Abstract:
Disclosed is a method for producing a highly concentrated, rice-derived lactic acid fermented product which tastes highly sweet and sour, and of which the flavor and texture are excellent. In this method, unground, grainy rice is gelatinized and then it is liquefied and saccharified by the primary treatment with α-amylase and glucoamylase. After sterilization, it is inoculated with lactic acid bacteria and undergoes a secondary treatment with α-amylase and glucoamylase. The glucose produced thereby continuously ferments to lactic acid. Finally, high concentrated lactic fermented product, tasting sweet and sour, and having excellent flavor and texture, is produced.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for producing a highly concentrated lactic-acid product fermented from rice, tasting sweet and sour, and having excellent flavor and texture. 
     In particular, the present invention relates to a method for producing the product comprising the steps of gelatinizing grainy rice which has not been ground, treating the gelatinized rice with α-amylase and glucoamylase thereby to producing glucose from rice-starch, which is a substrate for lactic acid fermentation, and producing lactic acid continuously by lactic acid fermentation. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method for producing a highly concentrated, rice-derived lactic acid fermented product which tastes sweet and sour comprising the steps of immersing unground, grainy rice in water to gelatinize it, treating the gelatinized rice with an amylolytic enzyme containing α-amylase and glucoamylase to liquefy and saccharify it, heating the saccharified solution to inactivate the amylolytic enzyme and to sterilize the solution at the same time, inoculating the sterilized solution with a starter containing Streptococcus and Lactobacillus, fermenting the solution at high concentration, and homogenizing the fermented solution. 
     The present invention is also directed to a method for improving the qualities of a rice-derived, lactic acid fermented product which tastes highly sweet and sour, and of which the flavor and texture are excellent, comprising the steps of immersing unground, grainy rice in water to gelatinize, treating the gelatinized rice with amylolytic enzyme including α-amylase and glucoamylase to liquefy and saccharify it, heating the saccharified solution to inactivate the amylolytic enzyme and to sterilize the solution at the same time, inoculating the sterilized solution with a starter containing Streptococcus and Lactobacillus, fermenting the solution at high concentration, simultaneously with the secondary addition of amylolytic enzyme including α-amylase and glucoamylase in a bacteria-free state to produce glucose, which is a substrate for lactic acid fermentation, and homogenizing the fermented solution, whereby the glucose produced by the secondary enzymatic treatment is continuously fermented into lactic acid providing the final product with plenty of oligoosaccharides, monosaccharides, and lactic acid. 
     The Streptococcus of both processes discussed above may be Streptococcus thermophilus. The Lactobacillus of both processes discussed above may be Lactobacillus acidophilus, Lactobacillus bulgaricus, or Lactobacillus plantarum. 
     The mixing ration of Streptobacillus and Lactobacillus is 1 to 1. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph showing the effect of acid-production in rice-derived, lactic acid fermented product by the secondary enzymatic treatment according to the present invention; 
     FIG. 2 is a graph showing the effect of overall preference in rice-derived, lactic-acid fermented product by the secondary enzymatic treatment according to the present invention; and 
     FIG. 3 is a photomicrograph showing micro-structure of rice-derived, lactic acid fermented product which has been treated secondarily with the enzyme according to the present invention. 
    
    
     Hereunder, the present invention is described in detail. In the specification, all % is by weight unless specified otherwise. 
     DETAILED DESCRIPTION OF THE INVENTION 
     (1) Gelatinization of rice 
     Rice used in the invention was &#34;Sunchang, Junrabuk-Do&#34; in Korea in 1989. When ground rice suspended in water is gelatinized, the viscosity of the suspension increases greatly and therefore, it is hard to stir the suspension. Accordingly, it is almost impossible to stir the suspension uniformly at a solution concentration of 10-15%. Furthermore, the heat transfer rate of the solution becomes decreased due to its high viscosity, thereby increasing energy consumption. 
     In the present invention unground, grainy rice is immersed in water at 40°-50° C. for 30 minutes (rice=1:1.25 part by weight), followed by heating the same at 100° C. for 30 minutes to gelatinize the starch thereof. 
     (2) Primary liquefaction, saccharification and sterilization 
     Enzymes used in the invention were α-amylase (90 units/mg, manufactured by Sigma Chemical Co., St. Louis, MO., U.S.A.) and glucoamylase (8400 units/g, Sigma Chemical Co., St. Louis, MO., U.S.A.) 
     A 50-75% enzyme solution containing 0.1% of amylase and 0.1% of glucoamylase is added to the gelatined rice at 55° C. for 1-2 hours, to simultaneously liquefy and saccharify the same simultaneously. Optimum sweetness after saccharification is determined as about 15°-20° Brix. If too much saccharification is conducted, the too much, sweetness of the solution increases too much, inhibiting the growth of lactic acid bacteria due to its high osmotic pressure. The inventors have tried to provide sweetness of the solution suitable for the growth of lactic acid bacteria, employing the enzymes for liquefaction and saccharification which are inactivated by heating at 95° C. for 30 minutes, which also results in sterilization of the solution. 
     (3) Fermentation with secondary enzymatic treatment 
     Lactic acid bacteria used in the invention are three species of lactobacillus and one species of Streptococcus. The lactic acid bacteria which can be used in the invention are selected from one or more species from the group consisting Strephococcus thermophilus, Lactoba cillus, acidophilus and L. bulgaricus. L. plantarum bacteria can also be used only if they conduct a lactic acid fermentation from rice. Lactic acid bacteria used in the invention were Lactobacillus acidophilus ATCC 11506, L. bulgaricus KCTC 2179 and L. Plantarum ATCC 8014, and Streptococcus thermophilus KCTC 2185. 
     The lactic acid bacteria can be used alone or a mixture of one of those three Lactobacillus and Streptococcus thermophilus KCTC 2185,a mixing ratio being 1:1, can be used, also. The lactic acid bacteria are added thereto in an amount of 2%, which have been activated in a rice-saccharified solution. 
     A starter of lactic-acid bacteria is inoculated therewith and the saccharified solution is treated with sterilized, bacteria-free enzym atic solution in which α-amylase alone or α-amylase mixed with gluco-amylase is contained. 
     By the treatment with the enzymatic solution, rice-starch is hydrolyzed into glucose, which is a substrate for lactic-acid fermentation, so that glucose is continuously fermented into lactic acid to obtain a fermented product tasting highly sweet and sour. The fermentation is conducted at 37° C. and completed after 15-30 hours. After the (fer) mentation the fermented solution is homogenized by a homogenizer (Nissei AM-8, manufactured by Nihonseiki Kaisha Ltd, Japan) to obtain the final product. 
     After fermentation for 18 hours, acid-production has been increased greatly due to the secondary enzymatic treatment, compared with a control plot. Also, acid-production with Lactobacillus plantarum appears small but production with the mixed culture (with Streptococcus thermophilus) improves greatly. Generally, acid-production appears larger when fermented by a mixture of Lactobacillus and Streptococcus thermophilus however, acid-production appears small when fermented by Lactobacillus  alone. 
     The results of the organoleptic test for the product fermented for 18 hours are shown in FIG. 2. The organoleptic test (sensory evaluation) which was repeated three times, was conducted by six trained panel members and was evaluated by a 5-point method. 
     
         ______________________________________  Very good     5 scores  Good          4 scores  Fair          3 scores  Bad           2 scores  Very bad      1 scords______________________________________ 
    
     In consideration of the effects of the secondary enzymatic treatment, the treatment plot with 0.02% of α-amylase reveals no significant difference compared with the control plot; however the treatment with each 0.02% of α-amylase and glucomylase brings about significant improvement in the overall preference (overall eating quality score). Furthermore, examining the overall preference according to the species of lactic acid bacteria, we have found out that the best preference is obtained when the fermentation is conducted by the mixed Lactobacillus bulgaricus and Streptococcus thermophilus, the mixing ratio being 1:1, and each 0.02% of each of α-amylase and glucoamylase are used. 
     The micro-structure of the product treated with the secondary enzymes, as shown in FIG. 3, brings about a conspicuous reduction in the size of insoluble solid particles, so that the texture in the mouth increases greatly. While not wishing to be bound by any theory, it is believed that insoluble solid portions are caused by the retrogradation of starch, and that the secondary enzymatic treatment prevents starch from retrogradation thereby reducing the size of the solid portion. 
     It is generally accepted that the critical particle size to be sensed extraneous in the mouth is about 20 μm. The insoluble solids which have been treated with the fermenting strain and the secondary enzymes according to the present invention were measured by a microscope equipped with a micrometer, and the results are shown in Table 1. 
     
                       TABLE 1______________________________________The effects of the particle size in insoluble solid portionsof rice-derived, lactic acid fermented product by the secondaryenzymatic treatment at fermentation.      particle size (μm)lactic acid          0.02%     0.02% α amylase +amylasea     control α        0.02% glucoamylase______________________________________L. acidophilus (L.a.)        27.0    11.8      8.2L. bulgaricus (L.b.)        17.4    14.5      8.9L. plantarum (L.p.)        23.0    21.0      12.8S. thermophilus (S.t.)        20.6    8.8       6.9L.a. + S.t.  22.7    10.5      12.5L.b. + S.t.  24.2    13.5      9.3L.p. + S.t.  19.8    10.3      8.1______________________________________ 
    
     The control, which has not been treated with the secondary enzymes, reveals 17-27 μm in average particle size, whereas the product which has been treated with 0.02% of α-amylase caused reduction of particle size to reach 9-21 μm. The product which has been treated with 0.02% of each of α-amylase and glucoamylase brings about a remarkable reduction of particle size to reach 7-13 μm thereby great contributing toward the improvement in texture. 
     Lactobacillus bulgaricus and Streptococcus thermophilus selected as improved strains, were used to ferment rice for 18 hours, and the effects of the secondary enzymatic treatment on organoleptic elements are shown in Table 2 with respect to the fermented product thereby obtained. 
     
                                           TABLE 2__________________________________________________________________________The effects to organoleptic elements of rice-derived, lacticacid fermented product with the secondary enzymatic treatmentat fermentationCondition forsecondary  Organoleptic elementsenzymatic                        overalltreatment  flavor           taste               smoothness                      consistency                            preference__________________________________________________________________________Control    3.2A 3.8BC               3.7A   3.3A  2.8Camylasealpha.      3.4A 2.9C               2.7B   3.0A  3.8BCamylase +pha.      3.6A 4.0AB               3.7A   2.8A  3.8AB0.02% glucoamylaseamylase +pha.      3.5A 4.3A               4.0A   2.7A  4.3A0.06% glucoamylase__________________________________________________________________________ *There is no significant difference at 5% level with respect to the score marked by the same character in each organoleptic element. 
    
     The effects of the secondary enzymatic treatment with respect to preference of flavor, smoothness and consistency reveals no significant difference but remarkable improvements were observed in taste and overall preference. 
     Significant difference was not recognized between the case treated with 0.02% of each of α-amylase and glucoamylase and the other case treated with 0.06% of each of the same; therefore, the proper level in which α-amylase and glucoamylase are added thereto is determined as 0.02% each of both enzymes. 
     The properties of the product treated with each of 0.02% of α-amylase and glucoamylase at fermentation using Lactobacillus bulgaricus and Streptococcus thermophilus according to said procedures are shown in Table 3. 
     
                       TABLE 3______________________________________Properties of rice-derived lactic acid fermented productItem______________________________________pH               2.95acidity (%)      0.53moisture content (%)            76.10solid (%)        23.90protein (%)      2.00sweetness (° Brix)            24.00number of viable 1.54 × 10.sup.8bacteria per mlapparent viscosity (mPa s)            35 (shear rate 100s.sup.-1)______________________________________ 
    
     EXAMPLE 
     Rice immersed in water or crushed rice obtained during milling (rice: water=4:5 parts by weight) was heated for 30 minutes to gelatinize, followed by the addition of 75% of amylase-solution containing 0.1% of α-amylase and 0.1% of glucoamylase to the gelatinized rice, thereby to saccharifying the same for 1-2 hours. Thereafter, a starter containing 1% of each of Lactobacillus bulgaricus and Streptococcus thermophilus which have been activated in a rice-saccharified solution was added and 0.02% of each enzyme (α-amylase and glucoamylase) sterilized by passing through a bacteria-free filter was added thereto at 37° C. to ferment for 15-30 hours. After homogenization, the highly concentrated, rice-derived lactic acid fermented product without any additives such as sugar and acid was obtained, which tasted sweet and sour, and of which the flavor and texture were excellent.