Potato (Solanum tuberosum L.) starch functionality during crisp making

Potato (Solanum tuberosum L.) crisps are manufactured from dry potato derivatives, i.e. potato flakes (PFs). In the process, PFs are mixed with water and low levels of emulsifier into a loose, relatively dry crumbly dough which is then sheeted, cut into uniform pieces, and deep-fried. During deep-frying, the dough expands as a result of intensive water evaporation. This results in the desired crispy texture. The growing consumer demand for low(er)-calorie food has stimulated research to lower oil uptake during manufacturing of deep-fried foods. However, minimizing oil absorption without losing the characteristic palatability of deep-fried products is challenging and requires fundamental knowledge of the factors determining product texture and oil uptake. PFs are produced by boiling, mashing, and drum-drying steam peeled potatoes. Starch makes up about 80% of their dry matter (dm) and consists essentially of two glucose polymers, i.e. amylose (AM; 18-21% of dm) and amylopectin (AP; 79-81% of dm). During PF manufacturing, starch gelatinizes within the potato tissue. Drum-drying partially disrupts the potato cell walls. This causes some gelatinized starch to enter the extracellular space. The functionality of the gelatinized PF starch constituents during crisp making, i.e. how they contribute to gelation, water-oil dynamics, and texture development, are poorly understood. However, an improved understanding of (potato) starch functionality would allow better control of product texture and oil absorption. Against this background, this doctoral dissertation aimed to examine the functional role of starch in PFs during the production of crisps. In a first part, the molecular features of starch in commercial PF samples were related to their viscosity building capacity. An extraction method was developed to examine leaching of starch from potato cells into the extracellular space. The molecular architecture of this extracted starch fraction was characterized by high performance size exclusion chromatography (HPSEC). PF gelation in a Rapid Visco Analyzer (RVA) model system was positively related to the level of relatively short extractable AM (E-AM) chains [degree of polymerization (DP) 150-1,500; E-AM150-1,500]. Moreover, the use of PFs with higher E-AM150-1,500 levels led to an increased extent of AM crystallization during dough making and stronger dough sheets. These relatively short AM chains probably readily associate during cooling which swiftly results in a strong gel network and, hence, dough structure. These stronger water containing starch networks resulted in slower water evaporation during air drying. Moreover, temperature-controlled time domain (TD) proton nuclear magnetic resonance (1H NMR) showed that the strength of the starchy gel network dictates the rate of the heat-induced transformations during deep-frying. More in particular, strong gel networks (as a result of the use of PFs with high levels of E-AM150-1,500) released water at a low rate and delayed the accompanying transition of starch to the glassy state. This and the fact that strong dough structures can partly resist expansion during deep-frying resulted in dense crisps with a hard texture. Moreover, the limited expansion of the food matrix reduced pore size and the associated oil absorption of crisps made from strong dough sheets. Thus, crisp structure, texture, and lipid content are directly related to the molecular fine structure of extractable starch (E-S) in PFs. To elaborate on the functional role of (extractable) starch in PFs, different strategies were used. In a first approach, PFs were physically (i.e. ball milling) or enzymatically (i.e. application of cellulase from Trichoderma reesei) treated to disrupt their cell walls. Further cell wall opening increased PF swelling and the release of starch, mainly AP, from their cellular matrix. This directly improved their instant viscosity development and gelation capacity, respectively, in an RVA model. Dough sheets made from ball milled PFs were stronger and resulted in denser and harder crisps. It was shown that the release of additional E-AM150-1,500 increased the extent of AM crystallization during dough making. Furthermore, pronounced reduction in PF particle size may have improved AM entanglement and increased the strength of the starch gel. As dough sheets made from ball milled PFs were stronger, they showed reduced expansion during deep-frying resulting in crisps with a lower lipid content. Next, the PF starch fine structure was tailored in situ during crisp making by using endo-α-amylase from Bacillus subtilis (BSuA) and exo-α-amylase from Bacillus stearothermophilus (BStA). Treating PFs with BSuA improved starch extractability from the potato cells and produced E-S chains with a DP lower than 150. Addition of BSuA during dough making by prior treatment of a fraction of the PF dm in the recipe with BSuA drastically weakened the dough sheet and increased product softness. TD 1H NMR analysis and texture analysis of dough sheets showed that, although starch chains with DP lower than 150 readily crystallize, they do not form relatively strong gel structures. The high lipid content of crisps made from BSuA treated PFs could most likely be attributed to structure collapse during deep-frying. BStA use allowed increasing the E-AM150-1,500 content of PFs mainly by hydrolyzing larger E-S fractions. Treating a fraction of the PF dm in the recipe with BStA prior to dough making increased the level of AM crystallization during dough making and strengthened the dough sheet. This resulted in crisps with a hard texture and reduced lipid content. In a final approach, sodium (Na+), calcium (Ca2+), or aluminum (Al3+) chloride were added to PF suspensions. Monovalent ions reduced PF swelling due to shielding of the negative charges of the phosphate monoesters on potato AP. Divalent and trivalent ions had an even more pronounced negative effect on PF swelling as they established ionic bridges between adjacent AP chains. Ionic cross-linking of AP reduced its extractability from the cellular matrix while AM extractability was not influenced. As a consequence, while addition of Ca2+ during dough making did not affect AM crystal formation, it did strengthen the dough sheet as a result of ionic cross-linking of potato AP in the amorphous regions of the water containing starch network. This reduced its expansion during deep-frying resulting in dense crisps with a hard texture and lower lipid content. In conclusion, this doctoral dissertation increased the understanding of the functional role of PF starch during crisp making. It not only shows that there is a direct relationship between the molecular structure of starch in PFs and product quality, it also provides feasible strategies for tailoring product texture and lipid content by impacting starch properties. The results thus are a scientific basis for expanding PF functionality and better controlling crisp manufacturing practices.

Publication year:2020

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