Heat Treatment for Single-Cell Materials and Resultant Products

1. Field of the Invention

This invention relates to processes for treating single-cell materials to yield a product having improved functional properties. More particularly, it relates to a heat treatment of an aqueous yeast slurry in which the yeast is held at a particular temperature for a predetermined period of time. The optimal time-temperature relationship happens to coincide with a segment of the invertase deactivation curve, which makes invertase a convenient indicator for the completion of the heat treatment.

2. Description of the Prior Art

In recent years, a number of different proteinaceous materials have been developed for use as substitutes for the more traditional sources of protein in the human diet. Included among these proteinaceous materials are the single-cell micro-organisms such as fungi, bacteria, and yeasts. They are generally used in small amounts in a variety of food applications, depending upon their individual properties.

For example, spray-dried Candida utilis is a highly nutritious food product which is high in protein, B vitamins, and essential minerals. It is commercially produced by a fermentation process wherein pure ethyl alcohol, nutrients, and water are sterilized and metered into an aseptic fermentation vessel. Active Candida utilis yeast is then added and grown continuously while the fermentor contents, or broth, is continuously withdrawn. The broth is centrifuged to concentrate the yeast cells into a cream having a yeast cell concentration of about 10-20 weight percent. The cream is then pasteurized and spray-dried to a powder form. The resulting powdered yeast is an effective food supplement by itself or when used in combination with other protein sources. Because of the high food value of such a product, there has been continual interest in improving or varying its functional properties in order to make it more acceptable in various food applications.

It has now been discovered that by subjecting single-cell materials to a controlled heat treatment having temperatures of from about 215.degree. to about 300.degree. F., some of the functional properties of the product are improved. In particular, the heat-treated product exhibits improved acid bite suppression which is of particular importance in salad dressings. It also heightens sour cream flavor and increases smooth mouthfeel. In addition, the heat treated product exhibits other functional properties such as fat sparing, fat binding, emulsification and stabilization control, and water binding, and also benefits a variety of food products such as taco seasoning mix, stroganoff sauce, pizza sauce, and others by heightening flavors and reducing ingredients costs.

In one aspect, the invention resides in a process for imparting improved functional properties to whole cell single-cell protein materials comprising heating an aqueous slurry of the single-cell materials to a temperature of from about 215.degree. to 300.degree. F. for from about 30 seconds to about 25 minutes. A more preferred temperature range would be from about 255.degree. to about 300.degree. F. A still more preferred temperature range would be from about 260.degree. to about 270.degree. F.

In another aspect, the invention resides in a process for imparting improved functional properties to whole cell single-cell protein materials, the improvement comprising heating an aqueous slurry of single-cell materials at a temperature of from about 215.degree. to about 300.degree. F., and preferably from about 255.degree. to about 275.degree. F. for a period of time about sufficient to deactivate invertase. Invertase, which is an enzyme generally present in most single-cell materials in significant amounts, can be deactivated by subjecting it to various time-temperature conditions. Deactivation times and temperatures are inversely related, i.e. at higher temperatures only a relatively short duration is necessary to deactivate the invertase. At lower temperatures, however, a much longer time is necessary. Coincidentally, those conditions which are most effective in producing the desired results of this invention fall along, or at least very close to, a segment of the invertase deactivation curve (see Example 3). In particular, the times necessary to deactivate invertase when subjected to temperatures between 215.degree. and 300.degree. F., and preferably between 255.degree. and 275.degree. F., are about the same as those necessary to achieve the optimal desired improvements in product properties. Therefore, invertase activity can be used as a convenient indicator as to whether or not a particular heat treatment is optimal. It must be pointed out that some improvement in functional properties can still be obtained when the single-cell material is either undertreated or overtreated, but optimum conditions appear to lie along the deactivation curve.

More particularly, the invention resides in the abovesaid process wherein the single-cell protein material is a yeast. The yeast can be active or inactive. Preferred yeasts include Candida utilis, Saccharomyces cerevisiae, Saccharomyces fragilis, and Saccharomyces carlsbergensis.

In a further aspect, the invention resides in an improved process for producing a food-quality yeast wherein a yeast broth is continuously withdrawn from a fermentation zone, said yeast broth being subsequently concentrated into a cream which is in turn spray-dried to a powder form, the improvement comprising heating the yeast cream to a temperature of from about 255.degree. to about 275.degree. F. for a period of time long enough to deactivate invertase. The yeast broth generally has a cell concentration between 1 and 5 weight percent (dry basis), whereas the yeast cream will generally have a cell concentration from between 5 and 25 weight percent (dry basis). The preferred temperature for the heat treatment is about 265.degree. F. and the preferred time of treatment is about 2-2.5 minutes.

In a further aspect, the invention resides in an improved process for the continuous production of food quality yeasts by aerobic growth of said yeasts in a fermentation zone on an aqueous ethanolic substrate in the presence of ammonia, an oxygen-containing gas, and an excess of essential nutrients, wherein said yeasts are withdrawn from said fermentation zone as a broth, concentrated to a cream by centrifugation, and spray-dried to powder form, the improvement comprising heating the yeast cream to a temperature of from about 260.degree. to about 270.degree. F. for a length of time of from about 2 to about 2.5 minutes. The preferred yeast is Candida utilis and the preferred temperature is about 265.degree. F.

In all aspects of this invention, the heat treatment insures full pasteurization.

The following examples will illustrate some of the features of this invention.

Invertase activity is essentially measured by reacting the invertase-containing sample with a sucrose solution and then colorimetrically analyzing for the invert sugar product.

For example, four 100 ml. samples of cream (15 weight percent live cell suspension) were separately heated in beakers with a constant agitation for different time periods: 0 min., 15 min., 30 min., and 45 min. at 93.degree. C. The cell suspensions were then centrifuged to remove cell debris. The clear supernatants were dialyzed to remove salts and then assayed for invertase activity. The assay solution contained 1 ml. of sucrose solution (400 mg. sucrose/ml.) and 0.1 ml. of enzyme solution (the supernatant). The sucrose solution was made by dissolving 40 mg. of sucrose in 100 ml. of 0.02 M phosphate buffer (pH 4.5). The reaction was initiated by addition of 0.1 ml. enzyme solution to the sucrose solution and was allowed to proceed at 55.degree. C. for 2 min. The reaction was terminated by addition of 1.0 ml. of Nelson's alkaline copper reagent. The reaction mixture, in a test tube, then was placed in boiling water and heated for 20 min. The tube was cooled and 1.0 ml. of arsenomolybdate reagent was added to the solution for color development. The color intensity was measured at 540 millimicrons after dilution of the solution with 7 ml. of water.

The results are shown below:

The invertase of the live cell suspension was completely inactivated in 45 min. at 93.degree. C.

As a comparison, invertase deactivation was measured for samples containing spray-dried inactive cells (10 weight percent slurry). The results, indicated below, show that the spray-dried cells can be deactivated in shorter times than the live cells.

It is believed that the spray drying process physically releases some of the bound invertase within the cells, leaving mostly free invertase. The free invertase is more readily deactivated than the bound invertase and therefore the spray-dried cells can be deactivated in a shorter period of time.

The following data represent heat treatment times at which the invertase in live yeast cells will be deactivated at the given temperatures. The numbers in parentheses indicate times which were insufficient to completely deactivate the invertase.

A plot of these points gives a curved line which, for purposes of this application, is called the "invertase deactivation curve". (The actual invertase deactivation curve lies slightly below this plot, but the plotted curve sufficiently serves the purpose of teaching those skilled in the art the essence of this invention.) It should be noted that the invertase is deactivated at any point on or above the curve. ##STR1##

To study the effects of heat treated single-cell protein materials, four samples of heat-treated Candida utilis yeast were incorporated into yellow cake and shortbread cookies as a replacement for 40% of the egg solids. Each sample was given a different time-temperature treatment. Samples 2 and 3 were treated at conditions closely corresponding to the invertase deactivation curve. The results are presented in TABLES I and II.

As shown in TABLE I, flavor improvement in yellow cakes was obtained at all levels of heat treatment, although the best results were achieved with Samples 2 and 3, which were treated more closely to the condition of the invertase deactivation curve. Sample 1 was undertreated, whereas Sample 4 was overtreated. Nevertheless, there was an overall flavor improvement over the control by all four samples. This illustrates the fact that improved functional properties of single-cell protein materials, and particularly yeast, can be achieved through a heat treatment, with optimal results occurring when the heat treatment corresponds to the conditions on the invertase deactivation curve.

TABLE II illustrates that Samples 2 and 3 again have better performance, this time in shortbread cookies.

The following Examples 6-16 will serve to further illustrate the advantages of using a heat-treated single-cell protein material (heat-treated yeast) in various food formulations. In producing this product, a normal yeast cell cream (Candida utilis), having a yeast cell concentration of about 17 weight percent, is heated to about 265.degree. F..+-.5.degree. with an intermediate preheat of 110.degree.-180.degree. F. to aid in bringing the cream up to the desired temperature. The heated suspension is held at temperature for about 2.0-2.5 minutes and then spray-dried to a dry powder with less than about 6 weight percent moisture. The heat treatment is carried out in a continuous, plug flow manner. Although the residence time of the preheat step is not critical, the residence time at 265.degree..+-.5.degree. F. must be maintained between 2.0-2.5 minutes if an optimal product is to be produced.

A typical analysis of the heat-treated yeast used herein is as follows:

DIRECTIONS:

1. Combine in steam kettle: 1/4 of cheese, 1/2 of water, stir until slightly melted.

2. Add salt, sodium aluminum phosphate, heat-treated yeast (blend with oil) and remainder of water and cheese.

3. Heat to 71.degree. C.--mold.

BENEFITS:

1. Heightens cheese flavor.

2. Promotes smooth mouthfeel.

Directions

1. Combine sugar, gums, and water; blend.sup.e for 15 minutes on speed 1.

2. Add seasonings, lemon juice, and vinegar, blend for 1 minute on speed 2.

3. Add egg and heat-treated yeast, blend for 2 minutes on speed 2.

4. Slowly add oil while mixing on speed 2.

5. Process through colloid mill.

Benefits

1. Lowers ingredient costs

2. Functionally replaces egg yolk

3. Balances flavor

DIRECTIONS

1. Combine sugar and gum; add to water blend.sup.d for 15 minutes on speed 1.

2. Add seasonings, vinegar, and lemon juice; mix for 1 minute on speed 2.

3. Add egg and heat-treated yeast; mix for 1 minute on speed 2.

4. Add oil slowly at first, blend well on speed 2.

5. Process twice through colloid mill.

6. Add sweet pickle relish.

BENEFITS

1. Functionally replaces egg yolk.

2. Lowers ingredient costs.

DIRECTIONS

1. Combine sugar and gum; add to water blend.sup.d for 15 minutes on speed 1.

2. Add seasonings, vinegar, and lemon juice; mix for 1 minute on speed 2.

3. Add egg and heat-treated yeast; mix for 1 minute on speed 2.

4. Add oil, slowly at first, blend well on speed 2.

5. Process twice through colloid mill.

BENEFITS

1. Heightens spice flavor.

2. Lowers ingredient costs.

DIRECTIONS

1. Combine sugar and gum; add to water, blend for 15 minutes on speed 1.

2. Add seasonings, vinegar, and lemon juice; mix for 1 minute on speed 2.

3. Add egg and heat-treated yeast; mix for 1 minute on speed 2.

4. Add oil slowly at first, blend well on speed 2.

5. Process twice through colloid mill.

6. Add onions and chives.

BENEFITS

1. Functionally replaces egg yolk.

2. Enhances onion flavor.

3. Lowers ingredient costs.

DIRECTIONS

1. Combine sugar and gum; add to water blend.sup.d for 15 minutes on speed 1.

2. Add seasonings, vinegar, and lemon juice; mix for 1 minute on speed 2.

3. Add egg and heated-treated yeast; mix for 1 minute on speed 2.

4. Add oil slowly at first, blend well on speed 2.

5. Process twice through colloid mill.

BENEFITS

1. Functionally replaces egg yolk.

2. Thickens dressing.

3. Lowers ingredient costs.

DIRECTIONS

1. Combine sugar and gum; add to water blend.sup.e for 15 minutes on speed 1.

2. Add seasonings, vinegar, and lemon juice; mix for 1 minute on speed 2.

3. Add egg and heat-treated yeast; mix for 1 minute on speed 2.

4. Add oil, slowly at first, blend well on speed 2.

5. Process twice through colloid mill.

BENEFITS

1. Lowers ingredient costs

2. Functionally replaces egg yolk

3. Balances flavor

Directions

1. Precook macaroni in water containing 2% salt. Boil for at least 12 minutes. Wash and rinse pasta in cold water and drain.

2. Grind all the beef through a 3/8" plate.

3. Blend the flour, salt, seasoning, heat-treated yeast and tomato paste with approximately 1/2 the water until a smooth paste is attained. Add the ground beef and remaining water and blend.

4. Add the cooked macaroni mix and can.

Benefits

1. Heightens chili flavor.

2. Reduces free surface fat after retorting.

3. Reduces spice use by 38 percent.

DIRECTIONS

1. Dry blend all the ingredients.

2. Package in 1.5 ounce packages.

3. Blend contents of one package with 1/2 cup of cold milk, allow mixture to stand for 15 minutes, mix and serve.

BENEFITS

1. Heightens sour cream flavor.

2. Replaces monosodium glutamate.

3. Partially replaces buttermilk solids.

4. Increases smooth mouthfeel.

5. Lowers ingredient cost.

6. Partially replaces sour cream solids.

DIRECTIONS

1. Dry blend all the ingredients.

2. Package in 1.5 ounce package.

3. Recommend browning 1 lb. of ground lean beef, adding 3/4 cup of water and the seasoning package ingredients. Bring this product to a boil and simmer.

BENEFITS

1. Heightens chili flavor.

2. Partially replaces monosodium glutamate.

3. Partially replaces garlic powder.

4. Partially replaces onion powder.

5. Lowers ingredient cost.

DIRECTIONS:

1. Mix all dry ingredients and emulsifier until homogeneous.

2. Add 3/5 of the water (ice water) and flavor, mix,*** scrape bowl after each mixing period as follows: 1/2 minute on speed 1, 1 minute on speed 2, 3 minutes on speed 3, and 3 minutes on speed 4.

3. Add balance of water and mix, scrape bowl after each mixing period as follows: 1/2 minute on speed 1, 2 minutes on speed 2, 4 minutes on speed 1, and 2 minutes on speed 2.

4. Specific gravity should be 0.510-0.525; if not, mix an additional 30 seconds on speed 3.

5. Scale 10 oz. in 61/2" ungreased tube pan or 8" round pan.

6. Bake at 375.degree. F. for 25-30 minutes.

Benefits:

1. Partially replaces egg yolk.

2. Improves bite quality--stronger bite quality.

3. Lowers ingredient cost.

These examples are only illustrative of some of the many applications to which the heat-treated single-cell materials of this invention are suited. It will be obvious to those skilled in the art that many variations from these examples, shown for purposes of illustration, can be made without departing from the scope of this invention.