Preparation method of probiotic nutrition fortified dried fruits

Abstract

The invention discloses a preparation method of probiotic nutrition fortified dried fruits, and belongs to the technical field of food processing. The preparation method disclosed by the invention comprises the following steps: inoculating probiotics into fruit juice added with desalted whey powder, and fermenting to obtain fermentation liquor; adding a freeze-drying protective agent into the fermentation liquid to serve as probiotic impregnation liquid to impregnate the fruit slices or blocks; and performing pre-freezing and vacuum freeze-drying to obtain the probiotic nutrition fortified dried fruits. The food raw materials are directly added into the probiotic fermentation liquor to prepare the steeping liquor, thallus separation is not needed, the process is simple and convenient, and the label cleaning concept is met; the impregnation liquid has a remarkable protection effect on probiotics in the vacuum freeze-drying process, and can strengthen nutritional and functional components such as ascorbic acid and total phenols in the dried fruits at the same time. The viable count of probiotics in the prepared dried fruits can reach 10 < 9 > CFU / g or above, and the dried fruits are balanced in nutrition, have the natural flavor of fruits and the functions of the probiotics, are convenient to carry and store and have good industrial application prospects.

Description

Technical Field

[0001] This invention belongs to the field of food processing technology, specifically relating to a method for preparing probiotic-fortified dried fruit. Background Technology

[0002] With the improvement of people's living standards and the enhancement of health awareness, consumers have put forward higher requirements for the safety, nutrition, and functionality of food. Fruits, as an important part of the daily diet, are rich in vitamins, minerals, dietary fiber, and phenolic compounds, among other nutrients and functional components. However, fresh fruits are prone to spoilage, have a short shelf life, and are inconvenient to carry, limiting their consumption scenarios. Dried fruit products, due to their advantages of easy storage, convenient carrying, and unique flavor, are gradually becoming one of the important directions in fruit processing.

[0003] Currently, the main processing methods for dried fruit products include hot air drying, microwave drying, and vacuum drying. However, these traditional methods usually involve high-temperature treatment, which can easily lead to serious loss of heat-sensitive nutrients in the fruit (such as ascorbic acid and phenolic substances). At the same time, the sensory quality of the product, such as color and texture, is also affected to varying degrees. Vacuum freeze-drying technology, because it is carried out under low temperature and low pressure conditions, can better preserve the original color, shape, and nutrients of the fruit, and has become an important technical means for processing high-quality dried fruit products.

[0004] Probiotics are widely used in the development of functional foods due to their various physiological functions, such as regulating intestinal flora balance, enhancing immunity, lowering lipids and blood sugar, anti-oxidation, and anti-allergy. However, probiotics are prone to decreased activity or even inactivation during processing, drying, and storage due to cell membrane damage and loss of enzyme activity. Freeze-drying, in particular, poses a severe challenge to the survival of probiotics. Currently, commonly used freeze-drying protectants for probiotics are mostly carbohydrates and proteins, but these often need to be added separately and formulated separately, and suffer from limited protective effects and poor compatibility with food systems.

[0005] Furthermore, current technologies for preparing probiotic functional dried fruits often involve separately culturing and collecting probiotic cells before combining them with a fruit matrix. This process is complex, costly, and may result in loss of activity during cell separation. Meanwhile, how to improve the survival rate of probiotics while further enhancing the natural nutrients (such as ascorbic acid and phenolic compounds) in the dried fruit remains a pressing issue that needs to be addressed.

[0006] Therefore, developing a simple, efficient, and food-grade raw material-oriented preparation method that can effectively protect the activity of probiotics during freeze-drying and achieve nutritional fortification of dried fruit is of great practical significance and application value. Summary of the Invention

[0007] In view of the problems existing in the prior art, the purpose of this invention is to provide a method for preparing probiotic-fortified dried fruit.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing probiotic-fortified dried fruit includes the following steps: (1) Probiotic fermentation: Probiotics are inoculated into a probiotic fermentation substrate consisting of fruit juice and 40~80g / L demineralized whey and fermented to harvest the fermentation liquid; (2) Preparation of probiotic impregnation solution: Add freeze-drying protectant to the fermentation broth obtained in step (1) and mix to obtain probiotic impregnation solution; the freeze-drying protectant is at least one of maltodextrin, skim milk powder, antioxidant and pectin; (3) Raw material pretreatment: Wash, select, peel and / or pit the fruit, and cut it into slices or chunks for later use; (4) Soaking: Place the fruit slices or pieces obtained in step (3) into the probiotic soaking solution prepared in step (2) for soaking treatment. After soaking, take them out and drain the liquid. (5) Pre-freezing and vacuum freeze-drying: After pre-freezing the fruit slices or pieces obtained in step (4), vacuum freeze-drying is carried out to obtain probiotic-fortified dried fruit.

[0009] Based on the above scheme, the juice in step (1) is apple juice with a soluble solids content of 10-15%.

[0010] Based on the above scheme, the probiotics are Lactiplantibacillus plantarum or Lactobacillus brevis; the fermentation temperature is 30~37℃ and the fermentation time is 48~72 h.

[0011] Based on the above scheme, the antioxidant mentioned in step (2) is ascorbic acid or apple polyphenols.

[0012] Based on the above scheme, the final concentration of each component in the probiotic impregnation solution in step (2) is: maltodextrin 40~80 g / L, skim milk powder 0~40 g / L, antioxidant 0~8 g / L, and pectin 0~5 g / L.

[0013] Based on the above scheme, the impregnation method in step (4) is atmospheric pressure impregnation or vacuum impregnation; the atmospheric pressure impregnation time is 20~40 min; the vacuum degree of the vacuum impregnation is 0.01~0.1 MPa, and the impregnation time is 5~20 min.

[0014] Based on the above scheme, in step (3), for fruits that are prone to enzymatic browning, after cutting and before soaking, they are soaked in 0.1-1% NaCl solution for 2-20 minutes, and then drained.

[0015] Based on the above scheme, the pre-freezing temperature in step (5) is -80℃ to -55℃, and the pre-freezing time is 12 to 24 h; the conditions for vacuum freeze drying are: cold trap temperature -80℃ to -40℃, vacuum degree 0.1 to 1 mbar, and partition temperature 0℃ to 40℃.

[0016] Based on the above scheme, the fruit is one or more of the following: apple, peach, pear, strawberry, hawthorn, mango, banana, or cantaloupe.

[0017] Based on the above scheme, the number of live probiotics in the dried fruit obtained in step (5) is ≥10. 9 The concentration is CFU / g and should be stored in a dry environment at 4°C.

[0018] Advantages of the technical solution of this invention: This invention uses apple juice with added demineralized whey powder as the probiotic fermentation medium, which not only increases the number of viable probiotics after fermentation but also eliminates the need for probiotic separation after fermentation, saving operational steps, ensuring probiotic activity, and aligning with the concepts of "clean label" and "food-grade fermentation." Maltodextrin, skim milk powder, ascorbic acid, pectin, and other substances are directly added to the re-fermented probiotic liquid. The components in the fermentation liquid and the added substances form a probiotic impregnation solution, which provides protection for the probiotics during freeze-drying and is beneficial for the nutritional fortification of dried fruit and the retention of important functional components such as phenolic substances. Vacuum impregnation of fruit slices (pieces) increases the number of viable probiotics and the nutritional and functional components in the product. Vacuum freeze-drying technology better preserves the original nutrients of the fruit. Storage at 4℃ further enhances the stability of the probiotics during product storage. Therefore, products produced using the technical solution of this invention combine the nutrition of fruits with the probiotic function of probiotics, enhancing both nutritional and functional components. They also have the advantages of being healthy, safe, and nutritionally balanced, and are easy to carry and consume. Attached Figure Description

[0019] Figure 1 The effect of probiotic fermentation substrate on probiotic growth; Figure 2 The effect of impregnation solution composition on the viable count of probiotics; Figure 3 The effect of organic acids and antioxidants in the impregnation solution on the viable count of probiotics; Figure 4 The effect of vitamin C in the impregnation solution on the content of phenolic substances; Figure 5 The effect of pectin in the impregnation solution on the number of Lactobacillus plantarum 21802 bacteria; Figure 6 The effect of different raw material pretreatment methods on color; Figure 7 The effect of soaking method on the number of live probiotics in dried apples; Figure 8 Different soaking methods for dried apples (V) C and phenolic content Figure 9 The effect of storage temperature on the number of live probiotics. Detailed Implementation

[0020] The terminology used in this invention, unless otherwise specified, generally has the meanings commonly understood by those skilled in the art. The invention is further described in detail below with reference to specific embodiments and data. The following embodiments are merely illustrative and are not intended to limit the scope of the invention in any way.

[0021] Unless otherwise specified, the experimental methods used in the following embodiments are conventional methods, performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Unless otherwise specified, the experimental materials, reagents, and chemicals used in the following embodiments can be obtained through general channels.

[0022] In the following examples, *Lactobacillus plantarum* 21802 was purchased from the China Industrial Microbial Culture Collection Center (CICC 21802); *Lactobacillus brevis* 6239 was purchased from the China Industrial Microbial Culture Collection Center (CICC 6239).

[0023] The method for preparing apple turbid juice with 12% soluble solids is as follows: wash the apples, remove the stems and cores, cut them, juice them using a Midea juicer (MJ-JS20A2), and dilute with distilled water to a soluble solids content of 12%.

[0024] Example 1 A method for preparing probiotic-fortified dried fruit, comprising the following steps: (1) Inoculate probiotics onto a probiotic fermentation substrate for fermentation and harvest the fermentation liquid; the composition of the probiotic fermentation substrate is: add desalted whey powder with a final concentration of 60g / L to apple turbid juice with 12% soluble solids as the probiotic fermentation substrate.

[0025] (2) Add 40 g / L maltodextrin, 40 g / L skim milk powder, 0.5% antioxidant and 2 g / L pectin to the fermentation broth obtained in step (1) to prepare a probiotic impregnation solution. The viable count of probiotics in the impregnation solution is ≥10. 9 CFU / mL The antioxidant is ascorbic acid or apple polyphenols.

[0026] (3) Wash, select, peel and cut the fruit into pieces, and soak it in the soaking solution prepared in step (2). After soaking, drain the solution in a biosafety environment. The soaking is atmospheric pressure soaking or vacuum soaking. For fruits that are prone to browning, soak them in 1% NaCl solution for 20 minutes before soaking after cutting to reduce browning. (4) The soaked fruit pieces are pre-frozen at -80℃ for 12 hours, and then freeze-dried under the conditions of cold trap temperature -80℃, vacuum degree 0.1mbar and partition temperature 4℃ to obtain probiotic fortified dried fruit.

[0027] Example 2 A method for preparing probiotic-fortified dried apples, comprising the following steps: (1) Add desalted whey powder to apple juice with 12% soluble solids to make a final concentration of 60 g / L, sterilize at 121℃ for 15 min, and use it as a fermentation medium for Lactobacillus plantarum 21802. (2) After activating Lactobacillus plantarum 21802 in MRS medium, it was inoculated onto the above fermentation medium at an inoculation amount of 5% and cultured at 37°C for 60 h to obtain the fermentation broth of Lactobacillus plantarum 21802. (3) Sterilized maltodextrin, skim milk powder, and ascorbic acid solution were added to the fermentation broth of *Lactobacillus plantarum* 21802 to achieve final concentrations of 40 g / L, 40 g / L, and 5 g / L, respectively; a probiotic impregnation solution was prepared, with a live probiotic count of 2.8 × 10⁻⁶. 9 CFU / mL; (4) Wash, select, peel, and cut the apples into cubes (2cm×2cm). Soak them in 1% NaCl solution for 20 minutes, drain them, and then soak them in the probiotic soaking solution prepared in step (3) under normal pressure for 20 minutes. Drain them in a biosafety environment. (5) The soaked apple pieces are pre-frozen at -80℃ for 12 hours, and then freeze-dried under the conditions of cold trap temperature -80℃, vacuum degree 0.1mbar and partition temperature 4℃ to obtain probiotic fortified dried apples.

[0028] The number of live lactic acid bacteria was 1.6 × 10⁻⁶. 9 CFU / g, ascorbic acid content 406.9mg / 100g, total phenol content 6.873g / kg.

[0029] Example 3 A method for preparing probiotic-fortified dried apples, comprising the following steps: (1) Add desalted whey powder to apple juice with 12% soluble solids to make a final concentration of 60 g / L, sterilize at 121℃ for 15 min, and use it as a fermentation medium for Lactobacillus plantarum 21802. (2) After activating Lactobacillus plantarum 21802 in MRS medium, it was inoculated onto the above fermentation medium at an inoculation amount of 5% and cultured at 37°C for 60 h to obtain the fermentation broth of Lactobacillus plantarum 21802. (3) Sterilized maltodextrin, skim milk powder, and ascorbic acid solution were added to the fermentation broth of *Lactobacillus plantarum* 21802 to achieve final concentrations of 40 g / L, 40 g / L, and 5 g / L, respectively; a probiotic impregnation solution was prepared, with a live probiotic count of 2.8 × 10⁻⁶. 9 CFU / mL; (4) Wash, select, peel and cut the apples into squares (2cm×2cm), soak them in 1% NaCl solution for 20min and drain them. Then vacuum impregnate them in the probiotic impregnation solution prepared in step (3) under the condition of 0.08MPa for 10min and drain them in a biosafety environment. (5) The soaked apple pieces are pre-frozen at -80℃ for 12 hours, and then freeze-dried under the conditions of cold trap temperature -80℃, vacuum degree 0.1mbar and partition temperature 4℃ to obtain probiotic fortified dried apples.

[0030] The number of live lactic acid bacteria was 2.0 × 10⁻⁶. 9 CFU / g, ascorbic acid content 512.49mg / 100g, total phenol content 6.720g / kg.

[0031] Example 4 A method for preparing probiotic-fortified dried apples, comprising the following steps: (1) Add desalted whey powder to apple juice with 12% soluble solids to make a final concentration of 60 g / L, sterilize at 121℃ for 15 min, and use it as a fermentation medium for Lactobacillus plantarum 21802. (2) After activating Lactobacillus plantarum 21802 in MRS medium, it was inoculated onto the above fermentation medium at an inoculation amount of 5% and cultured at 37°C for 60 h to obtain the fermentation broth of Lactobacillus plantarum 21802. (3) Sterilized maltodextrin, skim milk powder, apple polyphenols, and pectin solution were added to the fermentation broth of *Lactobacillus plantarum* 21802 to achieve final concentrations of 40 g / L, 40 g / L, 5 g / L, and 2 g / L, respectively; a probiotic impregnation solution was prepared, with a live probiotic count of 3.5 × 10⁻⁶. 9 CFU / mL; (4) Wash, select, peel and cut the apples into squares (2cm×2cm), soak them in 1% NaCl solution for 20min and drain them. Then vacuum impregnate them in the probiotic impregnation solution prepared in step (3) under the condition of 0.08MPa for 10min and drain them in a biosafety environment. (5) The soaked apple pieces are pre-frozen at -80℃ for 12 hours, and then freeze-dried under the conditions of cold trap temperature -80℃, vacuum degree 0.1mbar and partition temperature 4℃ to obtain probiotic fortified dried apples.

[0032] The number of live lactic acid bacteria was 2.1 × 10⁻⁶. 9 CFU / g, ascorbic acid content 7.88 mg / 100g, total phenol content 10.497 g / kg.

[0033] Example 5 A method for preparing probiotic-fortified dried apples, comprising the following steps: (1) Add skim milk powder to apple juice with 12% soluble solids to make a final concentration of 60 g / L, sterilize at 121°C for 15 min, and use it as a fermentation medium for Lactobacillus brevis 6239. (2) After activating Lactobacillus brevis 6239 in MRS medium, it was inoculated onto the above fermentation medium at an inoculation amount of 5% and cultured at 37°C for 60 h to obtain the fermentation broth of Lactobacillus brevis 6239. (3) Sterilized maltodextrin, ascorbic acid, and pectin solution were added to the fermentation broth of the above-mentioned Lactobacillus brevis 6239 to make their final concentrations 80 g / L, 5 g / L, and 2 g / L, respectively; a probiotic impregnation solution was prepared, and the number of viable probiotics in the impregnation solution was 2.5 × 10⁻⁶. 9 CFU / mL; (4) Wash, select, peel and cut the apples into squares (2cm×2cm), soak them in 1% NaCl solution for 20min and drain them. Then vacuum impregnate them in the probiotic impregnation solution prepared in step (3) under the condition of 0.08MPa for 10min and drain them in a biosafety environment. (5) The soaked apple pieces are pre-frozen at -80℃ for 12 hours, and then freeze-dried under the conditions of cold trap temperature -80℃, vacuum degree 0.1mbar and partition temperature 4℃ to obtain probiotic fortified dried apples.

[0034] The number of live lactic acid bacteria was 5.2 × 10⁻⁶. 8 CFU / g, ascorbic acid content 547.02 mg / 100g, total phenol content 9.151 g / kg.

[0035] Example 6 A method for preparing probiotic-fortified dried bananas, comprising the following steps: (1) Add desalted whey powder to apple juice with 12% soluble solids to make a final concentration of 60 g / L, sterilize at 121℃ for 15 min, and use it as a fermentation medium for Lactobacillus plantarum 21802. (2) After activating Lactobacillus plantarum 21802 in MRS medium, it was inoculated onto the above fermentation medium at an inoculation amount of 5% and cultured at 37°C for 60 h to obtain the fermentation broth of Lactobacillus plantarum 21802. (3) Sterilized maltodextrin, skim milk powder, apple polyphenol solution, and pectin solution were added to the fermentation broth of *Lactobacillus plantarum* 21802 to achieve final concentrations of 40 g / L, 40 g / L, 5 g / L, and 2 g / L, respectively; a probiotic impregnation solution was prepared, with a live probiotic count of 3.65 × 10⁻⁶. 9 CFU / mL; (4) Peel the bananas and cut them into round slices with a thickness of 10 mm. Place them in the probiotic soaking solution prepared in step (3) and vacuum soak them. The soaking conditions are 0.08 MPa for 10 min. Drain the solution in a biosafety environment. (5) The soaked banana slices are pre-frozen at -80℃ for 12 hours, and then freeze-dried under the conditions of cold trap temperature -80℃, vacuum degree 0.1mbar and partition temperature 4℃ to obtain probiotic fortified banana chips.

[0036] The number of live lactic acid bacteria was 1.9 × 10⁻⁶. 9 CFU / g, ascorbic acid content 21.69 mg / 100g, total phenol content 5.789 g / kg.

[0037] Example 7 A method for preparing probiotic-fortified dried pears, comprising the following steps: (1) Add desalted whey powder to apple juice with 12% soluble solids to make a final concentration of 60 g / L, sterilize at 121℃ for 15 min, and use it as a fermentation medium for Lactobacillus plantarum 21802. (2) After activating Lactobacillus plantarum 21802 in MRS medium, it was inoculated onto the above fermentation medium at an inoculation amount of 5% and cultured at 37°C for 60 h to obtain the fermentation broth of Lactobacillus plantarum 21802. (3) Sterilized maltodextrin, skim milk powder, apple polyphenol solution, and pectin solution were added to the fermentation broth of *Lactobacillus plantarum* 21802 to achieve final concentrations of 40 g / L, 40 g / L, 5 g / L, and 2 g / L, respectively; a probiotic impregnation solution was prepared, with a live probiotic count of 3.65 × 10⁻⁶. 9 CFU / mL; (4) Wash, select, peel, and cut the pears into pieces (2cm×2cm). Place them in the probiotic soaking solution prepared in step (3) for vacuum soaking. The soaking conditions are 0.08MPa for 10min. Drain the solution in a biosafety environment. (5) The soaked pear pieces are pre-frozen at -80℃ for 12 hours, and then freeze-dried under the conditions of cold trap temperature -80℃, vacuum degree 0.1mbar and partition temperature 4℃ to obtain probiotic fortified dried pear.

[0038] The number of live lactic acid bacteria was 9.3 × 10⁻⁶. 8 CFU / g, ascorbic acid content 4.72 mg / 100g, total phenol content 4.558 g / kg.

[0039] The effect of probiotic fermentation substrate on probiotic growth Demineralized whey powder (W) or skim milk powder (M) were added to apple juice (AJ) with 12% soluble solids to a final concentration of 60 g / L. Apple juice without any added substances served as a control. Comparisons were also made with two other culture media: one with a final concentration of 60 g / L skim milk powder and 15 g / L yeast extract (M+YE), and the other with a final concentration of 60 g / L demineralized whey powder and 15 g / L yeast extract (W+YE). *Lactobacillus plantarum* 21802, activated on MRS medium, was inoculated at a 5% inoculation rate into the aforementioned different culture media sterilized at 121°C for 15 min and cultured at 37°C for 60 h. The viable count of probiotics in the fermentation broth was then determined. The results are as follows Figure 1 As shown. From Figure 1 As can be seen, compared with simple apple juice fermentation (AJ), adding 60 g / L demineralized whey powder (W+AJ) to apple juice can promote the growth of *Lactobacillus plantarum* 21802 and significantly increase the total viable count in the fermentation broth. There was no significant difference between the W+AJ group and the W+YE and M+YE groups, but the W+AJ group had the highest viable count. Furthermore, by using common food ingredients instead of microbial culture medium components, no cell isolation is required after fermentation, and the broth can be directly used for the next production step, saving operational steps, ensuring the activity of probiotics, and better aligning with the concepts of "clean label" and "food-grade fermentation." Therefore, using apple juice with 12% soluble solids and adding 60 g / L demineralized whey powder as the fermentation medium for *Lactobacillus plantarum* 21802, and culturing at 37℃ for 60 hours, yields the best probiotic fermentation broth.

[0040] The protective effect of the composition of probiotic impregnation solution on probiotics Demineralized whey powder was added to apple juice with a soluble solids content of 12% to achieve a final concentration of 60 g / L, and the mixture was sterilized at 121°C for 15 min to obtain the fermentation medium for *Lactobacillus plantarum* 21802. *Lactobacillus plantarum* 21802 activated on MRS medium was inoculated into the above fermentation medium at a 5% inoculum size and cultured at 37°C for 60 h to obtain the fermentation broth of *Lactobacillus plantarum* 21802. Using physiological saline as a control (CK), the protective effects of adding trehalose (Tre), maltodextrin (Mal), skim milk powder (M), maltodextrin + trehalose (Mal + Tre), and maltodextrin + skim milk powder (Mal + M) on lactic acid bacteria were investigated. Add the above-mentioned single substance or 1:1 mixture to a final concentration of 80 g / L to the fermentation broth of *Lactobacillus plantarum* 21802 obtained above to obtain a probiotic infusion. After washing, sorting, and cutting, apples are immersed in the probiotic infusion, drained in a biosafety environment, pre-frozen, and vacuum freeze-dried. The viable probiotic count in the product is then performed according to the method in GB 4789.35-2023 "Microbiological Examination of Food - Examination of Lactic Acid Bacteria". The test results are shown in […]. Figure 2 .from Figure 2 As can be seen, compared with the control, all the preservatives in the experiment had a significant protective effect on probiotics during the freeze-drying process. Maltodextrin was superior to skim milk powder and trehalose. Replacing half of the maltodextrin with skim milk powder as a freeze-drying preservative also yielded the same effect, with the number of viable probiotics being approximately 1.6 times that of the control. Therefore, in the actual preparation of dried fruit soaking liquid, using maltodextrin alone or a 1:1 mixture of maltodextrin and skim milk powder both have excellent results.

[0041] Effects of organic acids and antioxidants in the impregnation solution on the viable count of probiotics Desalted whey powder was added to apple juice with 12% soluble solids to bring the final concentration to 60 g / L. The mixture was then sterilized at 121°C for 15 min to obtain the fermentation medium for Lactobacillus plantarum 21802. Lactobacillus plantarum 21802 activated by MRS medium was inoculated into the above fermentation medium at a 5% inoculum and cultured at 37°C for 60 h to obtain the fermentation broth of Lactobacillus plantarum 21802.

[0042] Maltodextrin with a final concentration of 80 g / L was added to the fermentation broth of *Lactobacillus plantarum* 21802 obtained above. Then, 5% citric acid (CA), ascorbic acid (Vc), or apple polyphenols (APP) were added as soaking solutions. Apples, after washing, sorting, and cutting, were soaked in the probiotic soaking solutions, drained in a biosafety environment, pre-frozen, and vacuum freeze-dried. The viable probiotic count in the product was performed according to GB 4789.35-2023 "Microbiological Examination of Food - Examination of Lactic Acid Bacteria". The results are shown in [Figure number missing]. Figure 3Adding citric acid (CA), ascorbic acid (Vc), or apple polyphenols (APP) to the impregnation solution also provides good protection for probiotics during the freeze-drying process. Figure 3 Ascorbic acid is a commonly used antioxidant in food processing. Its addition increased the number of viable probiotics to 5.9 times that of the control, which was not significantly different from apple polyphenols. Therefore, the types of antioxidants used in the impregnation solution can be selected according to the nutritional or functional ingredients that the product needs to be fortified with. Citric acid has a slightly lower protective effect than ascorbic acid.

[0043] The effect of organic acids and antioxidants in the soaking solution on the content of phenolic substances in dried fruit Desalted whey powder was added to apple juice with 12% soluble solids to bring the final concentration to 60 g / L. The mixture was then sterilized at 121°C for 15 min to obtain the fermentation medium for Lactobacillus plantarum 21802. Lactobacillus plantarum 21802 activated by MRS medium was inoculated into the above fermentation medium at a 5% inoculum and cultured at 37°C for 60 h to obtain the fermentation broth of Lactobacillus plantarum 21802.

[0044] Maltodextrin at a final concentration of 80 g / L was added to the fermentation broth of *Lactobacillus plantarum* 21802 obtained above. Then, 5% citric acid (CA) and ascorbic acid (Vc) were added as impregnation solutions, with the impregnation solution without added citric acid or ascorbic acid serving as a control (C). Apples were washed, sorted, and cut, then impregnated in the probiotic impregnation solution. The solution was drained in a biosafety environment, pre-frozen, and then freeze-dried under vacuum. The phenolic content in the probiotic dried apples prepared using the *Lactobacillus plantarum* 21802 impregnation solution was determined using the FC method. Specifically, a certain mass of the ground sample was weighed, ultrasonically extracted twice with 10 mL of methanol, and the extracts were combined and diluted to a 25 mL volumetric flask. After filtration through 0.22 μm micropores, the extracts were used for determination. Add 2.5 mL of FC reagent (Sigma, USA) to 0.5 mL of appropriately diluted sample, heat in a 50 °C water bath for 5 min, remove and cool, then add 2 mL of 75 g / L sodium carbonate solution, shake well, and place in the dark for 30 min before measuring the absorbance at 760 nm. Simultaneously measure the absorbance of the sample control. Using gallic acid as a standard, determine the standard curve using the same method. Calculate the total phenol content in the sample based on the standard curve. Results are as follows: Figure 4 As shown, from Figure 4 It can be seen that adding ascorbic acid (Vc) to the impregnation solution increases the total phenol content in the product by 33% compared to the control, while no beneficial effect of citric acid on the retention of phenolic substances was observed. Therefore, it is recommended to add ascorbic acid (Vc) to the impregnation solution, which not only provides protection for probiotics during freeze-drying, but also enhances the ascorbic acid content in the product and increases the retention of phenolic substances.

[0045] The effect of pectin in the impregnation solution on the viable count of probiotics Desalted whey powder was added to apple juice with 12% soluble solids to bring the final concentration to 60 g / L. The mixture was then sterilized at 121°C for 15 min to obtain the fermentation medium for Lactobacillus plantarum 21802. Lactobacillus plantarum 21802 activated by MRS medium was inoculated into the above fermentation medium at a 5% inoculum and cultured at 37°C for 60 h to obtain the fermentation broth of Lactobacillus plantarum 21802.

[0046] Maltodextrin (Mal) at a final concentration of 80 g / L, maltodextrin at a final concentration of 40 g / L, skim milk powder at a final concentration of 40 g / L (Mal+M), and pectin solution at a final concentration of 40 g / L, maltodextrin at a final concentration of 40 g / L, skim milk powder at a final concentration of 40 g / L, and pectin solution at a final concentration of 2 g / L (Mal+M+P) were added to the fermentation broth of *Lactobacillus plantarum* 21802 obtained above. Apples, after washing, sorting, and cutting, were immersed in probiotic soaking solution, drained in a biosafety environment, pre-frozen, and vacuum freeze-dried. The viable probiotic count in the product was performed according to the method of GB 4789.35-2023 "Microbiological Examination of Food - Examination of Lactic Acid Bacteria". The test results are shown in [Figure 1]. Figure 5 Adding pectin to the impregnation solution at a final concentration of 2 g / L significantly increased the number of viable Lactobacillus plantarum 21802 in probiotic dried apples. The number of viable probiotics was 3.1 times and 2.3 times that of maltodextrin alone (Mal) and maltodextrin-skimmed milk powder (Mal+M, 1:1), respectively.

[0047] The above studies indicate that adding 60 g / L demineralized whey powder as a fermentation substrate for *Lactobacillus plantarum* 21802 to apple juice sterilized at 120℃ for 15 min, followed by fermentation at 37℃ for 60 h, and then adding an impregnation solution with a final concentration of 40 g / L maltodextrin, 40 g / L skim milk powder, 0.5% ascorbic acid (or apple polyphenols), and 2 g / L pectin to the fermentation broth, not only provides better protection for probiotics during freeze-drying but also increases the content of phenolic substances in probiotic dried fruit.

[0048] The effect of fruit pretreatment on browning The phenolic substances and polyphenol oxidase contained in fruits make them highly susceptible to enzymatic oxidation after cutting, resulting in browning. This causes the product surface to turn yellow or even brown, greatly reducing its sensory quality. Therefore, for fruits that are prone to browning, choosing appropriate raw material pretreatment methods plays an important role in improving the sensory quality of the product.

[0049] Using apples as raw material, after washing, sorting, and cutting, they are pre-processed in the following ways: ① Soak in 1% NaCl solution for 2 minutes; ② Soak in 1% NaCl solution for 20 minutes; ③ Blanching at 95℃ for 1 minute; ④ Soak in 1% NaCl solution for 2 minutes, then blanch at 95℃ for 1 minute; ⑤ Soak in 1% NaCl solution for 20 minutes, then blanch at 95℃ for 1 minute; Using the untreated group as the control group (CK), the color parameters (L*, a*, and b*) of the products were measured using an UltraScan VIS colorimeter after different pretreatments. The results are as follows: Figure 6 As shown, except for the control sample which falls in the first quadrant (i.e., yellow with a slight reddish tinge), all other pretreated samples fall in the second quadrant of the colorimetric diagram (i.e., yellowish-green). Furthermore, their b-values ​​are lower than the control, indicating a lighter yellow hue and thus a lower degree of browning. The sample treated with 1% NaCl solution for 20 minutes followed by blanching at 95°C for 1 minute showed the smallest color difference (22.2), indicating the lightest color and lowest degree of browning. This was followed by apple slices treated with 1% NaCl solution for 2 minutes followed by blanching at 95°C for 1 minute and those treated with 1% NaCl solution for 20 minutes, with color differences of 22.3 and 22.6 respectively compared to the standard white board. These differences were significantly lower than other treatments and the control, and the color differences between these samples and the sample with the lowest degree of browning were 0.9 and 2.5 respectively. Studies show that ΔE* > 3 indicates that the colors of different samples can be easily distinguished by the observer. This result suggests that the colors of apple slices obtained by these two treatment methods are not easily distinguishable to the naked eye from the optimal treatment. This method can also be used for raw material pretreatment to reduce the browning degree of freeze-dried apple slices and improve the sensory quality of the product. Among these three better treatments, soaking in 1% NaCl solution for 20 minutes, while not the optimal treatment compared to the other two, is simple, gentle, and significantly reduces energy consumption, while achieving a color-protecting effect similar to the optimal treatment. It can be used as a suitable pretreatment method for probiotic fermented dried fruit processing. If higher sensory quality requirements are needed, the other two treatment methods can be used.

[0050] The effect of soaking method on the number of live probiotics, vitamin C and phenolic substances in dried apples Desalted whey powder was added to apple juice with 12% soluble solids to bring the final concentration to 60 g / L. The mixture was then sterilized at 121°C for 15 min to obtain the fermentation medium for Lactobacillus plantarum 21802. Lactobacillus plantarum 21802 activated by MRS medium was inoculated into the above fermentation medium at a 5% inoculum and cultured at 37°C for 60 h to obtain the fermentation broth of Lactobacillus plantarum 21802.

[0051] Add 40 g / L maltodextrin, 40 g / L skim milk powder, 0.5% ascorbic acid, and 2 g / L pectin to the fermentation broth of *Lactobacillus plantarum* 21802 obtained above as a probiotic impregnation solution. Immerse the chopped apple pieces in the probiotic impregnation solution using both atmospheric pressure impregnation (20 min, I) and vacuum impregnation (0.08 MPa, 10 min, VI). After impregnation, determine the viable probiotic count in the final product. Figure 7 As shown. From Figure 7 It can be seen that the number of live probiotics in the probiotic impregnant is 2.8 × 10⁻⁶. 9 At CFU / mL, the final product contains 10 live probiotics. 9 Compared with atmospheric pressure impregnation, vacuum impregnation can increase the number of live probiotics in probiotic fermented dried apples by 21.6% when the CFU / g is above a certain level.

[0052] The ascorbic acid (Vc) and total phenolic content (TPC) in dried apples treated with different soaking methods were determined. Ascorbic acid was determined using the 2,6-dichlorophenolindophenol method as specified in GB5009.86-2016, "National Food Safety Standard - Determination of Ascorbic Acid in Food". Results are as follows: Figure 8 As shown, from Figure 8 As can be seen, regardless of whether it is atmospheric pressure impregnation (I) or vacuum impregnation (VI), the probiotic impregnation solution of the present invention not only provides better protection for probiotics during the freeze-drying process, but also enhances the vitamin C content in the product. Compared with atmospheric pressure impregnation, the ascorbic acid content in probiotic dried apples prepared by vacuum impregnation under the same conditions increased by 25.9%. In addition, the total phenolic content in the dried fruit prepared by both impregnation methods also increased significantly, by 33.5% and 36.5% respectively compared with the control (ordinary freeze-dried apples, which were cut and not impregnated before freeze-drying), indicating that the probiotic impregnation solution provided by the present invention can also protect the phenolic substances in the probiotic dried fruit. Therefore, the use of vacuum impregnation to prepare probiotic dried fruit can increase the number of live probiotics in the final product, enhance the ascorbic acid content, and increase the phenolic substance content in the product.

[0053] The effect of storage temperature on probiotics in probiotic-fortified dried fruit Desalted whey powder was added to apple juice with 12% soluble solids to bring the final concentration to 60 g / L. The mixture was then sterilized at 121°C for 15 min to obtain the fermentation medium for Lactobacillus plantarum 21802. Lactobacillus plantarum 21802 activated by MRS medium was inoculated into the above fermentation medium at a 5% inoculum and cultured at 37°C for 60 h to obtain the fermentation broth of Lactobacillus plantarum 21802.

[0054] Add 40 g / L maltodextrin, 40 g / L skim milk powder, 0.5% ascorbic acid, and 2 g / L pectin to the fermentation broth of *Lactobacillus plantarum* 21802 obtained above as a probiotic impregnation solution. Vacuum impregnation (0.08 MPa, 10 min) was performed on diced apple pieces in the probiotic impregnation solution. After impregnation, the apple pieces were pre-frozen and freeze-dried to obtain probiotic-fortified dried apples. The dried apples were stored at 4℃, 25℃, and 37℃, and the changes in the number of viable probiotics in the dried apples with prolonged storage time under different storage temperatures were measured. The results are as follows: Figure 9 As shown, during the 28-day storage period, the number of live probiotics in dried apples stored at 4℃ remained relatively stable; storage at 25℃ showed relative stability for the first 14 days, but the number of live probiotics decreased significantly by day 28; while storage at 37℃ resulted in a linear decrease in the number of live probiotics throughout the entire storage period, although it remained at 7.8 log CFU / g after 28 days. Therefore, it is recommended to store probiotic-fortified dried fruit at 4℃, especially during the hot summer months, to minimize high-temperature exposure during storage and transportation and reduce the degree of reduction in live probiotic count.

[0055] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for preparing probiotic-fortified dried fruit, characterized in that, Includes the following steps: (1) Probiotic fermentation: Probiotics are inoculated into a probiotic fermentation substrate consisting of fruit juice and 40~80g / L demineralized whey and fermented to harvest the fermentation liquid; (2) Preparation of probiotic impregnation solution: Add freeze-drying protectant to the fermentation broth obtained in step (1) and mix to obtain probiotic impregnation solution; the freeze-drying protectant is at least one of maltodextrin, skim milk powder, antioxidant and pectin; (3) Raw material pretreatment: Wash, select, peel and / or pit the fruit, and cut it into slices or chunks for later use; (4) Soaking: Place the fruit slices or pieces obtained in step (3) into the probiotic soaking solution prepared in step (2) for soaking treatment. After soaking, take them out and drain the liquid. (5) Pre-freezing and vacuum freeze-drying: After pre-freezing the fruit slices or pieces obtained in step (4), vacuum freeze-drying is carried out to obtain probiotic-fortified dried fruit.

2. The method for preparing probiotic-fortified dried fruit according to claim 1, characterized in that, The juice in step (1) is apple juice with a soluble solids content of 10-15%.

3. The method for preparing probiotic-fortified dried fruit according to claim 1, characterized in that, The antioxidant mentioned in step (2) is ascorbic acid or apple polyphenols.

4. The method for preparing probiotic-fortified dried fruit according to claim 3, characterized in that, The final concentrations of each component in the probiotic impregnation solution in step (2) are: maltodextrin 40~80 g / L, skim milk powder 0~40 g / L, antioxidant 0~8 g / L, and pectin 0~5 g / L.

5. The method for preparing probiotic-fortified dried fruit according to claim 1, characterized in that, The impregnation method in step (4) is atmospheric pressure impregnation or vacuum impregnation; the atmospheric pressure impregnation time is 20~40 min; the vacuum degree of the vacuum impregnation is 0.01~0.1 MPa, and the impregnation time is 5~20 min.

6. The method for preparing probiotic-fortified dried fruit according to claim 1, characterized in that, In step (3), for fruits that are prone to enzymatic browning, after cutting and before soaking, soak them in 0.1-1% NaCl solution for 2-20 minutes, then take them out and drain them.

7. The method for preparing probiotic-fortified dried fruit according to claim 1, characterized in that, The pre-freezing temperature in step (5) is -80℃ to -55℃, and the pre-freezing time is 12 to 24 h; the conditions for vacuum freeze drying are: cold trap temperature -80℃ to -40℃, vacuum degree 0.1 to 1 mbar, and partition temperature 0℃ to 40℃.

8. The method for preparing probiotic-fortified dried fruit according to any one of claims 1 to 7, characterized in that, The fruit is one or more of the following: apple, peach, pear, strawberry, hawthorn, mango, banana, or cantaloupe.

9. The method for preparing probiotic-fortified dried fruit according to claim 8, characterized in that, The number of live probiotics in the dried fruit obtained in step (5) is ≥10. 9 The concentration is CFU / g and should be stored in a dry environment at 4°C.

Images