A method for preparing preserved strawberries by combining sugar and sugar alcohol and low-sugar preserved strawberries

Abstract

The invention discloses a method for making reduced-sugar strawberry preserves and the quality characteristics of the reduced-sugar strawberry preserves. The method uses quick-frozen Monterey strawberries, drains the water after thawing, adds 24%-32% of the mass of the strawberries to oligosaccharides, 8%-16% of trehalose and 0%-8% of sorbitol for candiing for 72-96 hours, and then dries at 50-60°C for 10-12 hours to obtain reduced-sugar strawberry preserves. Compared with sucrose-impregnated strawberry preserves, the present invention solves the problems of high sugar content, insufficient viscoelasticity and easy sand return caused by the impregnation of single sugar and sugar alcohol through the interaction of sugar and sugar alcohol. The strawberry preserves made by the present invention have the characteristics of low total sugar content, moderate hardness and no sand return.

Description

Technical Field

[0001] The invention belongs to the field of agricultural product processing, and specifically is a processing method for improving the sweetness, color and texture of preserved strawberries and reducing the sugar content by combined impregnation and dehydration of functional sugars and sugar alcohols. Background Art

[0002] Preserved strawberries that have been candied belong to the candied fruit category, and their sugar content can reach more than 60%. The most critical process in the processing of preserved strawberries is candied strawberries, which can not only remove about 30% of the water, but also increase the sugar content, thus forming the sweet and sour taste and full texture of the preserved strawberries. White sugar, also known as sucrose, is the most commonly used sugar. However, as people become more and more aware of the health risks of a high-sugar diet, reducing the sugar content in food and using healthier alternative sugars have become a trend in food processing.

[0003] Sugar flow, sand return and browning are common quality problems in preserved fruits, and these problems are all related to the conversion of sucrose into fructose and glucose and the carbonylamine reaction in which the two participate. In food processing, it is a common method to replace sucrose with more stable functional sugars or sugar alcohols, which can reduce sugar content and improve functional properties. However, foods in which sucrose is replaced often have reduced quality such as taste, sweetness, and texture. For example, mannitol and erythritol are prone to causing sand return in gel candies, while xylitol makes the candies too soft and not formed, and whether the quality will deteriorate requires an observation period of up to several months. The quality reduction problem caused by sugar alcohol replacing sucrose has also been reported in freeze-dried strawberries and jujube slices. There are many types of sugar alcohols, and there are differences in osmotic pressure, hygroscopicity, solubility and crystallinity. Therefore, whether the ideal effect can be achieved through the combination and ratio of sugar and sugar alcohols has become a direction worthy of attention.

[0004] Sugars and sugar alcohols both have a polyhydroxy structure. Sugar is a general term for polyhydroxy aldehydes or polyhydroxy ketones and their derivatives and polymers. Since most of them have the general formula of Cx(H2O)y, they belong to carbohydrates. Sugar alcohols refer to polyols generated by reducing the aldehyde and ketone carbonyl groups of sugars to hydroxyl groups. The general formula is HO-C n -H 2n-1 Sugar alcohols have a chemical structure similar to sugar and can activate the sweet taste receptors on the tongue, but they tend to have lower sweetness and calories, and they do not participate in the Maillard reaction, which can reduce browning.

[0005] The elastic structure of preserved fruit mainly depends on the formation of pectin gel. Pectin is a high molecular weight polysaccharide polymer, mainly composed of D-galacturonic acid or its methyl ester as units, and is an acidic heteropolysaccharide connected by α-1,4 glycosidic bonds. Strawberry pectin is a high methoxyl pectin, which requires a large amount of sugar and acid to form a gel. Only when the pH value is less than 3.5, the carboxylate group is converted into a carboxylic acid group, the pectin molecule is uncharged, and the intermolecular repulsion decreases, and then they can associate with each other. The role of sugar is to seize the water molecules on the pectin molecular chain, so that the solvation degree of the molecular chain decreases, and the molecular chains interact to form a three-dimensional network structure, fix water and solutes in the mesh, and thus form a gel. If dried in the absence of sugar and acid, strawberries cannot form a gel state, and water cannot be fixed. A large amount of water is lost during drying, and the product is shriveled and tough. On the other hand, the gel state formed by macromolecules such as sugar and pectin and water is not stable at room temperature, but when the balance between the three is broken, such as water loss causing sugar crystallization, sand will appear, and viscoelasticity will decrease.

[0006] Among the non-sucrose and sugar alcohols that are currently commercially available, isomaltooligosaccharide (IMO) is a type of starch sugar. Its main components are isomaltose, panose, isomaltotriose, and oligosaccharides above tetrasaccharide, which are linked by α-1,6 glycosidic bonds between glucose molecules. Its viscosity is higher than that of sucrose, which can inhibit the crystallization of sucrose. In addition, isomaltooligosaccharide is difficult to be digested by gastric enzymes, has low sweetness and calories, and basically does not increase blood sugar and blood lipids. Isomaltooligosaccharide products do not contain monosaccharides or have a very low monosaccharide content, and its thermal energy is only 1 / 6 of sucrose. Trehalose (α-D-pyranoglucopyranosyl-(1→1)-α-D-pyranoglucoside, TRE) is a naturally occurring non-reducing disaccharide composed of two glucose molecules. Trehalose has a special protective effect on biological macromolecules. It is often used to prevent the denaturation and inactivation of protein macromolecules and maintain life activity. It can prevent starch retrogradation and stabilize proteins and lipids when used in food processing. The molecular formula of sorbitol is C6H 14 O6, has a refreshing sweet taste, and its sweetness is about 50%-70% of sucrose. Sorbitol has good moisturizing properties, which can keep food moist and prevent it from drying out, and can also prevent sugar, salt, etc. from crystallizing.

[0007] Different sugars and sugar alcohols interact with other ingredients in food in different food systems in a complex dose-effect relationship, and their effects on quality vary. Moreover, for preserved fruit foods, they enter the preserved fruit through osmotic pressure, and are also affected by the spatial barriers of tissues such as the cell wall and cell membrane inside the preserved fruit. It is still unknown how small molecule sugars and sugar alcohols interact with macromolecules such as pectin and cellulose, how they change their macroscopic properties such as hardness and elasticity, what changes will occur during storage, and whether the sand and sugar will return. The complexity of these factors makes the application effect of alternative sugars and sugar alcohols more difficult to predict. Therefore, whether oligosaccharides, trehalose, and sorbitol can produce good results in the processing of strawberry preserved fruit requires a lot of experiments. Summary of the invention

[0008] The object of the present invention is to provide a method for processing preserved strawberries by using functional sugars and sugar alcohols instead of sucrose, which can avoid sugar flow and sand return of preserved strawberries, reduce sweetness, browning degree and sugar content, and have good viscoelasticity and taste.

[0009] Another object of the present invention is to provide reduced-sugar preserved strawberries. The preserved strawberries produced by the above method have a moisture content of 18%-21%, a total sugar content of 25%-40%, a solid increase rate of 22%-25%, a hardness of 140-350g, a viscosity of 10-18g, an elasticity of 0.2-0.4g, a chromaticity value L* of 22-32, a* of 5-18, and a b* of 4-14.

[0010] The specific technical solutions of the present invention are as follows:

[0011] A method for processing preserved strawberries by replacing sucrose with functional sugars and sugar alcohols comprises the following steps:

[0012] 1) Select cone-shaped Monterey strawberries with an individual mass of 5-8 g;

[0013] 2) Thawing by soaking in a 0.2% sodium sulfite or sodium metabisulfite aqueous solution for 2-4 hours;

[0014] 3) After the strawberries are thawed in step 2), the solution is drained, isomaltooligosaccharide, trehalose and sorbitol are added, and the strawberries are candied and dehydrated for 72-96 hours;

[0015] 4) Separate the candied strawberries from the sugar solution and place them in a hot air circulation drying oven at 50-60° C. for 10-12 hours.

[0016] The strawberries described in step 1) are quick-frozen strawberries in the semi-red stage, with an individual weight of 5-8 g. The hardness of the fruit at this maturity is conducive to the complete shape of the strawberry preserves. The fruit size range and sugar and acid content of this fruit are within the set immersion concentration and time, and the strawberries absorb sugar to make the fruit shape plump and uniform;

[0017] In step 2), the ratio of the volume of the soaking liquid to the mass of the strawberries is 3:2, and soaking and thawing simultaneously kills bacteria and inhibits enzymes, and avoids oxidation of anthocyanins;

[0018] In step 3), the isomaltooligosaccharide accounts for 24%-32% of the mass of the strawberry, the trehalose accounts for 8%-16% of the mass of the strawberry, and the sorbitol accounts for 0-8% of the mass of the strawberry. The candied time can be adjusted according to the temperature change, which can be extended to 96 hours at 0-15°C and 72-80 hours at 20-30°C.

[0019] In step 3), preferably, the isomaltooligosaccharide is 24% of the mass of the strawberry, the trehalose is 8% of the mass of the strawberry, and the sorbitol is 8% of the mass of the strawberry.

[0020] In step 4), the drying time and temperature are controlled so that the moisture content of the preserved strawberries is 18-21% at the end of drying.

[0021] After the production of the preserved strawberries is completed, in order to better reflect the sugar reduction effect and quality characteristics of the reduced-sugar strawberry preserved, the present invention investigates the quality changes of the preserved strawberries stored for 4 months.

[0022] The preserved strawberry provided by the invention is prepared by the method, and has a moisture content of 18%-21%, a total sugar content of 25%-40%, a solid increase rate of 22%-25%, a hardness of 140-350g, a viscosity of 10-18g, an elasticity of 0.2-0.4g, a chromaticity value L* of 22-32, a* of 5-18, and a b* of 4-14.

[0023] Compared with the prior art, the present invention controls the size of strawberry fruit through the mass range, promotes the association of pectin molecular chains through the compounding of oligomaltose, trehalose and sorbitol, and can form a three-dimensional network structure without adding acid. It has good viscoelasticity and fullness, and at the same time reduces the sweetness, sugar content and browning degree of strawberry preserves. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 AF are PAS-stained optical micrographs of the cross-section of preserved strawberry

[0025] Figure 2 AF is the X-ray diffraction pattern of preserved strawberries after 4 months of storage

[0026] A is the preserved strawberry prepared in Example 1;

[0027] B is the preserved strawberry prepared in Example 2;

[0028] C is the preserved strawberry prepared in Comparative Example 1;

[0029] D is the preserved strawberry prepared in Comparative Example 2;

[0030] E is the preserved strawberry prepared in Comparative Example 3;

[0031] F is the preserved strawberry prepared in Comparative Example 4. DETAILED DESCRIPTION

[0032] In order to make the purpose and technical solution of the present invention clearer, the present invention is further described below by examples. It should be understood that the examples of the present invention are only used for illustration and are not intended to limit the present invention. The reagents or instruments used without indicating the manufacturer are all food grade and are conventional products that can be purchased commercially. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making creative work belong to the protection scope of the present invention.

[0033] Example 1

[0034] The method for preparing reduced-sugar strawberry preserves by combined impregnation of functional sugars and sugar alcohols mainly comprises the following steps:

[0035] 1) Select 1.0 kg of Monterey strawberries with a mass of 5-8 g, without mechanical damage, removed the stems, washed and quick-frozen;

[0036] 2) Add 1500 mL of food-grade sodium sulfite or sodium metabisulfite solution and soak at room temperature for 2-4 hours until thawed;

[0037] 3) After the strawberries are thawed, remove them, drain the surface water, and transfer them to the candied jar;

[0038] 4) Weigh 240 g of isomaltooligosaccharide, 80 g of trehalose and 80 g of sorbitol, stir and mix, add to the candied jar and mix with the strawberries, and seal;

[0039] 5) After candied at room temperature for 72 hours, remove the strawberries and place them in a single layer on a circulating hot air drying oven tray at 50°C for 12 hours.

[0040] Example 2

[0041] The method for preparing reduced-sugar strawberry preserves by impregnation with a combination of functional sugars mainly comprises the following steps: the same method as in Example 1 is carried out, except that 320 g of oligomaltose and 80 g of trehalose are used in step 4).

[0042] Comparative Example 1

[0043] A method for producing preserved strawberries is carried out in the same manner as in Example 1, except that 400 g of sucrose is used in step 4).

[0044] Comparative Example 2

[0045] A method for producing preserved strawberries is carried out in the same manner as in Example 1, except that 400 g of isomaltooligosaccharide is used in step 4).

[0046] Comparative Example 3

[0047] A method for producing preserved strawberries is carried out in the same manner as in Example 1, except that 400 g of trehalose is used in step 4).

[0048] Comparative Example 4

[0049] A method for producing preserved strawberries is carried out in the same manner as in Example 1, except that 400 g of sorbitol is used in step 4).

[0050] The present invention determines the moisture content of preserved strawberries, and the specific determination method is: the preserved strawberries are subjected to vacuum drying according to the vacuum drying method in GB5009.3-2016 "Determination of Water Content in Foods".

[0051] The present invention measures the hardness of preserved strawberries, and the specific method is as follows: the strawberry fruit or preserved strawberries is placed sideways on the Brookfield CT3 texture analyzer base, and the probe TA39 is used for full spectrum analysis. The compression mode is measured, the fixture is TA-BT-KIT, the trigger point load is 7g, the test speed is 0.50mm / s, the compression distance is 1mm, and the number of cycles is 2. Nine preserved strawberries are measured for each group of samples.

[0052] The present invention determines the total sugar content of preserved strawberries, and the specific determination method is: GB / T 10782-2021 "General Rules for Quality of Preserved Fruits".

[0053] The present invention measures the solid increase rate of preserved strawberries. The specific measuring method is as follows: the mass of frozen strawberries is taken as the initial value M0, the mass of the initial strawberries dried to constant weight before treatment is recorded as X0, the surface sugar solution of the preserved strawberries after candied is quickly rinsed, the water is wiped off with absorbent paper, and then dried to constant weight, and the mass is recorded as X0. t , then the solid increase rate is (X t -X0) / M0×100%.

[0054] The present invention measures the chromaticity value of preserved strawberries, and the specific measuring method is: using a Japanese Minolta colorimeter CM-700d to measure and record the brightness value L*, red value a*, and yellow value b* on the surface of the preserved strawberries, measuring 9 strawberries for each sample and taking the average value.

[0055] The test results are shown in Table 1 and Table 2. The initial quality is the index value measured within 7 days after the drying is completed and the sealed package is completed.

[0056] Table 1 Initial quality characteristics of preserved strawberries in various embodiments and comparative examples

[0057]

[0058] The underlined data above do not meet the requirements of the present invention.

[0059] Compared with Examples 1 and 2, the preserved strawberries in Comparative Example 1 have a high total sugar content and low viscosity.

[0060] Compared with Examples 1 and 2, the solid increase rate of Comparative Example 2 is lower and the hardness is higher. When only isomaltooligosaccharide is used, due to its high molecular polymerization degree, there are trisaccharides, tetrasaccharides and even oligosaccharides with higher polymerization degrees, which have greater resistance to entering the internal tissue of strawberries, resulting in a low solid increase rate of strawberry preserved fruit.

[0061] Compared with Examples 1 and 2, Comparative Example 3 has a lower solid increase rate and a very high hardness, low viscosity and elasticity, an excessively high L* value, a low red value a*, and a high yellow value b*. These phenomena are related to the attachment and crystallization of trehalose outside the fruit cells. The product is hard and inelastic as a whole and is yellow-white in color.

[0062] Compared with Examples 1 and 2, Comparative Example 4 has a lower solid increase rate and a higher yellow value b*. In addition, sorbitol is not easily absorbed by the intestine and is not suitable for large-scale consumption.

[0063] Table 2 Hardness, adhesion, elasticity and chromaticity values ​​of Example 1, Example 2 and Comparative Example 1 after storage for 4 months

[0064] Hardness(g) Adhesion(g) Elasticity(g) L* a* b* Example 1 282.39 26.75 0.36 29.27 7.52 7.93 Example 2 227.39 20.5 0.35 27.58 6.72 5.46 Comparative Example 1 302.50 <![CDATA[ 6.50 ]]> 0.28 27.39 9.36 9.25

[0065] Table 2 shows that after 4 months of storage, the hardness values ​​of Examples 1 and 2 and Comparative Example 1 all increased, but the increase was less than that of Comparative Example 1. The viscosity values ​​of Examples 1 and 2 increased, while the viscosity value of Comparative Example 1 decreased. The initial viscosity value of the Comparative Example was low, and the viscosity continued to decrease after storage. Sucrose, as the most commonly used sugar, is often used in combination with citric acid in the processing of preserved strawberries to promote the formation of gel structure and promote its conversion into fructose and glucose to avoid sanding. If only sucrose is used for candiing, the initial product surface is dry, and it is easy to lose water and return to sand during storage.

[0066] The present invention performs optical microscopic observation on the cross section of preserved strawberries after PAS staining. The specific method is: fix the preserved strawberry tissue with formalin-propionic acid-alcohol fixative for 24 hours and then take it out. After the tissue is trimmed, dehydrate it with 75%, 85%, 90%, 95%, anhydrous ethanol I, and anhydrous ethanol II in sequence for 4 hours, 2 hours, 2 hours, 1 hour, 30 minutes, and 30 minutes respectively. Then dehydrate it with alcohol benzene, xylene I, and xylene II for 5-10 minutes, 5-10 minutes, and 5-10 minutes respectively. Finally, immerse it in paraffin for embedding, freeze and solidify, and cut it into slices with a thickness of 4 μm with a paraffin slicer. Then, flatten the tissue in 40°C warm water, pick up the tissue with a slide, and dry the slices in a 60°C oven. After drying, it can be observed under a microscope. The obtained test results are shown in Figure 1 .

[0067] Figure 1 It shows that the strawberry preserved obtained in Examples 1 and 2 (Figures A and B) and Comparative Examples 1-2 (Figures C and D) has a circular cross section, a spindle-shaped pith, and the distribution of sugars is consistent with the distribution of vascular bundles in strawberry tissues, while Comparative Examples 3-4 (Figures E and F) are significantly different, with trehalose and sorbitol extending from the pith to the periphery in the strawberry tissue in a branch-like manner. This indicates that sucrose, isomaltooligosaccharides, and the combination of sugars and sugar alcohols in Examples 1 and 2 are similar to the binding and distribution between macromolecular components of strawberries under the action of osmotic pressure, and may be deposited in the cell wall via vacuoles and membrane tissues, and still roughly maintain the shape of plant cells after dehydration, while trehalose and sorbitol may be deposited across the membrane wall, first enriched in the pith, and extending from the pith to the epidermis as water diffuses during the drying process.

[0068] The present invention analyzes the X-ray diffraction characteristics of freeze-dried strawberry preserved samples after storage for 4 months. The specific method is: the freeze-dried sample is ground and then spread on the X-ray diffraction detection table for testing. The X-ray diffraction analysis is performed at a diffraction angle 2θ of 5° to 80°, with a step interval of 0.01 and a scanning rate of 2° / min. The test results are shown in Figure 2 .

[0069] Figure 2 It shows that the X-ray diffraction patterns obtained by Example 1 and Example 2 (Figure A, B) and Comparative Example 2 (Figure D) are weak in intensity and are wide-arched, which are typical amorphous states, while Comparative Example 1 (Figure C) and Comparative Example 3 (Figure E) have obvious sharp diffraction peaks at 16.28°, 24.75°, 23.40°, 13.27°, etc., respectively. Comparative Example 4 (Figure F) is different from other samples, and its diffraction is at 1.53° and 12.29°. This shows that Comparative Example 1, Comparative Example 3, and Comparative Example 4 all have different degrees of crystalline state, that is, sand return, and the crystal components of the three are different.

Claims

1. A method for preparing reduced-sugar strawberry preserves by impregnation with functional sugar, characterized in that: The method comprises the following steps: 1) Select cone-shaped Monterey strawberries with an individual mass of 5-8 g; 2) Thawing by soaking in a 0.2% sodium sulfite or sodium metabisulfite aqueous solution for 2-4 hours; 3) After the strawberries are thawed in step 2), the solution is drained, isomaltooligosaccharide, trehalose and sorbitol are added, and the strawberries are candied and dehydrated for 72-96 hours; 4) separating the candied strawberries from the sugar solution and placing them in a hot air circulation drying oven at 50-60° C. for 10-12 hours; The conical Monterey strawberries in step 1) are quick-frozen strawberries at the half-red stage. The hardness of the fruit at this maturity is conducive to the complete shape of the strawberry preserves. The fruit size range and sugar and acid content of this fruit are within the set immersion concentration and time, and the sugar is absorbed to make the strawberry fruit plump and uniform. In step 2), the ratio of the volume of the soaking liquid to the mass of the strawberries is 3:2, and soaking and thawing simultaneously kills bacteria and inhibits enzymes, and avoids oxidation of anthocyanins; In step 3), the isomaltooligosaccharide is 24%-32% of the mass of the strawberry, the trehalose is 8%-16% of the mass of the strawberry, and the sorbitol is 8% of the mass of the strawberry; In step 4), the drying time and temperature are controlled so that the moisture content of the preserved strawberries is 18%-21% at the end of drying.

2. The method according to claim 1, characterized in that In the step 3), the isomaltooligosaccharide accounts for 24% of the mass of the strawberry, and the trehalose accounts for 8% of the mass of the strawberry.

3. A reduced-sugar preserved strawberry, characterized in that: The method according to any one of claims 1 to 2 is adopted, the moisture content of the preserved strawberry is 18%-21%, the total sugar content is 25%-40%, the solid increase rate is 22%-25%, the hardness is 140-350g, the adhesive force is 10-18g, the elasticity is 0.2-0.4g, the chromaticity value L* is 22-32, the a* is 5-18, and the b* is 4-14.

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