Plant Foodstuff By-Products as Valuable Products in Different Industries

1. The Impact of Farm and Industrial Feed Waste on the Safety Parameters of Tenebrio molitor Larvae

The increasing global demand for animal-based food is contributing to significant ecological challenges. Such challenges are further intensified by substantial food waste, which accounts for nearly one-third of all food produced [1]. This study investigated the feasibility of replacing conventional, nutritionally balanced feed with sustainable alternatives derived from agricultural, grain processing, and brewing by-products, thereby enhancing agricultural sustainability. Mealworm larvae were reared on four different substrates: (1) agar-agar gels, wheat bran, and brewer’s yeast; (2) carrots, wheat bran, and brewer’s yeast; (3) sprouted potatoes, wheat bran, and brewer’s yeast; and (4) carrots, brewers’ spent grain, and brewer’s yeast. The larvae were frozen, lyophilized, and analyzed for chemical safety in three accredited laboratories, with results indicating that pesticide levels in all samples were below the detection limit. Although polycyclic aromatic hydrocarbons (PAHs) have not been extensively studied in this context, this research detected PAHs in both the substrate and larvae. However, no significant accumulation was observed, with the sum of benzo(a)pyrene, benz(a)anthracene, benzo(b)fluoranthene, and chrysene reaching only 0.0007 mg/kg. Furthermore, the total PAH content, as well as benzo(a)pyrene and histamine levels, remained within the recommended safety limits. The highest concentration of heavy metals, particularly chromium (1.45 ± 0.02 mg/kg), was detected in larvae reared on brewer’s by-products. While plant-derived by-products are widely used in insect-rearing studies, glycoalkaloid accumulation has not been thoroughly explored. Among the glycoalkaloids analyzed—tomatidin, tomatine, α-solanine, α-chaconine, and solanidin—the highest concentrations were recorded in larvae fed sprouted potatoes, with α-solanine at 175.12 ± 0.21 mg/kg and α-chaconine at 139.32 ± 0.32 mg/kg. Additionally, mealworm larvae exhibited significantly higher levels of putrescine, tyramine, spermine, and spermidine compared to their respective substrates, while histamine levels were significantly lower in larvae than in the substrate. Despite the presence of certain toxic substances in by-product-based substrates, findings suggest that mealworms did not accumulate excessive toxin levels, ensuring compliance with safety regulations [2].

2. Effect of Storage Conditions on Physical Properties, Lipid Oxidation, Isoflavones and Antioxidant Capacity of Flour Prepared from Soy Milk By-Product

During soy milk production, a nutrient-rich by-product is generated that is often discarded as waste [3]. It contains high levels of dietary fiber, proteins, isoflavones, and antioxidants, making it a valuable functional ingredient when dried at 100 °C and milled into flour. However, its shelf-life stability under varying storage conditions, including temperature (2 °C, 20 °C, and 40 °C) and initial moisture content (9%, 12%, and 14%), remains largely unexplored, particularly when stored either packaged or exposed to the environment. This study examined the effects of a ten-week storage period on the physical properties, lipid oxidation, antioxidant capacity, and isoflavone stability of the flour. The results indicated that environmental exposure significantly impacted its stability, particularly affecting moisture content, water activity, isoflavone concentration, and lipid oxidation. Initial moisture content played a crucial role in determining both the initial and final color, as well as moisture and water activity, when stored in covered conditions at 2 °C and 20 °C. Storage temperature was a key factor influencing moisture retention, lipid oxidation, antioxidant capacity, and isoflavone conversion across all moisture levels. Notably, higher temperatures and extended storage durations accelerated the conversion of malonyl isoflavones to β-glucosides, while aglycone levels remained relatively stable. Overall, the study concluded that while initial moisture content had a limited impact on flour stability, high storage temperatures and exposure to the environment significantly reduced its shelf life [4].

3. Pilot-Scale Protein Recovery from Cold-Pressed Rapeseed Press Cake: Influence of Solid Recirculation

The agricultural sector contributes approximately 30% of global greenhouse gas emissions, highlighting the need for sustainable plant-based protein sources with a lower environmental footprint [5]. Rapeseed press cake, a by-product of rapeseed oil production, contains 30% high-quality protein, making it a promising alternative. This study aimed to recover protein from cold-pressed rapeseed press cakes on a pilot scale using a decanter and to assess the impact of recirculating the spent solid fraction on protein yield. Protein extraction was conducted under alkaline conditions (pH 10.5), followed by precipitation at pH 3.5. Recirculating the spent solid fraction once increased the accumulated protein yield from 70% to 83%, with the highest recovery efficiency observed in the first two cycles. However, additional (third and fourth) cycles contributed only a 2% increase in yield. The amino acid profile remained rich in essential amino acids throughout the process. Furthermore, glucosinolate and phytate levels were reduced in the precipitate after one cycle; however, additional processing steps are required to further lower phytate content and minimize its negative effects on mineral absorption [6].

4. Effect of Parinari curatellifolia Peel Flour on the Nutritional, Physical and Antioxidant Properties of Biscuits

This study examined the effects of incorporating Parinari curatellifolia peel flour on the nutritional, physical, and antioxidant properties of biscuits. Parinari curatellifolia is a fruit-bearing tree species native to tropical regions of Africa, with its natural distribution spanning from the western parts of the continent, such as Senegal, to the southern regions [7]. Formulations were prepared with 5%, 10%, 15%, and 20% P. curatellifolia peel flour, and their thermal, physicochemical, polyphenolic, and antioxidant properties were evaluated. The results showed that the addition of P. curatellifolia peel flour significantly increased (p < 0.05) its thermal properties, including onset, peak, and conclusion temperatures. However, the enthalpy of gelatinization, viscosity, and pH decreased. Nutritional analysis revealed higher ash (0.74% to 2.23%) and crude fiber (0.39% to 2.95%) contents with increasing levels of P. curatellifolia peel flour, while protein and carbohydrate contents declined. Moisture content remained largely unaffected. In terms of color, the L* and b* values, along with the whiteness index, decreased, whereas the a* value (10.76 to 21.89) and yellowness index (69.84 to 102.71) increased. Physical characteristics such as biscuit diameter (3.57 mm to 3.97 mm), spread ratio (2.67 to 3.45), and hardness (1188.13 g to 2432.60 g) increased, while weight and thickness decreased. The inclusion of P. curatellifolia peel flour significantly enhanced the polyphenolic content and antioxidant activity of the biscuits. Total flavonoid content ranged from 0.028 to 0.104 mg CE/g, total phenolic content increased from 20.01 mg to 48.51 mg GAE/g, ferric reducing antioxidant power (FRAP) improved from 108.33 mg to 162.67 mg GAE/g, and DPPH scavenging activity increased from 48.70% to 94.72%. These findings support the potential use of P. curatellifolia peel flour as a functional ingredient to enhance the nutritional value, polyphenolic content, and antioxidant activity of bakery products such as biscuits [8].

5. Reused Plant Fried Oil: A Case Study with Home-Made Soaps

The study investigated the feasibility of using waste frying oil for homemade soap production. Driven by technological and petrochemical advancements to meet growing population demands, in recent decades, the detergent industry has rapidly expanded, reaching a global market size of USD 175.6 billion in 2017, with continued annual growth projected [9]. Soaps were prepared from both unheated and fried rapeseed, sunflower, and palm oils, all of which had total polar material (TPM) values up to 24%. The physicochemical and microbial qualities of the produced soaps were assessed through various analyses. The results showed that soap hardness increased with the degradation of rapeseed and palm oils, while the opposite effect was observed for sunflower oil-based soaps. These soaps had the highest malondialdehyde (MDA) content, reaching a maximum of 6.61 µg/g, while palm oil soaps had the lowest, with a maximum of 0.94 µg/g. Microbial testing revealed no significant (p > 0.05) differences in antimicrobial activity between control soaps and those made with oils with the highest TPM values. The most sensitive microorganisms were Gram-positive bacteria, particularly methicillin-resistant Staphylococcus aureus (MRSA), when compared to Gram-negative bacteria and yeasts. The study found no significant differences between soaps made from fried or unfried oils, emphasizing the potential use of this by-product in homemade soap production [10].

6. Conclusions

The studies discussed here underscore the growing importance of sustainable food production and waste valorization in addressing global environmental and food security challenges. The safe rearing of Tenebrio molitor larvae on agricultural by-products highlights the potential of alternative feed sources in insect farming. Meanwhile, the stability of soy milk by-product flour under different storage conditions emphasizes the need for optimized preservation techniques to enhance its shelf life. The pilot-scale recovery of protein from rapeseed press cake demonstrates the feasibility of upscaling plant-based protein extraction, contributing to more efficient resource utilization. Additionally, the incorporation of Parinari curatellifolia peel flour in biscuit formulations enhances their nutritional and antioxidant properties, promoting the use of functional ingredients derived from food waste. Finally, the successful repurposing of used frying oils for homemade soap production supports circular economy principles and waste reduction strategies. Collectively, these findings emphasize the need for continued research and interdisciplinary collaboration to develop innovative, sustainable solutions that maximize the utilization of food industry by-products while ensuring safety and efficiency.