## 3 Results and Discussion
### Shelf Life Analysis of Produced Cookies
#### 3.4.3 Peroxide Value  
Peroxides are the primary oxidation products of fats, which are not perceptible organoleptically since they are tasteless and odorless but show high toxicity and promote biological oxidations in the human body. Therefore, the concentration of peroxides is one of the most important indicators of the quality of fats and oils. The maximum acceptable PV was set at 20 meq O\({}_{2}\)/kg oil [53]. The change in Peroxide Value (PV) of control, 4% SCG-added, and 7% SCG-added cookies as a function of storage time (over a period of 143 days) at three temperatures (25, 35, and 45 \({}^{\circ}\)C) is presented in Figure 5a-c. The plots show a characteristic form: initially, the oxidation products increase at a slow rate due to the \"resistance\" of the oil to oxidation, but then the slope of the resulting curve increases rapidly. The PV of control cookies for all temperatures was under the reported upper limit for the tested period. However, the PV for 4% SCG-added and 7% SCG-added cookies stored at 45 \({}^{\circ}\)C exceeded the PV limit at approximately 120 and 75 days, respectively.  
With the incorporation of the defatted SCG into the cookie recipe, it was attempted to develop a high-phenol and high-fiber bakery product. The contained polyphenols, together with their functionality for human health, could protect the food from oxidation and thus extend its shelf life. This was not observed, and it might be reasonable since the incorporated phenolics were highly polar, and therefore, their distribution in the lipid phase was unfavorable. The higher the concentration of SCG in the cookies, the faster the increase in PV. This may be attributed to the increase in the concentration of metal ions (e.g., Cu) in the coffee residue, resulting in pro-oxidation phenomena [54]. However, the above behavior was detected under accelerated oxidation conditions at 45 \({}^{\circ}\)C. At lower temperatures, all cookies were stable within the frame of 143 days of monitoring.  
[PERSON] et al. (2023) [55] incorporated SCG at concentrations between 2 and 10% in shortbread biscuits. They studied the change in PV of the extracted fat from shortbread samples with different SCG contents upon storage for three weeks at room temperature. They noted a gradual decrease in PV with the increasing level of SCG incorporation in shortbread biscuits. The same behavior can be observed in Figure 5, since the PV of control cookies, stored at 25 \({}^{\circ}\)C, for 39 days is higher than the respective values of 4% SCG-added and 7% SCG-added cookies. The differences presented by [PERSON] et al. (2023) (t = 21 d, \(\theta\) = 25 \({}^{\circ}\)C; PV\({}_{\text{control}}\) = 9.5, and PV\({}_{\text{SCG}\,8\%}\) = 3.5) as well as in the current research (t = 39 d, \(\theta\) = 25 \({}^{\circ}\)C; PV\({}_{\text{control}}\) = 7.0, and PV\({}_{\text{SCG}\,7\%}\) = 5.3) are rather insignificant, compared with the major changes of PVs occurring at higher storage times, as presented in Figure 5. To the best of our knowledge, there is no other published study examining the oxidation of bakery products with incorporated SCG for periods comparable to the current research.  
Figure 5: Peroxide Value (PV) of oil extracted from cookies vs. storage time for (**a**) the control cookie, (**b**) the 4% SCG-added cookie, and (**c**) the 7% SCG-added cookie at 25 \({}^{\circ}\)C, 35 \({}^{\circ}\)C, and 45 \({}^{\circ}\)C. Red horizontal dashed line: maximum acceptable Peroxide Value (20 meq O\({}_{2}\)/kg oil). The number of replications = 3.