Physicochemical and technological evaluation of flours made from fruit co-products for use in food products

Fruits and vegetables are widely processed, and during its processing, co-products are often discarded. Some co-products, such as peels, have a rich composition and could be used. However, the high perishability, due to high moisture content, limits this use. The reduction of moisture, achieved by drying, is an alternative to use these co-products. Flours, made from pineapple (Ananas comosus), banana (Musa sp.), lychee (Litchi chinensis) and papaya (Carica papaya) peels, through drying process, were evaluated for particle size distribution, water activity, hydrogenionic potential, titratable acidity, total soluble solids, solubility in water and in milk and water, milk and oil absorption, in order to characterize them and suggest potential applications in food products. Water activity (0.21 to 0.38) and pH (3.58 to 5.84) values were favorable for the preservation of flours, which also showed better solubility in water (17.57 to 53.52%) than in milk (4.46 to 11.22%), as well as a higher water uptake (2.16 to 6.35 g g -1 ) and milk (3.19 to 6.61 g g -1 ) compared to oil absorption (1.09 to 1.74 g g -1 ). These results indicated good potential for use in food processing, especially in instant products, meat and bakery, to develop new products or replace ingredients, representing an alternative to the use of co-products.


Introdução
The food industry is responsible for generating large quantities of waste or coproducts. Studies have shown that about 1.3 billion tons of foods are wasted worldwide per year, representing one-third of the total production of the food industry. Of this, about 0.5 billion tons are fruits and vegetables. In addition to losses in the field, great losses are observed during industrial processing (Gustavsson, et al., 2011).
In this context, there is a need to improve the full utilization of fruits and vegetables, thus minimizing the disposal and generation of co-products, and enabling their use in human nutrition rather than animal feed or organic fertilization only (Ayala-Zavala et al., 2010;Ayala-Zavala et al., 2011;Sun-Waterhouse, 2011). The use of fruit and vegetable processing co-products as raw material for the development of new food products has commercial importance, considering the interest in sustainable development of the food industry, and the consumer awareness of the benefits of natural foods (Canett-Romero et al., 2004;Yagci & Gogus, 2009;Ajila, et al., 2010;Betoret et al., 2011;Rosales Soto, Brown, & Ross, 2012).
However, these products are susceptible to microbiological degradation, mainly due to their high water content and chemical composition, impairing their utilization (Schieber, Stintzing, Research, Society and Development, v. 9, n. 3, e192932742, 2020 (CC BY 4. & Schieber, 2006). Thus, the reduction of moisture content after drying and subsequent transformation into flour can be a viable alternative for use of these coproducts (Ferreira et al., 2015).
Oven-drying is a simple technology to dry food products using moderate temperatures, below 65 °C, with no changes in the functional properties and nutritional content of fruits and vegetables and their co-products (Martínez et al., 2012). However, the use of dried products or flours as food ingredients requires the knowledge about their functionality, i.e., the properties that affect the behavior of the biomolecules, thus interfering with their use (Mizubuti et al., 2000).
Functional properties reflect the complex interaction between the composition, structure, molecular conformation and physicochemical properties of food components together with the nature of the environment in which these are associated and measured (Kinsella, 1976). However, studies that evaluate the technological properties of flours from fruit by-products are still limited.
Given the above, this study aimed to evaluate the chemical and technological characteristics of flours made from pineapple, banana, lychee, and papaya co-products and suggest possible applications in food products.

Material and methods
Ripe pineapple (Ananas comosus), banana (Musa sp.), and papaya (Carica papaya) fruits were provided by CEASA -GO (State of Goiás Supply Center), and lychee (Litchi chinensis) peels were provided by the ice cream company Frutos do Brasil.

Sampling
Ripe pineapple (Ananas comosus), banana (Musa sp.), and papaya (Carica papaya) fruits were transported in PVC (polyvinyl chloride) trays to the Laboratory of Food Chemistry and Biochemistry, Faculty of Pharmacy, Federal University of Goiás. The fruits were selected for the absence of defects, washed with mild soap and running water, sanitized with sodium hypochlorite solution 100 mL.L -1 for 15 minutes, and drained. Then, they were peeled manually with a knife. Pineapple and papaya co-products (peels + residual pulp), and banana peels were packed in low-density polyethylene bags and subjected to drying and physicochemical characterization.
Previously sanitized lychee (Litchi chinensis) peels from pulp processing were packed in low-density polyethylene bags, and transported to the Laboratory of Food Chemistry and Biochemistry, Faculty of Pharmacy, Federal University of Goias, for drying and physicochemical characterization.
Fruit peels and co-products were dried in forced air drying oven at 55 °C to constant weight. After drying, peels were ground in micro knife mill Willye type (Tecnal, TE-648), resulting in pineapple co-product flour (FCA), banana peel flour (FCB), lychee peel flour (FCL), and papaya co-product flour (FCM).

Particle Size
The particle size of flours was determined according to the methodology described by Zanotto and Bellaver (1996), with modifications. For that, 50 g sample was sieved in a set of sieves with 1000, 710, 500, 250, 150, and 106 µm in diameter (corresponding to 16, 24, 32, 60, 100 and 150 mesh, respectively) for 10 minutes, using an electromagnetic agitator (Bertel) under constant stirring and speed. The fraction retained in each sieve was calculated using Where R% = fraction retained on each sieve, and index = fixed value decreasing from 6 to 0, following the decreasing order of aperture size.

Hydrogenionic potential (pH)
The hydrogen ionic potential (pH) was measured using a digital potentiometer (pHTEK, PHS-3B) calibrated with pH 4.0 and 7.0 buffer solutions, by direct immersion of the electrode into the beaker containing the sample macerated with distilled water, according to the methodology proposed by the AOAC (2012).

Titratable acidity
The titratable acidity was determined by titration with 0.1N sodium hydroxide (NaOH) using phenolphthalein as an indicator (AOAC, 2012), and the values were expressed as grams of citric acid per 100 grams of sample.

Total soluble solids
The total soluble solids content was determined at 20 °C by readings of degree Brix in digital refractometer (Reichert, AR200), according to the method proposed by AOAC (2012).

Water solubility index and milk solubility index
The solubility of flours in water and in milk was determined according to adaptations by Okezie & Bello (1988). For water solubility index, a suspension with 25 mL water and 0.5 g flour was stirred by vortexing for 1 minute and then centrifuged at 5300 rpm for 20 minutes in a centrifuge (Eppendorf 5403). To determine the milk solubility index, a suspension with 25 mL water and 0.5 g flour was stirred and centrifuged at 3000 rpm for 10 minutes at 4 °C.
The supernatant was drained into a pre-weighed Petri dish and dried in an oven. The solubility index was calculated by the weight of dry solids in the supernatant expressed as a percentage of the original weight of the sample, according to Equation 3:

Water absorption index (WAI)
The water absorption index (WAI) of flours was determined according to adaptations by Okezie & Bello (1988). A suspension with 25 mL of water and 0.5 g flour was stirred by vortexing for 1 minute and then centrifuged at 5300 rpm for 20 minutes in a centrifuge (Eppendorf 5403). The supernatant was drained, and the remaining material was weighed.
The difference between the sample weight before and after water absorption represented the Research, Society and Development, v. 9, n. 3, e192932742, 2020 (CC BY 4.

Milk absorption index (MAI)
The milk absorption index (MAI) of flours was determined according to the methodology described in section WAI centrifuging the suspension containing 25 mL of milk and 0.5 g of sample at 3000 rpm for 20 minutes, 4 ºC. The milk absorption index was calculated according to Equation 5:

Oil absorption index (OAI)
The oil absorption index (OAI) was determined according to the methodology described in section WAI, using 25 mL of soybean oil. The oil absorption index was calculated according to Equation 6: (Equation 6)

Statistical Analysis
The experiment was carried out in a completely randomized design. The analyses were performed in triplicate, with four replications. The results were subjected to analysis of variance (ANOVA) and Tukey`s test to compare means, with the aid of SISVAR software, using a significance level of 5%. The values were expressed as mean ± standard deviation.

Results and Discussion
The results of the particle size of flours and fineness modulus are presented in Table 1.
According to the fineness modulus, flours can be classified as coarse (≥ 4.10 FM), medium (FM = 3.20), fine (FM = 2.30), and very fine (FM ≤ 1.50) (Ortolan, 2006). Thus, the samples FCA, FCB, and FCL (FM = 2.67, 2.91, and 2.74, respectively) were classified as fine to medium flours, while the sample FCM (FM = 3.32) was classified as medium to coarse flour. Flour particle size can vary depending on the milling process, and the heterogeneity can compromise the final quality of the manufactured products (Linden and Lorient, 1994).
The particle size homogeneity promotes proper and uniform cooking, and prevents hardness and partial cooking, which affect the quality of the product, both in appearance and palatability (Ramirez & Wanderlei, 1997). Although the particle size is influenced by the milling process, it can be easily standardized in the sieving step. However, Martínez et al. (2012) have emphasized that large particle sizes are more advantageous in maintaining the hydration and texture characteristics of the product.
All flours had low water activity (Aw), with values ranging from 0.21 to 0.38, as shown in Table 2. The free water, represented by the water activity, is directly related to the physicochemical and microbiological changes in foods during storage, which interferes with their conservation (Rockland & Nishi, 1980), once water molecules weakly associated with other food constituents can effectively participate in degradation reactions when compared to the strongly associated water molecules (Damodaran et al., 2010).
According to Alzamora et al. (2003), Aw values lower than 0.60 prevent microbial spoilage, once they prevent the growth of microorganisms. Thus, the samples of the present study are below the limit for microbial growth. Research, Society and Development, v. 9, n. 3, e192932742, 2020(CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i3.2742  The pH of the samples are shown in Table 2 and ranged from 3.58 to 5.84, which classify the samples as acidic or slightly acidic flours. The titratable acidity values are also presented in Table 2, and ranged from 1.36 to 2.62 g citric acid 100 g -1 .
The acidity of the flours analyzed in this work showed a result compatible with Brazilian legislation, which establishes a maximum limit of 5% for flour acidity (Brasil, 2005). Acidity is an important quality parameter, especially because it prevents microbial growth and enzymatic reactions, which affects the stability and quality of food products, as well as influencing food flavor (Cecchi, 2003;Souza et al, 2008). During storage, the assessment of acidity enables observation of product's conservation, since the decomposition process by hydrolysis or oxidation often changes the sensory and nutritional characteristics of the product (Ferreira et al., 2015). In addition, it is an essential measurement for use of a interactions between biomolecules, and between biomolecules and water. Flours were more soluble in water than in milk, since the milk solubility indices varied from 4.46 to 11.22%.
Solubility is a physical-chemical property influenced by the greater, or better, affinity of protein molecules for the solvent (Sgarbieri, 1996), which in the case of flour, was water, therefore, it is recommended to use it in products that need this characteristic, for example, liquid foods, instant soups, drinks, among others.