Development of Bergamot Yam Soymilk Yogurt

Yi TANG Mei LI Lulu LI Fang CAO Weixin WANG

Abstract [Objectives] This study was conducted to develop a bergamot yam soymilk yogurt.

[Methods]With bergamot yam and soybeans as the main raw materials， the nutritious bergamot yam soymilk yogurt was finally obtained through soymilk deodorization， strain acclimation and process optimization.

[Results] Soybeans were soaked in a 1∶1 mixture of 0.2% sodium chloride and 0.2% sodium bicarbonate for 10 h， and then added with water for grinding， and the obtained slurry was filtered. The filtrate was heated at 100 ℃ for 20 min on a low fire and mixed with skim milk in a ratio of 6∶4， and the obtained soymilk achieved the best deodorizing effect. Strains adapted to the soybean milk fermentation system were obtained by gradually increasing the ratio of soybean milk in the acclimation medium and performing passage. The curding time before the product acclimation was 6.5 h， and that after acclimation was 5.8 h， which meant that the time was shortened by 0.7 h， and the acidity increased significantly. Finally， an orthogonal experiment was carried out， determining that the optimal fermentation process was soybean-to-water ratio 1∶5， sucrose 8%， bergamot yam 8%， and fermentation time 6 h.

[Conclusions]This study provides a reference for the deep processing of bergamot yam.

Key words Bergamot yam; Soymilk yogurt; Soybean deodorization; Strain acclimation; Process optimization

Received： May 1 2021  Accepted： July 15， 2021

Supported by School-level Scientific Research Project of Huanggang Normal University （2011CA069）; Outstanding Young and Middle-aged Scientific and Technological Innovation Team of Colleges and Universities in Hubei Province （T201820）.

Yi TANG （1998-）， male， P. R. China， professional title， devoted to research about of yogurt of Foshou yam and soybean.

*Corresponding author.

Yam is a perennial herbaceous plant belonging to the Dioscorea of Dioscoreaceae， and the tuber shape has 3 types： long， flat and lump[1]. Guangji Bergamot yam is one of the main crops with traditional characteristics in Wuxue City， Hubei Province. Guangji bergamot yam has high starch content， rich nutrients， good taste， and is rich in vitamins， trace elements and amino acids[2-3]. Li Shizhens "Compendium of Materia Medica" pointed out that bergamot yam can cure various weaknesses， treat low back pain， promote memory， invigorate the spleen and stomach， lower blood pressure， and slow aging. Modern medicine has proved that yam has the functions of lowering blood sugar， blood lipids， regulating the stomach， and regulating or enhancing immunity[4-5]. In this study， with bergamot yam， soybeans and skimmed milk powder as materials， a bergamot yam soymilk yogurt product was developed， aiming to provide a reference for the deep processing of bergamot yam.

Materials and Methods

Reagents and Materials

Bergamot yam （produced in Wuxue City， Hubei Province）; skimmed milk powder; soybeans; sucrose （purchased from Huangshang Shopping Center supermarket）; strains （1∶1 direct injection of Lactobacillus bulgaricus and Streptococcus thermophilus from Angel Yeast Company）; food-grade sodium bicarbonate and sodium chloride.

Experimental methods

Technological process

Skimmed milk powder was prepared into cow milk through rehydration.

Soybeans were processed into soybean milk by the steps of soaking and skin removing， grinding， boiling and filtration.

Peeled yam was prepared into a yam juice by the steps of slicing， grinding， filtration and homogenization.

The strains were activated and acclimated.

The obtained cow milk and soybean milk were mixed， obtaining a mixture of soybean milk and cow milk， which was inoculated with the acclimated strains， and a soymilk yogurt product was obtained after fermentation and post curing.

Preparation of bergamot yam juice

Commercially available peeled bergamot yam was washed， and sliced into 5 mm. The cut yam was added with water at a ratio of 1∶3， grinded， and filtered， obtaining a yam juice.

Preparation of mixture of soybean milk and cow milk

Determination of soaking conditions

Soybeans were selected to remove impurities. After cleaning， they were soaked in three different soaking solutions at a ratio of 1∶1 for 8， 10， and 12 h， respectively. After soaking， water was added into the soybeans， which were then grinded to a slurry， which was boiled and filtered for sensory evaluation.

Determination of the cooking method

Three methods were used to boil the slurry， i.e.， heating on a low fire and then boiling for 20 min， heating on a medium fire and boiling for 15 min， and boiling on a high fire for 10 min. After boiling the slurry， it was filtered， and the soybean milk obtained was subjected to sensory evaluation.

Determination of the ratio of soybean milk and cow milk

Reconstituted milk was prepared by adding 180 ml of water for every 25 g of milk powder. Under the same other treatment conditions， soybean milk and cow milk were mixed in different proportions. The mixing ratios were set to five groups of 2∶8， 4∶6， 5∶5， 6∶4， and 8∶2 to obtain the mixtures of soybean milk and cow milk for sensory evaluation.

Acclimation experiment of strains for the fermentation of soymilk yogurt

Strain activation

The strains refrigerated in a refrigerator were taken out and placed at room temperature for 10 to 15 min， and added to sterilized skimmed milk test tubes， which were stirred and shaken for mixing. The strains were cultured at 43 ℃， and transferred and cultured every 8 h until they became vigorous， that is， the milk was fermented within 4 to 6 h.

Strain acclimation methods

Preparation of acclimation fluid： First， soybean milk was added with 8% of sucrose， and mixed with skimmed milk at volume ratios of 1∶10， 2∶8， 4∶6， and 6∶4， respectivley. The mixtures were added into test tubes according to 10 ml per test tube， respectively， and numbered ①， ②， ③ and ④， respectively. The mixtures were sterilized at 115 ℃ for 15 min， and then cooled to obtain culture liquids for strain acclimation[6].

Experimental method： The activated strains were inoculated to the acclimation liquid No. ① and cultured at 43 ℃. After curding， the culture liquid ① was inoculated into the No. ② culture liquid. Inoculation was performed in sequence to gradually increase the ratio of soybean milk in the soybean milk and cow milk system， and the acclimated culture liquid No. ④ was finally obtained.

Technological optimization experiment of bergamot yam soymilk yogurt

Single-factor experiments

（1） Effect of material-to-liquid ratio for grinding on soymilk fermentation： In the soybean grinding process， different ratios of material to water were set according to 1∶3， 1∶5， 1∶7. The prepared soybean milk was mixed with cow milk in a ratio of 6∶4， and added with 8% of sucrose and 10% of filtered yam juice. The mixtures were sterilized at 85 ℃ for 30 min， inoculated with the strains after cooling， and cultured at 43 ℃ for 6 h. The mixtures were then post-cured at 4 ℃for 12 h， and observed for the state and tasted.

（2） Effect of the amount of sucrose added on soymilk fermentation： Soybean milk was prepared with the material-to-water ratio of 1∶5 during grinding. It was mixed with cow milk in a ratio of 6∶4， and 10% of filtered yam juice was then added. Sucrose was added at different proportions of 6%， 7%， 8%， 9% and 10%， respectively. The mixtures were sterilized at 85 ℃ for 30 min， inoculated with the strains after cooling， and cultured at 43 ℃ for 6 h. The cultures were post-cured at 4 ℃for 12 h， and observed for the state and tasted.

（3） Effect of the amount of yam juice added on soymilk fermentation： Soybean milk was prepared with the material-to-water ratio of 1∶5 during grinding. It was mixed with cow milk in a ratio of 6∶4， and 8% of sucrose was then added. Yam juice was added at different proportions of 5%， 10%， 15%， 20% and 25%， respectively. The mixtures were sterilized at 85 ℃ for 30 min， inoculated with the strains after cooling， and cultured at 43 ℃ for 6 h. The cultures were post-cured at 4 ℃for 12 h， and observed for the state and tasted.

（4） Effect of fermentation time on the texture of soymilk yogurt： Soybean milk was prepared with the material-to-water ratio of 1∶5 during grinding. It was mixed with cow milk in a ratio of 6∶4， and 8% of sucrose and 10% of yam juice were then added. After mixing by stirring， the mixture was sterilized at 85 ℃ for 30 min， and inoculated with 4% of the strains after cooling. It was cultured at 43 ℃ for 5， 6 and 7 h， respectively. The cultures were post-cured at 4 ℃for 12 h， and observed for the state and tasted.

Orthogonal experiment

Based on the single factor experiments， this experiment optimized the four factors of fermented soybean milk， soybean-to-water ratio for grinding， amount of sucrose added， amount of bergamot yam juice added， and fermentation time. An orthogonal experiment was designed at the L9 （34） level. Other conditions were as followed： the ratio of soybean milk to cow milk at 6∶4， the sterilization condition of 85 ℃ for 30 min， the culture temperature at 43 ℃， and the post-curing condition of 4 ℃ for 12 h. Finally， the orthogonal results were analyzed to draw conclusions.

Sensory evaluation

Through tasting， a comprehensive evaluation of the four aspects of the products taste， texture， color and flavor was carried out. Five experienced tasters without special tastes were organized to taste the products. A 100-point scoring method was adopted， and the scoring standard was based on the scoring table[7].

Effects of different boiling conditions on soybean milk deodorization

It can be seen from Table 4 that for the heat treatment of soybeans， when the soybean milk was heated to 100 ℃ and then boiled for 20 min under a low fire， the soybean milk had a strong aroma and fine texture， and the best deodorizing effect was achieved.

Effects of soybean milk-to-cow milk ratio on deodorization

It can be seen from Table 5 that the addition of milk could significantly reduce the beany smell， and the most appropriate amount was 60∶40.

Analysis on the results of acclimation experiment on the strains for fermented soymilk yoghurt

Strain activation results

The strains used in this experiment were stored for a long time at -18 ℃， and their vitality was weakened after a long storage time， so they should be activated through several passages to recover their vitality for fermentation.

The newly purchased strains were stored at -18 ℃， and determined for their curding ability every week， 5 weeks in total. It can be clearly seen from Fig. 1 that the strains activity gradually decreased during the long-term low-temperature storage process and the fermentation time was prolonged. The initial curding time was 5 h， and after one week， the curding time increased to 5.5 h. Until the fifth week， the curding time reached 8 h. In order not to affect the taste and texture of soymilk， the strains were activated.

Domestication results of strains

In the process of fermenting soymilk with lactic acid bacteria， because soybean milk is greatly from the cow milk system and soybean milk lacks lactose that can be used by lactic acid bacteria， lactic acid bacteria grow poorly in soybean milk， and the number of viable bacteria and acidity after fermentation are both lower than those of cultured milk[7]. The curding speed is slow， the texture is not uniform enough， and the bad flavor produced directly affects the quality and taste of the product.

It can be seen from Fig. 2 that when the medium ratio was changed， the effect on the curding time was obvious. When the proportion of soybean milk in the whole soymilk continued to rise， the curding time was also significantly prolonged.

It can be seen from Fig. 3 that when the medium ratio was changed， the effect on pH value was also very significant. As the proportion of soybean milk in the whole soymilk continued to rise， the pH value increased， but it increased slowly after acclimation. The acidity increased significantly compared with before acclimation.

Analysis of orthogonal optimization experiment results

Single-factor experiment results

Effects of different soybean-to-water ratios on the quality of fermented soymilk yoghurt

It can be seen from Table 6 that the ratio of material to water was 1∶5.

Effects of different amounts of sucrose added on the quality of fermented soymilk yoghurt

It can be seen from Table 7 that when the sugar content was 8%， the taste of soymilk was the best， with suitable sweetness and sourness， and the curding ability was good.

Effects of amount of bergamot yam juice added on the quality of fermented soymilk yoghurt

It can be seen from Table 8 that when the amount of yam juice added was 10%， the taste of yam soymilk was the most suitable， and the texture is fine， without bad curding phenomenon.

Effects of fermentation time on the quality of fermented soymilk yoghurt

It can be seen from Table 9 that through sensory evaluation， it was known that fermentation time had a significant effect on the texture of soymilk， and the result was the best at the fermentation time of 6 h.

Orthogonal experiment results of bergamot yam soymilk yoghurt

It can be seen from the range table that the significant degrees of influencing factors ranked as material-to-water ratio>fermentation time>yam juice amount>sucrose amount， namely A>D>C>B. Among them， the material-to-water ratio had the greatest impact on the finished product. The main effects of the ratio of material to water were on the taste and color of the product. When the ratio of material to water was 1∶5， the score was highest. The amount of sugar added mainly affected the sweetness and sourness of the product. When the amount of sugar added reached 7%， the sweetness and sourness of the soymilk yoghurt were suitable. The amount of yam juice added mainly affected the texture and taste of the product. The fermentation time mainly affected the texture and flavor of the product.

Since the best bergamot yam soymilk yoghurt technology and formula was A2B2C3D2. It was not in the orthogonal table， so a verification test was carried out. The finished product was scored according to the scoring table， giving a score of 82.6， which did not reach the highest score in the orthogonal table. Therefore， A2B3C1D2 in the orthogonal table was selected as the best ratio， that is， the ratio of soybean to water at 1∶5， the addition amount of sucrose of 8%， the addition amount of bergamot yam of 8%， and the fermentation time of 6 h. The obtained product had the highest comprehensive score for taste， texture， color and flavor.

Conclusions and Discussion

In this study， deodorized soybean milk was mixed with reconstituted milk， added with appropriate amount of bergamot yam juice， and inoculated with an acclimated mixed starter of L. bulgaricus and S. thermophilus. After fermentation， a kind of bergamot yam soymilk yoghurt with fine texture and good taste without obvious beany smell was obtained. The successful development of this product can provide a reference for the deep processing of bergamot yam.

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