There are two main categories of high-pressure treatments: the first is a static type known as ultrahigh-pressure (UHP) or high-hydrostatic pressure (HHP), which is commonly utilized in food processing. The second type involves the use of homogenizers, which generate turbulence, high shear forces, and cavitation by pushing a starch slurry through a small orifice under high pressure. These two treatments produce distinct modifications in the starch.
Overview of UHP Treatment on Starch
Ultra-high pressure (UHP) treatment is a nonthermal food processing technique that can partially or completely gelatinize starch while maintaining its granular form. The process involves subjecting an aqueous slurry of starch granules to a pressure exceeding 400 MPa, with the type of starch, its concentration, and the pressure, temperature, and duration of treatment being variable factors. The degree of gelatinization at any combination of pressure, temperature, and time decreases as the slurry concentration increases, and different degrees of gelatinization can be achieved by manipulating these factors.
Factors Affecting Susceptibility to UHP Treatment
There is evidence that A-type starches are more susceptible to UHP treatment than B-type starches, but the type of crystallite packing alone cannot account for this difference. Other factors that may play a role in the gelatinization of starch granules under UHP treatment include the structural organization of amylopectin, different gelatinization behaviors related to different amylopectin interblock chain lengths, and the hydration and melting of crystallites.
Effects of UHP Treatment on Starch Properties
Reports on the effects of UHP treatment on starch properties have been inconsistent and likely arise from partially gelatinized products. UHP treatment can produce partially gelatinized products, where some granules are gelatinized while remaining intact, and others are not, if sufficient pressure or time for complete gelatinization is not applied. This results in an increase in viscosity and gel strength of the products.
Inhibition of Gelatinization by UHP Treatment
Inorganic salts have been found to inhibit gelatinization by UHP treatment, just as they inhibit thermal gelatinization. Additionally, the loss of crystallinity (gelatinization) affected by UHP treatment makes the granules more susceptible to attack by amylases.
Improvement of Food Quality by UHP-Treated Starch
The addition of UHP-treated mung bean starch to the batter before deep-fat frying has been found to improve the quality of deep-fried battered foods based on increased pick-up of batters, a higher moisture content, increased crispness, and reduced oil content of crusts.
High-Pressure Homogenizers and Jets
The treatment of starch with high-pressure homogenizers involves varying the type of starch, pressure, and number of cycles/passes. Several changes in starch have been reported, including granule deformation and fragmentation, partial gelatinization, increases in the onset, peak, and conclusion temperatures of gelatinization, and loss of crystallinity. All these changes have been observed to increase with increasing pressure. The increases in gelatinization temperatures are likely due to the more easily gelatinized granules being gelatinized first. Previous studies have employed high-pressure homogenizers and microfluidizers to prepare starch-fatty acid complexes.
High-Speed Jet Homogenizer
In the high-speed jet homogenizer, a starch suspension is passed through a small orifice, producing an ultrahigh-velocity jet that hits a target. Researchers have found that when cassava/tapioca starch was subjected to high-speed jet treatment, the particle size increased. This increase in particle size was attributed to granule aggregation caused by surface damage and gelatinization that led to granules sticking to each other. Similar observations were made when the same starch was subjected to microfluidization. It has also been reported that high-speed jet treatment can destroy both crystalline and granular structures, resulting in a partially gelatinized product, with the percentage of amorphous material increasing as the pressure increases. The treatment also causes decreases in molecular weights of the starch polysaccharide molecules, primarily amylopectin, and their root mean square radii of gyration, as well as a loss of viscosity. Solubility of the product increases, and the value of the storage modulus of resulting gels decreases. In one study, the use of water with added 0.24% ethanol in high-speed jet homogenization treatment of maize starch resulted in greater loss of granule crystallization than when pure water was used, a result that was attributed to greater cavitation.
Another technique involving pressure that has been applied to starches is instantaneous controlled pressure drop (DIC) (BeMiller and Huber, 2015).