Jan 20th Processing and GI.pptx

Impact of Processing in Plants Dr Marie-Ann van Ginkel Why is this important? • Recommended we eat around 50% of our energy from carbohydrates • Carbohydrates are the main energy store for a plant. • Very little carbohydrate is of animal origin • milk sugar, lactose, is an exception Carbohydrate...

Impact of Processing in Plants Dr Marie-Ann van Ginkel Why is this important? • Recommended we eat around 50% of our energy from carbohydrates • Carbohydrates are the main energy store for a plant. • Very little carbohydrate is of animal origin • milk sugar, lactose, is an exception Carbohydrates • By definition are made from sugar units BUT both starch and cellulose have the same chemical formula both being long chains of glucose units. • Our enzymes can break down starch but NOT cellulose. The way the glucose is bound to each other is slightly different. • The structure, NOT the chemical formula is what is important in carbohydrate digestion and utilisation. • Juice (sugars) or starch are stored inside each cell depending on the plant • The cell walls give plant foods their shape • Dietary fibre comes from plant cell walls. • Unlike animal cell walls, we do not fully know the structure of plant cell walls What happens when you eat a plant? • The assumed size of a chewed bite of food is 2mm x 2mm. • For an apple this would contain over 8000 cells. • Only those on the surface can be broken through chewing leaving a minimum of 5600 intact cells that are swallowed. • In some foodstuffs such as legumes (peas and beans), cells don’t break open during chewing • Anything inside the cells needs to escape through the multiple layers of plant cell walls to reach the intestinal lumen so it can be digested • Since sugars and starch are inside this is quite a barrier to get through • If you remove or break the barrier through processing then starches and sugar become accessible • Mill wheat grains and all walls inside the endosperm of each wheat grain are broken down. The stored starch is freed. • Crush apples for juice and walls are removed allowing easy access to the sugars inside the cell. • Bake a potato many cell walls burst with the starch swelling due to gelatinisation • Mash a potato and you break open cells • The quicker the starch is broken down to glucose and glucose is absorbed, the quicker glucose will reach the blood stream. Glycaemic Index • Is a ranking using a scale from 0 to 100 according to how much they raise blood sugars • ≥70 is high, the carbohydrate is rapidly digested and absorbed • ≤55 is low, the carbohydrate is slowly digested and absorbed • Cohort studies show a strong association between regular consumption of low glyacaemic index food and decreased risk of chronic disease including T2D. • Choosing foods with a low GI helps with management of diabetes as it helps reduce the blood glucose high excursions • It is a black box measure but it is reproducible between populations • Glycaemic index is a measure of how quickly carboydrate is absorbed • The slower the better • A low GI diet can be used to help control blood sugar levels • GI is not related to total dietary fibre content or to the polysaccharide content • GI can be an indication of the amount of processing Boiled Potatoes 72 Mashed Potatoes 90 In Summary • Carbohydrates are an important source of energy • Sugars and starch naturally occur within plant cell walls • Remove or break down the walls that contain these will lead to quicker absorption and therefore quicker rises in blood sugars • Glycaemic index is a useful measure of how quickly blood sugars rise after a meal.