2014 Volume 61 Issue 2 Pages 31-34
High hydrostatic pressure (HHP) processing is an attractive non-thermal technique because of its potential to achieve interesting functional effects. In spite of the rapid expansion of HHP application to food systems, limited information is available on effects of HHP on modification of starch and their structural and physicochemical properties. Therefore, functional roles of HHP in starch modification such as acid-hydrolysis, hydroxypropylation, acetylation, cross-linking and cationization of starch, as well as physicochemical properties of HHP-assisted modified starches were reviewed. HHP-assisted modified starches revealed similar or different physicochemical properties compared to conventionally modified starches, suggesting the consideration of HHP as a processing parameter for hydrolysis and modification of starch. Moreover, HHP-assisted starch modification would be an attractive technology and can be effectively used in starch industry with relatively low cost and short reaction time.
In high hydrostatic pressure (HHP) processing process, foods are subjected to pressures more than 100 MPa for certain time. The applications of HHP to food products can be traced back to the work of Hite (1899), who reported a significant reduction in microbial content in milk following pressurization at 680 MPa for 10 min and discovered that egg albumin was coagulated by HHP treatment.1) 2) Over the past several decades, the HHP process has been studied and applied to inactivate microorganisms and enzymes with minimal heating for the purposes of retaining the end-use qualities (e.g., sensory characteristics, nutritional values) of foods and extending their self-lives. Regarding a function of HHP in HHP process, it is independent of the morphological characteristics (size and shape) of the substrate, acting instantly and homogeneously from every direction. HHP reduces the volume of the reaction mixtures in the chemical reaction, accelerating the reaction rate and increasing the reaction products.3) HHP treatment can form or break off hydrogen bonds within or among biological macromoleculs without cleaving their covalent bonds, which ruptures and/or damages their secondary and tertiary structures.4) Accordingly, the treatment of foods with HHP altered their physicochemical properties, including the density, viscosity, thermal conductivity, ionic dissociation, pH and freezing point.5) Moreover, the recent researches have reported that HHP modifies the structural characteristics and reactivities of the protein-based hydrocolloids, impacting their physical and rheological functionalities. In general, an increase of pressure holding time at pressure levels above 300‒400 MPa non-thermally denatures the protein structures, and subsequently, their reaggregation and refolding irreversibly occurs, resulting in gel formation. Thus, excessive HHP treatment of foods, especially in protein-rich foods, may change their appearance and texture, resulting in their improved end-use qualities and vise versa. Nevertheless, the pressure-induced protein gels which is observed for some proteins (e.g., soybean, gluten, meat, fish and egg albumin) exhibit the smooth, glossy and soft texture with greater elasticity and maintain their natural color and flavor, compared to those of heat-induced protein gels.5) 6)
Furthermore, the investigations on the effects of HHP on the structural characteristics and physical functionalities of native starch granule have been studied with three standpoints of HHP application to native starch: the application of HHP levels not high enough to gelatinize starch,7) the application of HHP to starch in dry state, and starch suspended in ethanol or at low moisture level,8) and the treatment of starch suspension in excess water with pressure levels above 400 MPa. In the X-ray studies, the crystal packing arrangement of cereal starches converted A-type into B-type by HHP treatment.6) Pressure-induced gelatinization of starch is a pressure-temperature dependent phenomenon, but the process is incomplete unless a sufficient amount of water is present, similar to hydrothermal gelatinization.9) Pressure-induced starch gelatinization is highly dependent on both botanical origin and amylose content of starch, as well as HHP processing parameters, such as pressure, temperature and pressure holding time.6) More recently, HHP-assisted starch chemical modification reaction was designed and realized to prepare chemically-modified starch products that possessed the differentiated functionalities from those of traditional modified starches. In this review, thus, HHP-induced starch gelatinization and HHP-assisted starch chemical modification were briefly discussed to provide an understanding of HHP technology as a non-thermal processing way of preparing the processed starch products.
There have been many researches on the application of HHP to the varied starches for the purposes of improving and expanding their physical functionalities. HHP treatment of native starch granules in the presence of excess water causes the non-thermal starch gelatinization involving destruction of crystallite structures by the forced infiltration of water molecules into crystalline regions within starch granules by HHP. Degree of gelatinization of starch by HHP treatment was known to be varied depending on its botanical origins and genotypes as well as HHP processing parameters (pressure level, pressure holding time, starch concentration in the starch aqueous suspension). For cereal starch granules possessing a typical A-type crystal packing arrangement, wheat starch granules in excess water were completely gelatinized at 450 MPa and ambient temperature.10) Within a given moisture content of starch aqueous suspension, also, wheat starch granules began to gelatinize at 300 MPa, and a maximum degree of gelatinization was achieved at 600 MPa.11) In the case of rice starch granules, the crystalline structure of waxy and non-waxy starches in excess water were changed into the amorphous structure at 600 MPa and room temperature.12) However, potato starch granules possessing a typical B-type crystal packing arrangement are known to exhibit more resistance to pressure, i.e. potato starch required the pressure levels of 800‒1,000 MPa to achieve full gelatinization.13) 14) 18) It is also reported that depending on starch crystal packing arrangements, the resistance of native starch granules to pressure increased in the order: B-type > C-type > A-type.10) 15) The paths where water molecules infiltrated into starch granules are thought to be similar during heat- and pressure-induced starch gelatinization.6) In the case of pressure-induced starch gelatinization, however, the degree of gelatinization of starch granules is likely influenced by the distribution and arrangement of water molecules among amylopectin double-helices within starch granules.14) 16)
The manner in packing of starch polymer chains in the granular structure is regarded decisive for their behavior under HHP. HHP caused reversible hydration of the amorphous regions within starch granules, followed by irreversible distortion of the crystalline regions, which in turn led to the destruction of the granular structure.6) It was reported that when high-amylose corn starch (Hylon VII) in the aqueous suspension (starch concentration, 30%) were treated at 650 MPa, some granules appeared to have a hole-like matrix (likely filled with a gel-like network) inside and their maltese-crosses were observed only at their outer regions.17) The extent of gelatinization of starches induced by HHP depended on the pressure level, pressure holding time, temperature, starch concentration and starch type. Furthermore, the dissociation of crystallite structures and the unwinding of amylopectin double-helices might be restricted under HHP, due to possible stabilization of Van der Waals interactions and hydrogen bonds among amylopectin double-helices.6) 18) Different from heat-induced gelatinization, thus, the pressure-induced starch gelatinization may undergo the incomplete disintegration of crystalline regions within starch granules (though complete gelatinization of starch granules can be achieved at much severe HHP processing conditions). Accordingly, the gelatinization temperatures of HHP-treated (relative to native) starch granules were shifted upward, similar to common characteristics of annealed starch granules.19) 20) 21)
In summary, through many researches on HHP treatment of native starch granules in the aqueous suspension, some general conclusions were obtained as following: 1) all kinds of starch granules were gelatinized by HHP treatment even at subzero temperature, if the pressure is sufficiently high, 2) B-type (relative to A-type and C-type) starch granules were more resistant to pressure, 3) for pressure-induced starch gelatinization, applied pressure levels can be reduced in combination with mild heat treatment, 4) HHP-treated starch granules (not or partially present in the granular form) much less swells than heat-treated starch granules and 5) HHP-treated starches much more frequently maintains their granular structures than heat-treated starches.
Many researches to date have focused on investigating the impacts of HHP on characteristics of native starch granules. However, limited information is available on the influence of HHP on acid hydrolysis and chemical modification of native starch granules. Thus, an overview of some studies that have been conducted with respect to the acid hydrolysis, acetylation, hydroxypropylation, cross-linking and cationization of starch granules under HHP is given here.
HHP-assisted starch hydrolysis.
Hydrolyzed starch products were generally prepared using acid- or enzyme-catalyzed hydrolysis. In a case of acid hydrolysis, native (in the granular form) or pre-gelatinized starch was hydrolyzed with the diluted solutions of HCl, H2SO4, and oxalic acid under heat (or steam injection) and mild pressure conditions (0.3‒1.2 MPa) to facilitate the acid hydrolysis. However, the acid hydrolysis of starches can generate haze resulting from insoluble aggregates formed among linear starch hydrolysates, and the unfavorable color in the conversion products. Also, the steam continues to be injected into acid hydrolysis system to facilitate the starch hydrolysis. To resolve the noted shortcomings, application of HHP to acid hydrolysis of starch was attempted. Corn starch granules were treated with HCl, H2SO4 and oxalic acid under HHP, followed by characterization of non-thermally hydrolyzed starch products.22) 23) Hydrochloric acid (HCl) was most effective for HHP-assisted acid hydrolysis of corn starch granules among the tested acids.22) In HHP-assisted starch hydrolysis with HCl, the degree of hydrolysis of corn starch was highly dependent on pressure levels, and a maximum degree of starch hydrolysis was achieved at 600 MPa.23) Degree of starch hydrolysis increased with increasing HCl concentration up to 4 N at an applied pressure level. However, the pressure holding time did not greatly affect the degree of starch hydrolysis.23) Consequently, HHP treatment may be an effective way of facilitating the hydrolysis of starch granules with HCl, and HHP-assisted HCl hydrolysis may be a greater possibility to be utilized as a non-thermal starch hydrolysis process than a traditional scheme.
HHP-assisted starch acetylation.
Acetylation reaction of corn starch with acetic anhydride was combined with HHP technology, and potential roles of HHP in starch acetylation reaction were investigated with respect to reactivity and physical properties of HHP-assisted starch acetates.24) Both HHP-assisted and conventional acetylation of starch likely occurred predominantly at amorphous regions within starch granules. For HHP-assisted (relative to conventional) starch acetates, while the degree of substitution was about 25% lower, the reaction time was about 75% shorter. As the results of decreased degree of substitution of HHP-assisted starch acetates, their solubility, swelling power and pasting viscosity were reduced. Considering the process efficiency in acetylation reaction, however, HHP-assisted acetylation reaction appeared to be better than conventional one.24) To verify the impacts of HHP on physical properties of HHP-assisted starch acetates, the physical properties of HHP-assisted and conventional starch acetates with a similar degree of substitution were compared.25) At similar derivatization level, HHP-assisted starch acetates revealed the restricted starch solubility and swelling power, the reduced gelatinization temperatures and the lower pasting viscosities than conventional ones. These results suggested that HHP treatment in acetylation reaction may facilitate the formation of lipid-complexed amylose or alteration of reaction sites in/on the granules in the acetylation by acetic anhydride. Consequently, HHP treatment in starch acetylation may produce starch acetate differentiated in physical properties from those of conventional starch acetate.25)
HHP-assisted starch hydroxypropylation.
HHP was also applied to hydroxypropylation (HP) reaction of corn starch granules with propylene oxide, and the reactivity and physicochemical properties of HHP-assisted HP starches were investigated.26) Relative to conventional HP reaction, HHP accelerated hydroxypropylation reaction of starch, resulting in about 99% reduction in the reaction time, although HHP-assisted HP reaction generates HP starch possessing 19.7% reduced molar substitution. The pressure level was a critical factor to enhance the starch granule reactivity in HHP-assisted HP reaction, while the effects of pressure holding time on starch granule reactivity were not likely of practical importance. Based on comparison of physical and thermal properties of HHP-assisted HP starches to those of conventional ones, moreover, HHP-assisted HP starch properties appeared to be more influenced by HHP treatment than their derivatization degrees.26)
HHP-assisted cross-linking reaction of starch.
Cross-linking (XL) reaction of corn starch with phosphorus oxychloride were conducted under HHP, and investigated in regard to the physicochemical properties of HHP-assisted and conventional XL starches.27) 28) Although HHP-assisted XL reaction time was a 12.5% level of the conventional XL reaction time, the physicochemical properties of HHP-assisted XL corn starches were similar to those of the conventional ones.27) Thus, application of HHP to starch XL reaction may enhance the efficiency of the process to produce XL starch products. On the other hand, the physicochemical properties of HHP-assisted XL starches were not significantly influenced by pressure levels (100‒400 MPa). Nevertheless, their swelling powers and gelatinization temperatures were reduced relative to those of conventional one, and HHP-assisted XL starch reacted at 400 MPa exhibited very similar pasting viscosity profile to that of conventional ones.28)
HHP-assisted starch cationization.
There were recent attempts to produce the cationic derivatives of dextrin and native starch under HHP.29) 30) HHP-assisted cationization reaction successfully substituted ETMAC (2,3-epoxypropyl trimethyl ammonium chloride) onto hydroxyl groups of dextrin molecules.29) Cationic dextrin subjected to HHP-assisted cationization reaction with relatively higher ETMAC level and smaller volume of the reaction mixtures possessed higher values of degree of substitution (DS) than those of conventional cationic dextrin. Also, HHP-assisted cationization reaction likely required shorter reaction time to achieve similar DS values to those of conventional ones, suggesting that HHP-assisted cationization reaction can be an alternative way to produce cationic dextrin with an efficient and energy-saving scheme. Furthermore, granular and non-granular cationic starches were prepared via HHP-assisted cationization reaction.30) Similar to cationic dextrin, DS values of granular cationic starches did not differ for conventional and HHP-assisted reactions in tapioca and corn starches, although conventional reaction required longer reaction time than HHP-assisted reaction. On the other hand, both cationic dextrin and starches subjected to HHP-assisted cationization reaction revealed excellent flocculating activities, suggesting that those are promising materials to replace the synthetic flocculant as an environment-friend flocculant for waste water treatment.
This overview concluded that HHP can be used as a processing unit for hydrolysis and modification of starch. HHP-processed starch such as acid-hydrolyzed and modified starches revealed similar or different physicochemical properties compared to conventionally processed starches. If HHP-assisted reaction would be considered for chemical modificiation of carbohydrate-based hydrocolloids, the reaction parameters such as the molar ratio of carbohydrate substrates to reacting reagents, catalyst concentration, final volume of reaction mixtures, pressure and pressurizing time would be crucial factors. This review showed the potential and possibility of HHP-assisted acid hydrolysis and chemical modification of starch and provides the basic and scientific information on the physicochemical properties of non-thermally modified starch using HHP. Future investigation is warranted on the new application of high pressure processing on starches and other biopolymers.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2011-0021496).
