Protein-containing acidic foods and drinks

Provided are a method for producing a protein-containing acidic food and drink that comprises processing a protein emulsion having a pH value that is higher than the isoelectric point of the protein in the emulsion at a high temperature to make the emulsion having a pH value that is lower than the isoelectric point of the protein; the protein-containing acidic food and drink produced in the method; protein-containing acidic food and drink containing protein, fat and oil, and water-soluble polysaccharide, in which the particles dispersed have a mean particle size of not greater than 15 .mu.m; and a protein-containing acidic drink containing protein, fat and oil, and water-soluble polysaccharide, in which the particles dispersed have a mean particle size of not greater than 15 .mu.m and/or having a viscosity not greater than 20 centipoises (cPs). The protein-containing acidic food and drink is smooth, tasteful, palatable and has good storage stability.

FIELD OF THE INVENTION
 The present invention relates to protein-containing acidic foods and drinks
 which are smooth, tasteful, palatable and have good storage stability.
 BACKGROUND OF THE INVENTION
 Known examples of protein-containing acidic food and drink are acidic milk
 drinks. These acidic milk drinks are typically prepared by adding sours
 such as fruit juices, acidic electrolytes and the like to the main
 component of fermented milk that is obtained through fermentation of milk
 with lactic acid bacteria or yeasts. However, fermented milk has little
 taste for refreshment, and, in addition, it coagulates to a rough and
 non-smooth texture when electrolytes, fats and oils are added thereto.
 Known methods for producing smooth, acidic milk drinks, for example,
 include: a method of homogenizing a raw material mixture that comprises
 acidic milk, pectin and a calcium-containing component, and a method of
 homogenizing a raw material mixture that comprises acidic milk and pectin
 followed by adding calcium thereto (Japanese Unexamined Patent Publication
 No. 56567/96); and a method of forming fat-containing, condensed sour milk
 beverage containing lactic acid bacteria through high pressure
 homogenization, which are characterized by stable fat dispersion therein
 (Japanese Unexamined Patent Publication No. 20057/89). However, even in
 these methods, the smoothness of the acidic milk drinks obtained is still
 unsatisfactory.
 On the other hand, long-term storage of acidic milk drinks is difficult. In
 general, acidic milk drinks can only be distributed in the market at low
 temperatures and for a period of 2 weeks or so. In order to distribute
 acidic milk drinks in the market at room temperature for a longer period
 of time, they must be further treated. For example, they must be
 sterilized by ultra-high-temperature flash pasteurization (UHT) or
 retorting. Alternatively, fermented milk is processed with an acid added
 thereto, thereby having a pH of not higher than 4.0, and thereafter this
 is subjected to cold pasteurization. In those sterilization methods,
 however, acidic milk drinks generally coagulate.
 On the other hand, known methods for producing acidic milk drinks that are
 not derived from fermented milk include: a method of adding stabilizers
 (Japanese Examined Patent Publication No. 35470/73, Japanese Unexamined
 Patent Publication No. 280366/96, etc.), or a method of adding saccharides
 (U.S. Pat. No. 3,800,052); and a method of solubilizing protein in milk
 with strong acids (Japanese Unexamined Patent Publication No. 20508/74),
 etc.
 However, in those methods for producing acidic milk products that do not
 start with fermented milk, most proteins that are present in the starting
 milk pass through their own isoelectric point, and coagulate when they
 pass through the isoelectric point. This means that good products are
 difficult to produce.
 For these reasons, few protein-containing acidic drinks are known other
 than the acidic milk drinks that are produced from fermented milk.
 On the other hand, there is increasing a demand for liquid nutrients such
 as thick liquid diets that contain protein as the nitrogen source. Many of
 conventional liquid nutrients that contain protein as the nitrogen source
 are processed to a pH value that falls within a neutral range. However,
 the nutrients that are processed to have a pH value that falls within a
 neutral range are defective in that they often taste oily and their taste
 is bad. Therefore, it is desirable to develop protein-containing acidic
 foods and drinks such as acidic liquid nutrients, etc. However, even in
 the process of producing high-protein, acidic liquid nutrients, the same
 problems occur as in the process of producing acidic milk drinks not from
 fermented milk. In particular, when liquid nutrients are made and are
 supplemented with electrolytes such as minerals, water-soluble vitamins
 and the like, the coagulation of protein is promoted by the electrolytes
 when the protein passes through its isoelectric point. Therefore, in that
 case, good acidic liquid nutrients are difficult to obtain.
 On the other hand, as nutrient supplements for aged persons having lowered
 chewing and swallowing power, protein-containing semi-solid foods such as
 jellies, puddings, etc. are desirable. However, for the same reasons as
 those for protein-containing acid drinks noted above, it is difficult to
 produce acidic, protein-containing semi-solid foods.
 SUMMARY OF THE INVENTION
 The present invention provides a protein-containing acidic food and drink
 which is smooth, tasteful, palatable and have good storage stability, as
 well as a method for producing them.
 Specifically, the invention provides a method for producing a
 protein-containing acidic food and drink, which comprises processing a
 protein emulsion having a pH value that is higher than the isoelectric
 point of the protein in the emulsion at a high temperature to make the
 emulsion having a pH value that is lower than the isoelectric point of the
 protein; the protein-containing acidic food and drink obtainable by the
 method; a protein-containing acidic food and drink containing protein, fat
 and oil, and water-soluble polysaccharide, in which the particles
 dispersed have a mean particle size of not greater than 15 .mu.m; and a
 protein-containing acidic drink containing protein, fat and oil, and
 water-soluble polysaccharide, in which the particles dispersed have a mean
 particle size of not greater than 15 .mu.m and/or having the viscosity of
 not greater than 20 centipoises (cPs).
 DETAILED DESCRIPTION OF THE INVENTION
 A protein-containing acidic food and drink referred to herein means foods
 and drinks which are acidic and which contain proteins. Preferably, a food
 and drink has a pH value falling between 2.5 and 5.0 in order to have a
 good sour taste, but more preferably has a pH value of between 2.5 and 4.0
 in order to have a better taste for refreshment. The protein content of
 the protein-containing acidic food and drink can depend on the desired end
 product. However, the protein content is preferably from 0.1 to 10% by
 weight, more preferably from 0.5 to 7% by weight in order that the food
 and drink could be smooth.
 Protein-containing acidic food and drink includes, for example, acidic
 drink to be prepared by adding sour or the like to protein-containing
 drink such as milk, soy milk, etc.; protein-containing acidic drink, for
 example, acidic liquid nutrient such as acidic, thick liquid diet that
 contains protein as the nitrogen source; protein-containing acidic
 semi-solid food to be produced by semi-solidifying protein emulsion, which
 is prepared in the same manner as in the production of protein-containing
 acidic drink noted above, into jellie, paste, etc.
 To prepare the product of the present invention, any suitable protein
 usable in foods, drinks and medicines can be used, and preferably used are
 proteins usable in foods and drinks. Typical proteins for use in the
 present invention include, for example, any one or more of natural protein
 material having a high content of vegetable protein, animal protein, milk
 protein, etc.; as well as low-purity protein and high-purity protein
 derived from such natural protein material, etc. The protein for use in
 the invention may be any of those as processed through any one or more of
 chemical treatment, enzymatic treatment, physical treatment or the like
 (processed proteins), for example, hydrolyzed, acylated, alkylated,
 esterified, phosphorylated, glycosylated, hydroxylated, methylated,
 oxidized or reduced protein; or those in the form of salt with alkali
 metal, alkaline earth metal or the like (protein salt). Specific examples
 of the protein for use in the invention include soybean protein, wheat
 protein such as gluten, corn protein, plasma protein, blood cell protein,
 egg white protein, egg yolk protein, meat protein, fish protein, milk
 protein such as casein, whey protein, collagen, gelatin, albumin,
 globulin, fibrin, fibrinogen, etc.
 In accordance with a preferred aspect of the present invention, protein
 having an isoelectric point that falls within a pH range between 3.5 and
 7.0, such as soybean protein, milk protein, albumin, gelatin, etc. is
 used. For whey protein, egg white protein and other proteins that are
 easily denatured under heat, it is desirable that they are used in the
 form of their hydrolysates or partial hydrolysates.
 Protein emulsion having a pH value that is higher than the isoelectric
 point of the protein existing therein includes, for example, protein drink
 such as milk, soy milk, etc. Apart from those, protein emulsion of that
 type may be prepared by dissolving or dispersing a protein in an aqueous
 medium having a pH value that is higher than the isoelectric point of the
 protein, preferably in an amount of from 0.1 to 15.0% by weight, more
 preferably from 0.5 to 10.0% by weight to form a protein solution, then
 adding thereto any of fats and oils along with an emulsifier, and
 thereafter stirring and emulsifiying the resulting mixture.
 The aqueous medium as referred to herein is water or a solvent containing
 water as the major component. The solvent containing water as the major
 component is not specifically defined, and may contain any other
 components within the range not interfering with the formation of the
 intended protein emulsions.
 Any edible fat and oil is employable herein, which includes, for example,
 any one or more of vegetable fat and oil such as soybean oil, corn oil,
 etc.; animal fat and oil such as tallow, milk fat, etc.; MCT (middle-chain
 fatty acid triglyceride), etc.
 Fat and oil may be added to the protein emulsion in such a manner that the
 fat and oil content of the protein emulsion is preferably from 0.1 to 10%
 by weight, more preferably from 0.5 to 6% by weight. If the fat and oil
 content is smaller than 0.1% by weight, the final product to be obtained
 herein will lose a smooth taste; but if greater than 10% by weight, the
 protein emulsion will be unstable under an acidic condition and the final
 product will have an unpleasant fatty taste.
 As the emulsifier, any suitable edible emulsifier can be used. Typical
 examples of suitable emulsifiers include, for example, any one or more of
 lecithin, lysolecithin, glycerin fatty acid ester, sucrose fatty acid
 ester, organic acid monoglyceride, etc.
 The amount of the emulsifier to be added varies. The amount can depend on
 the protein solution to which it is added, the fat and oil to be added to
 the protein solution along with it, and even the type of the emulsifier
 itself. Anyhow, the emulsifier may be added to a protein solution to such
 a degree that the emulsified condition of the final product of
 protein-containing acidic food and drink of the invention can be kept
 stable.
 Fat and oil and an emulsifier are added to a protein solution, and the
 resulting mixture is stirred and emulsified using, for example, a colloid
 mill, a homo-mixer, a high-pressure homogenizer, an ultra-high-pressure
 homogenizer or the like, to obtain a protein emulsion. For example, when a
 high-pressure homogenizer is used for the emulsification, the mixture may
 be processed under a pressure of 100 kg/cm.sup.2 or higher.
 Where a protein contained in a protein-containing acidic food and drink is
 casein, the raw protein for those may be any of natural protein materials
 having a high casein content, for example, animal milk such as cow milk,
 goat milk, sheep milk, horse milk, etc.; and low-purity casein and
 high-purity casein derived from such natural protein material. Casein to
 be used herein may be in any form, for example, it may be processed
 through chemical treatment, enzymatic treatment, physical treatment or the
 like (processed caseins), or may be in the form of its salts (casein
 salt).
 Examples of casein emulsion having a pH value that is higher than the
 isoelectric point of casein of about pH 4.5 or so, include animal milk.
 Casein emulsion may be prepared by dissolving or dispersing a raw protein
 material in an aqueous medium having a pH value that is higher than the
 isoelectric point of casein to form a casein solution, then adding thereto
 any of fats and oils along with an emulsifier, and thereafter stirring and
 emulsifiying the resulting mixture.
 The protein content of the protein emulsion can be dependent on the
 desirable end product. However, typically, it is preferably from 0.1 to
 10% by weight, more preferably from 0.5 to 7% by weight, in order that the
 final product, protein-containing acidic food and drink of the invention
 can be smooth.
 Water-soluble polysaccharide, if added to the protein emulsion, may prevent
 the coagulation of the emulsion while the emulsion is processed at a high
 temperature so as to have a pH value lower than the isoelectric point of
 the protein therein. Therefore, it is desirable to add water-soluble
 polysaccharide to the protein emulsion prior to the high-temperature
 treatment of the emulsion.
 The water-soluble polysaccharide includes, for example, pectin, as well as
 hemicellulose derived from seeds of corn, rice, palm, coco palm, cotton,
 soybean, etc. Preferred examples of the water-soluble polysaccharide are
 those being capable of emulsifying protein solution and having a high
 viscosity in acidic condition. For example, water-soluble polysaccharide
 derived from soybean seed is preferably used.
 The amount of the water-soluble polysaccharide can depend on the desired
 end product. Preferably, the amount is such that the water-soluble
 polysaccharide content of the protein emulsion may fall between 0.1 and
 1.5% by weight, more preferably between 0.3 and 1.2% by weight.
 The protein-containing acidic food and drink of the invention may contain
 various electrolytes. However, if there is an excess of electrolyte, then
 the protein emulsion will often coagulate or gel when it is processed at a
 high temperature so as to have a pH value lower than the isoelectric point
 of the protein existing therein. Therefore, it is desirable that the
 solvent consisting essentially of water, which is used to prepare the
 protein emulsion, contains no or few electrolytes. Accordingly, for some
 applications it is preferable and desirable that no electrolyte is added
 to the protein emulsion before the emulsion is subjected to the
 pH-controlling treatment at a high temperature.
 Typical electrolytes include, for example, any one or more of mineral,
 water-soluble vitamin, amino acid, nucleic acid, etc.
 Typical minerals include, for example, any one or more of sodium, calcium,
 potassium, iron, magnesium, manganese, zinc, selenium, etc.
 Typical water-soluble vitamins include, for example, any one or more of
 ascorbic acid, thiamine, riboflavin, nicotinic acid, vitamin B.sub.6,
 pantothenic acid, folic acid, vitamin B.sub.12, biotin, choline, inositol,
 para-aminobenzoic acid, niacin, etc.
 Typical amino acids include, for example, any one or more of aspartic acid,
 glutamic acid, glycine, threonine, methionine, tyrosine, arginine, lysine,
 etc.
 Typical nucleic acids include, for example, any one or more of inosinic
 acid, guanylic acid, sodium ribonucleotide etc.
 Apart from water-soluble polysaccharide, non-electrolyte, such as other
 saccharides and fat-soluble vitamin, may be added to the protein emulsion
 before the emulsion is subjected to the pH-controlling treatment at a high
 temperature.
 The saccharide for use in the present invention which is not water-soluble
 polysaccharide can be typical saccharide. Typical saccharide include
 edible ones, including, for example, any one or more of dextrin, starch,
 cellulose, oligo-saccharide, di-saccharide, mono-saccharide, glycoalcohol,
 etc. The saccharide, if added to the protein emulsion, can reduce the sour
 taste of the final product, protein-containing acidic food and drink of
 the invention while improving the smoothness thereof. Therefore, the
 addition of saccharide is preferred.
 The amount of the saccharide to be added can be dependent on the desired
 end product. However, the amount is preferably such that the saccharide
 content of the protein emulsion could be from 0.5 to 30% by weight, more
 preferably from 5 to 25% by weight.
 Typical fat-soluble vitamins include, for example, any one or more of
 vitamin A, vitamin D, vitamin E, vitamin K, etc.
 The protein emulsion having d pH value that is higher than the isoelectric
 point of the protein therein is, after heated, processed with sour or the
 like at a high temperature, preferably falling between 50 and 150.degree.
 C., more preferably between 70 and 100.degree. C., thereby to have a pH
 value that is lower than the isoelectric point of the protein. The time
 for which the emulsion is kept at such high temperature can be dependent
 on the desired end product. However, generally the time falls between 5
 seconds and 30 minutes.
 Typical sours include, for example, any one or more of fruit juice such as
 orange juice, grape juice, apple juice, etc.; organic acid such as citric
 acid, malic acid, gluconic acid, tartaric acid, lactic acid, acetic acid,
 etc.; and inorganic acid such as hydrochloric acid, etc. Preferred sour is
 organic acid, especially citric acid, malic acid and gluconic acid.
 After the protein emulsion has been processed to have a pH value lower than
 the isoelectric point of the protein existing therein, any additive of
 electrolyte, non-electrolyte, sour and others, such as those noted above,
 and even flavoring, dye, etc. may be added thereto. After the additive has
 been added, the resulting emulsion is preferably homogenized in the same
 manner as in the emulsification noted above, for example, using any one or
 more of a colloid mill, a TK homo-mixer, a high-pressure homogenizer, an
 ultra-high-pressure homogenizer or the like.
 The protein-containing acidic food and drink of the invention may be
 produced by sterilizing, under heat in conventional manner, the protein
 emulsion that has been prepared in the manner mentioned above.
 The protein-containing, acidic semi-solid food of the invention may be
 produced by adding a gelling agent to the protein emulsion having been
 prepared in the manner mentioned above, followed by sterilizing under heat
 the resulting gelled product. The gelled product may be directly used as
 it is, or, if desired, may be formed into jelly, pudding, etc.
 As the gelling agent, any suitable edible gelling agent can be used.
 Typical gelling agents include, for example, any one or more of geranium
 gum, locust bean gum, guar gum, xanthane gum, carrageenan, konjak mannan,
 gelatin, agar, etc. In view of gelling ability, a preferred gelling agent
 is agar. The amount of the gelling agent to be added may be varied,
 depending on the desired taste and feel of the products to be produced.
 For example, when agar is used as the gelling agent, its amount may be
 generally from 0.1 to 1.0% by weight of the resultant product.
 In the method of the present invention, the protein emulsion, while being
 processed, is prevented from coagulating. Therefore, the method of the
 invention is favorable to the production of protein-containing acidic food
 and drink, especially those containing a large amount of electrolyte such
 as mineral, vitamin, amino acid, nucleic acid, etc.
 The present invention also covers, protein-containing acidic food and drink
 which contains protein, fat and oil, and water-soluble polysaccharide and
 in which the dispersed particles have a mean particle size of not greater
 than 15 .mu.m. Also covered are such foods and drinks containing protein,
 fat and oil, and water-soluble polysaccharide, in which the dispersed
 particles have a mean particle size of not greater than 15 .mu.m and/or
 and the having the viscosity of not greater than 20 cPs. Preferably, the
 food and drink is made by the method of the present invention.
 In the food and drink, the protein, fat and oil and water-soluble
 polysaccharide means the same as defined above. The protein content of the
 food and drink may typically fall between 0.1 and 10% by weight, but
 preferably between 0.5 and 7% by weight. The fat and oil content may
 typically fall between 0.1 and 10% by weight, preferably they fall between
 0.5 and 6% by weight. The water-soluble polysaccharide content may
 typically fall between 0.1 and 1.5% by weight, preferably, it falls
 between 0.3 and 1.2% by weight. For example, for thick liquid diet, it is
 desirable that the protein content thereof and the fat and oil content
 thereof are both not less than 1.0% by weight.
 The food and drink may contain, in addition to the one or more of the
 protein, fat and oil and water-soluble polysaccharide noted above, any one
 or more of electrolyte, non-electrolyte except water-soluble polymer,
 sour, gelling agent, etc. In such instances, the additional electrolyte,
 non-electrolyte except water-soluble polymer, sour and gelling agent may
 be the same as those defined above. For example, for thick liquid diet, it
 is desirable that it contains non-electrolyte except water-soluble
 polymer, especially saccharide in an amount of not less than 6.0% by
 weight. Where sour is added to the food and drink, organic acid is
 preferably used as sour. However, if the total normality of lactic acid
 and acetic acid existing in the food and drink is greater than 50% of the
 total normality of all organic acids existing therein, the food and drink
 will lose a good taste for refreshment. Therefore, it is desirable that
 the total normality of lactic acid and acetic acid to be in the food and
 drink is not greater than 50% of the total normality of all organic acids
 therein.
 The dispersed particles as referred to herein mean colloidal particles and
 the like that are in the food and drink in the form of a dispersion. The
 mean particle size of the dispersed particles may be measured, for
 example, by using a particle size distribution meter. Where the
 protein-containing acidic food and drink contains a gelling agent, it is
 heated and melted at a temperature not lower than the melting point of the
 gelling agent, and then diluted 500-fold or more with hot water, and
 thereafter the mean particle size of the particles dispersed in the
 resulting melt may be measured with a particle size distribution meter.
 The viscosity of the protein-containing acidic drink of the invention may
 be measured at 20.degree. C., using a B-type viscometer.
 Regarding the physical properties of protein-containing acidic food and
 drink, it is important that the particles dispersed therein have a mean
 particle size of not greater than 15 .mu.m, preferably not greater than 10
 .mu.m, in order that the food and drink is not rough to the throat or
 tongue but has a soft and smooth taste. In particular, for
 protein-containing acidic drink, it is preferable that the viscosity of
 the drink is not greater than 20 cPs in such instances the drink has a
 desirable smoothness. On the other hand, for protein-containing, acidic
 semi-solid food, it is desirable that the viscosity of the food, to which
 a gelling agent is not as yet added, is not greater than 20 cPs.
 The invention will now be described in more detail and with reference to
 the following Examples and Comparative Examples, which, however, are not
 intended to restrict the scope of the invention.

In the following Examples and Comparative Examples, "%" except the degree
 of water separation in Example 4 and molar ratio in Example 7 means "% by
 weight".
 EXAMPLE 1
 A solution having the following compositions was prepared by dissolving the
 following components in water; 14.6% dextrin, 3.25% sodium casein, 3.2%
 salad oil, 0.8% polyglycerin fatty acid ester, and 1.2% water-soluble
 soybean fiber (manufactured by Fuji Oil Co.). Citric acid was added to the
 solution, by which the solution was made to have a pH of 6.5. Then, the
 solution was pre-emulsified using a TK homo-mixer (manufactured by Tokushu
 Kika Kogyo KK) at 5000 rpm for 5 minutes, and then emulsified using a
 high-pressure homogenizer (manufactured by Raney Co.) under a pressure of
 300 kg/cm.sup.2. The resulting emulsion was heated at 80.degree. C. and
 kept at the temperature for 30 minutes, and, at the end of the heating,
 citric acid was added thereto by which the emulsion was made to have a pH
 of 3.9. After the pH control, the following components were added to the
 emulsion to have the following concentration; 1.47 g/liter calcium
 chloride, 2.06 g/liter magnesium sulfate and 1.7 g/liter
 dipotassiumhydrogen phosphate, which was then homogenized using a
 high-pressure homogenizer under a pressure of 300 kg/cm.sup.2. After
 homogenization, the emulsion was sterilized at 100.degree. C. for 10
 minutes, and then charged into bottles. Thus was produced a liquid
 nutrient according to the present invention.
 Comparative Example 1
 A solution having the following compositions was prepared by dissolving the
 following components in water; 14.6% dextrin, 3.25% sodium casein, 3.2%
 salad oil, 0.8% polyglycerin fatty acid ester, and 1.2% water-soluble
 soybean fiber. Citric acid was added to the solution, by which the
 solution was made to have a pH of 6.5. Then, the solution was
 pre-emulsified using a TK homo-mixer at room temperature at 5000 rpm for 5
 minutes, and then emulsified using a high-pressure homogenizer at room
 temperature under a pressure of 300 kg/cm.sup.2. The resulting emulsion
 was heated at 80.degree. C. and kept at the temperature for 30 minutes,
 and thereafter cooled to 20.degree. C. Then, citric acid was added thereto
 by which the emulsion was made to have a pH of 3.9. After the pH control,
 the following components were added to the emulsion to have the following
 concentration; 1.47 g/liter calcium chloride, 2.06 g/liter magnesium
 sulfate and 1.7 g/liter dipotassiumhydrogen phosphate, which was then
 homogenized using a high-pressure homogenizer under a pressure of 300
 kg/cm.sup.2. After homogenization, the emulsion was sterilized at
 100.degree. C. for 10 minutes, and then charged into bottles. Thus was
 produced a liquid nutrient.
 Comparative Example 2
 A solution having the following compositions was prepared by dissolving the
 following components in water; 14.6% dextrin, 3.25% sodium casein, 3.2%
 salad oil, 0.8% polyglycerin fatty acid ester, and 1.2% water-soluble
 soybean fiber. Citric acid was added to the solution, by which the
 solution was made to have a pH of 3.9. After the pH control, the solution
 was pre-emulsified using a TK homo-mixer at 5000 rpm for 5 minutes, and
 then emulsified using a high-pressure homogenizer under a pressure of 300
 kg/cm.sup.2. The resulting emulsion was heated at 80.degree. C. and kept
 at the temperature for 30 minutes, and thereafter cooled to 20.degree. C.
 Then, the following components were added to the emulsion to have the
 following concentration; 1.47 g/liter calcium chloride, 2.06 g/liter
 magnesium sulfate and 1.7 g/liter dipotassiumhydrogen phosphate, which was
 then homogenized using a high-pressure homogenizer under a pressure of 300
 kg/cm.sup.2. After homogenization, the emulsion was sterilized at
 100.degree. C. for 10 minutes, and then charged into bottles. Thus was
 produced a liquid nutrient.
 Comparative Example 3
 A solution having the following compositions was prepared by dissolving the
 following components in water; 14.6% dextrin, 3.25% sodium casein, 3.2%
 salad oil, 0.8 % polyglycerin fatty acid ester, and 1.2% water-soluble
 soybean fiber. Citric acid was added to the solution, by which the
 solution was made to have a pH of 3.9. The resulting solution was
 pre-emulsified using a TK homo-mixer at 5000 rpm for 5 minutes, and then
 emulsified using a high-pressure homogenizer under a pressure of 300
 kg/cm.sup.2. Then, the following components were added to the resulting
 emulsion to have the following concentration; 1.47 g/liter calcium
 chloride, 2.06 g/liter magnesium sulfate and 1.7 g/liter
 dipotassiumhydrogen phosphate, which was then homogenized using a
 high-pressure homogenizer under a pressure of 300 kg/cm.sup.2. After
 homogenization, the emulsion was sterilized at 100.degree. C. for 10
 minutes, and then charged into bottles. Thus was produced a liquid
 nutrient.
 Comparative Example 4
 A solution having the following compositions was prepared by dissolving the
 following components in water; 14.6% dextrin, 3.25% sodium casein, 3.2%
 salad oil, 0.8% polyglycerin fatty acid ester, and 1.2% water-soluble
 soybean fiber. Citric acid was added to the solution, by which the
 solution was made to have a pH of 6.5. The resulting solution was
 pre-emulsified using a TK homo-mixer at 5000 rpm for 5 minutes, and then
 emulsified using a high-pressure homogenizer under a pressure of 300
 kg/cm.sup.2. Then, the following components were added to the resulting
 emulsion to have the following concentration; 1.47 g/liter calcium
 chloride and 2.06 g/liter magnesium sulfate, which was then heated and
 kept at 80.degree. C. for 30 minutes. At the end of the heating, citric
 acid was added to the emulsion, by which the emulsion was made to have a
 pH of 3.9. After the pH control, dipotassiumhydrogen phosphate was added
 to the emulsion to have 1.7 g/liter dipotassiumhydrogen phosphate, which
 was then homogenized using a high-pressure homogenizer under a pressure of
 300 kg/cm.sup.2. After homogenization, the emulsion was sterilized at
 100.degree. C. for 10 minutes, and then charged into bottles. Thus was
 produced a liquid nutrient.
 Comparative Example 5
 A solution having the following compositions was prepared by dissolving the
 following components in water; 14.6% dextrin, 3.25% sodium casein, 3.2%
 salad oil, 0.8% polyglycerin fatty acid ester, and 1.2% water-soluble
 soybean fiber. Citric acid was added to the solution, by which the
 solution was made to have a pH of 6.5. The resulting solution was
 pre-emulsified using a TK homo-mixer at 5000 rpm for 5 minutes, and then
 emulsified using a high-pressure homogenizer under a pressure of 300
 kg/cm.sup.2. Then, the following components were added to the emulsion to
 have the following concentration; 1.47 g/liter calcium chloride and 2.06
 g/liter magnesium sulfate were added to the resulting emulsion. Next,
 citric acid was added thereto at room temperature, by which the emulsion
 was made to have a pH of 3.9. After the pH control, dipotassiumhydrogen
 phosphate was added to the emulsion to have 1.7 g/liter
 dipotassiumhydrogen phosphate was added to the emulsion, which was then
 homogenized using a high-pressure homogenizer under a pressure of 300
 kg/cm.sup.2. After homogenization, the emulsion was sterilized at
 100.degree. C. for 10 minutes, and then charged into bottles. Thus was
 produced a liquid nutrient.
 Test Results
 The viscosity of the liquid nutrients obtained in Example 1 and Comparative
 Examples 1 to 5 were measured. In addition, these liquid nutrients were
 tested for their condition as to whether or not the proteins therein
 coagulated, and for their roughness as to whether or not they are rough
 feel to the throat or tongue. Precisely, the viscosity of each sample was
 measured with a B-type viscometer (manufactured by Tokyo Keiki KK) at
 20.degree. C. For the protein coagulation, the samples were left for 1
 week, and checked for the presence or absence of precipitate formed
 therein. As a result of the visual observation, the samples were grouped
 into 6 ranking groups. In the first ranking designated by "-", the samples
 gave no precipitate; in the second ranking designated by "+", the samples
 gave minor precipitate; in the third ranking designated by "++", the
 samples gave a little precipitate; in the fourth ranking designated by
 "+++", the samples gave precipitate; in the fifth ranking designated by
 "++++", the samples gave much precipitate; and in the sixth ranking
 designated by "+++++", the samples gave great precipitate. For their
 roughness, the samples were sensually tested by 5 panelists. As a result
 of the sensual test, the samples were grouped into 6 ranking groups. In
 the first ranking designated by "-", the samples were not rough at all; in
 the second ranking designated by "+", the samples were slightly rough; in
 the third ranking designated by "++", the samples were somewhat rough; in
 the fourth ranking designated by "+++", the samples were rough; in the
 fifth ranking designated by "++++", the samples were much rough; and in
 the sixth ranking designated by "+++++", the samples were greatly rough.
 The test data obtained are shown in Table 1 below.
 TABLE 1
 Viscosity (cPs) Coagulation
 Roughness
 Example 1 9.0 - -
 Comparative Example 183.0 ++++ ++++
 1
 Comparative Example 190.0 ++++ ++++
 2
 Comparauve Example 870.0 ++++ ++++
 3
 Comparative Example 12.6 +++++ +++++
 4
 Comparative Example 380.0 ++++ ++++
 5
 As shown in Table 1, the liquid nutrient obtained in Example 1 had a low
 viscosity, and did not coagulate at all. In addition, it had a good and
 smooth taste. As opposed to this, the liquid nutrient obtained in
 Comparative Example 1, in which the protein emulsion was cooled before it
 passed through its isoelectric point; those obtained in Comparative
 Examples 2 and 3, in which the protein solution was emulsified after
 having passed through its isoelectric point; and those obtained in
 Comparative Examples 4 and 5, in which some mineral was added to the
 protein emulsion before the emulsion passed through its isoelectric point
 all had a high viscosity and greatly coagulated, and, in addition, they
 had a bad taste as being not smooth.
 EXAMPLE 2
 A solution having the following compositions was prepared by dissolving the
 following components in water; 14.6% dextrin, 3.25% sodium casein, 3.2%
 salad oil, 0.8% polyglycerin fatty acid ester, and 1.2% water-soluble
 soybean fiber. Citric acid was added to the solution, by which the
 solution was made to have a pH of 6.5. After the pH control, the solution
 was pre-emulsified using a TK homo-mixer at 5000 rpm for 5 minutes, and
 then emulsified using a high-pressure homogenizer under a pressure of 300
 kg/cm.sup.2. The resulting emulsion was heated at 80.degree. C. and kept
 at the temperature for 30 minutes, and, at the end of the heating, citric
 acid was added thereto by which the emulsion was made to have a pH of 3.9.
 After the pH control, the following components were added to the emulsion
 to have the following concentration; 1.47 g/liter calcium chloride, 2.06
 g/liter magnesium sulfate, 1.7 g/liter dipotassiumhydrogen phosphate and
 0.5% microcrystalline cellulose (manufactured by Asahi Chemical Industry
 Co.), which was then homogenized using a high-pressure homogenizer under a
 pressure of 300 kg/cm.sup.2. After having been thus homogenized, the
 emulsion was sterilized at 100.degree. C. for 10 minutes, and then charged
 into bottles. Thus was produced a liquid nutrient.
 The liquid nutrient obtained herein was stored at 30.degree. C. for a half
 year, and the sample thus-stored was tested for the emulsion stability and
 the suspension stability. For the emulsion stability, the sample was
 evaluated in three ranks, the first ranking being directed to the case
 with good emulsion condition, the second ranking to the case in which
 water separation and cream lines are seen, and the third ranking to the
 case in which oil-off layers, water separation and cream lines are all
 seen. As a result of the test, none of oil-off layers, water separation
 and cream lines is seen in the tested sample. This supports good emulsion
 stability of the liquid nutrient. For the suspension stability, the sample
 was checked for the viscosity change before and after the storage, and in
 addition, the stored sample was further checked for the presence or
 absence of precipitate formed therein. As a result of the test, no
 viscosity change was found before and after the storage, and no
 precipitate was found in the stored sample. This supports good suspension
 stability of the liquid nutrient.
 EXAMPLE 3
 A solution having the following compositions was prepared by dissolving the
 following components in water; 6.6% dextrin, 8.0% granulated sugar, 3.25%
 sodium casein, 3.2% salad oil, 0.8% polyglycerin fatty acid ester, and
 1.2% water-soluble soybean fiber. Citric acid was added to the solution,
 by which the solution was made to have a pH of 6.5. After the pH control,
 the solution was pre-emulsified using a TK homo-mixer at 5000 rpm for 5
 minutes, and then emulsified using a high-pressure homogenizer under a
 pressure of 300 kg/cm.sup.2. The resulting emulsion was heated at
 80.degree. C. and kept at the temperature for 30 minutes, and, at the end
 of the heating, citric acid was added thereto by which the emulsion was
 made to have a pH of 3.9. After the pH control, the following components
 were added to the emulsion to have the following concentration; 1.47
 g/liter calcium chloride, 2.06 g/liter magnesium sulfate, 1.7 g/liter
 dipotassiumhydrogen phosphate, 1 g/liter vitamin mix., 0.1% orange
 flavoring and 5% orange juice, which was then homogenized using a
 high-pressure homogenizer under a pressure of 300 kg/cm.sup.2. After
 homogenization, the emulsion was sterilized at 100.degree. C. for 10
 minutes, and then charged into bottles. Thus was produced a liquid
 nutrient.
 The liquid nutrient obtained herein was sensually tested by 5 panelists for
 its flavor and taste in a 5-point method in which the case with point 5 is
 the best. As a result, the liquid nutrient gained point 5, while a
 commercial product of neutral liquid nutrient gained point 3 and a
 commercial product of yogurt gained point 3. As shown in the test data
 obtained herein, even persons who do not like the flavor and taste of
 fermented milk and lactic acid drinks feel that the liquid nutrient
 produced herein has good flavor and taste.
 EXAMPLE 4
 A solution having the following compositions was prepared by dissolving the
 following components in water; 7.3% dextrin, 7.3% granulated sugar, 3.25%
 sodium casein, 3.2% salad oil, 0.8% polyglycerin fatty acid ester, and
 1.2% water-soluble soybean fiber. Citric acid was added to the solution,
 by which the solution was made to have a pH of 6.5. After the pH control,
 the solution was pre-emulsified using a TK homo-mixer at 5000 rpm for 5
 minutes, and then emulsified using a high-pressure homogenizer under a
 pressure of 300 kg/cm.sup.2. The resulting emulsion was heated at
 80.degree. C. and kept at the temperature for 30 minutes, and, at the end
 of the heating, gluconic acid was added thereto by which the emulsion was
 made to have a pH of 3.9. After the pH control, the following components
 were added to the emulsion to have the following concentration; 1.47
 g/liter calcium chloride, 2.06 g/liter magnesium sulfate, 1.7 g/liter
 dipotassiumhydrogen phosphate, 0.1% orange flavoring (manufactured by
 Kyowa Flavoring Chemical Co.) and 5% orange juice, and, in addition, 0.9%
 carrageenan and 0.6% (v/v) locust bean gum. Next, the resulting mixture
 was homogenized using a high-pressure homogenizer under a pressure of 300
 kg/cm.sup.2. After homogenization, the emulsion was sterilized at
 100.degree. C. for 10 minutes, then put into cups, and formed into
 jellies.
 The jellies were cut into columnar samples having a diameter of 25 mm and a
 height of 15 mm, which were put on filter paper spread over a laboratory
 dish, and kept as they were at 20.degree. C. for 60 minutes. The amount of
 water having transferred from each sample to the filter paper was then
 measured, and the degree of water separation from the jelly sample was
 obtained according to the following equation.
EQU Degree of Water Separation (%) =[weight of filter paper after test
 (g)-weight of filter paper before test (g)]/weight of sample (g).times.100
 The degree of water separation from the jellies obtained herein was 1%,
 which indicates that little water separated from the jellies. In addition,
 the jellies were sensually tested in the same manner as in Example 3, and
 all gained point 5. In the same test, however, commercial jellies gained
 point 3. The test data indicate that the jellies obtained herein had good
 flavor and taste.
 EXAMPLE 5
 A solution having the following compositions was prepared by dissolving the
 following components in water; 4.6% dextrin, 10.0% granulated sugar, 3.25%
 sodium casein, 3.2% salad oil, 0.8% polyglycerin fatty acid ester, and
 1.2% water-soluble soybean fiber. Citric acid was added to the solution,
 by which the solution was made to have a pH of 6.5. After the pH control,
 the solution was pre-emulsified using a TK homo-mixer at 5000 rpm for 5
 minutes, and then emulsified using a high-pressure homogenizer under a
 pressure of 300 kg/cm.sup.2. The resulting emulsion was heated at
 80.degree. C. and kept at the temperature for 30 minutes, and, at the end
 of the heating, gluconic acid was added thereto by which the emulsion was
 made to have a pH of 3.9. After the pH control, the following components
 were added to the emulsion to have the following concentration; 1.47
 g/liter calcium chloride, 2.06 g/liter magnesium sulfate, 1.7 g/liter
 dipotassiumhydrogen phosphate, 1 g/liter vitamin mix, 0.1% grape flavoring
 (manufactured by Kyowa Flavoring Chemical Co.) and 5% muscat juice, and,
 in addition, 0.2% agar. Next, the resulting mixture was homogenized using
 a high-pressure homogenizer under a pressure of 300 kg/cm.sup.2. After
 homogenization, the emulsion was sterilized at 100.degree. C. for 10
 minutes, then put into standing pouch, and formed into jellies.
 The jellies thus obtained herein were evaluated for their flavor and taste
 in the same manner as in Example 3, and tested for the roughness in the
 same manner as in Example 1. The results were that the jellies of this
 Example gained point 5 for their flavor and taste, while commercial
 jellies gained point 4. For the rough feel to the throat or tongue, all
 the panelists evaluate that the jellies of this Example were not rough.
 The test results indicate that the jellies obtained herein had a good
 taste and had no rough feel to the throat or tongue.
 EXAMPLE 6
 A solution having the following compositions was prepared by dissolving the
 following components in water; 14.6% dextrin, 3.25% sodium casein, 3.2%
 salad oil, 0.8% polyglycerin fatty acid ester, and 1.2% water-soluble
 soybean fiber. Citric acid was added to the solution, by which the
 solution was made to have a pH of 6.5. Next, the solution was
 pre-emulsified using a TK homo-mixer at 5000 rpm for 5 minutes, and then
 emulsified using a high-pressure homogenizer under a pressure of 300
 kg/cm.sup.2. The resulting emulsion was heated at 40, 50, 60, 70 or
 80.degree. C. and kept at the temperature for 30 minutes, and, at the end
 of the heating, citric acid was added thereto by which the emulsion was
 made to have a pH of 3.9. After the pH control, the following components
 were added to the emulsion to have the following concentration; 1.47
 g/liter calcium chloride, 2.06 g/liter magnesium sulfate and 1.7 g/liter
 dipotassiumhydrogen phosphate. Next, the resulting mixture was homogenized
 using a high-pressure homogenizer under a pressure of 300 kg/cm.sup.2.
 After homogenization, the emulsion was sterilized at 100.degree. C. for 10
 minutes, and then charged into bottles. Thus were produced liquid
 nutrients.
 The mean particle size and the viscosity of each liquid nutrient were
 measured, and, in addition, the liquid nutrients were tested for the taste
 including the rough feel, if any, to the throat or tongue. The overall
 evaluation of each liquid nutrient was derived from the test results
 obtained.
 Specifically, the mean particle size of the particles dispersed in each
 liquid nutrient obtained herein was measured, using a particle size
 distribution meter (HELOS, manufactured by Sympatech Co.). The viscosity
 at 20.degree. C. of each liquid nutrient was measured, using a B-type
 viscometer (manufactured by Tokyo Keiki KK). For the overall evaluation of
 each liquid nutrient, five panelists sensually tested all liquid nutrients
 for the taste and the rough feel to the throat or tongue. Regarding the
 rough feel to the throat or tongue, the samples tested were grouped into 6
 ranking groups in the same manner as in Example 1. Regarding the taste,
 the samples tested were grouped into 4 ranking groups. The taste of the
 samples in the fourth ranking was bad; that of the samples in the third
 ranking was relatively bad; that of the samples in the second ranking was
 relatively good; and that of the samples in the first ranking was good.
 Regarding the overall evaluation, the samples tested were grouped into 5
 ranking groups. The samples that gained point 5 were the best and in the
 first ranking, and those that gained point 1 were the worst and in the
 fifth ranking.
 The test results are in Table 2 below.
 TABLE 2
 Temper- Mean
 ature Particle Viscosity Rough- Overall
 (.degree. C.) Size (.mu.m) (cPs) ness Taste Evaluation
 40 16.2 28.7 +++++ bad 1
 50 13.4 17.2 +++ relatively bad 3
 60 12.2 11.2 ++ relatively 4
 good
 70 8.2 9.9 - good 5
 80 6.9 9.2 - good 5
 As shown in Table 2, the samples that had been heated at 50.degree. C. had
 a viscosity lower than 20 cPs, and their taste was evaluated to be
 acceptable by the panelists. On the other hand, the samples that had been
 heated at 70.degree. C. or higher had a viscosity of much lower than 20
 cPs and had a mean particle size of smaller than 10 .mu.m, and their taste
 was evaluated good by the panelists.
 EXAMPLE 7
 Liquid nutrients were produced in the same manner as in Example 1 except
 that a mixture of acids as prepared in the ratio indicated in Table 3
 below was used as the sour to be added to the emulsion, in place of citric
 acid.
 The liquid nutrients produced herein were sensually tested by 5 panelists
 for their taste for refreshment. The liquid nutrients tested were grouped
 into 5 ranking groups. The liquid nutrients that gained point 5 as having
 a good taste for refreshment were the best and in the first ranking, and
 those that gained point 1 as having no taste for refreshment were the
 worst and in the fifth ranking.
 The test results are in Table 3.
 TABLE 3
 Molar Ratio
 10/0 8/2 6/4 5/5 4/6 2/8 0/10
 malic acid/ 5 5 4 3 2 1 1
 lactic acid
 gluconic acid/ 5 4 3 2 1 1 i
 lactic acid
 citric acid/ 5 5 4 3 2 1 1
 lactic acid
 As shown in Table 3, when the sour comprising lactic acid in a molar ratio
 of not greater than 50% relative to the total mols of the organic acids
 constituting it was used, the liquid nutrients obtained had a good taste
 for refreshment.
 EXAMPLE 8
 To 500 g of water were added: 170 g of dextrin, 36 g of sodium casein, 16 g
 of soybean oil, 16 g of rape-seed oil, 1.8 g of polyglycerin fatty acid
 ester, and 12 g of water-soluble, -edible soybean fiber. The resulting
 solution (pH: about 6.5) was pre-emulsified using a TK homo-mixer at 5000
 rpm for 5 minutes, and then emulsified using a high-pressure homogenizer
 under a pressure of 300 kg/cm.sup.2. The resulting emulsion was heated at
 80.degree. C. and kept at the temperature for 30 minutes, and, at the end
 of the heating, 4.1 g of 90% lactic acid and 20.3 g of 50% gluconic acid
 were added thereto by which the emulsion was made to have a pH of 3.9.
 After the pH control, 1.11 g of calcium chloride, 2.06 g of magnesium
 sulfate, 2.26 g of dipotassiumhydrogen phosphate and 9 mg of iron sodium
 citrate were added to the emulsion, and 2.1 g of agar was added thereto.
 Then, water was added thereto to make it have a volume of one liter. This
 was then homogenized using a high-pressure homogenizer under a pressure of
 300 kg/cm.sup.2. After homogenization, the emulsion was sterilized at
 100.degree. C. for 10 minutes, and then charged into standing pouch. Thus
 was produced a liquid nutrient having a pH value of 3.9. The mean particle
 size of the particles dispersed therein was 6.9 .mu.m. The liquid nutrient
 contained the following components:

Dextrin 17.0%
 Sodium Casein 3.6%
 Soybean Oil 1.6%
 Rape-seed Oil 1.6%
 Polyglycerin Fatty Acid Ester 0.18%
 Water-soluble, Edible Soybean Fiber 1.2%
 90% Lactic Acid 0.41%
 50% Gluconic Acid 2.03%
 Calcium Chloride 0.111%
 Magnesium Sulfate 0.206%
 Dipotassiumhydrogen Phosphate 0.226%
 Iron Sodium Citrate 0.0009%
 Agar 0.21%
 As has been mentioned in detail hereinabove, the present invention provides
 protein-containing acidic food and drink which is smooth, tasteful,
 palatable and have good storage stability, and a method for producing
 thereof.
 While the invention has been described in detail and with reference to
 specific embodiments thereof, it will be apparent to one skilled in the
 art that various changes and modifications can be made therein without
 departing from the spirit and scope thereof.