Good quality food must be of the nature that consumer would always expect. The quality of the food combines all the aspects that build consumers perception of the product’s value. This includes aspects such as an appearance of the food: colour, shape, size, gloss, consistency. It also includes factors as chemical properties, texture, flavour and aroma. Food quality ensures consumers that the product is not contaminated, does not have any form of spoilage, discolouration and off-odours. Food quality is a manufacturing standard that also shows labelling and what ingredients are present in the product in terms of correct nutritional values, food allergies, diabetes, nutritional demands etc. (ORGANIZATION) The product that is delivered to the consumer must always meet requirements for its safety, quality and legality. Food Standard Agency has produced numbers of guidance and main food laws for example The Food Safety Act 1990 (First, 2011). In food science two main chemical processes affect food quality, the Maillard reaction and the lipid oxidation. Shelf-life is one of the aspects that have a significant impact on the quality of the food, especially in bakery products where the shelf-life is relatively short. The product’s shelf-life is determined mainly by the oxidation rate. Oxidation is a complex of reactions affecting mostly polyunsaturated fatty acids (Frankel, 1991). These reactions involve oxygen and lipid molecules forming species as reactive oxygen, peroxyl- fatty acid radicals and peroxides. Lipid oxidation in foods occurs during heating, processing and storage, and leads to formation of undesirable colour and flavour by rancidity, loss of quality and deterioration (Sproston and Akoh, 2016). Another chemical reaction that affects food quality is Maillard reaction. The Maillard reaction is a reaction between a reducing sugar and the amino group from amino acids (proteins or peptides) causing browning of foods by production of melanoids (Sproston and Akoh, 2016). This particularly involves desirable changes in aroma, flavour and nutritional values during processes like baking of bread, cooking of meat, roasting of coffee. From the other hand Maillard reaction can cause undesirable changes as loss of nutritive value of proteins that is related for example to decrease of digestibility, biological inactivation of amino acids and inhibition of proteolytic enzymes (Martins et al., 2000). It is also associated with the formation of toxic compound such as acrylamide (Van Boekel, 2006). Determination of lipid oxidation and Maillard reaction can be assessed by various methodology and parameter such as gas chromatography/ mass spectrometry, high-performance liquid chromatography, titration, peroxide value. Chemical reactions such as Miallard reaction and lipid oxidation in terms of their impact on quality of bakery products are going to be described in this report.
One of the most common method to measure extend of lipid oxidation is the Peroxide Value (PV). This methodology measures the peroxides R-OOH contained in the oil sample. It is obtained by titration of the oil dissolved in a suitable organic solvent with potassium iodine to liberate iodine. The iodine content is determined by titration against sodium thiosulfate. The AOCS has modified the methodology by using isooctane instead of chloroform as solvent (Steele, 2004). Fresh oils such as sunflower oil have peroxide value below 10 meq/kg. If the peroxide value is between 30-40 meq/Kg then the oil is considered as rancid. Another method for the determination of lipid oxidation is based on determination of thiobarbituric acid reactive substances. TBARS show the presence of lipid oxidation mainly in fried food. One of the secondary oxidation products are aldehydes that are responsible for products rancidity. The example of such aldehydes are malondialdehydes that are crucial markers in lipid oxidation that react with thiobarbituric acid TBA or thiobarbituric acid reactive species giving the results in colour of the compound. This further can be determine by gas chromatography mass spectrometry technique (Zeb and Ullah, 2016).
Maillard reaction products are ivolatile components and cannot be measured by gas chromatography method. The sample has to first undergo pyrolysis that requires rapid and controlled heating of the sample in temperature from 400-1000. This allows to separate any volatile molecules in GC column that can be further identified (Fayle and Gerrard, 2002).
Sugar is one of the most important ingredients in food industry, especially in sweet bakery products. Its main role is to preserve foods and to provide energy (Rodriguez et al., 2016). Under bakery conditions that require thermal processing reducing sugars participate in carmelisation and Maillard reaction to give desirable taste, aroma and colour. The reaction takes place between monosaccharides or disaccharides and compound with free amino group to produce N-substituted glycosylamine from aldose which reacts with a primary amino group of an amino acids. Maillard reaction is dependent on many factors: the type of reducing sugar and amino acid residue, the temperature, the pH, the water activity, buffers or the oxygen availability. (Newton et al., 2012) The glycosylamine forms the pH dependent Amadori rearrangement product, a 1-amino-1-deoxy-2-ketose (Zamora and Hidalgo, 2005). The Amadori compound can isomerise into different structures which then react in various ways. Depending on type of sugar each compound can react differently due to the position of the ring opening equilibrium in reducing sugar.
Figure 1. Showing Amadori compound formed from reducing sugar (glucose) and amino acid RNH2
source: Starovičová M., (2014) “The initial step of the Maillard reaction between glucose and an amino acid (RNH2), in which R is the amino acid side group”, www.food-info.net.
Depending on the pH the following degradation of the Amadori compound can undergo enolization, fragmentation or dehydration of the sugar giving products such as furanone via 2, 3-anolization (at higher pH) and furfural and hydroxymethylfurfural (HMF). Higher pH is suggested to enhance flavour formation (Martins et al., 2000). One of the formation of Amadori compound is Strecker degradation involving condensation of carbonyl groups with free amino acid groups and dicarbonyls with amino acids. This reaction forms alpha aminoketones and aldehydes. Set of other reactions follows formation of Amadori compounds that with condensation of some products produced will lead to formation of brown pigments called melanoidins.
Figure 2. formation of colour-forming compounds melanoidins
Source: Jongen W., Martins S., van Boekel M., (2000) A review of Maillard reaction in food and implications to kinetic modelling. Trends Food Sci Technol
The intermediates of Maillard reaction products are also carbonyl compound that are produced by addition of fat resulting in oxidation of lipids. Formation of carbonyl compounds enchances food flavour that can participate with other Mallard reaction prodcuts. Lipid degradation can decrease shelf-life producing rancid flavours and the presence of lipid oxidation products in Mallard reaction leads to producing antioxidants that removes rancind compounds extending the shelf-life. (Newton et al., 2012) Bressa et al. (1996) showed that browning products of Mallard reaction produced during the baking of buttered cookies have an antioxidative ability against peroxyl radicals (Bressa et al., 1996).
With the formation of Amadori compounds the decarboxylation of the products occurs where the amino acid asparagine molecules are formed. Amino acid asparagine goes through the condensation and forms the Schiff base, with the formation of N-glycosylasparagine. When the temepreture is high (over 120 degrees), for example during the cooking process, the Schiff base undergoes cyclisation producing oxazolidin-5-one. It is
found that the formation of oxazolidin-5-one leads to formation of acrylamide (Katsaiti and Granby, 2016). Acrylamide damages DNA froming DNA adducts and it is known as a genotoxis compound and carcinogen (Mottram et al., 2002). Labolatory tests proved that acrylamide causes cancer in animals and thus, it is a possible human carcinogen. Belan, et all. 2013 examined the possibility of the cancer in male and female mice and rats with acrylamide added to the drinking water for two years. They proved that acrylamide caused cancer in different organs of rats and male and female mice and thus, that the acrylamide is a genotoxic carcinogen (Beland et al., 2013). Bread, rolls, bisciuts and other bakery products contain relatively low level of acrylamide, and only high amounts consumed daily can cause the potential cancer risk. Concequently, the consumption of bakery products should be increased as much as it is possible.
Fats are one of the most important ingredients in bakery products. In food the main lipid components are triacylglycerols that contribute to foods flavour, palatability, mouthfeel, texture, colour and aroma. Shelf- life of the food is reduced during the heating, storage or processing by lipid oxidation which causes reduction of food quality. In this process important is content of polyunsaturated fatty acids that are regarded as healthy for consumers, but also take part in oxidative reactions (Jensen et al., 2011). Unsaturated fatty acids decompose to volatile products that contribute to production off-aromas decreasing the nutritional value of the product and causing rancidity of the food. Lipid oxidation consists of three stages: initiation, propagation and termination. The initiation stage is responsible for the break of covalent bonds by the heat and formation of free radicals. These are a highly reactive fatty acid molecules that contain unpaired electrons (Coultate, 2009). Free radicals are highly reactive and live for a very short time as they try to find a partner for their unpaired electron. Free radicals can also be formed by initiation of the reactive oxygen species that are a by-product of oxygen metabolism. They are catalysed by heat, light or other free radicals and peroxides and can be inhibited by antioxidants (Frankel, 2014). Reactive oxygen species are responsible for the damage to the cell structure and they level increases with the exposure to heat or UV. Unsaturated fatty acids produce destructive products such as alkyl radical (RO•), hydroxyl radical (HO•) and hydroperoxyl radical (HOO•) that subsequently react with oxygen and form peroxyl radicals (Saldaña and Martínez-Monteagudo, 2013) (dots indicates unpaired electron). In the propagation stage oxygen generates peroxyl (ROO•) and aloxyls radicals through degradation of hydroperoxides and primary oxidation products. In hydroperoxides the reduction of the hydroperoxyl group forms ketones. Another products formed via cleavage of the hydroperoxide chain are aldehydes, alcohols and hydrocarbons that are responsible for the off-flavour and rancidity. In results the increasing number of free radicals is formed where one reacts with one other and form stable end-products that are known as termination products (Coultate, 2009).
Source: ALDAÑA, M. 2013. Oxidative stability of fats and oils measured by differential scanning calorimetry for food and industrial applications.
There are many different factors that can affect lipid oxidation such as the temperature, oxygen, type of fat, UV, metals or pH. The temperature is the crucial factor that affects the rate of reaction. The rate of reaction increases when the temperature increases causing reduction of the induction period and decomposition of lipid peroxides into secondary oxidation products. Oxygen is necessary for oxidation to occur. It can be replaced by other gas for example by nitrogen to reduce the oxidation level. However, the decomposition of lipid hydroperoxides does not require oxygen and thus, production of off-flavour products will continue regardless the oxygen presence. Depending on type of fat the oxidative rancidity may differ. Generally, more unsaturated fatty acids are present in softer fat yielding in higher oxidative rancidity. Another factor that influences lipid oxidation is presence of metals such as copper, iron, chromium that can catalyse oxidation by two processes, the electron transfer and the decomposition of hydroperoxides. Metals can speed up the rate of reaction producing free radicals and secondary volatile oxidation compounds. Metal ions solubility mainly depends on pH causing that lipid oxidation increases in lower pH and decreases in higher pH (Jacobsen, 1999).
The autoxidation of free radicals can be retarded by antioxidants activity which are known as preserving agents. Antioxidants radical reacts with peroxyl radical and in result forms stable peroxides. One of the most important antioxidants are polyphenols that are also anti-inflammatory, antiviral, anticarcinogenic and anti-microbial activity (Frankel, 2014). Antioxidants can neutralize free radicals and retard staling of the bakery products. Kozlowska et al (2015) found that addition of thyme extract, known as a natural antioxidant, to the bakery cookies (with ratio bakery shortening/ sunflower oil 1:1) caused inhibition of free radicals formation during lipid oxidation. This result was comparable to the sample with added synthetic antioxidant BHA (Zawada et al., 2015).
Reactions that occurs during the lipid oxidation can be measured by the differential scanning calorimetry DSC that measures the oxidative temperatures and kinetic parameters to assess the oxidative stability. Other method to assess the oxidative stability of the sample is peroxide value (PV).
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