The Effect Of Hydrolyzed Soy Protein On Structure And Properties Of Ice Cream And Whipped Cream

Soy protein isolate has attracted much attention in recent years as a plant source protein due to its health benefits,cost,source and functionalities in food industry.Hence,soy protein has been considered as an alternative to or extension of milk protein in food systems such as ice cream and whipped cream.Soy protein has two major components–Glycinin（11S）andβ-Conglycinin（7S）,which account for 70%of total storage proteins in soybean seed.Glycinin（11S）consists of six subunits in which acidic and basic subunit linked by a disulfide bond as a hexamer with a molecular mass of 300-380 kDa.Hydrophobic interaction and hydrogen bonding were the major association for three subunits-α（67 kDa）,αˊ（71 kDa）andβ（50 kDa）to form a trimer inβ-Conglycinin（7S）.According to different processing condition,soy protein with different ratio of subunits demonstrated different emulsifying properties.Native soy protein isolates with a larger molecular weight and intact structure exhibits poor oil-water interfacial properties due to its slow diffusing property to oil-water interface and inflexibility.Thermal treatment to commercial soy protein isolates（CSPI）production deteriorated its solubility and produced some percentage of insoluble portions.Enzymatic modification of SPI was used for emulsifying properties with different functionalities.In order to identify the relationship of composition and properties of SPI hydrolysates,pepsin and papain driven modifications under controlled condition were conduced to get hydrolysates with different polypeptide profiles.Compared to native soy protein isolate（NSPI）,SDS-PAGE of CSPI indicated less bands corresponding to the basic subunit in glycinin andβ-subunit inβ-conglycinin.The main subunits in soy proteins hydrolyzed by pepsin（SPHPe）wereβ-conglycinin and basic subunit.Soy proteins hydrolyzed by papain（SPHPa）had peptides with less 20 kDa molecule weights with major subunits.SPHPa achieved the highest of degree of hydrolysis,following by SPHPe,CSPI and NSPI,which provided the information of molecular size at air-water/oil-water interface.Enzymatic hydrolysates altered the molecular weight distribution of soy protein.Compared to NSPI,the percentages of large molecular fractions（MW in the range of 200kDa to 1000kDa and aggregates with MW larger than 1000 kDa）in SPHPe and SPHPa were reduced.The fraction（MW in the 10-200 kDa）in CSPI was less than SPHPe due to the removal of insoluble portion during sample preparation.All samples（3%w/w solution）exhibited non-Newtonian fluids with n value ranging from 0.47 to 0.57 and SPHPa solution exhibited highest K and highest apparent viscosity at 50S^-1 and 100S^-1.The interfacial properties of NSPI,SPHPe and SPHPa demonstrated“gel-like”properties protein structure at the low frequency sweep（less 5Hz）and CSPI exhibited“fluid-like”behavior in the whole frequency sweep range at the oil-water interface.SPHPe has the highestξ-potential among all samples,indicating that it can be used for a stable emulsion.The lowest solubility of NSPI,CSPI and SPHPe were ahieved when pH was in the range 4.0-4.5.The solubility of SPHPa did not indicate strong pH dependence in the tested range.SPHPe with a higher ratio ofβ-conglycinin has the highest emulsifying activity and emulsifying stability.Although SPHPa had a good emulsifying activity due to the presence of high ratio of small peptides,its emulsifying stability was weaker than the others.The tested results above provided the foundation for their further studies.Small molecules emulsifiers such as monoglycerides have been used as surfactants in food industry.There are some studies conducted about the interaction of milk protein and monoglycerides at the water/oil interface.However,there is no study of monoglycerides and soy protein isolate at the water/oil interface.In order to expand the applications of soy protein and its hydrolysate,the effect of the adsorption behavior of soy proteins hydrolysates and monoglycerides on the properties of the film at oil-water interfaces in emulsions and emulsion stability was investigated.Surface tension,adsorbed protein composition and interfacial rheological properties were measured to gain insights into the relationships between molecular composition and properties.The interfacial film formed byβ-conglycine（7S）,Glycinin（11S）and NSPI exhibited high elasticity due to a high concentration of adsorbedβ-subunits,acidic and basic subunits.SPHPe and SPHPa had a high percentage of small peptides,forming a“fluid-like”interfacial film at the beginning of the time sweep.CSPI film without main soy protein subunits induced“fluid-like”properties.Monoglycerides displaced the small peptides,acidic and basic subunits from the interface and led to fat globule destabilization after 24h aging.Emulsions were stable with 7S and SPHPe with high proportion ofβ-subunits,which were not replaced by monoglycerides.This result provided the foundation for the preparation and application of soy protein isolate with different emulsion stability requirements paired with selected enzyme treatments.Base on the results above,soy protein isolate and its hydrolysates were used in ice cream as alternative source of milk protein.The effect of soy protein isolate and its hydrolysates on ice cream mix stability and melt-down properties of ice cream were investigated.Ice creams were made with 10%milk fat,3.5%protein and 36.8%total solids,and all contained 0.15%added monoglycerides.The proteins used were NSPI,CSPI,SPHPe,SPHPa and skim milk powder（SMP）.Ice cream with SPHPe containing the highest relative composition ofβ-subunit showed good mix emulsion stability and rapid melting rate becauseβ-subunit cannot be displaced by the monoglycerides,leading to lack of fat partial coalescence in the ice cream.SPHPa ice cream exhibited comparable functionality to SMP in rheological and meltdown properties.SDS-PAGE results indicated thatαsubunit,α’subunit,acidic subunit,basic subunit and small molecule polypeptide were displaced by monoglycerides during ice cream aging.Selective hydrolysis of soy protein can be used for ice cream with sufficient fat partial coalescence and good melt-down rates.The interaction and synergetic effect of SPI and its hydrolysates with different concentration monoglycerides were explored at air-water/oil-interface in low fat whipped cream（20%）.The effect of soy protein isolate and its hydrolysates with different concentration of monoglycerides（0.25-1.00%）on the overrun,stability,rheological behaviour and texture of low fat whipped cream were studied.Low-fat whipping creams were made with 20%palm oil,18%carbohydrate,0.22%stabilizers and 0.25-1.00%monoglycerides.The proteins used were NSPI,CSPI,SPHPe,SPHPa and sodium caseinate（SC）.The results indicated that increasing concentration of monoglyceirdes added was effective in improving the textural,whipping properties and stability of low-fat whipping cream.Increasing concentration of monoglycerides in the mix prompted the displacement of adsorbed protein from fat globules,built up a stronger structure of fat aggregate and stabilized the trapped air bubbles in the structure of low-fat whipping cream.At same level of monoglycerdies,SPHPa whipped cream indicated similar properties such as overrun,stability and texture as the control（sodium caseinate）and can be alternative protein source in building a desirable whipped cream structure.Due to the presence of high proportion ofβ-conglycinin in SPHPe,the low degree of fat globules partial coalescence occurrence led to low overrun and weakened the structure low-fat whipped cream.In conclusion,controlled enzymatic medication with pepsin and papsin can manipulate the peptides profiles in SPI hydrolysates for different surface properties.SPHPe had a good emulsifying stability in the presence of monoglycerides,which cannot provide desirable melting properties for ice cream and build up a comparable structure of low-fat whipping cream as milk protein.SPHPa were displaced by monoglycerides at oil-water interface,which contributed to the formation partial fat coalescence.The partial fat coalescence and formation of fat aggregates play the role in trapping dispersed tiny air bubbles in the structure of ice cream and whipping cream.Surface properties of SPHPa were comparable to milk protein（skim milk protein in ice cream and sodium caseinate in whipping cream）.The melting properties of ice cream and whipping properties of whipping cream made by NSPI and CSPI were inferior to SPHPa due to their intact strucuture.Moverover,increasing monoglycerides concentration in low-fat whipping cream improved the whipping properties and stability of low-fat whipping cream.This study found high ratio ofβ-subunit in SPHPe were not displaced by monoglycerides and small molecules weight of peptides in SPHPa were displaced by monoglycerides at oil-water interface during aging.The study broadens the knowledge of emulsifying properties of soy protein subunits and their interaction with monoglycerides at oil-water interface in fresh and aged emulsion.It also provides the insight of the role of SPI and SPI hydrolysates in the structure formation of ice cream and low-fat whipping cream and impact on their properties.