Estudios relacionados con las concepciones de currículo

1.5.1.1. CHO chemistry, dietary sources and digestion

Carbohydrates are a neutral compounds that contain carbon, hydrogen and oxygen with the formula (CH2O)n, where n is three or more. Carbohydrates are classified into simple

sugars and non-sugars (McDonald et al., 2011). Simple sugars are subdivided according to the number of carbon atoms presents in the molecule, into monosaccharides and oligosaccharides (Whistler and Smart, 1953). Monosaccharides include trioses, tetroses, pentoses, hexoses and heptoses, whereas oligosaccharadies include disaccharides, trisaccharides and tetrasaccharides (Mussatto and Mancilha, 2007). Non- sugars are divided into polysaccharides (glycans) and complex carbohydrates. Polysaccharides or glycans are polymeric carbohydrate molecules consisting of a long chain of monosaccharide units bonded together by the glyosidic bonds (Mussatto and Mancilha, 2007). These are divided into two groups, homoglycans that have only one unit of monosaccharide (starch, cellulose, glycogen), and the heteroglycans which contain more than one type of monosaccharide unit (pectic substances and hemicellulose) (Cui, 2005). The complex carbohydrates include those compounds that contain CHO and non- carbohydrate molecules (lipids and proteins). These types include glycolipids and glycoproteins which have structural and biological importance (Whistler and Smart, 1953). The primary carbohydrate sources of ruminants are fibrous feeds which contain a variable amount of cellulose, hemicellulose, starch and water soluble carbohydrates (WSC) (Mussatto and Mancilha, 2007). Young pastures contain approximately 400 g/kg DM of cellulose and hemicellulose and 200 g/kg DM of water soluble carbohydrate (WSC), whereas, mature pasture, straw and hay, contain a higher content of cellulose and hemicellulose and a lower content of WSC (Coleman et al., 2002). The proportion of lignin in ruminant diets varies between 20 to 120 g/kg DM (Coleman et al., 2002). Its concentration limits digestibility of diets as it is indigestible by rumen microbes. The rumen microorganisms mainly Fibrobacter succinogenes, Ruminococcus flavefaciens, and R. albus, and also anaerobic fungi attack carbohydrates (Hungate et al., 1997). In general, >90% of digestible CHOs are digested in the rumen, and 10% are digested in the small and large intestine (Nocek et al., 1991). Digestion of CHOs in ruminants is divided into two stages (Niwiska, 2012); first is the breakdown of complex CHOs into simple sugars by extra- cellular microbial enzymes, with the second stage being digestion and metabolism of simple sugars which is similar in many aspects to the metabolism of CHO by the animal itself (Niwiska, 2012)(Figure 1.7).

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Cellulose is hydrolysed into cellobiose by β-1, 4-glucosidases, which is then converted either to glucose-6-phosphate by the action of phosphorylase or to glucose. Starch is first converted to maltose and isomaltose by amylases, then both of them are converted to either glucose or glucose-6-phosphate by maltases, maltose phosphorylases or 1, 6- glucosidases.

Figure 1. 7. Conversion of CHOs into pyruvate (Baldwin, 1995).

Fructose is produced from the digestion of sucrose and fructans by the enzymes attacking 2, 1 and 2, 6 linkage of fructans. Pentoses are mainly produced by enzymes attacking β-1, 4 linkages of hemicellulose. Pectins and pentosans are also converted indirectly into pentoses by involved enzymes pectinesterase and polygalacturonidases (Whistler and Smart, 1953).

The intermediate product of the first stage of CHO digestion is pyruvate which is a precursor of the endproducts of rumen CHO digestion which are; acetate, propionate, butyrate, carbon dioxide and methane (Figure 1.8). Small quantities of additional VFAs are also produced such as valerate, isobutyrate, 2-methyl butyrate and 3-methyl butyrate by deamination of proline, valine, isoleucine and leucine, respectively (Nafikov and Beitz, 2007).

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Acetate is produced from pyruvate through a pathway of acetyl phosphate with methane and carbon dioxide also being produced. Propionate is produced from pyruvate through two alternative pathways. First, through lactate when the proportion of concentrate in the diet is high or through a second pathway, where succinate is involved when the diet is rich in fibrous forage (Bergman, 1990). Lactate produced from the first pathway may accumulate in the rumen when the diet is too high in concentrates and can lead to acidosis (Harmon et al., 1985). The volatile fatty acids (VFAs) produced from microbial fermentation in the rumen are absorbed through the rumen wall.

Figure 1. 8. Conversion of pyruvate into volatile fatty acids (Dijkstra et al., 1993).

The relative proportions of VFAs very according to the source of CHOs. Generally, the proportions of acetate: propionate: butyrate ratios derived from hexoses are 65: 21: 14, respectively (McDonald et al., 2011). However, these ratios differ from the actual ratios as some amino acids are also fermented in the rumen (Suárez et al., 2006). The total concentration of VFAs in the rumen varies according to the ruminant’s diet and time of feeding as the absorption of individual VFAs is different (Doreau et al., 1997). In general, the relative molar proportion of acetic: propionic ratio is 70: 20 for ruminants fed mature herbage. This ratio is reduced to 60: 30 when the ruminant fed on a less mature herbage and especially with diets high in concentrates (60%) (McDonald et al., 2011; Dijkstra et al., 2012).

1.5.1.2. Effects on meat quality

It has been found that feeding sheep with different sources of carbohydrates can modify the chemical composition and eating quality of meat (Fraser and Rowarth, 1996; Díaz et al., 2002). Olfaz et al. (2005) studied lambs fed on control diet (60% commercial concentrate + 40% grass hay) or a mixture of 40% and 60% sugar beet pulp that was partially substituted with grass hay. It was reported, that the inclusion of 60% SBP significantly reduced stearic, oleic and arachidonic acids, but increased the palmitic and linoleic acid content of LD

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muscle compared to the control diet (Olfaz et al., 2005). It was also found that the ultimate pH and cooking loss decreased and lightness increased compared to the control diet (Olfaz

et al., 2005). However, dietary inclusion of SBP did not affect sensory attributes reported by a trained taste panel.

Oliveira et al. (2017) evaluated the effect of different starch levels (mid 35 % and high 50% DM) and rumen degradable starch (mid 70% and high 80%) on the chemical composition of lamb meat. Meat from lambs fed on high starch had a lower shear force value compared to mid degradability starch (Oliveira et al., 2017). The total lipid content of meat was not affected by treatment, however, saturated FA and cis MUFA increased, and trans MUFA decreased in lambs fed on the mid starch diet (Oliveira et al., 2017).

Pre slaughter muscle glycogen stores have been recognised to be crucial for meat quality characteristics (Immonen et al., 2000). Lambs fed on pasture and concentrate have different levels of glycogen (Santé-Lhoutellier et al., 2008). A high ultimate pH is primarily found in undernourished animals as these animals are unable to store sufficient glycogen reserves in muscles (Pethick et al., 1999). Ruminants fed on pastures, which are typically low in starch and rich in fibre, and where the ratio of acetate: propionate is high tend to have a lower muscle glycogen content than those raised on concentrate diets which are rich in starch, and where the ratio of acetate: propionate is low (Martin et al., 2004; De Brito et al., 2017a). Propionate is a glycogenic VFA, therefore, pasture finished animals generally have a higher ultimate pH than concentrate finished animals, although usually within the normal range (Priolo et al., 2002). There is a high correlation between muscle ultimate pH and meat colour (Calnan et al., 2016). Feeding ruminants with a high level of digestible carbohydrate sources or sugars for a few weeks or days pre slaughter has been shown to increase glycogen stores in muscle and reduce ultimate pH (Andersen et al., 2005).

Sheep fed on concentrate diets (rich in starch) tends to have increased levels of branch chain FAs, especially 4-methyloctanoic acid in muscle (Sinclair, 2007). This FA is related to the pastoral flavour and species flavour characteristics of sheep meat, that also tends to be higher in rams than castrates (Young et al., 2003). This is caused by changes in the ruminal fermentation patterns that result in an increase in propionate and the oxidative deamination of branched chain amino acids (Young et al., 2003).

In an experiment investigating the effect of replacing cereal concentrates with dried citrus pulp (24% and 35%) on the shelf life of lamb meat, Inserra et al. (2014) reported no treatment effects on ultimate pH and lightness, but redness, yellowness, chroma and lipid oxidation values all reduced after 4 days of ageing in vacuum pack. This was attributed to the presence of a high content of phenolic compounds in dried citrus pulp rather than the effect of carbohydrate sources. In addition, Caparra et al. (2007), reported that the inclusion of dried citrus pulp (30% and 45%) did not affect the chemical analysis of meat.

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