A method for low-temperature high-efficiency annealing by using starch glass transition temperature window

Abstract

The application discloses a method for realizing low-temperature and high-efficiency annealing by using starch glass transition temperature window, and belongs to the technical field of deep processing of grains. In the method, starch-containing raw materials are adjusted to moisture content of 20%-35%, so that the glass transition temperature of the starch-containing raw materials is reduced to the interval of 45-55 DEG C; then, annealing is performed in the window of 50-65 DEG C which is higher than the glass transition temperature and lower than the peak temperature of starch gelatinization for 8-20 minutes, so that the starch molecular chains are fully movable above the glass transition temperature and the natural microcrystals are not fused, linear starch of the gelatinized part is orderly rearranged along the surface of the residual microcrystals, and RS3 type resistant starch with melting temperature of greater than or equal to 120 DEG C is formed; and then, cooling, setting and low-temperature drying are performed. The method is a pure physical zero-addition process, the glass transition temperature of the starch is used as the theoretical basis for the lower limit of the annealing temperature for the first time, is suitable for all starch-containing raw materials, and the obtained product has GI value of less than or equal to 65 and resistant starch content of greater than or equal to 7%.

Description

Technical Field

[0001] This invention belongs to the field of deep grain processing technology, specifically involving a low-temperature, high-efficiency annealing method based on the glass transition temperature of starch, which is used to reduce the glycemic index while retaining the processing performance of grains. Background Technology Starch is a semi-crystalline polymer material, and the glass transition temperature (Tg) of its amorphous region is a key physical parameter that determines the mobility of starch molecular chains. Below Tg, starch molecular chains are in a frozen state and cannot undergo ordered rearrangement; only above Tg do the molecular chains acquire sufficient mobility to perform conformational adjustment and lattice repair. Current starch annealing processes typically rely solely on experience to set annealing temperatures, never systematically lowering the glass transition temperature (Tg) through moisture control based on the glass transition theory, thus failing to achieve efficient annealing at lower temperatures. The applicant has discovered and proposed for the first time that by precisely controlling the conditioning moisture content, the Tg of starch can be lowered to the 45-55℃ range. Annealing can then be performed within a temperature window 3-15℃ above this Tg (i.e., 50-65℃), ensuring both sufficient molecular chain mobility and preventing the melting of natural microcrystals—a golden window for achieving short-time, efficient annealing. Summary of the Invention Purpose of the invention The purpose of this invention is to provide a starch annealing method based on the glass transition theory, which reduces the starch Tg by controlling the moisture content, and achieves efficient RS3 generation under conditions lower than the traditional annealing temperature. Technical solution The present invention includes the following steps: (1) conditioning to a moisture content of 20%-35% to reduce Tg to 45-55℃; (2) annealing for 8-20 minutes in a window (50-65℃) that is 3-15℃ above Tg and below the peak gelatinization temperature; (3) cooling and setting; (4) drying at low temperature. Core Mechanism The glass transition temperature (Tg) of starch exhibits a strict negative correlation with moisture content. When the moisture content increases from 10% to 28%, the Tg of starch decreases from 60-70℃ to 45-50℃. This invention precisely controls the conditioning moisture content to lower the starch Tg to the 45-55℃ range, and then anneales it at a temperature 3-15℃ above this Tg (i.e., 50-65℃). This temperature window is both higher than the Tg (ensuring sufficient molecular chain mobility) and lower than the gelatinization peak temperature (ensuring the natural crystallites do not melt), representing a golden window that simultaneously satisfies both "molecular mobility" and "crystal collapse prevention." The core contribution of this invention lies in the fact that it is the first time that the glass transition temperature (Tg) has been used as the theoretical basis for the lower limit of starch annealing temperature, and the first time that a specific effective driving force window of 3-15°C above Tg has been determined. Existing technologies have never proposed setting the annealing window by adjusting Tg, let alone proposing this specific preferred range of 3-15°C above Tg. Below this window, the molecular chains freeze, and annealing is ineffective (as shown in Comparative Example 3); above this window, the microcrystals melt and collapse, losing their seeding function (as shown in Comparative Example 2). Beneficial effects (1) The glass transition theory is applied to the starch annealing process design for the first time, and the annealing temperature window has a clear theoretical basis and strong repeatability. (2) By controlling the moisture content to reduce Tg, efficient RS3 generation is achieved under conditions lower than the traditional annealing temperature, and the annealing temperature window is lower than that of the traditional process. (3) Pure physical process with zero additives. (4) Annealing time is 8-20 minutes, which is suitable for industrial production. Detailed Implementation Example 1 (Wheat starch, high moisture content reduces Tg) 1000g of wheat starch was adjusted to 28% with water at 48℃. Its glass transition temperature (Tg) was determined using differential scanning calorimetry (DSC). The temperature was increased from -20℃ to 100℃ at a rate of 10℃ / min, and the midpoint of the heat flow curve step was taken as Tg, confirming that the Tg had dropped below 50℃. Based on this measurement, an annealing temperature higher than Tg but lower than the gelatinization peak temperature was selected. Annealing was performed at 51℃ and 85% humidity for 14 minutes. The starch was then gradually cooled to 33℃ and dried at 42℃ until the moisture content reached 12.0%. The resulting product exhibited a significantly lower GI value, an RS3 melting temperature exceeding 120℃, and a significantly higher resistant starch content. This demonstrates that by adjusting the moisture content to lower the temperature range (Tg) to an appropriate level, annealing within a temperature window above the Tg can achieve efficient RS3 formation. Example 2 (Corn Starch) Take 1000g of corn starch, add water to adjust to 25%, and heat at 48℃. Measure its Tg using the same method as in Example 1 to confirm that its Tg has dropped to an appropriate range. Based on this measurement, an annealing temperature higher than Tg but lower than the gelatinization peak temperature was selected. Anneal at 58℃ and 85% humidity for 12 minutes. Rapidly cool to 23℃ and dry at 45℃. The resulting product exhibits a significantly reduced GI value and an RS3 melting temperature exceeding 120°C. This demonstrates that the method is applicable to starches from various sources. Comparative Example 1 (insufficient moisture, excessively high Tg, annealing temperature below Tg) The moisture content of Example 1 was reduced to 12%. At this point, its Tg was higher than the annealing temperature of 51°C and the annealing temperature was lower than the Tg, while the other conditions remained unchanged. The resulting product had a low resistant starch content and no significant decrease in the GI value. This demonstrates that molecular chains cannot move when the annealing temperature is below Tg, rendering annealing ineffective. Comparative Example 2 (annealing temperature too high, exceeding the gelatinization peak) The annealing temperature of Example 1 was changed to 72°C, which exceeds the peak temperature of starch gelatinization, while the other conditions remained unchanged. The resulting product exhibited complete microcrystal destruction, low resistant starch content, and an RS3 melting temperature below 120°C. This demonstrates that annealing temperatures exceeding the gelatinization peak caused microcrystal melt collapse. Comparative Example 3 (annealing temperature below Tg+3℃ window) The annealing temperature of Example 1 was set to be only slightly higher than its Tg (temperature difference less than 3°C), while the other conditions remained unchanged. The resulting product showed only a small increase in crystallinity and low annealing efficiency. This demonstrates that the annealing temperature needs to be significantly higher than the annealing temperature (Tg) to obtain sufficient driving force. Detection methods Glass transition temperature: Determined by differential scanning calorimetry. The sample was sealed in an aluminum crucible and heated from -20℃ to 100℃ at a rate of 10℃ / min. The midpoint of the heat flow curve step was taken as Tg. Resistant starch content: AOAC 2002.02 standard method was used. GI value: based on the ISO 26642:2010 standard method. RS3 melting temperature: Differential scanning calorimetry was used, with a heating rate of 10℃ / min and a nitrogen atmosphere. Industrial applications This invention can be implemented on existing continuous grain processing production lines. The core equipment includes a dual-shaft differential atomizing conditioning tank, a closed annealing tower with a temperature control accuracy of ±0.5℃ across the entire range, and a multi-stage air-cooling or fluidized bed quenching system.

Claims

1. A method for achieving low-temperature, high-efficiency annealing using the glass transition temperature window of starch, characterized in that, Includes the following steps: a. Conditioning: Add water to starch-containing raw materials to condition the moisture content to 20%-35%, thereby reducing the glass transition temperature to the range of 45-55℃; b. Isothermal annealing: The conditioned material is kept in a constant humidity environment with a relative humidity of 80%-95% for 8-20 minutes within a temperature window of 50-65℃. The annealing temperature is 3-15℃ higher than the glass transition temperature of the material and lower than the starch gelatinization peak temperature, so that the starch molecular chains can obtain sufficient mobility and the natural microcrystals do not melt. The straight-chain starch of the gelatinized part rearranges orderly along the surface of the residual microcrystals to form high melting point RS3 type resistant starch with a melting temperature ≥120℃. c. Cooling and shaping: Cool the annealed material to below 35°C; d. Low-temperature drying: Dry the cooled material at a temperature not exceeding 60°C until the moisture content is below 13%.

2. The method according to claim 1, characterized in that, The moisture content mentioned in step a is negatively correlated with the glass transition temperature. The glass transition temperature can be controlled within a predetermined range by adjusting the moisture content.

3. The method according to claim 1, characterized in that, The starch-containing raw material is any one or more of rice, wheat, corn, potato, sweet potato, cassava, oats, and buckwheat, or starch processed from them.

4. The method according to claim 1, characterized in that, The cooling and shaping process in step c adopts a gradient slow cooling method, wherein the gradient slow cooling is a first stage of cooling from the annealing temperature to 40-42℃ at a rate of 3-5℃ / min, and a second stage of cooling from 40-42℃ to below 35℃ at a rate of 1-2℃ / min.

5. The method according to claim 1, characterized in that, The cooling and shaping process described in step c uses a fluidized bed rapid cooling method, which uses cold air at -5°C to 0°C to cool the material from the annealing temperature to below 25°C within 90 seconds.

6. The method according to claim 1, characterized in that, The annealing time mentioned in step b is 10-15 minutes.

7. The method according to claim 1, characterized in that, The temperature for low-temperature drying in step d is 42-50℃.

8. The method according to claim 1, characterized in that, When the raw material is wheat starch, it is adjusted to 28% moisture content, and after measuring its glass transition temperature, an annealing temperature higher than that temperature but lower than the gelatinization peak temperature is selected for annealing.

9. A low glycemic index cereal powder or starch, characterized in that, Prepared by the method described in any one of claims 1-8, and simultaneously satisfying the following conditions: GI value ≤ 65, resistant starch content ≥ 7%, and RS3 type resistant starch melting temperature ≥ 120℃.

10. The use of the cereal powder or starch according to claim 9 in the preparation of food, pet food, and biodegradable packaging materials.