Enhancing the hardness of potato slices after boiling by combined treatment with lactic acid and calcium chloride: Mechanism and optimization

Highlights

• The hardness of boiled potato was significantly increased through the combined use of LA and CC.

• Pre-soaking of the potato slice in LA promoted gelation of the pectin.

• Pre-soaking of the potato slice in LA and CC inhibited PG activity.

Abstract

Potatoes usually suffer from greatly decrease of hardness after boiling, which limits their processing potential in food industry. Moreover, methods for enhancing the hardness of potatoes after boiling are underexplored. In this study, the hardness of potato slices after boiling were increased from 288 g to 2342 g by the combined treatment of lactic acid (LA) and calcium chloride (CC). Through the analysis of the microstructure of the potato cells, the molecular weight distribution and natural sugar ratio of different soluble pectin fractions, and the enzymatic activities (polygalacturonase, PG and pectin methylesterase, PME), the possible mechanism behind the hardness enhancement by LA and CC pretreatment, namely the direct link between pectin and potato structure was revealed. The obtained results confirmed the target spot for enhancing the hardness of potatoes after boiling lay in PG activity and gelation of the pectin, which also could be used to help other plants resist the heat process if pectin existed in their cell wall.

Introduction

Potato (Solanum tuberosum L.) is a tuber widely consumed throughout the world typically in forms such as mashed potatoes, potato soups and salads, French fries and potato chips (Willard, 1993). Heat processing of potatoes in oil (frying) or water (boiling) before serving on the table is necessary to gelatinize the starch. The different evaporation rates of water at the surface and in the core of potato strips during frying make the outer shell hard and the inner core soft (Bouchon, 2009). While, boiling makes the bulk of the potato soft for preparing mashed potatoes. Thus, the processing of potato was limited because potato easily became soft after heat processing. In China, there is popular snack in Sichuan and Chongqing province called “leng chuan chuan”. This snack is prepared as follows. The vegetables and meat are first boiled and then immersed in a mixture of oil, flavouring and spices. After a few hours, the vegetables and meat absorb the taste compounds from the mixture, and the snack is ready to serve. However, it is strange that the potato slices are crisp after boiling (i.e., their soft mouth feel is lost). The secret of the recipe is soaking the potato slices in the pickle water (fermented juice in pickle) to obtain such a unique texture before boiling. The dominant microbe in pickles is lactic acid bacteria (LAB) (Liu, Han, & Zhou, 2011). Lactic acid (LA) is the main product of LAB metabolism and is responsible for preserving and improving the texture of vegetables (Leroy & De Vuyst, 2004). Therefore, exploring whether LA is involved in improving the texture of potato slices after boiling is reasonable.

Normally, significant changes in potato texture occur a few days after slicing because the integrity of plant cells decreases, and the endogenous enzymes can act on the plant cell walls (decomposing the structural components) (Rocculi et al., 2007). However, the texture changes quickly when potato slices are heated, and these changes can be attributed to two factors: starch and pectin. The gelatinization of starch granules during heating caused earlier researchers to believe that starch swelling was the reason that the potato texture changed more quickly. Jarvis and Shomer proved that the swelling of the cell wall and the starch in the potato cells impacted the texture of the potato (Jarvis et al., 1992, Shomer, 1995, Shomer et al., 1995). However, subsequent studies proved that there were other factors in addition to starch gelatinization that affected the texture of the potato tissue after boiling. Ng found that heat-induced softening of potato tissues resulted in the solubilization of pectic polysaccharides (Ng & Waldron, 1997). Parker found that the edge-of-face structures could be strongly labelled with JIM5 but not with JIM7 (JIM5 and JIM7 were anti-homogalacturonan monoclonal antibodies, which linked with esterified and non-esterified pectin), indicating that the faces contained polygalacturonic acid with a low ester content (Parker, Parker, Smith, & Waldron, 2001). In addition, the adhesion of the middle lamella to the face of the primary wall differed from the adhesion at the edge of each cell face. Ross found that when potato tubers contained higher levels of total pectin methylesterase (PME) activity, the degree of methylation of pectin in the cell wall was lower and consistently higher values of peak force and work done were observed during the fracture of cooked tuber samples (Ross et al., 2010). Bordoloi found that the cell walls of parenchyma cells partially degraded upon boiling, resulting in loosening of the cell wall microfibrils (Bordoloi, Kaur, & Singh, 2012). Ormerod considered that the weakening of the potato tissue upon boiling was primarily controlled by thermal degradation of the middle lamella (Ormerod, Ralfs, Jobling, & Gidley, 2002).

Taken together, most researchers agree that the texture change in potatoes after boiling is caused by changes in the polysaccharides in the cell wall, such as pectin degradation. Namely, the change of potato structure was accompanied with the change of pectin during or after heat processing. However, why the change of pectin take place and how pectin influence the potato structure during or after heat processing is still a mystery. Through the tracking of the change of pectin chemical structure and relevant enzyme activity, hardness enhancement after boiling of potato slices pre-soaked with LA was elucidated in this study. Besides, calcium could affect the fruit quality and preservation based on the knowledge that calcium in cell could interact with the uronic acid carboxyl functions in pectin polysaccharide chains to create the so-called pectin ‘egg-box’ to maintain the cell wall stabilization and integrity (Vicente, Saladié, Rose, & Labavitch, 2007). In order to maximumly enhance the hardness of potato after boiling, calcium chloride (CC) in combination with LA was used in the optimization experiment to achieve the greatest hardness of potato slices after boiling.