**4. Discussion**

Of all the raw materials, tomato pomace powder (TPP) contained the highest fat, ash, and crude fiber content. The total phenolic content (TPC) was maximum for finger millet flour (FMF), followed by TPP and potato flour (PF) while TPP had the maximum total flavonoid content (TFC). The same pattern was observed for the DPPH radical scavenging activity. TPP retains its bioactive potential even after the tomatoes have undergone processing [3,37]. TPP also has maximum content of iron and zinc, while FMF is a rich source

of minerals, especially calcium, phosphorus, potassium, and magnesium. An estimated 17.3% of the world population is at risk of inadequate zinc intake [38] which is important for the immune system and metabolic activities. A combination of these raw materials in product development will help to meet the requirements of these micronutrients, which have established implications in various metabolic processes and health benefits, in diet.

The addition of TPP had a defined effect on the color values of the RTC-GF snack (Figure 2). The a\* values, which depict the redness of the product, increase significantly (*p* ≤ 0.05) with the gradual addition of TP due to the presence of lycopene in it. A higher a\* value hinders the acceptability of a sample by consumers [39,40]. Hue angle is used for perception of color while chroma indicates the degree of departure of a color from a grey of the same lightness [25]. The increase in L\* values with frying may be attributed to the darkening resulting from Maillard's reaction products [41].

The frying time decreased with the increase in the TPP levels in the snack (Figure 3a). This might be attributed to the fact that the crust achieved a brown color faster due to the higher lycopene content, which in turn is due to an increased percentage of TPP, which might affect one's perception of the quick cooking time, due to the darkening of the crust. A similar trend was also observed for the cooking loss per cent and oil up-take per cent of the snack (Figure 3b). This could be due to the increased moisture binding capacity of TPP, attributed to its fiber content, as evident by the highest cooking loss in T1 with no TPP. Oil uptake by a fried food is primarily due to the formation of pores by evaporation of water from the food surface [42]. The uptake of absorbed oil in food can range from 4 to 14% of the total weight, depending upon the food and type of frying medium, as reported by Andrikopoulos et al. [43] and the results are in conformity with this observation. A number of factors, like the time in and temperature of the oil, type of oil, and the shape and surface of the food and coatings, affect the oil uptake in fried foods [44]. Surface starch gelatinization due to the added gelatinized finger millet paste might have formed a layer that protects the food from oil absorption, as was reported by Califano and Calvelo [45]. Low oil absorption has also been reported by Kim et al. [46] by increasing the addition of preharvest-dropped apple pomace in instant fried noodles. Oil uptake while frying the snack was observed to be less compared to other fried products. This may be due to the reason that since the snack has been formulated with raw materials which have already undergone a preliminary cooking procedure, the chosen frying temperatures were high, both for par-frying as well as finish-frying. Moyano and Pedreschi [47] and Rojas-Gonzalez et al. [48] have argued that lower frying temperatures result in longer frying times and higher oil uptake. The acceptability of the ready-to-cook snack is attributable also to the crispness of the crust that developed due to deep frying, which is a factor of quality and freshness [49]. The increase in hardness may be attributed to the increase in fiber content of the product as well as the decrease in oil content of the fried snack as the level of TPP supplementation increases.

The lower scores for appearance are probably due to the dark color of the crust, which in turn is due to the lycopene content and dark color of FMF. The texture scores also showed a decrease after 15% supplementation with TPP, which might be attributed to the increased fiber content as well as less oil absorption by the snack. The snacks with 10% TP supplementation showed maximum overall acceptability, due to having highest scores for appearance, taste, and texture; the taste parameter had decreased scoring for supplementation beyond 10% due to the increased sourness of the product because of the tomato pomace. Based on these results, the RTC-GF snack with 10% TPP supplementation (T3) had the highest overall acceptability, and was therefore taken up for further storage studies.

FTIR spectroscopy helps to identify the presence or absence of specific functional groups as well as to support the results of chemical analysis [50]. The selected RTC-GF snack (T3) was subjected to FTIR spectroscopy (Figure 5a) for composition determination. The presence of a dominant peak at 2925 cm<sup>−</sup><sup>1</sup> confirms the presence of carbohydrates, being due to aliphatic C-H stretches [51], while the presence of phenolic compounds is confirmed by dominant peaks at 1376.6 cm<sup>−</sup><sup>1</sup> and 1378.5 cm<sup>−</sup><sup>1</sup> which are due to the presence of CH3 stretch. Similarly, work by Jebitta et al. [52] had a peak at 2931 cm<sup>−</sup>1,

which they reasoned was associated with asymmetric and symmetric stretching modes of alkane C–H. The peak at 1261.1 cm<sup>−</sup><sup>1</sup> and 1241.4 cm<sup>−</sup><sup>1</sup> denotes an asymmetrical C-O-C stretch asserting the presence of phenolic compounds. The absorption bands at 1013.5 cm<sup>−</sup><sup>1</sup> and 1011.3 cm<sup>−</sup><sup>1</sup> are due to C-C and C-O stretching in addition C-O-H bending which are attributed to the structural changes in starch [53]. These bands arise mainly from carbohydrates of cellulosic origin [54]. The peak at 1745.2 cm<sup>−</sup><sup>1</sup> is due to C-O (esters) which indicates lipid characteristics [52]. The absorption bands in the range of 3290 cm<sup>−</sup><sup>1</sup> to 3373 cm<sup>−</sup><sup>1</sup> indicate the presence of hydroxyl groups and denote the OH bond stretch [55] which shows a decrease in percent transmittance in the finished product because of a loss of moisture in the sample product due to the extended frying time and high temperature. Similar data were reported by Gowthamraj et al. [56] and Gull et al. [57]. The presence of an absorption band at 1376–1378 cm<sup>−</sup><sup>1</sup> (C-CH3 stretching) due to the CH bending vibration indicates the presence of cellulose and hemicellulose chemical structures which are the established components of tomato pomace [58]. Some stretching vibrations at 2854 cm<sup>−</sup><sup>1</sup> and 1745 cm<sup>−</sup><sup>1</sup> might be due to presence of non-starch constituents such as protein and fat [59]. The presence of phenolic compound peaks in the finished product confirms the positive contribution towards antioxidant potential of the final product. Most of the functional groups are retained in the RTC-GF (T3) product, as confirmed from the FTIR spectra of the finished product.

The complete nutritional profile of the RTC-GF snack (T3) and the control snack (T1) is enlisted under Table 3. Most of the vegetarian ready-to-cook frozen snacks available in the market have a protein content in the range of 2 to 3% whereas the RTC-GF snack (T3) has a protein content which is more than double this (6.9%) along with an adequate amount of fiber. As opposed to T1 having no TPP, the RTC-GF snack (T3) delivers a substantial amount of lycopene, which has 10 times the singlet-oxygen-quenching ability as that of α-tocopherol [60]. The processing temperature enhances the absorption of lycopene by altering the trans isomers to cis form [61], which is further enhanced by the addition of oil in the product, since lycopene is fat soluble. The enrichment with TPP in T3 resulted in a higher TPC and TFC of the snack as also reflected in significantly higher antioxidant activity (as DPPH, FRAP, ABTS and MCA). Similar results are reported by Isik and Topkaya, who also observed increased TPC and antioxidant activity in supplemented crackers in their work on tomato pomace supplementation [62]. Dewanto et al. [63] inferred from their studies that thermal processing enhanced the nutritional value of tomatoes and produced no significant changes in the TPC and TFC of tomatoes which helps in enhancing the antioxidant potential as well. The T3 sample also delivers good amounts of minerals, especially calcium, phosphorus, and potassium. Positive calcium content maintains healthy bones [64] while potassium prevents the onset of diabetes, renal, and cardiovascular diseases [65]. 'Western' diets have ω6/ ω3 fatty acids in a ratio of up to 20:1, whereas this ratio should be low as this helps in the managemen<sup>t</sup> of obesity [66]. The RTC-GF snack (T3) has a ratio of 7.1:1, which is quite reasonable and low, as a high ratio of ω6/ ω3 is known to be a risk factor in cancers and coronary heart disease [67]. The essential amino acid composition profile of T3 shows that this RTC product is a source of good quality protein as it has valine, lysine, methionine, threonine, and tryptophan in good amounts, as recommended by the WHO [68]. Overall, it also provides sufficient energy (248.2 kcals) and hence good satiation level; thus, the RTC-GF snack (T3), besides being high on energy, also provides good nutritional value.

Lipid oxidation is often the determining factor during shelf studies of foods, since it is the cause of adverse changes in flavor and nutritive value, and has health implications as well. The peroxide value as well as the free fatty acid values of the RTC-GF snack (T3) show a very slight increase during the storage period but the values are well within the acceptable range for both the packaging materials (Figure 6a). Free fatty acids are the result of hydrolytic rather than oxidative rancidity and are used as an indicator of the storage stability of fried foods. A peroxide value of 2.0 meqO2/kg of fat is normally considered to be low and fats are free of oxidative flavor at this point [69].

The sensory evaluation of the RTC-GF snack during the storage period shows little decrease in the overall acceptability scores of the product. During par frying, the amylose and amylopectin present in the raw material form a gel by losing their crystalline structure, which when cooled at low temperature undergoes retrogradation [70]. During the finish frying of the ready-to-cook frozen snack the retrogradation is reversed and the snack regains its original crispy nature, and this is corroborated by the good overall acceptability scores obtained by the snack which show a non-significant decrease in the values (Figure 6b). The overall acceptability of the product stored in HDPE pouches had higher scores. Hence, it is assumed that the product will show good acceptance beyond the storage study period as well, but further studies are needed to corroborate this.
