The glycaemic index of fresh and processed potatoes

Potatoes are an important staple food, which provide vital nutrition to millions of people globally every year. However, potatoes have a relatively high carbohydrate content as well as being generally considered as a high Glycaemic Index (GI) food. Research suggests that the consumption of potatoes or high GI foods can contribute to the onset of certain chronic conditions such as cardiovascular disease and type II diabetes. The supposed link between potatoes and negative health outcomes has led to a decline in their consumption in the developed world. The aim of this thesis was to assess the GI, glycaemic load (GL), and carbohydrate parameters of selected potato cultivars using an in-vitro method. The main objectives of the research were to employ various interventions in an attempt to reduce the GI and GL of the potatoes. Five potato cultivars commonly consumed in Ireland (Cultra, Gemson, Kerr’s Pink, Maris Piper, and Rooster) were tested. All varieties were found to have a medium or high GI. Evidence suggests that certain types of processing can reduce GI therefore each of the potato cultivars were then subjected to high pressure processing (HPP) at either 400Mpa or 600Mpa. In the potatoes which had the highest GI a decline in the GI was observed as the pressure was increased, however this decrease in GI was not seen in the potato varieties with a lower GI. Roosters were selected for further testing as they are the most widely produced and consumed potato variety in Ireland. To determine the effect of added fat on GI, a dose response was conducted by adding 10%, 15%, 20% or 25% (w/w) of rapeseed oil to potato and measuring GI. The GI of potato was also assessed following addition of butter, coconut oil, or olive oil at a concentration of 10% (w/w). No significant changes in GI or GL of the potato were observed following the addition of fat at any concentration; nor did the degree of saturation of the added fat impact GI or GL. The impact of combining Roosters with either cheese, peas, beans, or tuna on the GI of the resultant meal was investigated. Each meal consisted of 50g of available carbohydrates. Roosters alone had a medium GL, this was reduced to a low GL when they were included as part of a meal. The GI was also reduced for every meal in comparison to Rooster alone, apart from the potato-tuna meal. The greatest decrease in the GI was observed when Rooster were combined with beans which are a rich source of fibre. Consequently, the addition of three fibres; pectin, HPMC (hydroxypropylmethylcellulose), or inulin was investigated as a means of reducing the GI of Rooster potatoes. The fibres were tested at three concentrations 5%, 7% or 10% (w/w). The viscosity of the digesta as well as the carbohydrate parameters were measured. Pectin induced the greatest reduction to the GI out of all three fibres, whilst causing the highest increase in viscosity. HPMC had a similar but less pronounced effect, whilst inulin did not affect the GI. Finally, the impact of HPMC on carbohydrate parameters in sweet potatoes as well as the antioxidant potential of both Roosters and sweet potatoes was investigated. Ferric reducing antioxidant potential (FRAP), Oxygen radical absorbance capacity (ORAC), total phenol content (TPC), as well as ascorbic acid content were quantified. The addition of the fibre caused reductions in the FRAP and ascorbic acid content of the tubers however, ORAC and TPC values remained unchanged. Overall, our findings have identified methods to potentially reduce the GI of potatoes which could be useful for the food industry and have also demonstrated that in-vitro methods can be a convenient tool for the determination of GI and GL in potato and potato-based meals