WALNUT YOGURT AND PREPARATION METHOD THEREOF

Abstract

A preparation method of a walnut yogurt includes roasting peeled walnut kernels at 145° C. to 165° C. for 5 to 25 min, degreasing the roasted walnut kernels until the fat content is 20% to 50% to obtain semi-degreased walnut kernels, mixing the semi-degreased walnut kernels with a stabilizer, a carbon source and water and then milling the resultant to obtain a walnut slurry, and subjecting the walnut slurry to fully mixing, sterilization, inoculation, and fermentation to obtain the walnut yogurt. This preparation method can effectively improve the aroma of walnuts after roasting at high temperature. On this basis, the ideal flavor of the system can be ensured after removing part of the fat. Moreover, a yogurt with stable quality and rich nutritional value can be obtained by removing fat to the above degree.

Description

CROSS REFERENCE

The present application claims the priority of the Chinese patent application No. 202110459504.X, entitled “Walnut yogurt and preparation method thereof”, filed on Apr. 27, 2021, the entire disclosure of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to the technical field of food processing, specifically, to a walnut yogurt and preparation method thereof.

BACKGROUND ART

Walnut, also known as Juglans regia, is one of the four major nuts. It has high nutritional value and economic value, and is a kind of medicine and food homologous food. The main components of walnut kernels are fat and protein, and the fat content is up to 60% to 70%, wherein unsaturated fatty acids such as oleic acid, linoleic acid and linolenic acid account for more than 90% of the total fatty acids, and the protein content is up to 15% to 20%. Moreover, walnut protein contains 18 amino acids, of which the content of 8 essential amino acids is very high. In addition, walnut also contains a variety of trace elements such as potassium, calcium, iron, manganese, zinc, copper, and selenium, as well as vitamins such as folic acid, niacin, and pantothenic acid. Therefore, walnut has the effects of anti-oxidation, anti-fatigue, preventing cardiovascular and cerebrovascular diseases, and reducing the risk of diabetes.

Yogurt is one of the favorite foods of consumers, but milk contains relatively high cholesterol and saturated fatty acids. Today, when the whole people are pursuing health, yogurt can no longer fully meet the needs of consumers. However, plant-based yogurt has the characteristics of high protein content and zero cholesterol, and has broad prospects in future fermented beverage products.

If yogurt can be prepared from walnut as a plant-based raw material, due to the action of microorganisms, the macromolecular substances such as fat and protein in walnut yogurt will be partially decomposed into small molecules that are easier to absorb and have strong functional activity, and meanwhile, the acid- and aroma-producing characteristics of lactic acid bacteria make the products have obvious fermentation flavor and have great development potential. However, due to the high fat content in walnut, there are problems of instability and easy layering of yogurt when fermentation is directly performed with walnut as a raw material. Moreover, with walnut as a raw material of plant origin, it is difficult to ferment and prepare a pure fermentative yoghurt with ideal flavor, taste and color. For most of the existing technologies, the pure fermentative yoghurt is used in a fermented milk beverage, which is added with various additives such as sugar and stabilizers and is far inferior to the pure fermentative yoghurt in terms of nutrition and health.

SUMMARY OF THE INVENTION

The present invention provides a preparation method of a walnut yogurt with ideal flavor, stable quality and high nutritional value, which comprises the following steps:

• • 1) roasting peeled walnut kernels at 145° C. to 165° C. for 5 to 25 min;
  • 2) degreasing the roasted walnut kernels until the fat content is 20% to 50% to obtain semi-degreased walnut kernels;
  • 3) mixing the semi-degreased walnut kernels with a stabilizer, a carbon source and water and then milling the resultant to obtain a walnut slurry; and
  • 4) subjecting the walnut slurry to sufficient mixing, sterilization, inoculation, and fermentation to obtain the walnut yogurt.

It is found in the present invention that by degreasing the walnut to control the fat content in the above range further assisted by the roasting treatment as mentioned above on the materials before degreasing, it can be ensured that the obtained yogurt has a good aroma and flavor on the basis of ensuring the stability of the obtained yogurt, and a walnut yogurt that is close to the traditional animal-derived yogurt in terms of taste, flavor, stability and the like can be prepared.

Preferably, the following operation is also included: adding 0.5% to 3% by mass of walnut oil obtained by hot pressing at 150° C. to 200° C. and then cooling to the walnut slurry. Hot pressing of walnut oil will cause a certain loss of nutrients therein. Therefore, cold pressing is generally used in the prior art, but the aroma of the oil will be significantly enhanced after hot pressing. With the above addition of a small amount of walnut oil, the aroma of yogurt can be significantly improved.

Further preferably, after adding the walnut oil, the slurry is subjected to high-speed shearing under the condition of 5,000 to 20,000 r/min for 2 to 15 min.

Preferably, the stabilizer is one or a combination of at least two of diacetyl tartaric acid ester of mono(di)glycerides, hydroxypropyl distarch phosphate, sodium carboxymethyl cellulose, agar, pectin or gellan gum. After high-temperature roasting treatment, part of the protein in the walnut will be denatured, so the stability of the system after fermentation will be deteriorated than that without roasting. The addition of a stabilizer can further improve the stability of the system, especially the addition of the above stabilizer can effectively improve the stability of the system.

Further preferably, based on the mass of the walnut slurry, the stabilizer is added at the following amount: 0.05 to 0.5 wt % for diacetyl tartaric acid ester of mono(di)glycerides, 0.5 to 3 wt % for hydroxypropyl distarch phosphate, 0.05 to 0.5 wt % for sodium carboxymethyl cellulose, 0.05 to 0.5 wt % for agar, 0.05 to 0.5 wt % for pectin, and 0.05 to 0.5 wt % for gellan gum.

More preferably, based on the mass of the walnut slurry, the stabilizer comprises 1.5 wt % of hydroxypropyl distarch phosphate, 0.3 wt % of sodium carboxymethyl cellulose and 0.1 wt % of agar; or 0.05 wt % of diacetyl tartaric acid ester of mono(di)glycerides, 0.08 wt % of hydroxypropyl distarch phosphate and 0.3 wt % of pectin; or 0.15 wt % of pectin and 0.3 wt % of gellan gum.

Preferably, in the step 4), the sufficient mixing is performed by high-pressure-homogenization, and after a first homogenization is performed under the pressure condition of 20 to 70 MPa, a second homogenization is performed under a pressure which is 10 to 20 MPa higher than the pressure in the first homogenization. The stability of the system can be effectively improved, and the color of yogurt can be effectively improved by the above-mentioned homogenization treatment.

Preferably, the degreasing treatment in the step 2) is specifically as follows: hydraulic pressing is performed under the condition of a pressure of 6 to 22 MPa. Hydraulic pressing will not have a significant impact on the quality of fat, and can ensure the nutritional value of yogurt.

Preferably, in the step 3), the carbon source is sucrose or glucose or a combination of the two; and the addition amount of the carbon source is 1% to 8% by mass of the walnut slurry.

Further preferably, the mass ratio of the walnut kernels to the water is 1:(4-15).

Preferably, the milling rotational speed is 20,000 to 40,000 r/min and the temperature of milling is 40° C. to 75° C. during the milling process in the step 3).

Preferably, the specific operation of the sterilization is as follows: heating materials to to 95° C. and sterilizing for 15 to 20 min.

Preferably, in the inoculation process, strains Streptococcus thermophilus and Lactobacillus plantarum are used. The addition of Lactobacillus plantarum can enhance the acid-producing performance in walnut yogurt.

Further preferably, the Streptococcus thermophilus and Lactobacillus plantarum are compounded at a cell number ratio of 1:(1-5).

Preferably, the specific operation of the fermentation is as follows: adding a starter culture at 3×10⁷ to 7×10⁷ CFU/mL to the sterilized materials, and fermenting at 35° C. to until the acidity reaches 70° T.

Preferably, an after-ripeness operation is further included after the fermentation is completed, and specifically, the after-ripeness operation is as follows: rapidly cooling the walnut yogurt to 4° C., and storing at 4° C. to 6° C. for 12 h. By the above operation, the further growth of microorganisms can be inhibited, and meanwhile, the activity of enzymes can be reduced to prevent excessive acid production; such that, the state of yogurt can be more stable, the production of flavor substances can be facilitated, and the taste of yogurt can be increased.

The present invention also protects a walnut yogurt, which is prepared by the method of the present invention.

The present invention has the following beneficial effects:

• • 1) The present invention uses partial degreasing technology to retain walnut fat to the greatest extent and increase the relative protein content of raw materials. In order to compensate for the influence of lipid deficiency on aroma, roasting treatment is introduced in the pre-treatment, so that the aroma of walnut in the product is activated. Meanwhile, because the walnut itself is rich in fat, protein and sugar, the Maillard reaction is promoted by roasting, so that the final yogurt obtains a special roasting aroma and light caramel color, which further improves the sensory quality of the product. In addition, the inventor found that because the early roasting will cause partial denaturation of protein and variation of sugar content, which will have a certain impact on the later fermentation, the roasting condition is controlled at 145° C. to 165° C., and the degree of degreasing is degreasing to the fat content of 20% to 50%, which can effectively take into account the overall quality of the product such as stability, flavor, color and taste, to obtain a walnut yogurt of ideal quality.
  • 2) The walnut oil obtained by pressing by semi-degreasing technology in the present invention can be reprocessed and utilized in the form of raw materials of walnut yogurt or further prepared into walnut oil finished products, which reduces the cost of product raw materials and enriches the utilization of by-products.
  • 3) The fermentation temperature and the fermentation time involved in the present invention are consistent with the fermentation conditions required for traditional yogurt, which is easy to industrialize.
  • 4) The semi-degreased roasted walnut yogurt prepared by the present invention has a smooth mouthfeel, a fine texture close to that of traditional yogurt, and can meet the needs of consumers.

SPECIFIC MODES FOR CARRYING OUT THE EMBODIMENTS

The following Examples are intended to illustrate the present invention, but are not intended to limit the scope of the present invention.

Example 1

The present Example relates to a preparation method of a walnut yogurt, comprising the following steps:

• • (1) peeled walnut kernels were roasted at 160° C. for 15 min, and cooled to room temperature for later use;
  • (2) the roasted walnut kernels were pressed at 6 MPa for 15 min to obtain semi-degreased roasted walnuts with a fat content of 49.61%;
  • (3) the semi-degreased roasted walnuts obtained in step (2) were mixed with auxiliary materials, carbon source and water and then milled to obtain a walnut slurry, wherein the mass ratio of semi-degreased roasted walnuts to water in the walnut slurry was 1:13; based on the mass of the walnut slurry, the auxiliary materials were added at the following amount: wt % of diacetyl tartaric acid ester of mono(di)glycerides, 0.08 wt % of hydroxypropyl distarch phosphate, and 0.3 wt % of pectin; the carbon source was 8 wt % of sucrose; the water was purified water; the milling rotational speed was 36,000 r/min; and the milling temperature was 40° C.;
  • (4) after heating hot-pressed walnut oil to 140° C. and cooling, the hot-pressed walnut oil was added at an amount of 0.5% by mass of the walnut slurry obtained in step (3), and high-speed shearing was performed at 10,000 r/min for 3 min to obtain a walnut emulsion;
  • (5) the walnut emulsion obtained in step (4) was subjected to high pressure homogenization under a pressure of 20 MPa, then homogenization again under a pressure of 40 MPa to obtain a walnut fermentation broth;
  • (6) the walnut fermentation broth obtained in step (5) was heated to 90° C. and sterilized for 15 min;
  • (7) a compounded starter culture was added at 4×10⁷ CFU/mL into the sterilized fermentation broth obtained in step (6), and fermentation was performed at 40° C. until the acidity reached 70° T; the compounded starter culture was composed of Streptococcus thermophilus and Lactobacillus plantarum at a ratio of 4:3; and
  • (8) the walnut yogurt from step (7) was rapidly cooled to 4° C., and stored at 4° C. for 12 h for after-ripeness to obtain the semi-degreased roasted walnut yogurt.

The semi-degreased roasted walnut yogurt prepared according to the above method had a protein content of 4.4%, a fat content of 4.04%, and a carbohydrate content of 6.2%.

Example 2

The present Example relates to a preparation method of a walnut yogurt, comprising the following steps:

• • (1) peeled walnut kernels were roasted at 145° C. for 20 min, and cooled to room temperature for later use;
  • (2) the roasted walnut kernels were pressed at 8 MPa for 40 min to obtain semi-degreased roasted walnuts with a fat content of 40%;
  • (3) the semi-degreased roasted walnuts obtained in step (2) were mixed with auxiliary materials, carbon source and water and then milled to obtain a walnut slurry, wherein the mass ratio of semi-degreased roasted walnuts in the walnut slurry to water was 1:10; based on the mass of the walnut slurry, the auxiliary materials were added at the following amount: 1.5 wt % of hydroxypropyl distarch phosphate, 0.3 wt % of sodium carboxymethyl cellulose, and 0.1 wt % of agar; the carbon source was 5 wt % of glucose; the water was purified water; the milling rotational speed was 30,000 r/min; and the milling temperature was 50° C.;
  • (4) after heating hot-pressed walnut oil to 200° C. and cooled, the hot-pressed walnut oil was added at an amount of 3% by mass of the walnut slurry obtained in step (3), and high-speed shearing was performed at 13,000 r/min for 10 min to obtain a walnut emulsion;
  • (5) the walnut emulsion obtained in step (4) was subjected to high pressure homogenization under a pressure of 30 MPa, then homogenization again under a pressure of 70 MPa to obtain a walnut fermentation broth;
  • (6) the walnut fermentation broth obtained in step (5) was heated to 92° C. and sterilized for 20 min; a compounded starter culture was added at 5×10⁷ CFU/mL into the sterilized fermentation broth obtained in step (6), and fermentation was performed at 37° C. until the acidity reached 70° T; the compounded starter culture was composed of Streptococcus thermophilus and Lactobacillus plantarum at a ratio of 1:2;
  • (7) the walnut yogurt in step (6) was rapidly cooled to 4° C., and stored at 4° C. for 12 h for after-ripeness to obtain the semi-degreased roasted walnut yogurt; and
  • (8) the semi-degreased roasted walnut yogurt prepared according to the above method had a protein content of 4.2%, a fat content of 6.64%, and a carbohydrate content of 5.3%.

Example 3

The present Example relates to a preparation method of a walnut yogurt, comprising the following steps:

• • (1) peeled walnut kernels were roasted at 165° C. for 5 min, and cooled to room temperature for later use;
  • (2) the roasted walnut kernels were pressed at 22 MPa for 30 min to obtain semi-degreased roasted walnuts with a fat content of 21%;
  • (3) the semi-degreased roasted walnuts obtained in step (2) were mixed with auxiliary materials, carbon source and water and then milled to obtain a walnut slurry, wherein the mass ratio of semi-degreased roasted walnuts to water in the walnut slurry was 1:10; based on the mass of the walnut slurry, the auxiliary materials were added at the following amount: 0.15 wt % of pectin, and 0.3 wt % of gellan gum; the carbon source was 7 wt % of sucrose; the water was purified water; the milling rotational speed was 36,000 r/min; and the milling temperature was 65° C.;
  • (4) after heating hot-pressed walnut oil to 150° C. and cooling, the hot-pressed walnut oil was added at an amount of 2% by mass of the walnut slurry obtained in step (3), and high-speed shearing was performed at 15,000 r/min for 12 min to obtain a walnut emulsion;
  • (5) the walnut emulsion obtained in step (4) was subjected to high pressure homogenization under a pressure of 25 MPa, then homogenization again under a pressure of MPa to obtain a walnut fermentation broth;
  • (6) the walnut fermentation broth obtained in step (5) was heated to 88° C. and sterilized for 20 min;
  • (7) a compounded starter culture was added at 6×10⁷ CFU/mL into the sterilized fermentation broth obtained in step (6), and fermentation was performed at 45° C. until the acidity reached 70° T; the compounded starter culture was composed of Streptococcus thermophilus and Lactobacillus plantarum at a ratio of 2:3; and
  • (8) the walnut yogurt in step (7) was rapidly cooled to 4° C., and stored at 4° C. for 12 h for after-ripeness to obtain the semi-degreased roasted walnut yogurt.

The semi-degreased roasted walnut yogurt prepared according to the above method had a protein content of 5%, a fat content of 3.9%, and a carbohydrate content of 7.4%.

Example 4

Compared with Example 1, the present Example differs in that the cooled hot-pressed walnut oil was not added, that is, the operation of step 4) was omitted.

The semi-degreased roasted walnut yogurt prepared according to the above method had a protein content of 4.4%, a fat content of 3.5%, and a carbohydrate content of 6.2%, with relatively light walnut flavor.

Comparative Example 1

A walnut yogurt was prepared according to preparation process of a walnut yogurt described in CN201810888504.X. Refer to Example 2 described in CN201810888504.X for specific preparation steps.

Comparative Example 2

A walnut yogurt was prepared according to a pure walnut yogurt and preparation method thereof described in CN201510329886.9. Refer to Example 2 described in CN201510329886.9 for specific preparation steps.

Comparative Example 3

A walnut yogurt was prepared according to a fermentation method of a pure walnut yogurt described in CN201911005491.8; Refer to Example 2 described in CN201911005491.8 for specific preparation steps

Comparative Example 4

Compared with Example 1, the present Comparative Example differs in that the walnut kernels were not roasted at a high temperature, that is, the operation of step 1) was omitted.

Comparative Example 5

Compared with Example 1, the present Comparative Example differs in that the roasting temperature in step 1) was 170° C.

Comparative Example 6

Compared with Example 1, the present Comparative Example differs in that in the step 2), degreasing was performed until the fat content was 15%.

Comparative Example 7

The yogurt already on the market was purchased, and the ingredients of the yogurt were raw milk, Lactobacillus thermophilus, Lactobacillus bulgaricus and sucrose.

Experimental Example

The tissue morphology of the semi-degreased roasted walnut yogurt prepared in Examples 1-4 and the walnut yogurt prepared in Comparative Examples 1-7 were subjected to sensory evaluation, respectively. The sensory evaluation was performed by 10 panelists according to the standards in Table 1, and the color, smell, taste and texture were described and analyzed.

The scoring standard was as follows: five items including color, smell, taste, texture, and overall acceptability were scored on a 10-point scale, wherein 10 represents the best and 1 represents the worst; and it was ensured that other conditions are consistent.

The sensory evaluation results of the tissue morphology of the semi-degreased roasted walnut yogurt prepared in Examples 1-4 and Comparative Examples 1-7 were shown in Table 2.

TABLE 1
Sensory scoring standard for semi-degreased roasted walnut yogurt
Items	Features	Score
Color	Uniform color, creamy white or creamy	8-10
	yellow
	Nonuniform color, brown or gray.	4-7
	Nonuniform color, colored spots or other	1-3
	abnormal colors
Smell	Strong walnut milk aroma, no peculiar	8-10
	smell
	Light walnut milk aroma, no peculiar	4-7
	smell
	No walnut milk aroma, slight other smell	1-3
Taste	Moderate sweet-sour ratio, walnut flavor,	8-10
	no bitterness and astringency
	Slightly sour or sweet, light walnut flavor,	4-7
	light bitterness and astringency
	Too sour or too sweet, slight walnut	1-3
	flavor, slight bitterness and astringency
Texture	Fine and uniform tissue, smooth surface,	8-10
	no precipitated whey
	Uniform tissue, small amount of	4-7
	precipitated whey
	Rough tissue, precipitated whey	1-3
Overall	Very good	8-10
acceptability	Fair	4-7
	Very poor	1-3

TABLE 2
Sensory index comparison of semi-degreased roasted walnut yogurt
prepared by different Examples and Comparative Examples
					Overall
					accept-
Items	Color	Smell	Taste	Texture	ability	Tissue status
Example	8	8	9	9	9	Fine tissue, smooth
1						texture, sticky,
						substantially no
						precipitated whey
Example	9	9	9	9	9	Fine tissue, soft texture,
2						smooth surface, no
						precipitated whey
Example	8	8	8	9	8	Relatively fine
3						tissue, soft
						texture, relatively
						refreshing, small amount
						of precipitated whey
Example	8	5	6	8	7	Relatively fine
4						tissue, but
						slightly hard texture,
						small amount of
						precipitated whey
Compar-	8	8	9	8	9	Fine tissue, smooth
ative						texture, no whey
Example						separation
1
Compar-	8	6	7	5	6	Fine tissue, soft texture,
ative						relatively thin overall
Example						morphology, relatively
2						serious whey
						precipitation
Compar-	8	8	6	6	7	Bad curd formation
ative						status, relatively thin
Example						overall morphology,
3						obvious whey
						precipitation
Compar-	6	3	4	7	5	Relatively fine tissue,
ative						soft texture,
Example						substantially no
4						whey precipitation
Compar-	4	4	4	3	2	Substantially no curd,
ative						fluid texture
Example
5
Compar-	7	5	5	3	4	Rough tissue, hard
ative						texture, whey
Example						precipitation
6						and obvious
						granular sensation
Compar-	9	9	9	9	9	Fine tissue, soft and
ative						elastic texture,
Example						substantially no whey
7						precipitation

It can be seen from Table 2 that the semi-degreased roasted walnut yogurt prepared in Examples 1-4 of the present invention has better sensory evaluation and higher overall acceptability than the walnut yogurt prepared in Comparative Examples 2-5. The semi-degreased roasted walnut yogurt prepared in Examples 1-3 of the present invention has creamy white or creamy yellow color, uniform color, soft texture, fine tissue, substantially no whey precipitation, and smooth mouthfeel, and its mouthfeel, texture, color and the like are close to the commercially available yogurt (such as Comparative Example 7). However, the walnut yogurt prepared in Example 4 lacks the addition of high-temperature walnut oil, so it is not good in smell and taste. The walnut yogurt prepared in Comparative Example 1 has fine texture, smooth texture and no whey precipitation, and its better texture formation may be related to the use of skimmed milk powder as an ingredient. Therefore, in essence, the yogurt prepared in Comparative Example 1 is not pure walnut yogurt. Although the pure walnut yogurt prepared in Comparative Example 2 has fine texture and soft texture, the overall morphology is relatively thin and the whey precipitation is relatively serious, leads to obvious layering of emulsion. The pure walnut yogurt prepared in Comparative Example 3 is not good in curd formation status, the overall morphology is relatively thin and there is a relatively obvious problem of whey precipitation. Due to lacking of the roasting step, the semi-degreased roasted walnut yogurt prepared in Comparative Example 4 has gray product color, and there are unpleasant flavors such as raw odor and obvious grassy odor in smell and taste. The roasting conditions of Comparative Example 5 are relatively intense, so the color of the product is dark brown and there is an obvious burnt smell. Meanwhile, the excessively high roasting temperature results in denaturation of basically all of the raw proteins, and curd is not formed at the end of fermentation, which is very different from the desired sensory properties of the product. Comparative Example 6 has the lowest fat content, so the relative content of protein is greatly increased. After fermentation, the texture of the product is too hard, and the powdery sensation and granular sensation are obviously enhanced, which has a negative impact on the sensory experience.

TABLE 3
Comparison of quality indexes of walnut yogurt prepared
in different Examples and Comparative Examples
			Water holding
Items	Acidity (°T)	pH	capacity (%)
Example 1	73.24 ± 0.34	4.26 ± 0.25	79.04 ± 0.66
Example 2	75.12 ± 0.22	4.19 ± 0.08	80.23 ± 1.43
Example 3	74.32 ± 1.55	4.23 ± 0.56	77.56 ± 0.22
Example 4	73.74 ± 1.23	4.21 ± 0.43	78.73 ± 0.35
Comparative Example 1	76.23 ± 2.64	4.13 ± 0.88	85.34 ± 2.61
Comparative Example 2	64.98 ± 0.79	4.45 ± 0.73	56.98 ± 2.33
Comparative Example 3	36.39 ± 1.64	4.74 ± 0.96	68.42 ± 0.96
Comparative Example 4	77.40 ± 1.31	4.16 ± 0.49	77.46 ± 1.42
Comparative Example 5	49.02 ± 0.45	4.75 ± 0.05	46.33 ± 1.84
Comparative Example 6	76.51 ± 0.97	4.18 ± 0.34	80.39 ± 0.61
Comparative Example	78.90 ± 0.15	4.17 ± 0.02	64.38 ± 0.23
7

It can be seen from Table 3 that the degreased roasted walnut yogurt prepared in Examples 1-4 of the present invention has similar acidity, pH and water holding capacity to those of the walnut yogurt prepared in Comparative Examples 1, 4 and 6, indicating that semi-degreasing and roasting treatment can reduce the fat content of the raw materials, increase the flavor of the product without affecting the acid-producing capacity of the lactic acid bacteria, and can increase the water holding capacity of the product, which has a positive effect on the improvement of product stability. Compared with the commercially available animal yogurt of Comparative Example 7, Examples 1-4 of the present invention have similar acidity and higher water holding capacity, indicating that the product can simulate traditional yogurt in terms of texture, tissue state and taste, and has better stability. Meanwhile, compared with the pure walnut yogurt prepared in Comparative Examples 2, 3 and 5, the semi-degreased roasted walnut yogurt prepared in Examples 1-4 of the present invention has higher acidity, lower pH value and better water holding capacity, indicating that the semi-degreased roasted walnut yogurt prepared in Examples 1-4 of the present invention can compensate for the defects of insufficient acid production, poor taste and poor texture of walnut yogurt by technical improvements. Meanwhile, it is also proved that the technical means of semi-degreasing in combination with roasting in Examples 1-4 of the present invention can solve the problem that the raw materials must contain animal derived components in order to obtain better texture, sensory and rheological properties, and thus the raw material cost in the production process is reduced, and a new possibility for the development of pure plant-based yogurt is provided.

Although the present invention has been described in detail with general description, specific embodiments and experiments above, it is obvious to a person skilled in the art that some modifications or improvements can be made on the basis of the present invention. Therefore, these modifications or improvements made without departing from the spirit of the present invention belong to the scope of protection claimed by the present invention.

INDUSTRIAL APPLICABILITY

The present invention provides a walnut yogurt and a preparation method thereof. The preparation method of the present invention comprises the following steps: 1) roasting peeled walnut kernels at 145° C. to 165° C. for 5 to 25 min; 2) degreasing the roasted walnut kernels until the oil content is 20% to 50% to obtain semi-degreased walnut kernels; 3) mixing the semi-degreased walnut kernels with a stabilizer, a carbon source and water and then milling the resultant to obtain a walnut slurry; and 4) subjecting the walnut slurry to sufficient mixing, sterilization, inoculation, and fermentation to obtain the walnut yogurt. The present invention can effectively improve the aroma of walnuts after roasting at high temperature. On this basis, the ideal flavor of the system can be ensured after removing part of the fat. Moreover, a yogurt with stable quality and rich nutritional value can be obtained by removing fat to the above degree, which has a good economic value and application prospects.

Claims

1. A preparation method of a walnut yogurt, comprising: 1) roasting peeled walnut kernels at 145° C. to 165° C. for 5 to 25 min;2) degreasing the roasted walnut kernels until the fat content is 20% to 50% to obtain semi-degreased walnut kernels;3) mixing the semi-degreased walnut kernels with a stabilizer, a carbon source and water and then milling the resultant to obtain a walnut slurry; and4) subjecting the walnut slurry to sufficient mixing, sterilization, inoculation, and fermentation to obtain the walnut yogurt.

2. The preparation method according to claim 1, further comprising an operation of adding 0.5% to 3% by mass of walnut oil obtained by hot pressing at 150° C. to 200° C. and then cooling to the walnut slurry in said 3).

3. The preparation method according to claim 1, wherein the stabilizer is one or a combination of at least two of diacetyl tartaric acid ester of mono(di)glycerides, hydroxypropyl distarch phosphate, sodium carboxymethyl cellulose, agar, pectin or gellan gum.

4. The preparation method according to claim 1, wherein, in the said 4), the sufficient mixing is performed by homogenization, and after a first homogenization is performed under the pressure condition of 20 to 70 MPa, a second homogenization is performed under a pressure which is 10 to 20 MPa higher than the pressure in the first homogenization.

5. The preparation method according to claim 1, wherein, the degreasing treatment in the said 2) is specifically as follows: hydraulic pressing is performed under the condition of a pressure of 6 to 22 MPa.

6. The preparation method according to claim 1, wherein, in the said 3), the carbon source is sucrose or glucose or a combination of sucrose or glucose; and the addition amount of the carbon source is 1 to 8% by mass of the walnut slurry.

7. The preparation method according to claim 1, wherein, the specific operation of the sterilization is as follows: heating materials to 85° C. to 95° C. and sterilizing for 15 to 20 min.

8. The preparation method according to claim 1, wherein, strains Streptococcus thermophilus and Lactobacillus plantarum are used for the inoculation.

9. The preparation method according to claim 1, wherein, an after-ripeness operation is further included after the fermentation is completed, and specifically, the after-ripeness operation is as follows: rapidly cooling the walnut yogurt to 4° C., and storing at 4° C. to 6° C. for 12 h.

10. A walnut yogurt, which is prepared by the method of claim 1.

11. The preparation method according to claim 2, wherein the stabilizer is one or a combination of at least two of diacetyl tartaric acid ester of mono(di)glycerides, hydroxypropyl distarch phosphate, sodium carboxymethyl cellulose, agar, pectin or gellan gum; preferably, based on the mass of the walnut slurry, the stabilizer is added at the following amount: 0.05 to 0.5 wt % for diacetyl tartaric acid ester of mono(di)glycerides, 0.5 to 3 wt % for hydroxypropyl distarch phosphate, 0.05 to 0.5 wt % for sodium carboxymethyl cellulose, 0.05 to wt % for agar, 0.05 to 0.5 wt % for pectin, and 0.05 to 0.5 wt % for gellan gum.

12. The preparation method according to claim 2, wherein, in the said 4), the sufficient mixing is performed by homogenization, and after a first homogenization is performed under the pressure condition of 20 to 70 MPa, a second homogenization is performed under a pressure which is 10 to 20 MPa higher than the pressure in the first homogenization.

13. The preparation method according to claim 2, wherein, the degreasing treatment in the said 2) is specifically as follows: hydraulic pressing is performed under the condition of a pressure of 6 to 22 MPa.

14. The preparation method according to claim 2, wherein, in the said 3), the carbon source is sucrose or glucose or a combination of sucrose or glucose; and the addition amount of the carbon source is 1 to 8% by mass of the walnut slurry; preferably, the mass ratio of the semi-degreased walnut kernels to water is 1:(4-15).

15. The preparation method according to claim 2, wherein, the specific operation of the sterilization is as follows: heating materials to 85° C. to 95° C. and sterilizing for 15 to 20 min.

16. The preparation method according to claim 2, wherein, strains Streptococcus thermophilus and Lactobacillus plantarum are used for the inoculation, preferably, the Streptococcus thermophilus and Lactobacillus plantarum are compounded at a cell number ratio of 1:1 to 1:5.

17. The preparation method according to claim 2, wherein, an after-ripeness operation is further included after the fermentation is completed, and specifically, the after-ripeness operation is as follows: rapidly cooling the walnut yogurt to 4° C., and storing at 4° C. to 6° C. for 12 h.

18. The preparation method according to claim 2, wherein the walnut slurry added with the walnut oil is subjected to high-speed shearing under the condition of 5,000 to 20,000 r/min for 2 to 15 min.

19. The preparation method according to claim 3, wherein, based on the mass of the walnut slurry, the stabilizer is added at the following amount: 0.05 to 0.5 wt % for diacetyl tartaric acid ester of mono(di)glycerides, 0.5 to 3 wt % for hydroxypropyl distarch phosphate, to 0.5 wt % for sodium carboxymethyl cellulose, 0.05 to 0.5 wt % for agar, 0.05 to 0.5 wt % for pectin, and 0.05 to 0.5 wt % for gellan gum.

20. The preparation method according to claim 6, wherein the mass ratio of the semi-degreased walnut kernels to water is 1:4-15.

21. The preparation method according to claim 8, wherein the Streptococcus thermophilus and Lactobacillus plantarum are compounded at a cell number ratio of 1:1 to 1:5.