Dairy and Food Engineering

Lesson 26. Changes undergone by the food components during Thermal Processing

26.2 CHANGES IN FOOD COMPONENTS DURING THERMAL PROCESSING

Although thermal processes may cause desirable changes to foods, like cooking and improvement of eating quality of foods, some heat-induced chemical and biochemical changes are undesirable, e.g., non-enzymatic browning, and vitamin, taste, texture, and colour deterioration. Most thermal damage reactions are described by first-order kinetics, similar to the inactivation of microorganisms and enzymes. The rate of thermal damage to food components is much slower than the thermal inactivation of the heat-resistant microorganisms and enzymes.

Important nutritional aspects of the thermal treatment of food are the inactivation of enzyme activities and the elimination/reduction and inactivation of microbial contamination and microbial toxins. These changes in general lead to an increase in the ability to store and use food. Thermal treatment also changes the physico-chemical structure of macronutrients, e.g. starches and proteins, with the generalized effect of a better gastro-intestinal digestion. Regarding the nutritional quality of heat-treated food one should discriminate between the effects on essential nutrients and non-nutrient bioactive components. While some water-soluble vitamins are heat-sensitive, e.g. vitamins C, B1, B2, B6, and folic acid, the lipid-soluble vitamins are not. A decrease in essential nutrients by heat treatment thus reduces the nutritional value of certain foods. Thermal treatment also causes changes in plant cell wall structure and thus modifications of the food matrix. These changes may lead to a marked increase in the bioavailability of bioactive food components, e.g. secondary plant metabolites (phytochemicals). Such effects have been demonstrated for carotenoids, e.g. β-carotene, lycopene, and lutein, but also for other substances like the isoflavonoids. By changing the tertiary structure of proteins thermal treatment may also lead to a change in epitopes responsible for allergenicity of certain foods. However, not all allergens can be inactivated by heat treatment.

However, during thermal treatment new compounds are formed and these compounds may have nutritional relevance. The Maillard reaction products are important for the colour and sensory properties of foods. The major nutritional benefits of thermal treatment of food are:

• Reduction/Elimination of Harmful Components

- Microbial Contamination/Parasites

- Toxins

- Enzyme Inhibitors

- Allergens

• Preservation of Food
• Changes in Food Matrix Structure and Texture

- Improvement of Digestibility

- Increased Bioavailability

• Generation of Beneficial New Compounds

- Aroma

- Antioxidants

For the nutritional quality of foods and the respective impact on health nutrients and bioactive non-nutrients should be considered separately. For the nutrients thermal effects on macronutrients and micronutrients are of importance and since the effects are rather different they will be discussed individually.

26.3 NUTRIENTS: MACRONUTRIENTS:

26.3.1 PROTEINS

Thermal treatment of proteins leads to denaturation and corresponding changes in three-dimensional structure, which in general leads to a better digestibility by the action of proteolytic enzymes in the human digestive system. Protein denaturation, on the other hand, also imposes a greater risk of rapid subsequent microbial contamination under poor hygienic or storage conditions, respectively, imposing the risk of food poisoning. Immunological properties of food proteins may be altered by heat denaturation as has recently been demonstrated with bovine β-lactoglobulin. Further, heat-denatured β-lactoglobulin was less efficiently transported than the native form.

26.3.2 COMPLEX CARBOHYDRATES

During thermal treatment, in the presence of water, starches, e.g. amylose and amylopectin undergo gelatinization and during subsequent cooling retrogradation, both processes greatly affect the digestibility of these compounds. The degree of gelatinization depends on temperature, proportion of water, and time of cooking. Gelatinized starches are digested much more rapidly than raw ones. With retrogradation resistant starches are produced which are poorly degraded by α-amylase and thus generally reach the large intestine, where microbial fermentation takes place. Resistant starches thus may affect the microbial flora composition and the microbial metabolism which may have consequences for gut health by changing for example the short-chain fatty acid patterns. Short-chain fatty acids (SCFA) and particularly butyrate are considered to be important substrates for colonic epithelial cells and relations between butyrate concentration and the risk for colorectal carcinogenesis have been observed. Thus foods which lead to a higher production of butyrate in the colon may have beneficial effects. Therefore the generation of resistant starches by thermal treatment of starchy foods may also be associated with beneficial nutritional and health effects.

26.3.3 FATS

The physicochemical properties of fats are highly affected by temperature. Unsaturated fatty acids (mono-unsaturated fatty acids: MUFA; poly-unsaturated fatty acids: PUFA) are rapidly oxidized in the presence of oxygen and in the absence of food antioxidants. Furthermore MUFA and PUFA may undergo isomerisation with heat treatment. Fat oxidation compounds in foods are nutritionally undesirable and have not been shown to have any beneficial effects.

26.3.4 AMINO ACIDS AND SUGARS

Amino acids either in free or in protein-bound form and sugars undergo Maillard-type reactions during thermal treatment and thus change the nutritional and sensory properties of food. Advanced glycation end products (AGE) in food have been demonstrated to show undesired nutritional and health effects, for example they seem to play a role in cellular signal transduction pathways involved in inflammatory processes, cell proliferation, tumor growth and metastasis.

 26.4 ESSENTIAL MICRONUTRIENTS: VITAMIN AND MINERALS

In relation to micronutrients it is known since several decades that particularly the water-soluble vitamins are rapidly degraded by thermal treatment. Thus thermal treatment of foods may cause a nutritionally relevant decrease of heat-labile vitamin content. The magnitude of these thermal processing losses of course depends on the processing variables, e.g. temperature and exposure time. The processing related losses of vitamins are creating nutritional problems only for those vitamins where the recommended dietary intakes in the population or in population subgroups are not achieved, this is of particular importance in specific subgroups with a high requirement, for example folic acid supply in pregnant women. Folic acid is one of the heat labile vitamins and is rapidly destroyed with increasing temperature. Furthermore the bioavailability of folates from natural sources is highly variable and may depend on the food matrix structure and stabilizing factors in the food to reduce folate degradation for example by thermal treatment. Most nutrition societies promote a high consumption of fresh fruit and vegetables and also encourage the use of frozen vegetables which maintain a high level of vitamins if properly processed and stored.

On the other hand the bioavailability of minerals and trace elements may be enhanced by changes in the food matrix upon heat treatment. A recent human dietary intervention study investigating the bioavailability of selenium from various food sources, however, could not find a difference in apparent absorption or retention of selenium from either cooked or salted fish, respectively. In general, information is lacking on the effects of thermal treatment on the bioavailability of essential micronutrients in human nutrition.

 26.5 THERMAL EFFECTS ON NON-NUTRIENTS

Non-nutrients are those bioactive compounds in food that do not seem to be essential for avoiding a specific disease or a clinical condition associated with a deficiency for a specific substance, e.g. a nutrient. However, non-nutrients may have important nutritional and biochemical functions by acting for example as antioxidants or having antimicrobial or anticarcinogenic properties. Thus the effects of thermal treatment on these food components are of importance and highly relevant to human health. There are a number of bioactive compounds in food and in the human diet, probably the largest group being the phytochemicals. Phytochemicals are secondary plant metabolites which are present in small and varying amounts in plants. Phytochemicals belong to different chemical classes among them carotenoids, flavonoids, isoflavonoids, phenolic acids, glucosinolates, monoterpenes, phytosterins, and saponins to name the most important and prominent classes.

26.5.1 POLYPHENOLS

Polyphenols are highly reactive compounds and good substrates for various enzymes, including polyphenoloxidases, peroxidases, glycosidases, and esterases. They undergo numerous enzymatic and chemical reactions during postharvest food storage and processing. The effects of thermal treatment on these compounds so far have not been studied in detail: Particularly the interactions between different bioactive compounds present in the same food during thermal treatment. Different polyphenols react differently to thermal treatment as has been shown for example in a study with virgin olive oils. Virgin olive oils contain hydroxytyrosol and tyrosol-like substances and they also contain the lignans 1-acetoxypinoresinol and pinoresinol. The lignans were much less affected by heating to 180°C for 25 hours, microwave heating for 10 minutes or boiling in a pressure cooker for 30 minutes than hydroxytyrosol and tyrosol-like substances. Thus thermal treatment differentially affects bioactive compounds present in the same food and thus very likely changes the nutritional characteristics of that food considerably.

26.5.2 ANTHOCYANINS

Anthocyanins are present in different fruits, particularly in berries, and vegetables. They are water-soluble plant pigments and they occur naturally as glycosides. The large variety of different anthocyanins results from the differences in glycosylation patterns. Some anthocyanins occur also in the acylated form, for example in the skin of red radishes or in purple carrots. Anthocyanidins, the aglycon forms of anthocyanins are chemically labile with respect to pH, temperature and oxidation. Anthocyanins are considered to be potent anti- oxidants, at least in vitro. However, several studies have investigated their bioavailability from different foods sources and have generally found a very low bioavailability. Thermal processing and high-pressure treatment was shown recently to have no effects on the stability of different anthocyanins from grape juices, but another study investigating the effects of processing on different strawberry products found a loss of 27–39% after pasteurization of juice or heating of nectar, respectively.

26.5.3 CAROTENOIDS

Carotenoids are also a large group of plant pigments. It is estimated that in nature about 500–600 different carotenoids exist. However, plants used for human nutrition contain only approximately 50 different carotenoids. The major carotenoids in the human diet are α- and β-carotene, lycopene, lutein and zeaxanthin. A general feature of carotenoids is their ability to quench singulett-oxygen and thus act as antioxidants. The main food sources of carotenoids are vegetables and fruits. In recent years the most studied carotenoids have been β-carotene, lycopene, and lutein. The most important dietary sources of lycopene are tomato and tomato products. Furthermore tomato products usually undergo intense processing including thermal treatment during their production. During thermal processing isomerisation of lycopene from all-trans-lycopene to cis-lycopene isomers occurs depending on the temperature and the time of heating. Thermal processing enhances the nutritional value of tomatoes by increasing total antioxidant activity; despite a decrease in vitamin C content with increased duration of processing at 88°C, lycopene content and total antioxidant activity increased. The bioavailability of lycopene has been shown to be much higher from processed tomato products (tomato paste) as compared to fresh tomato in a human dietary intervention study. These initial findings with tomato products have also been confirmed in a human dietary intervention study using lycopene-rich carrot products. It was found that lycopene uptake from carrots treated for 30 minutes at 130°C was increased 10-fold. A high consumption of tomatoes and tomato products leading to a high supply of lycopene has been shown in several epidemiological studies to be associated with a decreased risk of cancers and in particular of prostate cancer. Thus increasing the bioavailability of lycopene by thermal processing of tomatoes offers a great health benefits.

26.5.4 ISOFLAVONOIDS

Isoflavonoids, an example of bioactive phytochemicals and a group of compounds, are often also designated as phytoestrogens because of their structural similarity to human endogenous which has capacity to bind to human estrogen receptors. The main dietary isoflavonoids are genistein and daidzein and these compounds occur primarily in soy and soy products. Soy foods are traditional foods in Asian countries, particularly in Japan and China, however, in the past years they have become more and more accepted also in the Western world, particularly as a good source of vegetable protein as an alternative to animal protein. Soy protein has specific properties and a health claim in relation to cardiovascular health approved by FDA. The influence of thermal treatment on the stability of soy isoflavones, genistein and daidzein, was studied in model solutions. Rapid degradation of genistein and daidzein was observed at pH 7 and thermal treatment at 120°C for 20 minutes. It should be emphasized, however, that the degradation rates may be different in an intact food matrix as compared to a model system with isolated compounds.

26.6 OTHER THERMAL EFFECTS

The allergenicity of certain food allergens may be affected by thermal processing. Besides the allergens themselves, however, other modifying factors present in food may play an important role with respect to thermal treatment and heat inactivation of allergens. The presence of sugars in fruits may contribute to the thermostability of the allergenic activity of LTP (Lipid Transfer Protein) in heat-processed foods. In some foods, particularly in legumes, natural components such as lectins, amylase, and trypsin inhibitors are present that may adversely affect their nutritional properties. Because of their protein nature, enzyme inhibitors and lectins are inactivated under conditions leading to irreversible protein denaturation. Thus heat treatment may lead to the destruction of these antinutritional factors and thus may improve the nutritional quality of the respective foods.

26.7 CONCLUSION

The nutritional effects of thermal treatment of foods are very diverse. The effects of thermal treatment can be nutritionally beneficial for example by increasing the bioavailability of bioactive components from an altered food matrix. However, also some of these bioactive components, particularly water-soluble vitamins and some heat-sensitive phytochemicals, will be lost during heat treatment of foods. Although the heating of food has a very long history during the evolution of modern man still the knowledge about the nutritional consequences of chemical and physical changes in food induced by thermal processing are scarce. With the modern analytical methods, further understanding of the interactions between the various food components is become possible during thermal treatment and the respective consequences for nutrition and health.