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Coke, McDonald's sales slips: blame the millennials?

Two big-cap companies, McDonald's and Coca-Cola, reported disappointing earnings Tuesday. One fear among investors is a disinterest among millennials in such iconic brands.

McDonald's reported its worst monthly comparable sales decrease in the U.S. and Europe since early 2003, while Coca-Cola unveiled a cost-cutting program as its worldwide soda volume was flat and revenue slipped in the third quarter.

Ilana Stern, a millennial retail expert and founder of Weddington Way, an e-commerce platform for millennial consumers, told CNBC that being connected and giving context should be top of mind for attracting the generation to spend. She said brands need to understand that the generation interacts differently than previous ones.

"It's not about push-marketing and brand endorsement. They value the opinions of their peers and community," she said in an interview with "Power Lunch."

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Customers order food at a McDonald's Corp. restaurant in the pedestrianized Dongmen area of Shenzhen, China.
Brent Lewin | Bloomberg | Getty Images
Customers order food at a McDonald's Corp. restaurant in the pedestrianized Dongmen area of Shenzhen, China.

"Millennials have a unique dynamic and will splurge on items important to them or for brands with similar values, but will also look for the budget-friendly options," she added, pointing to a recent Goldman Sachs and Teen Vogue study that found that women ages 19-36 value both Target and Louis Vuitton as top 20 brands.

Of course, McDonald's and Coke have been trying.

McDonald's recently released YouTube videos "Our Food. Your Questions" to keep the consumers informed on their food processes and ingredients as a response for losing the millennial customer to "fast-casual" places such as Panera and Chipotle.

Read MoreCoke is 'not a well-run company,' says Cramer

Meanwhile, Coke's recent "Share a Coke" campaign allowed consumers to find beverages with their name and share photos on social media, a move that Stern says was good for the brand.

"Seeing your name on a brand provides an instant personal connection," she said. However, "can they create an ongoing conversations or is it just one-off campaigns?"

Read MoreA bigger, better consumer market than millennials

Stern suggested that companies remember millennials' traditional values, which she sees as happiness, passion, diversity, sharing and discovery.

"Don't just market to them: Millennials want to feel informed and involved. They're more likely to engage in group activities as they shop with friends and coworkers, to whom they look for validation that they've made the right decisions," she said.

If there is in fact a shift in consumer spending among big-cap companies, Stern said it's heading toward newer companies that are more trustworthy. "Look at a brand like Chobani and how it disrupted a company like Yoplait."