sieve of the following numbered series of stainless steel sieves (Endcotts, London, England): mesh no. 40, 50, 60, 100, and 230 (equivalent to opening sizes of 63, 150, 250, 300 and 450 μm, respectively). The sieves were shaken for 20 min, and the retained contents in each sieve were carefully removed. The water holding capacity and swelling properties were then determined following the procedures of Robertson et al.[20]

To determine the water holding capacity (WHC), 1 g of sample from each particle size range was hydrated in 30 ml of distilled water at room temperature (∼30°C). After equilibration (for 18 h), sample was centrifuged at 3000 × g for 20 min and supernatant was removed. The weight of the residue was recorded both prior to drying at 105°C and after drying until constant weight was obtained. WHC was calculated as the amount of water retained by the sample (g/g dry weight) as follows:

(2)

For swelling capacity (SWC) determination, 0.1 g of sample of each particle size ranges was hydrated in 10 ml of distilled water in a calibrated cylinder (15-cm diameter) at room temperature (∼30°C). After equilibration for 18 h, the bed volume was recorded and expressed as volume/g of original substrate dry weight. The SWC was then calculated as follows:

(3)

Experimental Design

Data were collected with a 4 × 3 full factorial experimental design. The factors consist of 4 levels of particle size and 3 levels of drying temperature. Particle size levels were 63–150, 150–250, 250–300, and 300–450 μm. Drying temperature levels were 60°, 70°, and 80°C.

Data Analysis

Results of 3 replications were analyzed using the ANOVA. Tukey's test was used to establish the differences among treatments at a confidence level of 95%. Two-way ANOVA was performed using MINITAB Statistical Software, Release 14 for Windows (Minitab Inc., PA, USA).

RESULTS AND DISCUSSION

Drying Kinetics

The drying curves of lime residues (initial moisture contents of 460.45 ± 26.06% dry basis) and outer leaves of cabbage undergoing drying at various temperatures are shown in Fig. 1 and Fig. 2, respectively. The moisture ratio (MR) of the samples was calculated as follows:

(4)

where MR is the moisture ratio; M _t is the moisture content at time t (kg/kg); M _eq is the equilibrium moisture content (kg/kg); and M _i is the initial moisture content (kg/kg). The equilibrium moisture contents of the samples are shown in Table 1. As expected, increasing the drying temperature resulted in the higher drying rates. The drying time required to obtain the dried samples, which had moisture contents in the range of 6–8% (wet basis) or 6.4–8.7% (dry basis), is shown in Table 2. This range of moisture content (lower than 9% (wet basis) or 9.89% (dry basis)) is recommended as the desired moisture content of commercial dietary fiber powder.[1] The results showed that the required time for drying lime residues was shorter than that required for drying the outer leaves of cabbage at every drying condition. This may be due to the effect of pretreatment. Boiling lime residues at high temperature (95°C) could soften the plant tissue, hence increasing the rate of moisture removal.[1,21]

Figure 1 Drying kinetics of lime residues at temperatures of 60°C (▴), 70°C (•), and 80°C (▪).

Figure 2 Drying kinetics of outer leaves of cabbage at temperatures of 60°C (▴), 70°C (•), and 80°C (▪).

Table 1 Equilibrium moisture content of samples undergoing different drying temperatures

Samples	Temperature (°C)	Equilibrium moisture content (% dry basis)
Lime residues	60	2.62 ± 1.29
	70	2.42 ± 0.21
	80	0.70 ± 0.49
Outer leaves of cabbage	60	3.26 ± 0.04
	70	2.36 ± 0.13
	80	1.27 ± 1.11

Table 2 Drying time of samples to obtain final moisture content in range of 6.4–8.7% (dry basis)

Samples	Temperature (°C)	Drying time (h)
Lime residues	60	7.0
	70	5.0
	80	4.5
Outer leaves of cabbage	60	7.5
	70	6.0
	80	5.5

Dietary Fiber Contents

The effect of pretreatment on the dietary fiber contents of lime residues were monitored along the preparation steps. Table 3 shows the values of the TDF, IDF, and SDF of lime residues. The results showed that TDF of fresh lime residues after removing seeds was 82% (dry basis); fresh lime residues also contained a significant amount of SDF. The proportion of IDF to SDF was 1.11 indicating that lime residues are good sources of dietary fiber.[2,3] However, a marked loss of DF was observed during blanching; 10.07% and 25.76% (dry basis) losses of IDF and SDF, respectively, resulted in 17.52% loss of TDF. Higher loss was found for SDF because washing with water at 90°C for 15 min led to a removal of such soluble components as soluble pectin.[22] However, no significant losses were found during further processing steps.

Table 3 Dietary fiber content (dry basis) of lime residues during processing steps

Processing step	IDF	SDF	TDF	SDF:IDF
Removing seed	43.40 ± 0.76^a	39.24 ± 6.49^a	82.64 ± 6.28^a	1:1.1
Blanching	39.03 ± 2.22^b	29.13 ± 1.90^b	68.16 ± 0.32^b	1:1.3
Soaking in NaCl solution	36.27 ± 1.47^b	29.11 ± 2.46^b	65.38 ± 1.10^b	1:1.2
Chopping	35.62 ± 1.53^b	28.29 ± 0.68^b	63.91 ± 1.39^b	1:1.3
Same letters in the same column indicate that values are not significantly different (P ≤ 0.05).

Well-balanced compositions of fiber (SDF:IDF) were found for the lime residues used in this study. It is indeed recommended that the ratio of IDF to SDF should be in the range of 1.0–2.3 in order to obtain the physiological benefits from both fractions of fiber.[4] For using as a food ingredient, it is generally accepted that the SDF:IDF ratio should be close to 1:2.[2]

The DF contents of lime residue powder are presented in Table 4. Drying temperature in the selected ranges did not affect the DF content of the powder. The total loss of DF during the pretreatment processes was approximately 27.91% for SDF and 22.66% for TDF. After drying the SDF of lime residues was in the range of 29.12–29.55% while the TDF was in the range of 62.94–64.30%. SDF fraction of the sample was higher than those reported previously in the cases of Mexican and Persian lime residues, although the TDF was lower. It has been reported that dietary fiber powder produced from Mexican and Persian lime residues contained 21.86 and 20.26% SDF and 70.4 and 66.7% TDF, respectively.[23]

Table 4 Dietary fiber content (dry basis) of lime residues powder

Drying temperature (°C)	IDF	SDF	TDF	IDF:SDF
60	34.15 ± 1.28^a	29.55 ± 0.69^a	63.70 ± 0.70^a	1:1.2
70	33.82 ± 1.47^a	29.12 ± 0.73^a	62.94 ± 1.84^a	1:1.2
80	34.99 ± 1.48^a	29.31 ± 1.32^a	64.30 ± 1.73^a	1:1.2
Same letters in the same column indicate that values are not significantly different (P ≤ 0.05).

The values of the IDF, SDF, and TDF of powder of cabbage outer leaves are given in Table 5. Drying temperature did not have a significant effect (P ≤ 0.05) on the TDF contents of outer leaves of cabbage (40.50–43.15% TDF). IDF contents were much higher than the SDF contents leading to the SDF:IDF ratios of 1:7.5 to 1:7.8. Comparing with the previous works IDF contained in this white cabbage cultivar was much higher than in other cultivars. However, the SDF fractions were comparable. Wennberg et al.[18] reported that SDF and IDF contents of two cultivars of white cabbage were in the ranges of 17.3–20.0% and 4.7–6.2% (dry basis), respectively. This discrepancy may be due to the use of different cultivar along with the use of different plant portion for the analysis. In this work only discarded outer leaves were used as raw materials. The IDF of more matured leaves would be higher than the younger ones.

Table 5 Dietary fiber content (dry basis) of dried powder of cabbage outer leaves

Drying temperature (°C)	IDF	SDF	TDF	SDF:IDF
60	38.27 ± 5.24^a	4.88 ± 0.33^a	43.15 ± 5.56^a	1:7.8
70	37.82 ± 2.94^a	4.04 ± 0.19^a	41.86 ± 3.03^a	1:9.4
80	35.48 ± 1.43^a	5.03 ± 0.71^a	40.50 ± 1.65^a	1:7.1
Same letters in the same column indicate that values are not significantly different (P ≤ 0.05).

Hydration Properties

The effects of particle size and drying temperature on the water holding capacity (WHC) of dietary fiber powder are shown in Table 6. The effect of particle size on the WHC of lime residue powder was not observed in this study. These results were in contradiction with some previous works, which reported the strong effect of particle size on WHC.[17,24] However, not all cases exhibited such relationship. Parrott and Thrall[21] reported that WHC values of rice bran with different particle sizes were similar. No effect of particle size on the WHC of microcrystalline cellulose was also observed by Cadden.[15] The results suggested that different fiber possessed different hydration properties. Moreover, the process history also contributed to these values.

Table 6 Water holding capacity of high dietary fiber powder

	Drying temperature (°C)
	60		70		80
Particle size	Lime residues	Outer leaves of cabbage	Lime residues	Outer leaves of cabbage	Lime residues	Outer leaves of cabbage
300–450	11.59 ± 0.90^a	17.02 ± 2.71^c	11.33 ± 1.56^a	17.39 ± 2.32^c	11.15 ± 0.83^a	18.21 ± 1.52^d
250–300	11.40 ± 1.21^a	15.59 ± 1.96^c	12.00 ± 1.89^a	15.58 ± 0.61^c	11.12 ± 0.91^a	15.43 ± 1.20^c
150–250	12.49 ± 2.60^a	11.60 ± 1.34^b	11.46 ± 1.21^a	12.34 ± 0.74^b	10.78 ± 1.18^a	10.20 ± 0.93^b
63–150	11.74 ± 2.96^a	6.50 ± 1.05^a	10.28 ± 0.92^a	7.13 ± 1.43^a	11.04 ± 1.06^a	6.27 ± 0.28^a
Same letters in the same column indicate that values are not significantly different (P ≤ 0.05).

WHC values of powder of both lime residues and outer leaves of cabbage were in the range of 10.28–12.49 g/g. These high WHC values could aid gel formation and enhance texture stability of food products such as yoghurt and jams.[25] WHC values obtained in this study were similar to those of Mexican lime peel[23] and citrus pulp.[20]

Citrus fruits contain high amount of pectin, which influences the WHC of plant tissues.[22] Loss during preparation, however, leads to lower WHC of dietary fiber powder. Moreover, lime residues consist of many parts such as sack, peel and albedo, which are not homogeneous in nature; different individual parts compose of varying dietary fractions, which relate to WHC. Therefore, the effect of particle size on WHC was not clearly observed.

For the powder of cabbage outer leaves, WHC values were significantly higher than those of lime residues, except for the smallest particle size in the range of 63–150 μm. WHC of cabbage leave powder decreased with a decrease of the particle size. The results were in agreement with the previous reports. This phenomenon may be due to the damage of the fiber matrix and the collapse of the pore during grinding.[26]

The effect of particle size on SWC of lime residue powder was not observed. This may be due to from the heterogeneous fiber fraction and pretreatment methods mentioned earlier. The swelling capacities of lime residue powder were in the range of 13.66–15.87 ml/g (Table 7). Similar values were found for Maxican lime peel[23] and citrus fruits.[20]

Table 7 Swelling capacity of high dietary fiber powder

	Drying temperature (°C)
	60		70		80
Particle size	Lime residues	Outer leaves of cabbage	Lime residues	Outer leaves of cabbage	Lime residues	Outer leaves of cabbage
300–450	14.10 ± 1.88^a	21.38 ± 1.70^b	14.38 ± 2.01^a	20.73 ± 0.44^b	14.79 ± 0.80^a	21.41 ± 1.82^c
250–300	13.69 ± 1.19^a	19.82 ± 1.36^b	15.13 ± 0.75^a	19.45 ± 1.62^b	13.66 ± 0.77^a	20.32 ± 1.22^bc
150–250	13.87 ± 1.00^a	17.31 ± 1.22^a	14.55 ± 1.07^a	17.12 ± 1.32^a	14.50 ± 0.62^a	18.69 ± 0.68^ab
63–150	15.87 ± 1.01^a	16.84 ± 0.46^a	14.85 ± 1.11^a	16.58 ± 0.61^a	14.72 ± 0.69^a	16.37 ± 0.92^a
Same letters in the same column indicate that values are not significantly different (P ≤ 0.05).

The swelling capacities of cabbage outer leave powder were also significantly higher than those of lime residue powder and decreased with the particle size (Table 7). These high values could be related to the IDF content of the cabbage samples. The SWC values of powder of cabbage outer leaves were slightly higher than those of cauliflower fiber, which were in the range of 16.9–17.5 ml/g (dry basis).[28]

Although it is known that higher drying rates lead to lower hydration properties of fiber;[1,9] this phenomenon was not observed in the present study. Considering at similar particle sizes, drying temperature in the selected range did not significantly affect WHC and SWC of the samples (P ≤ 0.05). The results implied that drying within such temperature range, i.e., 60–80°C, did not affect much the structure of the powder samples.

CONCLUSIONS

The results demonstrated that pretreatment had a significant effect on the fiber composition of the samples. Boiling lime residues affected their dietary fiber content, especially SDF. However, the DF content of lime residues and outer leaves of cabbage were not affected by different drying temperatures. For hydration properties, WHC and SWC of lime residues and outer leaves of cabbage fiber powder were not significantly different among 60–80°C drying temperatures. Particle size of lime residues did not affect the WHC and SWC of their fiber powder whereas the particle size of cabbage outer leaves affected the WHC and SWC of their fiber powder. Decreasing the particle size of outer leaves of cabbage resulted in decreased WHC and SWC. It was concluded that type of raw materials and proper process establishment are very important factors in order to produce high-quality dietary fiber powder. The bioactive substances contained in the samples should also be monitored to ensure added value of the product.

ACKNOWLEDGMENTS

The authors express their sincere appreciation to the National Center for Genetic Engineering and Biotechnology, Thailand (BIOTEC) for the financial support. The authors also wish to thank Thai Q.P. Co., Ltd. for kindly providing the vegetable residue samples for use in the study.