Effect of a new canning process on cell wall pectic substances, calcium retention and texture of canned carrots

A new process was developed for the canning of carrots. The main features of this process are: filling the cans with carrots and a minimum amount of brine (ca.8.0\% of the canned carrots), vacuum sealing, preheating at a certain temperature and time (67 degrees C for 30min) followed directly by sterilization. Quality attributes of the resultant canned carrots were superior to that conventionally canned in respect to texture, colour, flavour and nutritional value. In this research the effect of preheating time (15, 30 and 60 min) followed by sterilization as well as the comp arisen between the new and the conventional canned carrots was studied with regard to the texture, Ca++ retention and cell wall pectic substances. Texture of canned carrots was improved when the time of preheating was increased, while the amount of Ca++ bound in the cell wall was not significantly different from that of fresh carrots. Degree of methylation(DM; measured by HPLC) of total pectin reduced when the time of preheating was increased. This demethylation was reflected on the DM of the oxalate-soluble pectin, while that of the water-soluble pectin remained unchanged. Conventional blanching did not alter the DM of total pectin which was lowered after sterilization. Texture of conventionally canned carrots was considerably lower than that produced by the new process. Addition of CaCl2 resulted in similar improvement in texture of carrots canned by both processes. However, the amount of Ca++ retained in the cell wall material of conventionally canned carrots was twice as much as that found in carrots canned by the new process. Both canning processes resulted in the conversion of the major part of acid-and alkali-soluble pectin into water-and oxalate-soluble pectin. This conversion was accompanied by alteration in DM, degree of acetylation and in the amount of individual neutral sugars. The results suggest that the improvement in texture of canned carrots can be partly ascribed to the activation of the native pectinesterase during preheating, consequently the quantity of free carboxyl groups increased which were cross-linked with Ca++ Additionally, the improvement in texture was correlated with higher amount of alkali soluble pectin fraction.