Chuanbin WEI, Ping ZHANG

College of Life Science, Jianghan University, Wuhan 430056, China

Purple sweet potatoes(Solanum tuberdsm) are rich in starch and can be used as raw materials in grain and food industries.Moreover, purple sweet potatoes contain more essential trace elements and lysine than ordinary sweet potatoes[1]. In particular, purple sweet potatoes are abundant in anthocyanins which are natural food pigments[2],thus exhibiting certain health care functions[3]. Therefore, purple sweet pota toes have been widely grown and popularized in recent years and have gradually become novel favorite health care foods.In food processing,starches should be gelatinized before being digested by saccharifying enzyme for better digestion, absorption and utilization by the body[4]. Anthocyanins extracted from the raw materials can be absorbed by the body, thus producing health effects. However, anthocyanins can be easily lost or extracted inadequately in the processing of purple sweet potatoes,thereby limiting the application and health functions of purple sweet potatoes.

Purple sweet potatoes exhibit high production seasonality, inconvenient storage and high transportation costs. Therefore, purple sweet potatoes are generally prepared into raw materials before processing. In this study, purple sweet potatoes were dried at two specific temperatures and prepared into preliminarily-processed products(purple sweet potato powder)to analyze the extraction characteristics of anthocyanins under simulated gelatinization conditions and investigate an efficient method for the development and utilization of purple sweet potatoes with high anthocyanin retention amount and extraction rate, aiming at providing theoretical reference for the deep processing of purple sweet potatoes.

Materials and Methods

Materials

Experimental purple sweet potatoes(Yuzi 263)were planted and provided by Wuhan Runnong Technology Co.,Ltd.

Methods

Preparation of preliminarily-processed samples of purple sweet potatoes According to the thermal stability properties of anthocyanins,anthocyanins are relatively stable below 60 ℃[2]. The amount of dry matter in cereals, grains and other plant tissues was measured. All plant materials were rapidly dehydrated and dried at 110 ℃[5]. Thus, in this study, purple sweet potatoes were dried at 60 ℃and 110 ℃to prepare preliminarilyprocessed samples.

Fresh purple sweet potatoes were washed, air-dried, peeled, sliced into 0.3-0.4 mm pieces, dried at 60 ℃and110 ℃to a constant weight (the weight before and after drying was weighed to calculate the moisture content) and bottled (as sample 1 and sample 2)before use.

Due to the high activity of anthocyanins, temperature, pH, oxygen,ascorbic acid, metal ions and other factors can affect the stability of anthocyanins. Therefore, in the preparation process of preliminarily-processed samples, purple sweet potatoes should be sliced rapidly using plastic cutting boards and ceramic cutting tools.

Determination of anthocyanin content in preliminarily-processed samples of purple sweet potatoes

In accordance with the method proposed by Liu et al.[6], anthocyanin content in preliminarily-processed samples of purple sweet potatoes was determined by citric acid-disodium hydrogen phosphate buffer and calculated according to the following formula:

Where, C indicates anthocyanin content (mg/kg fresh potatoes);1/958 is the empirical absorption coefficient of anthocyanins; V indicates the final volume (100 ml); m indicates the weight of fresh potatoes equivalent to that of experimental samples (g);A525indicates the absorbance of anthocyanin extract at 525 nm;1 000 000 is the correction value for unit conversion.

Approximately 1 g of preliminarily-processed samples of purple sweet potatoes (sample 1 and sample 2) were weighed, transferred into 50 ml graduated vitreous centrifuge tubes with stoppers, added with extraction solution (0.1 mol/L citric acid standard solution∶0.2 mol/L disodium hydrogen phosphate standard solution=15.89 ∶4.11, V/V) to 50 ml, stoppered, incubated in a water bath at 60℃for 2 h,cooled to room temperature,set to a constant volume using a 100 ml volumetric flask, and placed in the dark for 2 h with shaking. Subsequently, an appropriate amount of mixture was transferred into a centrifuge tube,and centrifugated at 4 000 r/min for 3 min; the supernatant was collected to measure the absorbance A525at 525 nm(using a cuvette 1 cm in thickness) with the extract as a reference. Each sample was measured three times. The anthocyanin content was calculated and averaged as the final anthocyanin content in experimental samples.

Gelatinization of preliminarily-processed samples of purple sweet potatoes and determination of extraction amount of anthocyanins during the gelatinization process

Simulated gelatinization of experimental samples Preliminarily-processed samples were gelatinized in accordance with the method proposed by Chang et al[7]. Respectively 20 g of experimental samples (sample 1,sample 2) were weighed, added with 200 ml of distilled water (the ratio of material to water=1:10 g/ml)in a 500 ml flask,stirred, and placed into a programmed thermostatic water bath for gelatinization according to the following procedures. ①Incubation at 50 ℃for 30 min→②heating(with a heating rate of 1.5 ℃/min)to 90 ℃→③incubation at 90 ℃for 20 min→④heating(with a heating rate of 1.5 ℃/min) to 95 ℃→⑤incubation at 95 ℃for 15 min→⑥natural cooling to 50 ℃.

Extraction amount, extraction rate and extraction efficiency of anthocyanins from gelatinized samples

An appropriate amount of gelatinized samples in the programmed thermostatic water bath were collected using a straw after incubation at 50 ℃for 30 min(the end of gelatinization stage 1),before heating to 90 ℃ (the beginning of gelatinization stage 2),after incubation at 90 ℃for 20 min (the end of gelatinization stage 3), before heating to 95 ℃(the beginning of gelatinization stage 4), after incubation at 95 ℃for 15 min (the end of gelatinization stage 5), and after natural cooling to 50 ℃(gelatinization stage 6), respectively,transferred to a centrifuge tube, centrifugated at 4 000 r/min for 3 min(samples with extremely high viscosity were diluted with deionized water by a certain ratio, which should be corrected in subsequent calculations); after centrifugation, the supernatant was collected to measure the absorbance A525at 525 nm.

The extraction amount of anthocyanins from gelatinized samples was calculated according to the following formula:

Where,C1indicates the extraction amount of anthocyanins in the supernatant of gelatinized samples at different gelatinization stages (mg/kg fresh potatoes); m1indicates the weight of experimental samples (g); V1indicates the total volume of prepared gelatinized samples(ml);A′525indicates the absorbance of the supernatant of gelatinized samples at different gelatinization stages(this formula was obtained based on the method proposed by Liu et al.[6]with modifications according to sample dilution).

The maximum extraction rate of anthocyanins and extraction efficiency of anthocyanins at different gelatinization stages were calculated according to the following formulae:

Maximum extraction rate of anthocyanins (%) = Maximum extraction amount of anthocyanins (mg/kg fresh potatoes)/Anthocyanin content (mg/kg fresh potatoes)×100

Extraction efficiency of anthocyanins[mg/(kg fresh potatoes·min)]=Difference in extraction amount of anthocyanins/Extraction time

Results and Analysis

The anthocyanin content in sample 1 and sample 2, and extraction amount of anthocyanins from gela-tinized samples at different gelatinization stages were shown in Fig.1. The extraction rate of anthocyanins from sample 1 and sample 2 at different gelatinization stages was shown in Fig.2. The extraction efficiency of anthocyanins at different gelatinization stages was shown in Fig.3.

Comparative analysis of the extraction amount of anthocyanins during the simulated gelatinization process

As shown in Fig.1, during the simulated gelatinization process, total content of anthocyanins in sample 1 was 180.83 mg /kg (fresh potatoes),and the maximum extraction amount(incubation at 95 ℃for 15 min) of anthocyanins from sample 1 reached 158.26 mg/kg (fresh potatoes); total content of anthocyanins in sample 2 was 80.83 mg/kg(fresh potatoes),and the maximum extraction amount (incubation at 95 ℃for 15 min) of anthocyanins from sample 2 reached 66.54 mg/kg (fresh potatoes). The maximum extraction amount of anthocyanins from two samples were both present after incubation at 95 ℃for 15 min.

Comparative analysis of the extraction rate of anthocyanins during the simulated gelatinization process

As shown in Fig.2, anthocyanins were extracted from two samples. To be specific, the extraction rate of anthocyanins from sample 1 reached the highest during heating from 90 ℃to 95℃ [the extraction amount of anthocyanins was 93.20 mmg/kg (fresh potatoes), accounting for 58.89% of the total extraction amount]; the extraction rate of anthocyanins from sample 2 reached the highest during incubation at 95 ℃for 15 min [the extraction amount of anthocyanins was 31.39 mmg/kg (fresh potatoes), accounting for 47.17% of the total extraction amount]. Although other gelatinization stages exhibited longer duration, the extraction rate of anthocyanins was relatively lower.

Comparative analysis of the extraction efficiency of anthocyanins during the simulated gelatinization process

As shown in Fig.3, during the simulated gelatinization process, the maximum extraction rate of anthocyanins from sample 1 reached 27.96 mg/(kg·min)during heating from 90 ℃to 95 ℃,followed by 1.66 mg/(kg·min)during incubation at 50 ℃for 30 min,and 1.11 mg/(kg·min) during incubation at 95 ℃for 15 min; the maximum extraction rate of anthocyanins from sample 2 reached 3.63 mg/(kg·min)during heating from 90 ℃to 95 ℃,followed by 2.09 mg/(kg·min) during incubation at 95 ℃for 15 min, and 0.80 mg/(kg·min)during incubation at 50 ℃for 30 min. The extraction rate of anthocyanins from two samples both reached the maximum during heating from 90 ℃to 95 ℃.

In addition, during heating to 90℃, the extraction amount of anthocyanins was reduced, and the extraction rate was negative, because the degradation rate of anthocyanins was higher than the extraction rate of anthocyanins due to temperature factor.

Conclusion and Discussion

Raw starches in raw-food materials are present in the starch granules.Heating in preliminary processing will change the structure of starch granules (particle compactness). At different temperatures, the compactness of starch granules varies.These changes will influence the dissolution (gelatinization) of starches during the gelatinization process[8]and the dissolution(extraction) of anthocyanins in starch granules.Starch granules dried rapidly at 110 ℃exhibit higher compactness,longer extraction time,lower extraction efficiency and lower maximum extraction rate of anthocyanins during the gelatinization process compared with those dried at 60 ℃.Non-extracted anthocyanin residues in starch granules are difficult to be absorbed and utilized, which are essentially ineffective although the biological activity remains high.

Moreover, different preliminary processing temperatures will change the retention amount of anthocyanins.Chen et al.[9]determined anthocyanin content in purple sweet potato samples dried at different temperatures and found that the determined values varied extremely significantly. After rapid drying at 60 ℃, the retention amount of anthocyanins in purple sweet potato powder was 180.83 mg/kg(fresh potatoes); after rapid drying at 110 ℃, the retention amount of anthocyanins in purple sweet potato powder was 80.43 mg/kg(fresh potatoes).

In addition, at the end of the gelatinization process, gelatinized sample 1 and sample 2 were cooled from 95 ℃to 50 ℃; results show that anthocyanin content in the supernatant was lower than the maximum extraction amount, because cooled starch granules form starch gels and re-adsorb anthocyanins extracted from the solution.

According to the experimental results,during the gelatinization process,the extraction effect of anthocyanins is not significant at the initial gelatinization stage below 90 ℃. However, the effects of initial gelatinization stage on the extraction of anthocyanins in sub-sequent stages remain unknown. Furthermore,the ratio of material to water exerts certain effects on the extraction of anthocyanins[10], but the specific effects of changes in ratio of material to water on extraction characteristics of anthocyanins during the gelatinization process require further investigation.

In the development of novel health-care foods using purple sweet potatoes, the content of active substances and whether these active substances can be effectively utilized should both be considered. Thus, according to the experimental results,under the premise of full consideration of production cycle and production costs,purple sweet potatoes should be preliminarily-processed at low temperatures to improve the retention amount of anthocyanins in preliminarily-processed products. In subsequent processing of preliminarily-processed products of purple sweet potatoes, under the premise of ensuring starch gelatinization and anthocyanin extraction effects, the gelatinization stage at 90-95 ℃is indispensable,and the initial gelatinization stage below 90 ℃can be appropriately shortened.

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