Practical Food Applications of Differential Scanning Calorimetry (DSC)

Abstract

This note describes a number of important food applications utilising the PerkinElmer DSC demonstrating

the versatility of the technique as a tool in the food industry.

Introduction

Food is often a complex system including various compositions and structures. The characterization

of food can therefore be challenging. Many analytical methods have been used to study food,

including differential scanning calorimetry (DSC).

DSC is a thermal analysis technique to measure

the temperature and heat flows associated with phase transitions in materials, as a function of

time and temperature. Such measurements can provide both quantitative and qualitative informa-

tion concerning physical and chemical changes that involve endothermic (energy consuming) and

exothermic (energy producing) processes, or changes in heat capacity.

DSC is particularly suitable for analysis of food systems because they are often subject to heating

or 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. DSC

is easy to operate and in most cases no special sample preparation is required. With a wide range

of DSC sample pans available, both liquid and solid food samples can be studied. Typical food

samples and the type of information that can be obtained by DSC are listed in Table 1. These tests

can be used for both QC and R&D purposes. DSC applications are used from troubleshooting up to

new product developments.

Differential Scanning Calorimetry

application note

Authors

Patricia Heussen

Unilever

Research & Development

Vlaardingen, The Netherlands

Peng Ye, Kevin Menard, Patrick Courtney

PerkinElmer, Inc.

Shelton, CT 06484 USA

Practical Food Applications of

Differential Scanning Calorimetry (DSC)

DSC is used to study fat phase transitions and melting range.

It is one technique to explain the physical and textural

properties of fats in bulk and final products. The combination

of 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 the

melting behaviour indicating differences in crystallinity of

the fat or composition of the end product. Different scanning

rates are used to investigate the cooling effect on the

crystallisation of a specific fat. The solid fat content (SFC) of

a fat system can be determined over a given melting range.

The solid fat content values are calculated through the

partial areas of DSC heating curves usually between 5-60 °C

and compared to NMR (Minispec) data.

4,5

To study the aging of a fat or end product the sample is

kept at an isothermal temperature to mimic e.g. refrigerator

conditions. Comparing the DSC thermograms of a fresh

sample and after a known storage time gives information

on phase transitions during these storage conditions.

Other studies

involve tempering to investigate the influence

on the final product after temperature abuse or due to

transport at ambient. Tempering consisted of warming the

systems up to a temperature between 15 and 30 °C and

cooling down to 5 °C. These results can be correlated with

the storage modulus (G’).

DSC melting and crystallisation behaviour of different types

of oils and fats are studied when replacing them in a product.

In a factory and also at lab scale, different ingredients are

added at different stages of the production process. Adding

an ingredient which is not at the correct temperature can

cause encapsulation of other ingredients or may stay present

in the product as a particle. The filling temperature of a

product is important for example to obtain the desired

firmness of a product and to prevent graininess.

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 are

given 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 (ΔH), the identification of polymorphic

behaviour and crystallisation of oils and fats can be deter-

mined. An isothermal method or scanning method with

an oxygen atmosphere can also be used to determine the

oxidation induction time (OIT), in which case a heat-cool-heat

method is applied to hydrogenated vegetable oils. Sometimes

additional information about the sample is necessary for

data interpretation, as for example in combination with XRD

analysis which provides information on the specific polymorphic

transitions. Most triglycerides

exist at least in three crystalline

forms, a (alpha), b’ (beta-prime), and b (beta) that can be

identified according to their X-ray diffraction patterns.

In Figure 1 it can be observed that a a-modification is

formed after a heat-cool treatment. This will be transformed

into a b’-modification and after a certain time at room

temperature partially to the b-modification. In Figure 2 the

influence of storage time at room temperature is shown.

The first heating of day 8 shows a better resolved peaks

due to the transition of the less stable b’ to a more stable

polymorphic fraction, as it was also confirmed by XRD.

Figure 1. Heat influence on emulsifier.

Figure 2. Time influence on palmkernel oil melting behaviour.

The composition of plain rice

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-lipid

complex endotherms.

DSC of vegetable powders

Since food products are complex mixtures of several

compounds, it is often difficult to determine their glass

transition (Tg) temperatures accurately. Understanding the

glass transition

phenomenon provides an insight into the

causes of the cohesiveness of many important powders and

influencing the wetability or solubility of the powder, which

is important for new product development. Food material

often contains water which can be present as free or bound

water. The free water is related to the wateractivity (Aw).

The plasticization effect of water leads to depression of the

glass transition temperature causing significant changes in

the physicochemical and crystallization properties during

storage. Loss of physical stability by the effect of moisture

and temperature will reduce flowability and increase caking

tendency and, to a smaller extent, affect other physical

properties such as colour. A Tg is only observed for amorphous

matter. Sugars in a powder can undergo a phase transition

from amorphous to crystalline at a given relative humidity

during storage and thus have an effect on the glass transition

temperature.

An AOCS

method can be carried out for quality control of fats

to analyse these raw materials used in food products. This is

a “fingerprint” method whereby the sample is melted, subse-

quently cooled down with a predefined scanning rate to a low

temperature. After crystallisation for a specific time, a heating

curve is obtained also with a predefined scanning rate.

DSC of starch samples

Starch

8,9

, a major structure-forming food hydrocolloid

is a polymeric mixture of essentially linear (amylose) and

branched (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 be

used as stabilizers for oil emulsions in for instance dressings.

Native starch or modified starch used in these types of food

products can show different endothermic peaks in the DSC

thermograms respectively, retrogradation (recrystallized

amylopectin), gelatinization (50 < T < 80 °C depending on

the type of starch), amylose-lipid complex (T > 100 °C) or

recrystallized amylose (T > 140 °C) can be observed.

Retrogradation is only possible in processed (cooked or

modified starch) materials which have been stored at lower

temperatures. Retrogradation can expel water from a polymer

network also known as syneresis but it can also cause dough

to harden.

The hydrogen bond arrangement of amylopectin and amylose

makes it difficult for water to penetrate into intact starch

granules. When the water is heated the granules swell and

gelatinization is observed. DSC measures the temperature at

which irreversible changes occur in the granule. This process

can also be observed by polarised light microscopy during

heating.

The starch powders can be analysed dry to obtain information

about the pure sample. Additionally, after adding a known

amount of water, information is obtained about the degree

of gelatinization. The level of water used is of influence on

the gelatinization degree and peak shapes. Starch with low

and intermediate water content can show more melting

endotherms. The gelatinization information can be used to

determine the temperature and time necessary for e.g. rice

which is used in instant soups. If the rice has a too high

amount of gelatinization left in the product, this will result

in hard uncooked rice in the instant soup.

Most starches and rice products contain a lipid (fat) which

can form an amylose-lipid complex. This complex can be

formed during gelatinization.

It is also a thermo reversible

complex and should show an exothermic peak on cooling.

Sometimes the modification of the amylose with a lipid is

performed to control the texture of the final starch.

Figure 3. Native rice dry sample showing a retrogradation peak around 45 °C

and a gelatinization peak around 70 °C.

Figure 4. Native rice wet sample showing a gelatinization peak at around 70 °C

and some amylose-lipid complex at 112 °C.

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 calorimetry

studies of b’ → b transitions in fat mixtures, Szydlowsak-

Czerniak, A et al, Food chemistry, 2005, 92, 133-141.

4. Solid fat content determination: Comparison between

pNMR and DSC techniques, Nassu, R.T. et. al., Grasas y

Aceites, 1995, V46, N°6, 337-343.

5. Modern magnetic resonance (3rd edition), Graham A.

Webb, 2006, chapter Time-Domain NMR in quality

control.

6. Influence of tempering on the mechanical properties of

whipped dairy creams, Drelon, N. et. al., International

dairy journal, 2006, 16, 1454-1463.

7. AOCS Official Method Cj 1-94, Reapproved 2009, DSC

Melting Properties of Fats and Oils.

8. Carbohydrates in food, Eliasson A., CRC press, 2006.

9. Starch chemistry and technology (3rd edition), Bemiller

J., Whistler R., 2009, Chapter 8 and 20.

10. Texture in Food; Semi-Solid Foods, McKenna B., CRC,

2003.

11. The structural and hydration properties of heat-treated

rice 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 of

food systems, Kaletunc G., Wiley, 2009.

DSC is widely used to study glass transition phenomena.

The effect of water as a plasticizer on Tg was studied for

vegetable powders stored at different Aw values (humidity).

At a higher Aw value the samples take up more water. In

Figure 5 it is shown that the Tg drops to lower temperatures

as the amount of water in the sample increases. The knowledge

of Tg in combination with the water activity is important

in predicting the physical state of the powder at various

conditions, from free flowable to stickiness or phase transi-

tions to crystalline matter.

Proteins denaturation is also intensively studied by DSC. The

influences of pH, salt and polysaccharides were investigated

for food proteins.

Conclusion

DSC is an essential tool to reveal the underlying phase-

compositional principles of food systems. For systems with a

clearly established phase-composition-functionality relation,

DSC can contribute to the development of novel food products.

Figure 5. Water influence on Tg of tomato, the Aw 0.86 also shows an

endothermic peak which is due to the melting of free water.

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