Abstract:
Functional yeast protein products with good thermogelability, emulsification capacity, foaming ability, solubility, and whippability are prepared by an alkali extraction of whole yeast cells preceded by a hot water extraction and/or a dilute alkaline extraction. These products are good substitutes for either casein, sodium caseinate, or egg white, and can also be used in making stabilized acidic protein beverages.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to processes for preparing yeast food products. More particularly, it relates to an alkaline extraction process for preparing yeast whippable proteins and/or acid-soluble proteins. 
     2. Description of the Prior Art 
     Protein ingredients such as egg white, casein, sodium caseinate, and dried milk solids, are useful in food applications primarily because of their functional properties such as whippability, emulsification capacity, gelability, solubility, etc. Unfortunately, they are expensive and in short supply. Yeast materials, especially those processed products having a bland flavor and reduced purine content, have the potential to replace some of these protein ingredients in various food applications. It is necessary, however, to efficiently extract the proteins from the yeast cell and process them properly to obtain the desired functional properties. 
     One method of removing the proteins from the yeast cells is by alkaline extraction. An example of this process is provided in U.S. Pat. No. 3,862,112. The efficiency of such a process is determined generally by the combined effect of alkalinity, reaction temperature, reaction time, and to a certain extent the cellular material concentration. Both the yield and the functional properties of the recovered protein are closely related, and after digestion in the hot alkali solution, the processed yeast material contains a complex mixture of various cellular components and hydrolysis products in both soluble and insoluble forms. These various materials greatly affect the quality of the protein product in terms of composition and functional properties. 
     Accordingly, it is an object of this invention to develop a process for preparing functional yeast proteins. 
     It is a further object of this invention to produce a yeast whippable protein. 
     These and other objects will become apparent upon further reading of this specification. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention resides in a process for preparing functional food yeast proteins comprising: heating an aqueous slurry of whole yeast cells to an elevated temperature to remove undesirable flavor and color bodies; separating the yeast cells from the aqueous extract; slurrying the separated yeast cells with a dilute alkaline solution at an elevated temperature to remove nucleotidic materials; separating the yeast cells from the alkaline extract; reslurrying the separated yeast cells in a solution of strong alkali at an elevated temperature to extract the proteins; neutralizing the slurry; separating the undigested cell residue from the supernatant containing the extracted protein; acidifying the supernatant, preferably to a pH of from about 3.5 to 4.5, to precipitate proteins at the isoelectric point from the mother liquor solution containing soluble proteins; neutralizing the mother liquor solution; and drying the mother liquor solution to yield a yeast whippable protein. As a coproduct, a yeast protein isolate can be produced by neutralizing and drying the isoelectrically precipitated proteins. Optionally, the isoelectrically precipitated proteins can be washed with a solvent such as water, ethanol, or acetone prior to drying to improve the flavor characteristics of the yeast protein isolate product. Also, the undigested cell residue separated after neutralization of the slurry can be washed with water if desired and the wash water returned to the process by combination with the supernatant prior to acidification. 
     More specifically, the invention resides in a process for preparing functional food yeast proteins comprising: heating an aqueous slurry of whole yeast cells to a temperature of from 60° to 100° C., preferably about 90° to 95° C. for from 1 to 5 minutes to extract flavor and color bodies; separating the yeast cells from the aqueous extract; slurrying the separated yeast cells in a dilute alkaline solution having a pH of from 8.5 to 10.0, preferably about 9.5, at a temperature of from 85° to 95° C., preferably about 90° C. for about 10 minutes to extract nucleotidic materials; separating the yeast cells from the resulting alkaline extract; reslurrying the separated yeast cells in a solution of from 0.1 to 0.3N sodium hydroxide or potassium hydroxide, preferably 0.15N sodium hydroxide, at a temperature of from 85° to 100° C., preferably about 95° C. for about 30 minutes; neutralizing the slurry; separating the undigested cell residue from the supernatant which contains the extracted proteins; acidifying the supernatant to a pH of from about 3.5 to 4.5, preferably about 4.0, to precipitate the isoelectrically precipitable proteins; separating the precipitated proteins from the mother liquor solution containing the soluble proteins; neutralizing the mother liquor solution to a pH of from 6.7 to 7.5, preferably 7.0; and spray-drying the mother liquor solution to yield a yeast whippable protein. The precipitated proteins can be neutralized, dissolved in water, and spray-dried to yield a yeast protein isolate coproduct. Optionally, the precipitated proteins can be washed with a solvent such as water, ethanol, or acetone prior to dissolution in water for spray-drying. Also, as previously mentioned, the undigested cell residue can be washed with water. 
     In another aspect, the invention resides in a similar process for preparing functional food yeast proteins, but without the initial hot water extraction step. The process comprises the steps of: slurrying whole yeast cells in a dilute alkaline solution, preferably having a pH of from 8.5 to 10.0, at an elevated temperature, preferably from 85 to 95° C., to extract both nucleotidic materials and undesirable flavor and color bodies; separating the yeast cells from the alkaline extract; reslurrying the separated yeast cells in a solution of strong alkali, preferably from 0.1 to 0.3N sodium hydroxide, at an elevated temperature, preferably from 85 to 100° C., to extract proteins; neutralizing the slurry; separating the undigested cell residue from the supernatant containing the extracted proteins; acidifying the supernatant, preferably to a pH of from about 3.5 to 4.5, to precipitate the isoelectrically precipitable proteins; separating the precipitated proteins from the mother liquor solution which contains the soluble proteins; neutralizing the mother liquor solution; and drying the mother liquor solution to yield a yeast whippable protein. (The conditions described for previous aspects of this invention are also applicable to this aspect of the invention insofar as the steps are the same). By eliminating the hot water extraction step, the process is somewhat simplified and a by-product (aqueous extract) is eliminated. On the other hand, the single extract does not contain the B-vitamins present in the aqueous extract because they are decomposed when exposed to hot alkaline solutions. Therefore, in choosing between the two processes, it is necessary to consider the end use of the extract(s) obtained, which include such uses as flavorants and food or nutritional supplements. 
     In a further aspect, the invention resides in the products produced by the aforementioned processes. 
     All aspects of this invention are applicable to food yeasts in general, and more particularly, to those yeasts selected from the group consisting of Candida utilis, Saccharomyces cerevisiae, Saccharomyces fragilis, and Saccharomyces carlsbergensis. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A culture of Candida utilis yeast cells (ATCC-9256) was grown on an ethanol substrate under oxygen-limiting growth conditions. The whole cells were harvested and concentrated into a 10-14 weight percent (dry weight) aqueous slurry or cream. The aqueous slurry was heated to a temperature of 90° C. for about 5 minutes to remove materials which would give the product poor color and flavor characteristics. The slurry was centrifuged to separate the aqueous extract from the cells, which were reslurried into a 10 weight percent suspension with a dilute alkaline solution of 0.03N sodium hydroxide having a pH of 9.5. The alkaline slurry was heated to a temperature of about 90° C. for about 10 minutes to extract primarily nucleotidic materials. The slurry was centrifuged to separate the alkaline extract from the cells. The separated cells were then reslurried in a 0.15N sodium hydroxide solution at a temperature of about 95° C. for about 30 minutes to extract the proteinaceous materials. Thereafter, the slurry was neutralized to lower the viscosity and the undigested cell residue was separated from the supernatant by centrifugation. The supernatant containing the extracted proteins was acidified to a pH of 4.2 to precipitate the isoelectrically precipitable proteins, which were separated from the mother liquor solution or whey by centrifugation. The mother liquor solution was then neutralized to about pH 7.0 and spray-dried to yield a yeast whippable protein. Also, the precipitated portion of the proteins was neutralized, dissolved in water, and spray-dried to yield a yeast protein isolate. 
     Product samples based on 1 kg. starting material (dry weight) were prepared with and without dialysis to show the effect of NaCl. The composition of the two primary products are summarized in Table I. 
     
                       TABLE I______________________________________PRODUCT COMPOSITIONProduct       Dialysis Ash, %  N, %  Protein, %*______________________________________Yeast Protein Isolate         -        2.4     14.0  87.5Yeast Protein Isolate         +        2.6     14.2  88.7Yeast Whippable Protein         -        24.8**   5.2  32.8Yeast Whippable Protein         +        3.2      5.7  35.6______________________________________ *(N × 6.25) **Mainly NaCl 
    
     The product samples were further tested for their functional properties. These results are summarized in Tables II, III, IV, and V. 
     
                       TABLE II______________________________________FOAMING AND EMULSIFYING PROPERTIES                                Emulsifying                Foaming  Foam   Capacity,**         Dia-   Ability, Stability,                                ml. oil/g.Product       lysis  ml.*     ml.*   sample______________________________________Yeast Protein Isolate         -      265      208    300Yeast Protein Isolate         +      130       50    330Yeast Whippable Protein         -      265      180    172Yeast Whippable Protein         +      260       30    160______________________________________ *Testing procedure on &#34;Foaming Ability&#34; and &#34;Foam Stability&#34;: A 1 per cen aqueous solution of the test sample is agitated at 1000 rpm in a Virtis 4 mixer for 60 seconds at 32° F. The mixture is placed in a measurin cylinder and the measurement of volume in ml. is the &#34;Foaming Ability.&#34; The foam is allowed to settle for 30 minutes, and the volume of foam remaining at that time, in ml., is the &#34;Foam **Testing procedure on &#34;Emulsifying Capacity&#34;: A one g. sample of test material is mixed in a Waring blender with 50 ml. of a 0.9 per cent NaCl solution. 50 ml. of vegetable oil is added to the contents of the blender and mixed for ten seconds. A stream of oil is continuously added to the mixture at a rate of 20 ml./minute, with the blender mixing, until the emulsion breaks. The amount of oil added when the emulsion breaks is the &#34;Emulsifying Capacity. 
    
     
                       TABLE III______________________________________THERMOGELABILITY*      Di-     Diameter, Height,Sample     alysis  cm.       cm.    Observations______________________________________Yeast Protein      -       7.0       3.6    cake,Isolate                             crystalline                               mild flavor,                               brown colorYeast Protein      +       7.0       4.1    cake,Isolate                             crystalline,                               soft, mild                               flavor, cream                               color.Yeast Whippable      -       12.8      0.9    crystalline,Protein                             brittle,                               tan color.Yeast Whippable      +       13.2      0.8    crystalline,Protein                             crunchy.Promosy 100      -       7.7       2.5    soft, crystal-                               line, tough,                               cream color.Sodium caseinate      -       9.0       2.5    crystalline,                               tough, holes,                               cream color.*Testing procedure: Mix the following ingredients (given in weight per cent) into a ball and bake at 350° F.for 45 minutes:Protein sample         10.00Salt                    0.75Sugar                  30.00Flour                  30.00Water                  25.25______________________________________ 
    
     Determine the dimensions, textures, and organoleptic properties. 
     
                       TABLE IV______________________________________WHIPPABILITY IN FRAPPE SYSTEM*Product    Dialysis Whippability                           Color______________________________________Soy Isolate      -        Yes         WhiteEgg White  -        Yes         WhiteYeast Protein      -        Yes         Dark tanIsolate             (low volume)Yeast Protein      +        Yes         Slightly tanIsolate             (low volume)Yeast Whippable      -        Yes         WhiteProteinYeast Whippable      +        Yes         WhiteProteinControl    -        No          Slightly tan______________________________________ *Testing procedure: Heat corn syrup to 245° F. and add it to a slurry of invert sugar, water, and the protein sample. Beat in a Hobert mixer with whip for five minutes. Observe if foam is produced. The proportions of the ingredients are as follows (expressed as weight percent): Corn syrup 47.3 Invent sugar 47.3 Water 3.6 Protein sample 1.8 
    
     
                       TABLE V______________________________________PER CENT SOLUBIITY IN ACID pH*pH   Yeast Protein Isolate                 Yeast Whippable Protein______________________________________4.0  0                1003.8  19.8             1003.6  81.1             1003.3  100              100______________________________________ *One per cent of a protein sample in water was adjusted to the desired pH and dispersed for thirty minutes at room temperature. The suspension or solution was centrifuged for twenty minutes at 18,000 rpm. The concentration of nitrogen in the supernatant was determined and the percentage of soluble nitrogen calculated. 
    
     The results of these tests indicate that the yeast protein isolate appears to be responsible for the good emulsification characteristics and has about twice the emulsion capacity of soy isolate. In addition, the yeast protein isolate has thermogelability comparable to that of sodium caseinate. This property may be improved to some extent by mixing the yeast protein isolate with an amount of yeast whippable protein. In addition, the yeast whippable protein product is an excellent whippable protein material having a performance comparable to that of egg white. The functional performance of the yeast whippable protein product is not impaired by the high content of NaCl formed from neutralization. Furthermore, some hydrolyzed proteinaceous components were apparently lost through dialysis as indicated by the composition data in Table I. Hence, desalting is not necessary. Thus, the products produced by this process have the ability to replace the high-cost functional protein ingredients such as soy isolate, sodium caseinate, and egg white. Also, the yeast whippable protein is soluble at all acid pH values and the yeast protein isolate can be completely dissolved at pH 3.3. These properties demonstrate their potential for use in making acidic protein beverages. 
     It will be obvious to those skilled in the art that many variations from the preferred embodiment chosen for purposes of illustration can be made without departing from the scope of this invention.