植物油中量热检测方案

智能文字提取功能测试中

009742_01 APPLICATIONNOTE Differential Scanning Calorimetry Authors Patricia HeussenUnileverResearch & DevelopmentVlaardingen， The Netherlands Peng Ye， Kevin Menard， Patrick Courtney PerkinElmer， Inc.Shelton， CT 06484 USA Practical Food Applications ofDifferential Scanning Calorimetry (DSC) Abstract This note describes a number of important food applications utilising the PerkinElmer DSC demonstratingthe versatility of the technique as a tool in the food industry. Introduction Food is often a complex system including various compositions and structures. The characterizationof food can therefore be challenging. Many analytical methods have been used to study food，including differential scanning calorimetry (DSC).1 DSC is a thermal analysis technique to measurethe temperature and heat flows associated with phase transitions in materials， as a function oftime and temperature. Such measurements can provide both quantitative and qualitative informa-tion concerning physical and chemical changes that involve endothermic (energy consuming) andexothermic (energy producing) processes， or changes in heat capacity. DSC is particularly suitable for analysis of food systems because they are often subject to heatingor cooling during processing. The calorimetric information from DSC can be directly used to under-stand the thermal transitions that the food system may undergo during processing or storage. DSCis easy to operate and in most cases no special sample preparation is required. With a wide rangeof DSC sample pans available， both liquid and solid food samples can be studied. Typical foodsamples and the type of information that can be obtained by DSC are listed in Table 1. These testscan be used for both QC and R&D purposes. DSC applications are used from troubleshooting up tonew product developments. Table 1. Typical food samples and their application by DSC. Type of Samples Type of Information Oils， fats and spreads Onset temp of melt/crystallisation /polymorphic behaviour/oxidation stability Flour and rice starch Retrogradation/gelatinization/glass transition Tg Vegetable powders Glass transition Tg Pastes and gels containing Specific heat Cp， onset temp of polysaccharides or gums melt and crystallisation Protein Denaturation/aggregation In this note， several samples of food material systems aregiven to illustrate the versatility of DSC. DSC of oils and fats Using a heat-cool-heat DSC program， the onset temperature，the heat of fusion (AH)， the identification of polymorphicbehaviour and crystallisation of oils and fats can be deter-mined. An isothermal method or scanning method withan oxygen atmosphere can also be used to determine theoxidation induction time (OIT)， in which case a heat-cool-heatmethod is applied to hydrogenated vegetable oils. Sometimesadditional information about the sample is necessary fordata interpretation， as for example in combination with XRDanalysis which provides information on the specific polymorphictransitions. Most triglycerides? exist at least in three crystallineforms， α (alpha)， '(beta-prime)， and β (beta) that can beidentified according to their X-ray diffraction patterns.3 In Figure 1 it can be observed that a o-modification isformed after a heat-cool treatment. This will be transformedinto a B'-modification and after a certain time at roomtemperature partially to the B-modification. In Figure 2 theinfluence of storage time at room temperature is shown.The first heating of day 8 shows a better resolved peaksdue to the transition of the less stable p'to a more stablepolymorphic fraction， as it was also confirmed by XRD. Figure 2. Time influence on palmkernel oil melting behaviour. DSC is used to study fat phase transitions and melting range.It is one technique to explain the physical and texturalproperties of fats in bulk and final products. The combinationof DSC and XRD is often used to identify the stable B-form，which can result in grainy mouth feel in final products. DSC is used to compare batches of a product to study themelting behaviour indicating differences in crystallinity ofthe fat or composition of the end product. Different scanningrates are used to investigate the cooling effect on thecrystallisation of a specific fat. The solid fat content (SFC) ofa fat system can be determined over a given melting range.The solid fat content values are calculated through thepartial areas of DSC heating curves usually between 5-60 ℃and compared to NMR (Minispec) data.4.5 To study the aging of a fat or end product the sample iskept at an isothermal temperature to mimic e.g. refrigeratorconditions. Comparing the DSC thermograms of a freshsample and after a known storage time gives informationon phase transitions during these storage conditions. Other studies involve tempering to investigate the influenceon the final product after temperature abuse or due totransport at ambient. Tempering consisted of warming thesystems up to a temperature between 15 and 30 °C andcooling down to 5 C. These results can be correlated withthe storage modulus (G'). DSC melting and crystallisation behaviour of different typesof oils and fats are studied when replacing them in a product.In a factory and also at lab scale， different ingredients areadded at different stages of the production process. Addingan ingredient which is not at the correct temperature cancause encapsulation of other ingredients or may stay presentin the product as a particle. The filling temperature of aproduct is important for example to obtain the desiredfirmness of a product and to prevent graininess. An AOCS’ method can be carried out for quality control of fatsto analyse these raw materials used in food products. This isa "fingerprint" method whereby the sample is melted， subse-quently cooled down with a predefined scanning rate to a lowtemperature. After crystallisation for a specific time， a heatingcurve is obtained also with a predefined scanning rate. DSC of starch samples Starch8.9， a major structure-forming food hydrocolloid10，is a polymeric mixture of essentially linear (amylose) andbranched (amylopectin) molecules. Small amounts of non-carbohydrate constituents (lipids， phosphorus， and proteins)present in native starch also contribute to its functionality.Starch is used as thickening agent in e.g. dry sauce bases，instant soups， mayonnaise， spreads. Starch pastes can beused as stabilizers for oil emulsions in for instance dressings. Native starch or modified starch used in these types of foodproducts can show different endothermic peaks in the DSCthermograms respectively， retrogradation (recrystallizedamylopectin)， gelatinization (50 100°C) orrecrystallized amylose (T>140°C) can be observed. Retrogradation is only possible in processed (cooked ormodified starch) materials which have been stored at lowertemperatures. Retrogradation can expel water from a polymernetwork also known as syneresis but it can also cause doughto harden. The hydrogen bond arrangement of amylopectin and amylosemakes it difficult for water to penetrate into intact starchgranules. When the water is heated the granules swell andgelatinization is observed. DSC measures the temperature atwhich irreversible changes occur in the granule. This processcan also be observed by polarised light microscopy duringheating. The starch powders can be analysed dry to obtain informationabout the pure sample. Additionally， after adding a knownamount of water， information is obtained about the degreeof gelatinization. The level of water used is of influence onthe gelatinization degree and peak shapes. Starch with lowand intermediate water content can show more meltingendotherms. The gelatinization information can be used todetermine the temperature and time necessary for e.g. ricewhich is used in instant soups. If the rice has a too highamount of gelatinization left in the product， this will resultin hard uncooked rice in the instant soup. Most starches and rice products contain a lipid (fat) whichcan form an amylose-lipid complex. This complex can beformed during gelatinization. It is also a thermo reversiblecomplex and should show an exothermic peak on cooling.Sometimes the modification of the amylose with a lipid isperformed to control the texture of the final starch. The composition of plain ricell is starch (76.5%)， water(12%)， protein (7.5%)， fat (1.9%) and minors (2.1%).An example of a native rice (Figure 3) and rice slurry (Figure 4)show the presence of retrogradation and amylose-lipidcomplex endotherms. Figure 3. Native rice dry sample showing a retrogradation peak around 45℃and a gelatinization peak around 70°C. Figure 4. Native rice wet sample showing a gelatinization peak at around 70℃and some amylose-lipid complex at 112 °C. DSC of vegetable powders Since food products are complex mixtures of severalcompounds， it is often difficult to determine their glasstransition (Tg) temperatures accurately. Understanding theglass transition12 phenomenon provides an insight into thecauses of the cohesiveness of many important powders andinfluencing the wetability or solubility of the powder， whichis important for new product development. Food materialoften contains water which can be present as free or boundwater. The free water is related to the wateractivity (Aw).The plasticization effect of water leads to depression of theglass transition temperature causing significant changes inthe physicochemical and crystallization properties duringstorage. Loss of physical stability by the effect of moistureand temperature will reduce flowability and increase cakingtendency and， to a smaller extent， affect other physicalproperties such as colour. A Tg is only observed for amorphousmatter. Sugars in a powder can undergo a phase transitionfrom amorphous to crystalline at a given relative humidityduring storage and thus have an effect on the glass transitiontemperature. DSC is widely used to study glass transition phenomena.The effect of water as a plasticizer on Tg was studied forvegetable powders stored at different Aw values (humidity).At a higher Aw value the samples take up more water. InFigure 5 it is shown that the Tg drops to lower temperaturesas theamount of water in the sample increases. The knowledgeof Tg in combination with the water activity is importantin predicting the physical state of the powder at variousconditions， from free flowable to stickiness or phase transi-tions to crystalline matter. Figure 5. Water influence on Tg of tomato， the Aw 0.86 also shows anendothermic peak which is due to the melting of free water. Proteins denaturation is also intensively studied by DSC. Theinfluences of pH， salt and polysaccharides were investigated13for food proteins. Conclusion DSC is an essential tool to reveal the underlying phase-compositional principles of food systems. For systems with aclearly established phase-composition-functionality relation，DSC can contribute to the development of novel food products. References 1. Phase transitions in foods， Roos Y.H.， Academic Press，1995. 2. Physical properties of fats， oils and emulsifiers， Widlak N.，AOCS press， 1999. 3. X-Ray diffraction and differential scanning calorimetrystudies of β’>B transitions in fat mixtures， Szydlowsak-Czerniak， A et al， Food chemistry， 2005， 92， 133-141. 4. Solid fat content determination： Comparison betweenpNMR and DSC techniques， Nassu， R.T. et. al.， Grasas yAceites， 1995，V46， N°6， 337-343. 5. Modern magnetic resonance (3rd edition)， Graham A.Webb， 2006， chapter Time-Domain NMR in qualitycontrol. 6. Influence of tempering on the mechanical properties ofwhipped dairy creams， Drelon， N. et. al.， Internationaldairy journal， 2006， 16，1454-1463. 7.AOCS Official Method Cj 1-94， Reapproved 2009， DSCMelting Properties of Fats and Oils. 8. Carbohydrates in food， Eliasson A.， CRC press， 2006. 9. Starch chemistry and technology (3rd edition)， BemillerJ.， Whistler R.， 2009， Chapter 8 and20. 10. Texture in Food； Semi-Solid Foods， McKenna B.， CRC，2003. 11. The structural and hydration properties of heat-treatedrice studied at multiple length scales， Witec， M. et. al.，Food Chemistry，2010， V120， N4， 1031-1040. 12. The glassy state in food， Blanshard J.， Lfillford P.，Nothingham University Press 1993. 13. Calorimetry in food processing； analysis and design offood systems， Kaletunc G.， Wiley， 2009. For a complete listing of our global offices， visit www.perkinelmer.com/ContactUs ( C opyright O2011， P e rk i nElm e r， Inc. A ll rig h t s rese r ved. Perk i nElme r @ is a re gist e red tradem a rk of Perki n Elm e r， Inc . All o t h er tr a demar k s ar e the p roperty of t h eir respect i ve o wners. )