Preparation method of yak hand-made yogurt

Abstract

The application relates to a preparation method of yak handmade yoghourt and belongs to the technical field of yak dairy product processing. Pure fresh yak milk is heated to 80 DEG C for sterilization, and then cooled to 60 DEG C for constant temperature standby; sterilized pure cow milk stored at minus 5 DEG C is added into handmade old yoghourt for stirring until complete fusion, so that the bacterial strain is pre-activated at low temperature; the activated bacterial strain milk is all poured into the 60 DEG C main milk for continuous stirring and mixing, sealed and covered with cotton quilts after being filled into a barrel, and fermented at an environment of 43 DEG C for 3 hours or at an environment of 45 DEG C for 4 hours, so that the finished product is obtained. The gradient temperature control process of low-temperature milk pre-activated bacterial strain and 60 DEG C constant temperature bacterial mixing effectively avoids high-temperature inactivation of the bacterial strain, guarantees the bacterial strain activity and the fermentation stability; the cotton quilt natural heat preservation fermentation does not need industrial constant temperature equipment and has strong environmental adaptability; no food additive and industrial bacterial powder is added in the whole process, and the prepared yak yoghourt is fine and smooth in organization, stable in coagulation state and pure and natural in flavor.

Description

Technical Field

[0001] This invention relates to the field of yak dairy product processing technology, specifically a method for preparing handmade yak yogurt. Background Technology

[0002] Yak yogurt is a traditional dairy product made from the milk of yaks raised on the Qinghai-Tibet Plateau through lactic acid bacteria fermentation. Yak milk has a high nutrient density, with significantly higher levels of milk protein, milk fat, lactose, and non-fat milk solids than regular cow's milk. Traditional yak yogurt production typically employs a natural fermentation process: fresh yak milk is filtered through gauze, heated to boiling for sterilization, and then, when the temperature drops to around 40°C, stored yogurt is added as a starter. The mixture is then sealed and allowed to ferment slowly at ambient temperature for 6 to 8 hours until it coagulates. This traditional process does not add any additives, preserving the natural flavor and nutritional value of yak milk. Building upon this foundation, researchers have recently explored and improved the industrial production of yak yogurt. Current techniques for preparing yak yogurt typically include steps such as raw material standardization, addition of white sugar and stabilizers, homogenization, high-temperature sterilization, cooling to 37°C–43°C for inoculation with industrial starter cultures, and constant-temperature fermentation. Regarding sterilization conditions, some studies have used 90℃ for 10 minutes; fermentation temperatures are mostly controlled between 40℃ and 44℃, and fermentation times are 6 to 9 hours. The starter cultures used include commercially available mixed lactic acid bacteria powders or specific, screened lactic acid bacteria strains.

[0003] However, the aforementioned existing technologies still have many shortcomings. First, regarding the activation of the microbial strains, current industrial methods typically involve directly adding commercial starter cultures or traditional yogurt starters to raw milk at 37℃–43℃ for fermentation. Since the microbial strains undergo a sudden temperature change when transitioning directly from low-temperature storage to the fermentation temperature environment, some strain cells may become inactive or damaged due to heat stress, resulting in a lower actual number of viable bacteria and a prolonged curd lag period, thus affecting fermentation efficiency and the stability of the finished product quality. Second, regarding the temperature transition between sterilization and inoculation, current processes often employ high-temperature, long-duration sterilization (e.g., 90℃ / 10min) followed by a significant cooling to the fermentation temperature before inoculation. High-temperature treatment causes significant thermal damage to heat-sensitive nutrients and the functional properties of milk proteins in yak milk, and the large cooling amplitude and long processing time increase energy consumption and the risk of microbial contamination during production. Third, existing yak yogurt preparation methods largely rely on industrial constant-temperature fermentation equipment to maintain precise fermentation temperatures, requiring significant equipment investment and energy supply, making them unsuitable for the relatively weak infrastructure, such as electricity, in remote pastoral areas like the Qinghai-Tibet Plateau. Furthermore, existing patented technologies commonly add food additives such as white sugar and stabilizers to improve the flavor and texture of the product, which deviates from the pure, natural, and additive-free quality characteristics of traditional yak yogurt. Therefore, there is an urgent need to develop a method for preparing pure, natural, handmade yak yogurt that can effectively protect the activity of bacterial strains, reduce heat damage to the nutritional value of yak milk, and eliminate dependence on industrial temperature-controlled equipment, enabling stable year-round production. Summary of the Invention

[0004] In order to solve the problems of the prior art, the present invention provides a method for preparing yak handmade yogurt.

[0005] To solve the above-mentioned technical problems, the present invention is achieved through the following technical solution: Firstly, a method for preparing yak handmade yogurt, comprising a main milk sterilization step, a main milk temperature control step, a bacterial inoculum activation step, a mixing and stirring step, and a filling and fermentation step: The main milk temperature control step involves cooling the sterilized pure fresh yak milk to 60°C and maintaining it at a constant temperature for later use. The activation step involves taking sterilized pure milk stored at -5°C, adding it to homemade yogurt starter, and stirring until completely dissolved to obtain activated starter milk. The mixing step involves pouring all the obtained activated bacteria milk into pure yak milk at a constant temperature of 60°C and continuously stirring and mixing it. The filling and fermentation step involves filling the obtained mixed milk into a fermentation tank, sealing the tank, covering it with an insulation blanket, and fermenting it at a constant temperature of 43°C for 3 hours or at a constant temperature of 45°C for 4 hours to obtain yak handmade yogurt.

[0006] In one specific embodiment of the first aspect, in the main milk sterilization step, the amount of pure fresh yak milk used is 100 parts by weight, and it is heated to 80°C for sterilization.

[0007] In one specific embodiment of the first aspect, in the strain activation step, the amount of sterilized pure milk stored at -5°C is 10 parts by weight, and the amount of handmade old-fashioned yogurt starter is 0.3 parts by weight.

[0008] In one specific embodiment of the first aspect, the stirring time in the strain activation step is 2 minutes.

[0009] In one specific embodiment of the first aspect, the mixing and stirring step involves continuous stirring for 10 to 15 minutes.

[0010] In one specific embodiment of the first aspect, in the filling and fermentation step, when the ambient temperature is 43°C, the fermentation time is 3 hours.

[0011] In one specific embodiment of the first aspect, in the filling and fermentation step, when the ambient temperature is 45°C, the fermentation time is 4 hours.

[0012] In one specific embodiment of the first aspect, the thermal blanket is a cotton blanket.

[0013] In one specific embodiment of the first aspect, the preparation method is characterized by not adding any food additives and not adding industrial bacterial powder.

[0014] Secondly, a type of handmade yak yogurt, prepared using a method for making handmade yak yogurt.

[0015] The beneficial effects of this invention are as follows: 1. This invention effectively solves the problem of bacterial activity damage caused by traditional high-temperature inoculation methods through an innovative pre-activation and gradient inoculation process. First, pasteurized pure milk stored at -5°C is thoroughly mixed with a traditional homemade yogurt starter, allowing the bacteria to gradually regain metabolic activity and achieve uniform dispersion under low-temperature conditions. Then, this activated starter milk is added to the main milk at a constant temperature of 60°C. The cooling buffer effect of the low-temperature milk allows the temperature of the mixture to transition naturally, thus avoiding heat stress inactivation caused by direct exposure of the bacteria to high temperatures. This technique significantly improves the survival rate and fermentation activity of the bacteria, shortens the coagulation lag period, and ensures the robustness of the subsequent fermentation process. Simultaneously, because the bacteria are uniformly dispersed during the pre-activation stage, the consistency of bacterial distribution in the mixed milk is effectively guaranteed, resulting in a uniform and stable coagulation state in the final product and significantly improved batch-to-batch quality consistency. In addition, compared with industrial starter culture powder, this invention uses traditional handmade yogurt starter as a starter, which retains the complex metabolic characteristics of natural lactic acid bacteria. During the fermentation process, it can produce richer flavor metabolites, giving the finished product the rich flavor characteristics unique to traditional handmade yogurt. 2. This invention employs a process route combining mild sterilization at 80℃ with natural heat preservation fermentation using cotton quilts. Compared to high-temperature, long-duration sterilization, this effectively reduces the thermal damage to heat-sensitive nutrients and functional properties of milk proteins in yak milk, allowing for better preservation of the casein micelle structure and whey protein conformation. This provides a high-quality protein matrix environment for subsequent fermentation and curdling. Simultaneously, this sterilization temperature, combined with subsequent temperature-controlled inoculation at 60℃, ensures the product's microbial safety while maximizing the stability of the natural fatty acid composition in yak milk. Furthermore, this invention eliminates reliance on industrial constant-temperature fermentation equipment, utilizing only the physical heat of the environment through the cotton quilts for fermentation. The fermentation time is precisely matched to seasonal temperature variations, ensuring sufficient accumulated temperature under different external temperature conditions. This allows for the coordinated completion of lactic acid bacteria metabolism and acid production, as well as the isoelectric point precipitation of casein, ultimately resulting in a fine, smooth, and crack-free typical set-type yogurt texture. The synergistic effect of the above-mentioned technologies enables the industrial production of pure, natural, high-quality, and stable year-round set-type yak yogurt without the addition of any food additives or exogenous industrial bacteria. It has significant advantages in terms of process simplicity, environmental adaptability, and superior product quality. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the preparation method of the present invention. Detailed Implementation

[0017] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0018] like Figure 1 This illustrates a method for preparing handmade yak yogurt.

[0019] I. Raw Materials and Proportions The raw materials and their proportions used in this invention are as follows: 1. Pure Fresh Yak Milk: Fresh yak milk collected daily from pastoral areas of the Qinghai-Tibet Plateau is used directly after filtration to remove impurities. Yak milk has a high nutrient density. Studies have shown that the main nutrient content of Meiren yak milk is 6.43% fat, 5.45% protein, 5.06% lactose, 3.97% casein, 9.57% non-fat milk solids, and 13.97% total solids. The milk protein and milk fat content of yak milk varies at different altitudes, with higher altitudes having higher content. This invention does not have special restrictions on the fat and protein content of pure fresh yak milk; natural raw yak milk is sufficient. The dosage is 100 parts by weight (equivalent to 100 catties).

[0020] 2. Low-temperature pasteurized pure milk: Use commercially available pasteurized pure milk, refrigerated at -5°C for later use. The purpose of low-temperature storage is to provide a low-temperature environment to protect the activity of the bacteria when mixing with the homemade yogurt starter later. The dosage is 10 parts by weight (equivalent to 10 catties).

[0021] 3. Homemade Yogurt Starter: A traditionally made yogurt starter containing live lactic acid bacteria (mainly Streptococcus thermophilus and Lactobacillus bulgaricus). Its preparation method is as follows: Take 5 parts by weight of fresh yak milk or pure cow milk, heat to 80℃ to sterilize, then cool to 43℃~45℃. Add 0.15 parts by weight of the previously successfully fermented yogurt as a starter, stir well, and ferment at a constant temperature of 43℃~45℃ for 4~6 hours, until the milk is completely coagulated into a yogurt-like consistency. This is your homemade yogurt starter. This starter should be used within 24 hours of preparation, or refrigerated at 4℃ for no more than 72 hours. The dosage is 0.3 parts by weight (equivalent to 3 liang).

[0022] II. Detailed Process Steps Step 1: Sterilize the main milk Procedure: Take 100 parts by weight of pure fresh yak milk, filter it through an 80-mesh filter to remove mechanical impurities, and then place it in a stainless steel sterilizer and heat it to 80℃.

[0023] Process parameters: Sterilization temperature: 80℃±2℃; Heating method: water bath heating or steam jacket heating, with the heating rate controlled at 3-5℃ / min; Heat preservation time: Heating will be stopped immediately after reaching 80℃, without additional heat preservation (instant sterilization).

[0024] Technical Principle: Instant sterilization at 80℃ effectively kills most pathogenic and spoilage bacteria in raw yak milk. Studies have shown that pasteurization promotes the stability of saturated fatty acids in yak milk during storage and maximizes the retention of easily absorbed unsaturated fatty acids. Low-temperature treatment better maintains the function of yak milk proteins and preserves their nutritional value. Compared with high-temperature sterilization above 85℃, sterilization at 80℃ causes less denaturation of yak milk proteins, which is beneficial for the formation of casein gel networks during subsequent fermentation.

[0025] Input: 100 parts by weight of pure yak milk (raw milk, total bacterial count ≤ 2 × 10⁻⁶) 5 (CFU / mL) Output: 100 portions of sterilized pure yak milk (total bacterial count ≤300 CFU / mL).

[0026] Step Two: Main Milk Temperature Control Procedure: Quickly cool the sterilized yak milk to 60°C and keep it at a constant temperature for later use.

[0027] Process parameters: Cooling methods: using cold water jacket cooling or plate heat exchanger cooling; Target temperature for cooling: 60℃±1℃; Temperature maintenance methods: water bath temperature control or jacket insulation; Maintain constant temperature for no more than 30 minutes (to prevent microbial resuscitation).

[0028] Technical Principle: The main milk is cooled to 60℃ and maintained at a constant temperature to ensure the temperature of the mixture remains within a suitable range when subsequently mixed with milk containing activated microbial cultures. 60℃ is a critical temperature point—higher than the conventional fermentation temperature (40℃~45℃) for most lactic acid bacteria. However, this invention employs a "low-temperature milk pre-activated microbial culture" strategy. The microbial cultures are already activated in milk at -5℃. The main role of the 60℃ main milk is to provide a suitable mixing temperature environment, rather than being directly used for microbial fermentation. Research shows that yak milk proteins begin to undergo disulfide bond-mediated aggregation and denaturation above 75℃, and 60℃ is far below the temperature threshold for severe protein denaturation.

[0029] Input: 100 parts by weight of sterilized pure yak milk (80℃); Output: 100 parts by weight of pure fresh yak milk at a constant temperature of 60℃.

[0030] Step 3: Pre-activation of microbial strains Instructions: Take 10 parts by weight of pasteurized pure milk stored at -5℃, add 0.3 parts by weight of homemade yogurt starter, and stir continuously until fully combined and dissolved.

[0031] Process parameters: Low-temperature milk temperature: -5℃±2℃ (use immediately after being taken out of the cold storage); Mixing method: manual mixing or low-speed mechanical mixing; Stirring time: 2 minutes ± 0.5 minutes; End point judgment: The bacteria and milk are completely mixed, with no obvious particles or clumps.

[0032] Technical Principle: This is one of the core innovative steps of this invention. Traditional yogurt preparation processes typically involve directly adding the starter culture to milk at 40℃~45℃, exposing the starter culture to a high-temperature environment. In contrast, this invention uses -5℃ low-temperature milk pre-mixed with the starter culture, allowing it to gradually regain activity and disperse evenly under low-temperature conditions. When mixed with the 60℃ main milk, the introduction of the low-temperature milk lowers the local mixing temperature, preventing the starter culture from being instantly exposed to a high-temperature environment and becoming inactive. Simultaneously, the low-temperature environment helps inhibit the growth of unwanted bacteria, ensuring the purity and vitality of the starter culture.

[0033] Input: 10 parts by weight of pasteurized pure milk at -5°C; 0.3 parts by weight of homemade traditional yogurt starter.

[0034] Output: 10.3 parts by weight of activated cultured milk (cultured bacteria evenly dispersed in low-temperature milk). Step 4: Mixing and stirring. Procedure: Pour all the activated bacteria milk obtained in step three into the 60℃ constant temperature pure yak milk obtained in step two, and stir continuously to mix.

[0035] Process parameters: Mixing temperature: The temperature of the system after mixing is approximately 55℃~58℃ (because 10.3 parts by weight of -5℃ low-temperature milk is mixed with 100 parts by weight of 60℃ main milk). Mixing method: manual mixing or low-speed mechanical mixing; Mixing time: 10-15 minutes; End point judgment for stirring: The milk has a uniform color and no layering.

[0036] Technical Principle: The activated starter culture milk is thoroughly mixed with the 60℃ main milk, ensuring the starter culture is evenly distributed throughout the milk system. Thorough stirring for 10-15 minutes ensures a consistent starter culture concentration in each batch of milk, guaranteeing uniformity in subsequent fermentation. After mixing, the system temperature is lowered to 55℃-58℃. This temperature prevents the starter culture from becoming inactive (as it has been pre-activated and adapted to the low-temperature milk) while providing a suitable temperature base for subsequent filling and fermentation.

[0037] Input: 100 parts by weight of pure fresh yak milk at a constant temperature of 60℃; 10.3 parts by weight of activated bacterial culture milk; Output: 110.3 parts by weight of mixed milk (with uniformly distributed bacteria, at a temperature of 55℃~58℃).

[0038] Step 5: Filling and Fermentation Operating procedure: Pour the well-stirred milk mixture into a yogurt fermentation container, seal the container, cover it with an insulation blanket, and ferment at a constant temperature under the set ambient temperature until the end.

[0039] Process parameters: ; Technical Principle: This is another core innovative step of the invention. Traditional yogurt fermentation usually relies on temperature-controlled fermentation equipment (such as yogurt makers, fermentation boxes, etc.), while the present invention uses a cotton quilt for natural heat preservation fermentation. It utilizes the ambient temperature (43℃ in summer / 45℃ in winter) combined with the heat-insulating effect of the cotton quilt to achieve natural temperature-controlled fermentation. The cotton quilt has excellent heat-insulating properties, effectively reducing heat loss and keeping the temperature inside the fermentation tank relatively stable during fermentation.

[0040] The matching relationship between fermentation temperature and time is based on the results of extensive experimental optimization. Fermentation at 43℃ for 3 hours: Lactic acid bacteria are metabolically active, the acid production rate is moderate, and the curdling endpoint is reached after 3 hours. Fermentation at 45℃ for 4 hours: In winter, the ambient temperature is low, and although the cotton quilt has a good insulation effect, the temperature inside the container rises slowly. Therefore, it is necessary to extend the fermentation time by 1 hour to ensure complete fermentation. The criteria for determining the fermentation endpoint are: the milk completely coagulates into a tofu-like consistency, with a smooth surface, no whey separation or only a small amount of whey, and has the typical sweet and sour flavor of yogurt.

[0041] Input: 110.3 parts by weight of mixed milk (temperature 55℃~58℃); Fermentation tank (pre-cleaned and disinfected); Cotton quilt (clean and dry); Output: Approximately 110 servings of handmade yak yogurt (finished product, set form).

[0042] Step 6 (optional): Post-cooking in cold water Procedure: After fermentation is complete, remove the insulation blanket and move the yogurt container to a 4℃ cold storage for further maturation.

[0043] Process parameters: Refrigeration temperature: 4℃±1℃; Refrigeration time: 8-12 hours.

[0044] Technical principle: Post-fermentation ripening allows the flavor of yogurt to develop and improve further, with a slight increase in acidity and a richer texture. At the same time, low temperature inhibits the continued growth and acid production of lactic acid bacteria, preventing excessive acidification.

[0045] Input: Freshly fermented yak yogurt; Output: Post-ripened yak yogurt (with better flavor and richer texture).

[0046] IV. Finished Product Quality Standards The handmade yak yogurt prepared according to the method of this invention should meet the following quality indicators: Sensory indicators: Color: Uniform milky white or pale milky yellow; Organizational state: solidified type, fine and smooth texture, no bubbles or cracks, and neat cut surface; Odor: It has the unique milky aroma of yak milk and the typical fermented aroma of yogurt, with no off-odors; Taste: A perfect balance of sweet and sour, with a rich and mellow flavor.

[0047] Physicochemical properties: Fat content: ≥3.5% (based on yak milk raw material); Protein content: ≥3.0% (based on yak milk raw material); Titration acidity: 70–100°T; pH value: 4.3~4.6; Microbiological indicators: Lactic acid bacteria live count: ≥1×10 7 CFU / mL.

[0048] V. Examples Example 1 (Summer Standard Process) Raw material: 100 jin of pure fresh yak milk (collected on the same day, fresh and uncontaminated); Low-temperature sterilized pure milk: 10 jin (store at -5℃); Handmade old-fashioned yogurt starter: 3 liang (traditional homemade, made within 24 hours).

[0049] Steps: (1) Sterilize the main milk: Take 100 catties of pure fresh yak milk, filter it through an 80-mesh filter, place it in a stainless steel sterilizer and heat it in a water bath to 80°C. Stop heating immediately after reaching the temperature.

[0050] (2) Main milk temperature control: The sterilized pure yak milk is cooled to 60°C by a cold water jacket and kept at a constant temperature for later use.

[0051] (3) Pre-activation of bacteria: Take out 10 catties of sterilized pure milk refrigerated at -5℃ from the cold storage, add 3 liang of handmade old-fashioned yogurt starter, stir by hand for 2 minutes until completely blended and dissolved to obtain activated bacteria milk.

[0052] (4) Mixing and stirring: Pour all the activated bacteria milk into yak pure fresh milk at a constant temperature of 60℃, and stir by hand for 12 minutes until the milk has a uniform color.

[0053] (5) Filling and fermentation: Pour the mixed milk into a pre-cleaned and sterilized yogurt fermentation container, seal the container and cover it with a clean cotton quilt. Ferment at a constant temperature of 43℃ for 3 hours in summer.

[0054] (6) Finished product: After fermentation, remove the cotton quilt to obtain yak handmade yogurt. Testing showed that the finished product had a titratable acidity of 78°T, a pH of 4.45, and a viable lactic acid bacteria count of 2.3 × 10⁻⁶. 7 CFU / mL, with fine texture and good coagulation.

[0055] Example 2 (Standard Process in Winter) Raw materials: Same as in Example 1.

[0056] Steps: Steps (1) to (4) are the same as in Example 1.

[0057] (5) Filling and fermentation: Pour the mixed milk into the fermentation tank, seal the tank and cover it with a cotton quilt. Ferment at a constant temperature of 45℃ for 4 hours in winter.

[0058] (6) Finished product: Remove the cotton quilt after fermentation. Testing showed that the finished product had a titratable acidity of 82°T, a pH of 4.40, and a viable lactic acid bacteria count of 1.8 × 10⁻⁶. 7 CFU / mL, tissue condition is good.

[0059] Example 3 (with cold post-ripening process) Raw materials: Same as in Example 1.

[0060] step: Steps (1) to (5) are the same as in Example 1 (summer mode, fermentation at 43°C for 3 hours).

[0061] (6) Cold storage and ripening: After fermentation, remove the cotton quilt and move the yogurt container to a 4℃ cold storage for cold storage and ripening for 10 hours.

[0062] (7) Finished product: Removed after post-ripening. Testing showed the finished product had a titratable acidity of 85°T, a pH of 4.35, and a viable lactic acid bacteria count of 1.5 × 10⁻⁶.7 CFU / mL, with a richer flavor and a more balanced sweet and sour ratio.

[0063] Example 4 (Verification with different ratios) Ingredients: 100 jin (50 catties) of pure fresh yak milk; Low-temperature sterilized pure milk: 10 jin (refrigerated at -5℃); Homemade yogurt starter: 3.5 liang (approximately 17% more than the standard amount).

[0064] Steps: Same as in Example 1.

[0065] Results: Due to a slight increase in the amount of starter culture, the fermentation speed was slightly accelerated, reaching the curdling endpoint in approximately 2 hours and 45 minutes. The finished product had a slightly high acidity (85°T), a slightly sour taste, and a good texture. This indicates that a qualified product can be obtained with a starter culture amount ranging from 0.25 to 0.4 parts by weight, with 0.3 parts by weight being the optimal choice.

[0066] VI. Comparative Example Comparative Example 1 (Traditional direct inoculation method – strain not pre-activated) Preparation method: Take 100 catties of pure fresh yak milk, heat it to 80℃ to sterilize it, then cool it down to 43℃, add 3 liang of handmade old-fashioned yogurt starter directly (without the -5℃ low-temperature milk pre-activation step), stir evenly, fill it, and ferment it at a constant temperature of 43℃.

[0067] result: ; Analysis: In Comparative Example 1, the bacterial strain was directly added to milk at 43°C. Although 43°C is the optimal growth temperature for lactic acid bacteria, the strain underwent a sudden temperature change from a low-temperature (preserved) environment to a 43°C environment, resulting in the inactivation or damage of some bacterial cells due to heat stress. In contrast, this invention pre-activates the bacterial strain in milk at -5°C, allowing it to gradually regain its metabolic activity before mixing it with the main milk at 60°C. This significantly improves the survival rate of the bacterial strain, shortens the fermentation time, and results in a superior product quality.

[0068] Comparative Example 2 (High-temperature inoculation method – direct inoculation at 60℃, without pre-activation) Preparation method: Take 100 catties of pure fresh yak milk, heat it to 80℃ to sterilize it, then cool it down to 60℃, add 3 liang of handmade old-fashioned yogurt starter (without pre-activation with milk at -5℃), stir evenly, fill it, and ferment it at a constant temperature of 43℃.

[0069] result: ; Analysis: In Comparative Example 2, the bacterial strains were directly added to milk at 60°C. 60°C is far above the optimal growth temperature for lactic acid bacteria (40°C–45°C). Most strains were inactivated due to the high temperature, leading to fermentation failure or extremely slow fermentation. This fully demonstrates the necessity and ingenuity of the "low-temperature milk pre-activated bacterial strains" step in this invention—by pre-activating the strains in milk at -5°C before mixing them with the main milk at 60°C, the cooling effect of the low-temperature milk lowers the temperature of the mixture to 55°C–58°C, avoiding direct exposure of the bacterial strains to lethal high temperatures.

[0070] Comparative Example 3 (Conventional Industrial Microbial Powder Method) Preparation method: Take 100 catties of pure fresh yak milk, heat it to 80℃ to sterilize it, then cool it down to 42℃, add commercially available yogurt starter powder (about 5 grams according to the product instructions), stir evenly, fill the container, and ferment at a constant temperature of 42℃ for 6 hours.

[0071] result: ; Analysis: Comparative Example 3, using industrial starter culture and constant-temperature fermentation equipment, produced a stable quality yogurt product, but the flavor was monotonous, lacking the complex aroma characteristic of traditional handmade yogurt. Furthermore, industrial starter culture typically contains carrier additives (such as maltodextrin), and is not a purely natural fermentation process. This invention uses a purely handmade yogurt starter culture, without adding any food additives or industrial starter culture, preserving the natural flavor and nutritional value of traditional yak yogurt.

[0072] Comparative Example 4 (different combinations of fermentation temperature and time) Preparation method: Using the same raw materials and process as in Example 1, but changing the combination of fermentation temperature and time, the following experimental groups were set up: ; Analysis: Experimental results show that 43℃ / 3 hours (summer) and 45℃ / 4 hours (winter) are the optimized combinations of fermentation conditions. Too low a temperature (40℃) or too short a time (2.5 hours) leads to incomplete curdling; too high a temperature (46℃) causes some microbial strains to become inactive; and too long a time (4 hours @ 43℃) leads to over-fermentation and excessive sourness. This invention precisely differentiates fermentation parameters according to seasonal differences, ensuring consistent product quality throughout the year.

[0073] To more clearly demonstrate the technical advantages of the present invention, the core comparative data between the above comparative examples and the embodiments of the present invention are summarized as follows: ; Experimental conclusion: The pre-activation step of the bacterial strain is a key innovative technical feature. Comparison of Comparative Example 1 and Comparative Example 2 shows that directly inoculating the bacterial strain into milk at 43℃ or 60℃ without pre-activation leads to prolonged fermentation time, reduced viable cell count, and decreased product quality. Direct inoculation at 60℃ almost completely causes fermentation failure.

[0074] Pre-activation of milk at -5°C is fundamentally different from traditional inoculation methods. Although Comparative Example 1 (direct inoculation at 43°C) can also produce yogurt products, it is significantly inferior to the present invention in terms of coagulation time, viable bacteria count, and flavor quality.

[0075] The process combination of this invention produces a synergistic effect. The synergistic effect of pre-activation at -5℃ + constant temperature main milk at 60℃ + natural heat preservation fermentation with cotton quilts enables the stable production of high-quality yak handmade yogurt under conditions of no additives, no industrial starter culture, and no constant temperature equipment.

[0076] Setting fermentation parameters according to seasonal differences is of practical significance. Both Example 1 (43℃ / 3h in summer) and Example 2 (45℃ / 4h in winter) yielded high-quality finished products, demonstrating that the present invention's scheme of adjusting fermentation parameters according to environmental temperature differences is effective and ensures the feasibility of year-round production.

[0077] The method for preparing yak yogurt provided by this invention has the following advantages in industrial applicability: Low equipment requirements: No expensive constant temperature fermentation equipment is needed, only simple facilities such as sterilizers, cooling equipment and cotton quilts are required, making it suitable for promotion and implementation in yak breeding areas such as the Qinghai-Tibet Plateau.

[0078] Low energy consumption: It ferments naturally using ambient temperature, without the need for additional heating or insulation, making it energy-saving and environmentally friendly.

[0079] Raw materials are widely available: pure fresh yak milk is a common raw material in the Qinghai-Tibet Plateau region, and low-temperature sterilized pure milk and handmade yogurt starters can be obtained locally or made at home.

[0080] High added value: Purely handmade, additive-free yak yogurt has distinctive traditional characteristics and a high-quality positioning, making it highly competitive in the market.

[0081] Scalable for production: Although the method of this invention is called "manual", each process step can be scaled up proportionally. Scalable production can be achieved by increasing the number of fermentation tanks and batches, making it suitable for small dairy processing enterprises or cooperatives.

[0082] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing yak handmade yogurt, comprising a main milk sterilization step, a main milk temperature control step, a bacterial inoculum activation step, a mixing and stirring step, and a filling and fermentation step, characterized in that: The main milk temperature control step involves cooling the sterilized pure fresh yak milk to 60°C and maintaining it at a constant temperature for later use. The activation step involves taking sterilized pure milk stored at -5°C, adding it to homemade yogurt starter, and stirring until completely dissolved to obtain activated starter milk. The mixing step involves pouring all the obtained activated bacteria milk into pure yak milk at a constant temperature of 60°C and continuously stirring and mixing it. The filling and fermentation step involves filling the obtained mixed milk into a fermentation tank, sealing the tank, covering it with an insulation blanket, and fermenting it at a constant temperature of 43°C for 3 hours or at a constant temperature of 45°C for 4 hours to obtain yak handmade yogurt.

2. The preparation method according to claim 1, characterized in that, In the main milk sterilization step, the amount of pure fresh yak milk used is 100 parts by weight, and it is heated to 80°C for sterilization.

3. The preparation method according to claim 1, characterized in that, In the strain activation step, the amount of sterilized pure milk stored at -5°C is 10 parts by weight, and the amount of handmade old-fashioned yogurt starter is 0.3 parts by weight.

4. The preparation method according to claim 1, characterized in that, In the strain activation step, the stirring time is 2 minutes.

5. The preparation method according to claim 1, characterized in that, In the mixing and stirring step, the continuous stirring time is 10 to 15 minutes.

6. The preparation method according to claim 1, characterized in that, In the filling and fermentation step, when the ambient temperature is 43°C, the fermentation time is 3 hours.

7. The preparation method according to claim 1, characterized in that, In the filling and fermentation step, when the ambient temperature is 45℃, the fermentation time is 4 hours.

8. The preparation method according to claim 1, characterized in that, The thermal blanket is a cotton blanket.

9. The preparation method according to any one of claims 1 to 8, characterized in that, The preparation method does not add any food additives or industrial bacterial powder.

10. A type of handmade yak yogurt, characterized in that, It is prepared by any one of claims 1 to 9.

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